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The deep structure and reactivation of the Kyrgyz Tien Shan: Modelling the past to better constrain the present
Abstract
In the Tien Shan belt, Cenozoic to active deformation is guided by structures and rheological contrasts partly inherited from the Paleozoic Variscan orogeny and reactivated by the India-Asia collision. Cenozoic deformation net estimates are constrained by lithospheric scale geological cross-sections, which rely on reconstructions of the Paleozoic Central Asian Orogenic Belt (CAOB). However, several geodynamic scenarii have been proposed for its formation including a south or a north dipping subduction, which are still debated due to lack of constraints. Here, we designed numerical experiments to test different hypotheses about the initial geometry and the effective rheology of the paleo-sutures and especially the one located between the Tien Shan and the Tarim basin, where most Cenozoic deformation localizes. The different geometries of Paleozoic structures are used as input variables in the thermo-mechanical models, which are then run forward in time. After a finite amount of shortening, the structures that develop self-consistently out of the proposed heterogeneities are then compared to the current Cenozoic finite and active deformations. We find that a crustal south-dipping suture zone lying on a resistant lithospheric mantle best explains the localization of the deformation, the current geometry of the structures and the Moho depth variations. Using the model results, we propose a mechanically consistent depth-interpolated crustal cross-section of the Kyrgyz Tien Shan, which incorporates both geological and geophysical data.
Supplementary resource (1)
... The southwestern Tian Shan, which was reactivated due to the north-dipping subduction of the Tarim Basin and the south-dipping subduction of the Kazakh Platform (Jourdon et al., 2017;Yu et al., 2017), is one of the most intense areas of tectonic deformation within the Eurasian continent. The N-S crustal shortening rate across the western Tian Shan has been estimated to be 18-20 mm/yr (e.g., Abdrakhmatov et al., 1996;Molnar and Ghose, 2000;Yang et al., 2008;Zubovich et al., 2010). ...
... All rights reserved. (Yin et al., 1998;Allen et al., 1999;Burchfiel et al., 1999;Qu et al., 2003;Jourdon et al., 2017). An amount of crustal shortening of 23-45 km in the foreland thrust fault and fold belts was reported based on the balanced geological section (Yin et al., 1998;Allen et al., 1999). ...
... Previous studies have determined a sinistral strike-slip rate of ~2 mm/yr for this fault (e.g., Zhou, 2013). Adjacent thin-skinned thrust belts reach only a few tens of kilometers into the Kazakh Platform, the thin-skinned back-thrust also extends into the mountains for a few tens of kilometers ( Figure 17; Selander et al., 2012;Goode et al., 2014;Jourdon et al., 2017). The Kemin-Chilik fault acts as a central structure of a dominantly north-south contractional system, and its sinistral slip occurs together with adjacent thrust faulting (Selander et al., 2012). ...
The Maidan fault, which is an east‐northeast trending fault in the southwestern Tian Shan, is a sinistral reverse fault that extends more than 400 km in length and constitutes the boundary between the southwestern Tian Shan and the Tarim Basin. Here we quantify its late Quaternary activity based on the interpretations of high‐resolution remote sensing images and detailed field investigations. In the Aheqi valley, an ~150‐km‐long active fault can be divided into northeastern and southwestern segments based on variations in its strike and geometry. Based on the analysis of its offset geomorphological features and the dating of Quaternary sediments, we estimate the late Quaternary shortening rate across the fault to be 1.19 ± 0.25 mm/year, the sinistral strike‐slip rate to be 1.56 ± 0.64 mm/year, and the oblique thrust rate to be 1.96 ± 0.69 mm/year. Active tectonics, GPS crustal deformation data, and seismic activity indicate that the deformation in the southwestern Tian Shan is characterized by out‐of‐sequence thrust faulting and folding. Late Quaternary deformation has been partitioned into low‐angle thrust faulting along the Kalpin Tagh foreland fold and thrust system and sinistral reverse faulting along the high‐angle range‐front Maidan fault. The sinistral Maidan fault acts as a nucleation point for slip partitioning system, which can be viewed as positive flower structure with its surrounding thrust faults.
... The impact of inherited mechanical properties of the lithosphere is recognized to be a major driver in the evolution of orogenic belts (Beaumont et al., 2009;Calignano et al., 2015a;Chenin et al., 2017;Heron et al., 2016aHeron et al., , 2016bJourdon et al., 2018a;Manatschal et al., 2015;Velasque et al., 1989;Yamato et al., 2008). Geological, geophysical and numerical modelling studies show for instance that age-dependent properties of the lithospheric mantle and crust control first-order patterns of shortening distribution, cyclical evolution and seismicity in orogens (Audet and Bürgmann, 2011;Mouthereau et al., 2013). ...
... Geological, geophysical and numerical modelling studies show for instance that age-dependent properties of the lithospheric mantle and crust control first-order patterns of shortening distribution, cyclical evolution and seismicity in orogens (Audet and Bürgmann, 2011;Mouthereau et al., 2013). While lateral heterogeneities in the lithospheric mantle control the location of intracontinental orogenic belt (Calignano et al., 2015b;Heron et al., 2016aHeron et al., , 2016b, the local mechanical coupling between crust and mantle or within the crust controls the tectonic style (Jourdon et al., 2018a;Lafosse et al., 2018). ...
... Numerous numerical modelling studies have been performed to understand the role of the rheology and of the properties and architecture of continental lithosphere during collision (Beaumont et al., 2009(Beaumont et al., , 2001Burg and Gerya, 2005;Burov et al., 2014Burov et al., , 2011Burov et al., , 2001Duretz et al., 2012;Duretz and Gerya, 2013;Dymkova et al., 2016;Jourdon et al., 2018a;Warren et al., 2008;Yamato et al., 2008). However, the majority of these models did not account for the complexity of rifted margins. ...
Many relics of passive margin are found in mountain belts sometimes together with pristine exhume mantle. The reasons why these structures are preserved from tectonic reworking and whether the piece of mantle that accompanies them is exhumed during extension or collision remains a question. Using thermo-mechanical modelling we find on the one hand that, the strain distribution in the orogenic wedge is controlled by the original position of crustal domains across the rifted margin. In the upper plate, the structural relationships acquired during the rifting phase between the upper mantle and surrounding units are never reactivated during compression due to crustal strain localization. Mantle exhumation on the other hand can occur either during the collision or rifting phase depending on the thermal regime at the onset of the inversion. Applying these general rules to the Pyrenees, we show that exhumed mantle domain in the former passive margin could not exceed a width of 50 km to 70 km; otherwise obduction would have occurred due to the high thermal regime at the onset of the inversion.
... Large NW-SE transcurrent shear zones compartment the Tien Shan orogeny in blocks (Choulet et al., 2012;Laurent-Charvet, Charvet, Shu, Ma, & Lu, 2002;Peltzer & Saucier, 1996) which display cylindrical deformation. The Kyrgyz Tien Shan, located between 75°E and 81°E (Figure 1), is one of them Jourdon, Le Pourhiet, Petit, & Rolland, 2017;Macaulay, Sobel, Mikolaichuk, Kohn, & Stuart, 2014). In this region, Cenozoic N-S to NW-SE shortening has been accommodated by a double-vergent orogen bordered by a main north-dipping thrust located at the border of the Tarim (Makarov et al., 2010), and two south-dipping thrusts. ...
... In this region, Cenozoic N-S to NW-SE shortening has been accommodated by a double-vergent orogen bordered by a main north-dipping thrust located at the border of the Tarim (Makarov et al., 2010), and two south-dipping thrusts. A late Oligocene thrust separates the North Tien Shan unit and the Lake Issyk-Kul basin (Macaulay et al., 2014) and a Miocene thrust reactivates the South Tien Shan Palaeozoic suture (Buslov, De Grave, Bataleva, & Batalev, 2007;Glorie et al., 2010Glorie et al., , 2011Jourdon, Le Pourhiet, et al., 2017). At a large scale, this orogen surrounded by two parallel flexural basins is locally quite cylindrical (Avouac et al., 1993;De Grave et al., 2011;Macaulay et al., 2014). ...
... Yet, the structures' original orientation is largely debated (Biske, 1995;Charvet et al., 2011;Gao, Li, Xiao, Tang, & He, 1998;Windley, Alexeiev, Xiao, Kr€ oner, & Badarch, 2007). Jourdon, Le Pourhiet, et al. (2017) In pTatin2d, like in many thermo-mechanical codes, the sedimentation and erosion is approximated by Culling's (1965) law: Comp. ...
Evolving mountain belts dynamics is very sensitive to surface processes. They affect tectonics by enhancing crust exhumation and thermal weakening, and depositing soft yet cold sediments in surrounding basins. While 2D plane strain models approximate cylindrical tectonic structures well, simple 1D mass transfer cannot capture erosion‐sedimentation complexity. The Eastern Kyrgyz Tien Shan where structures, basins and exhumation rates are well constrained is used here to illustrate this issue. Thermo‐mechanical models demonstrate that 1D transport cannot adjust both basin geometry and AFT exhumation ages. When out‐of‐plane sediment transfer is considered, the amount of evacuated sediment delays or accelerates the formation of new faults, affecting the relative timing of exhumation. For our case study, lateral drainage must evacuate 80% of the sediments to match the geological constraints, which is consistent with other source to sink analyses. This indicates that lateral drainage should not be neglected in regional 2D models.
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... We do not introduce a source term, which can be used to account for local carbonates production or out-of-plane sediment infill [e.g. Jourdon et al., 2018b, Perron et al., 2021. The basal boundary is prescribed an inflow to compensate for the outflow imposed by the extensional velocity varying along the lateral boundaries. ...
... We use the thermo-mechanical numerical code pTatin [May et al., 2014[May et al., , 2015 in its 2D version [Jourdon et al., 2018b, Perron et al., 2021, where two new components are implemented, respectively a restart module and a kinematic module. This version of the open-source code is available publicly at https://bitbucket.org/ptatin/ptatin2d/src/restart/. ...
... Numerous previous studies have shown the potential for mantle lithosphere structures to control the evolution of shallow tectonics (Balázs et al., 2018;Heron et al., 2019;Jourdon et al., 2017;Pysklywec & Beaumont, 2004;Phillips et al., 2018;Salazar-Mora et al., 2018;Schiffer et al., 2018), highlighting a deep genesis for lithosphere-scale deformation (e.g., Holdsworth et al., 2001;Vauchez et al., 1997). Reactivation of features formed through previous collisional or rifting events (Wilson, 1966) is well established and thought to occur along well-defined, preexisting structures such as faults, shear zones, or lithological contacts (Holdsworth et al., 1997). ...
... It should be noted that it is indeed unexpected that applying a North Atlantic Craton mantle suture (Figure 3b) in the presence of an extension field that is relevant in velocity and orientation to the Paleogene (Figure 4) would produce appropriate rift dynamics for the Davis Strait system ( Figure 6). However, the study here complements a growing body of work that highlights the potential of the mantle lithosphere to play an important role in tectonic processes (Pysklywec & Beaumont, 2004;Babuška & Plomerová, 2013;Jourdon et al., 2017;Salazar-Mora et al., 2018;Phillips et al., 2018;Balázs et al., 2018;Heron et al., 2019). ...
Mesozoic‐Cenozoic rifting between Greenland and North America created the Labrador Sea and Baffin Bay, while leaving preserved continental lithosphere in the Davis Strait, which lies between them. Inherited crustal structures from a Palaeoproterozoic collision have been hypothesized to account for the tectonic features of this rift system. However, the role of mantle lithosphere heterogeneities in continental suturing has not been fully explored. Our study uses 3‐D numerical models to analyze the role of crustal and subcrustal heterogeneities in controlling deformation. We implement continental extension in the presence of mantle lithosphere suture zones and deformed crustal structures and present a suite of models analyzing the role of local inheritance related to the region. In particular, we investigate the respective roles of crust and mantle lithospheric scarring during an evolving stress regime in keeping with plate tectonic reconstructions of the Davis Strait. Numerical simulations, for the first time, can reproduce first‐order features that resemble the Labrador Sea, Davis Strait, Baffin Bay continental margins, and ocean basins. The positioning of a mantle lithosphere suture, hypothesized to exist from ancient orogenic activity, produces a more appropriate tectonic evolution of the region than the previously proposed crustal inheritance. Indeed, the obliquity of the continental mantle suture with respect to extension direction is shown here to be important in the preservation of the Davis Strait. Mantle lithosphere heterogeneities are often overlooked as a control of crustal‐scale deformation. Here, we highlight the subcrust as an avenue of exploration in the understanding of rift system evolution.
... Here, the ML scar for the reference model ML is 10 km thick, dipping at an angle of 15° from the horizontal ( Fig. 2A), and rheologically weak by having a reduced angle of internal friction compared to the surrounding material. The influence of changing shape and dip angle of generic styles of such weak scars is explored in detail in Heron et al. (2016b), Jourdon et al. (2017), and the Data Repository. ...
... Our mechanism for Ouachita deformation has broad implications for lithosphere geodynamics and supplements recent studies on the role of mantle heterogeneities. In keeping with an extensive suite of simulations analyzing the importance of lithosphere rheology in the role of mantle lithosphere scars (Heron et al., 2016b), Jourdon et al. (2017) also found that the reactivation of a mantle suture strongly influences the localization and deformation of tectonics. Hansen and Nielsen (2003) presented numerical models of shortening in the presence of a preexisting crustal rift basin, which developed "marginal troughs" in a flexure response to crustal thickening (e.g., Sydorenko et al., 2017). ...
Mantle lithosphere heterogeneities are well documented, are ubiquitous, and have often been thought to control lithosphere-scale deformation. Here, we explore the influence of deep scarring in crustal deformation in three dimensions by considering the Ouachita orogeny in the southeastern United States, an example of a continental collision where mantle structure is present but not previously linked to the regional crustal tectonics. We present state-of-the-art continental compressional models in the presence of inherited three-dimensional lithospheric structure. Our models find that the surface expression of the Ouachita orogeny is localized by, and projected from, the controlling mantle scarring, in keeping with geological and geophysical observations. We are able to produce a large-scale arcuate orogeny with associated basin development appropriate to the Ouachita orogeny, alongside smaller-scale crustal faulting related to the region. This study offers a new and alternative hypothesis to the tectonic history of the Ouachita orogeny, with previous research having focused exclusively on crustal structures. The findings have broad implications, demonstrating the important potential role of the mantle lithosphere in controlling crustal dynamics and highlighting the requirement to consider deeper structure and processes when interpreting tectonic evolution of lithospheric-scale deformation.
... In contrast, intraplate orogens form within a continental plate by localization of strain along preexisting weaknesses (e.g., Vilotte et al., 1982;Ziegler et al., 1995;Raimondo et al., 2014). Some intraplate orogens reactivate weaknesses inherited from past collisions (e.g., the Tien Shan [Central Asia]; Jourdon et al., 2018), whereas others exploit weaknesses developed during continental rifting and thus are considered the result of rift inversion (Fig. 1;e.g., Cooper et al., 1989;Beauchamp et al., 1996;Marshak et al., 2000). A common presumption seems to be that the structural style of intracontinental rift-inversion orogens should be distinct from that of plate-boundary orogens, because during rift inversion, convergence is expected to occur by reactivation of extensional structures, resulting in distributed lithospheric thickening (e.g., Buiter et al., 2009;Vincent et al., , 2018. ...
Although many collisional orogens form after subduction of oceanic lithosphere between two continents, some orogens result from strain localization within a continent via inversion of structures inherited from continental rifting. Intracontinental rift-inversion orogens exhibit a range of structural styles, but the underlying causes of such variability have not been extensively explored. We use numerical models of intracontinental rift inversion to investigate the impact of parameters including rift structure, rift duration, post-rift cooling, and convergence velocity on orogen structure. Our models reproduce the natural variability of rift-inversion orogens and can be categorized using three endmember styles: asymmetric underthrusting (AU), distributed thickening (DT), and localized polarity flip (PF). Inversion of narrow rifts tends to produce orogens with more localized deformation (styles AU and PF) than those resulting from wide rifts. However, multiple combinations of the parameters we investigated can produce the same structural style. Thus, our models indicate no unique relationship between orogenic structure and the conditions prior to and during inversion. Because the style of rift-inversion orogenesis is highly contingent upon the rift history prior to inversion, knowing the geologic history that preceded rift inversion is essential for translating orogenic structure into the processes that produced that structure.
... In view of the results of this study, Cretaceous reactivation may have extended from the Tianshan belt (including the Beishan) (e.g., and this study), through the East Junggar (this study) to the Siberian Altai of the interior Eurasian plate in the north (e.g., Glorie et al., 2012a;. The role of inherited Paleozoic major fault zones in the strain partitioning was paramount as they were (repeatedly) reactivated and thus controlled the exhumation process (e.g., Jourdon et al., 2018b). and this study). ...
The Central Asian Orogenic Belt (CAOB) lies between the Baltica, Siberia and Tarim-North China cratons, and is one of the largest Phanerozoic orogenic belts on Earth. The development of the CAOB initiated in the Neoproterozoic and it further grew during the Paleozoic via the accretion of various island arcs, seamounts, accretionary wedges and micro-continents. This vast orogenic system was eventually amalgamated by the final closure of the Paleo-Asian Ocean during the late Paleozoic, resulting in the docking of the Tarim-North China craton from the south. Since the late Paleozoic, parts of the CAOB (e.g., the Tianshan and Altai) have served as typical examples of intra-continental orogens where the relationship between plate margin processes and the occurrence of compressive intra-plate deformation can be studied. Throughout the Mesozoic, the southwestern (SW) part of the CAOB experienced several major periods of intra-continental deformation, which have been interpreted to be related with a series of Cimmerian collisions (e.g., the collisions of Qiangtang, Lhasa and Karakorum blocks with Eurasia) occurring along the southern Eurasia margin. The evolution of the SW CAOB continued with active deformation in response to far-field effects of the convergence between the Indian plate and the Eurasia continent throughout the Cenozoic.
Stress-fields as a result of these distal tectonic events propagated through the inherited Paleozoic structures of the CAOB resulting in progressive and punctuated exhumation and mountain building events that shaped the prominent Tianshan and Altai-Sayan mountainous landscapes that are seen today. This study focuses on the intricate intra-continental evolution of the Chinese Tianshan and Junggar orogenic collage, a key component of the SW CAOB. After the initial establishment in the late Paleozoic, this orogenic belt was immediately reworked by the movement of several deep-rooted strike-slip faults probably until the earliest Triassic, then subjected to large-scale reactivation events during the Meso-Cenozoic. As the architecture of the Tianshan and Junggar orogenic belt is complicated and its intra-plate evolution long-lasting, several issues regarding its thermo-tectonic history since the late Paleozoic remain unclear. Main objectives of this research are to better unravel late Paleozoic tectonic wedging due to strike-slip movements and to further elucidate the Meso-Cenozoic reactivation history of the Tianshan and Junggar systems, focusing on some of their uninvestigated or poorly constrained key regions.
Regarding the late Paleozoic strike-slip faults system developed along the Chinese Tianshan belt, we carried out structural and geochronological studies on the poorly investigated Xiaergou and Wulasitai shear zones around and in the Central Tianshan block (Chapter 4). The Xiaergou shear zone is the connecting segment between the North Tianshan fault and Main Tianshan shear zone along the northern margin of the Yili - Central Tianshan blocks, it strikes NW-SE with a width of ~3-5 km and shows predominant dextral kinematics. Zircon U-Pb ages of pre- and syn-kinematic granitic dykes within the Xiaergou shear zone indicate that the dextral shearing was active at ~312-295 Ma. The Wulasitai shear zone is a high-strain belt occurring in the interior of the Central Tianshan block, it extends NW-SE for more than 40 km with variable widths of ~1-5 km, steep mylonitic foliations and sub-horizontal stretching lineation are well developed and various kinematic indicators suggest prevailing sinistral shearing. New biotite 40Ar/39Ar ages of two meta-sedimentary rock units, together with the published metamorphic zircon U-Pb ages constrain the timing of the sinistral shearing at ~312-301 Ma. Our new results combined with the previous studies reveal that the dextral strike-slip shear zones framing the Central Tianshan formed almost simultaneously in the latest Carboniferous (~310 Ma) and lasted until the middle to late Permian. They resulted from the eastward tectonic wedging and relative rotations between continental blocks in the SW CAOB. The sinistral shearing of the Wulasitai shear zone within the Central Tianshan was likely generated due to differential eastward motions of the northern and southern parts of the Central Tianshan.
New apatite fission track (AFT) data on the Paleozoic rocks in and adjacent to the Chinese Central Tianshan were obtained, including two age-elevation profiles in the Alagou and Gangou areas. Inverse thermal history modeling reveals that the basement of the Central Tianshan experienced regional slow to moderate cooling during most of the Mesozoic, and that the present-day topography was mainly built by Cenozoic surface uplift and erosion. Geomorphological observation reveals several remnant fragments of flat, low-relief surfaces within the Central Tianshan, which were likely to have formed in the Mesozoic as evidenced by thermal history modeling of the Alagou age-elevation profile. Furthermore, the new data suggests that the Chinese Central Tianshan and its adjacent terranes did not undergo intensive relief building during its long-term Mesozoic evolution, as several pre-Mesozoic deep-rooted regional faults did not record evidence for a significant Mesozoic reactivation. Finally, differential exhumation of the basement in the western Chinese Tianshan and Junggar has been studied, and shows that the development of regional brittle faults significantly influences the processes of intra-continental deformation (Chapter 5).
The Chinese Eastern Tianshan and East Junggar orogenic belts are major constituents of the SW CAOB, and low-temperature thermochronology was applied to constrain the thermo-tectonic history of these two domains (Chapter 6). AFT dating of Paleozoic basement samples from the region dominantly yields Cretaceous (~126-70 Ma) AFT ages, except for two granitic samples from the East Junggar with older ages of ~239 and ~157 Ma, respectively. Thermal history modeling reveals that the Eastern Tianshan and southern part of the East Junggar experienced moderate to rapid basement cooling throughout the Cretaceous. We interpret this as a far-field effect of accretion and collision along the southern Eurasia margin since the Early Cretaceous. Major faults were reactivated and thus may have played an important role in controlling localized rapid basement uplift and cooling. We also dated seven Mesozoic sandstone samples collected from the eastern margin of the Junggar Basin. The detrital AFT age peaks, together with inverse thermal history modeling of the basement, reveal that the East Junggar underwent late Permian to Early Jurassic basement cooling episodes. These cooling events are thought to be related to post-orogenic transpression along major faults as a distal effect of the coeval Qiangtang-Eurasia collision. Combined with already published evidence, our new data suggests that the Eastern Tianshan and East Junggar did not undergo significant exhumation (> ~2-3 km) during the Cenozoic.
The Yili block in the western Chinese Tianshan forms the easternmost part of the Kazakhstan paleocontinent, and exploring its thermo-tectonic history is important to reconstruct the intra-continental evolution of the Tianshan belt. We report new AFT data from the basement rocks from the northern (i.e. the Wenquan complex) and southern (i.e. the Dahalajunshan - Nalati range) margins of the Yili block (Chapter 7). Thermal history modeling reveals that the Wenquan complex underwent moderate basement cooling in the Cretaceous, possibly due to far-field effects of the Tethys closure and convergent deformation and the ensuing Lhasa-Qiangtang collision. These events at the southern Eurasian margin propagated tectonic stress to the northern Yili and triggered localized deformation. Early Triassic-middle Jurassic moderate cooling is also identified in the Dahalajunshan - Nalati range, and is interpreted to be related to the post-orogenic strike-slip motion along the major shear zones and the effects of the Qiangtang and Kunlun-Qaidam collision. Combined with the published thermochronological data, it is suggested that the northern and southern parts of the Yili block experienced a distinctly different Mesozoic thermo-tectonic evolution. Basement cooling of the northern Yili block generally took place before the Cretaceous, exhuming shallower crustal levels as compared with the southern one. The intermontane Yili basin may have accommodated substantial propagated contraction induced by the Cretaceous collisional events, resulting in less strain reaching the northern Yili. Based on our new results and the previously published thermochronological data, it is suggested that the intra-continental reactivation of the North Tianshan and Nalati faults probably did not invoke significant regional exhumation during the Meso-Cenozoic. Instead, small-scale brittle faults controlled localized enhanced denudation.
In general, the research conducted in this dissertation provides new constraints and valuable improvements on our knowledge of the timing and nature of intra-continental deformation and reactivation of the Chinese Tianshan and Junggar orogenic collage since the late Paleozoic. Meanwhile, it lays the framework for a systematic review of low-temperature thermochronological data that could now be undertaken as many of the regional gaps in the Tianshan-Junggar have been filled (Chapters 8 and 9).
... It is worth mentioning that a series of east-west left-lateral strike-slip faults also developed in the hinterland of Tianshan. In northern Tianshan, the left-lateral strike-slip rate of the Kemin-Chilik Fault since the Late Quaternary is approximately 2 mm/a (Zhou, 2013), and the adjacent foreland thrust belt also extends tens of kilometers to the Kazakh platform ( Figure 1a, Goode et al., 2014;Jourdon et al., 2017;Selander et al., 2012). The Kemin-Chilik Fault not only acts as the central structure of the north-south contractional tectonic system of the Tianshan, but also absorbs the clockwise rotation deformation of the Tarim Block by the left-lateral strikeslip motion. ...
On 24 March 2021, an Mw 5.3 earthquake struck northwest Baicheng, located in the Kuqa fold-and-thrust belt (FTB), northwest China. In the current study, interferometric synthetic aperture radar (InSAR) data were used to investigate the associated fault rupture solution (dip, dip direction, and slip sense), to determine the geometry of the seismogenic structure; the high-resolution images of the surface rupture were obtained using an unmanned aerial vehicle (UAV). This geometric model, along with the co-seismic slip distribution from the InSAR data, and deformation characteristics of co-seismic surface rupture revealed that: (a) it was a shallow event, the focal depth was 0.5–2 km, and the co-seismic slip was distributed in the 0–7 km
range with high dip angles (66°, 70°). (b) A 4-km long surface rupture with obvious left-lateral strike-slip was distributed on the surface, and the maximum strike-slip displacement and width were 0.79 and 0.7 m,
respectively. This left-lateral strike-slip event indicates that the relative motion between the Tianshan and Tarim blocks is a continuous and diffuse deformation process. The study shows that, when evaluating the earthquake risk of the Kuqa FTB, we should consider both the front position of the fault outburst and the internal compressional salt-related structures, which can also produce moderate to strong earthquakes.
... Viscosity evaluated on these material points is then averaged (in our models using a geometric averaging scheme), before it is projected onto the finite elements of the mesh. Details of the 2D version of the code are given in Appendix A, as well as in Jourdon et al. (2018) and Ioannidi et al. (2021). We consider the structures observed in the field to be scale independent, as suggested by recent studies (Fagereng, 2011;Grigull et al., 2012;Ioannidi et al., 2021;Le Pourhiet et al., 2013), therefore we upscale field outcrop scales to map scales (from a cm-scale to a m-scale). ...
Frictional failure is the dominant deformation mechanism for rocks in the upper crust while in the middle crust rocks begin to deform viscously. Within this transition, brittle and viscous phases coexist, forming semi-frictional materials. While semi-frictional deformation on large scales might play an important role in understanding the transition between earthquakes and slow slip/creep, it can also be observed at smaller scales. Here, we use field observations of the Papoose Flat pluton in eastern California to study deformation of heterogeneous materials during shearing. Clast concentration varies between 2-12% by area. Field and microscopic observations show that the matrix deforms viscously, while the clasts fail in a brittle manner. We systematically document clast concentration and spacing with respect to clast fracturing and observe increasing frictional failure of clasts with increasing clast concentration. To test which matrix viscosities impose enough stresses on the clasts to lead to frictional deformation, we complement field observations with 2D numerical models. Maps with 7% by area randomly placed circular clasts are created and deformed under simple shear kinematic conditions. We test different matrix viscosities, from constant low and high viscosity (10 17 and 10 19 Pa.s, respectively), to dislocation creep for granite. Clasts in the vicinity of other clasts are affected by stresses around their neighbors. This effect decreases with increasing clast distance. Our field observations and numerical results suggest that the viscous phase can impose significant stresses onto the brittle phase, causing failure even at very low clast concentrations and in the absence of clast-clast interactions.
... In order to study the influence of accreted lithospheric heterogeneities on the architecture and the low long-lived subsidence of intracratonic basins, we use the thermomechanical numerical code pTatin (May et al., 2014(May et al., , 2015 in its 2D version (Jourdon et al., 2017). The code relies on PETSc library Balay et al. (2017) to solve conservation of momentum ∇:s ¼ rg; ...
Intracratonic basins tend to subside much longer than the timescale predicted by thermal relaxation of the lithosphere. Many hypotheses have been suggested to explain their longevity, yet few have been tested using quantitative thermo-mechanical numerical models, which capture the dynamic of the lithosphere. Lithospheric-scale geodynamic modelling preserving the tectono-stratigraphic architecture of these basins is challenging because they display only few kilometres of subsidence over 1000 of km during time periods exceeding 250 Myr. Here we present simulations that are designed to examine the relative role of thermal anomaly, tectonics and heterogeneity of the lithosphere on the dynamics of intracratonic basins. Our results demonstrate that initial heterogeneity of accretionary continental lithosphere explains long-term subsidence and the arches-basins architecture of Saharan type intracratonic basins at first order. The simulations show that initially heterogeneous lithospheres inherited from accretion are strong enough to resist local isostatic re-equilibration for very long period of time. Indeed, the lateral density variations store potential gravitational energy that is then slowly dissipated by differential erosion and slow vertical movements. For relatively well-accepted coefficient of erosion of 10 À6 m 2 /s, the subsidence last longer than 250 Myr. Extensional tectonic forcing and thermal anomalies both result in an effective strength drop of the lithosphere, which allows a temporal acceleration of local isostatic re-equilibration. Periodic changes in far field tectonic forcing from extension to compression complicate the tectono-stratigraphic architecture (intra-basin arches, sub-basins) introducing stratigraphic unconformities between different neighbouring basins such as the ones observed in North Africa.
... However, the crustal-scale used in the thermo-mechanical models does not permit the reproduction of the detailed structural evolution of the STS at the scale of studied metamorphic units, and to include all their rheological complexity (serpentinized layers, continental units, metasedimentary rocks, etc.). Alternative tectonic models proposed north-dipping subduction, mainly based on the current structure of STS belt, which is thrust towards the south onto the Tarim basin (e.g., Makarov et al., 2010;Windley, Allen, Zhang, Zhao, & Wang, 1990), but most of these top-to-the-south thrusts are Cenozoic in age (Jourdon, Le Pourhiet, et al., 2017). In the alternative north-dipping subduction scenario, arc-magmatism dated at 390-340 Ma in the Central Tien Shan-Yili Block (Gao et al., 2009;Han, He, Wang, & Guo, 2011;Han et al., 2016;Konopelko, Seltmann, Biske, Lepekhina, & Sergeev, 2009;Seltmann, Konopelko, Biske, Divaev, & Sergeev, 2011) could be related to the north-dipping subduction. ...
The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐pressure rocks along the South Tien Shan suture but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (i) a high‐pressure tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (ii) a large (> 100 km long) high‐pressure Metasedimentary Unit (HPMU) and (iii) a lower blueschist facies accretionary prism. Raman Spectroscopy on Carbonaceous Material combined with phengite and chlorite multi‐equilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P‐T conditions of 23‐25 kbar and 560‐570°C along the whole unit. 40Ar/39Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during Tmax conditions at a pressure range of 25 to 20 kbar. Thermobarometry on samples from the high‐pressure tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P‐T conditions of 5‐6 kbar and 290‐340°C, highlighting a 17‐20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the Tmax (328‐319 Ma) and the final exhumation of the HPMU (303‐295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70‐75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (1) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390‐340 Ma, and (2) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma. This article is protected by copyright. All rights reserved.
... Tien Shan under the Tarim Craton (e.g. Charvet et al., 2011;Loury et al., 2015;Jourdon et al., 2017b). ...
The Permian history of the Central Asian Orogenic belt is marked by large-scale strike-slip faults that reactivate former Paleozoic structures, delineated by widespread alkaline magmatism. The genetic link between the syn-kinematic granitoids emplaced in the Tien Shan range and magmas emplaced within the Tarim Large Igneous Province, and the interaction between this plume and transcurrent tectonics, are still unsolved issues. We investigated the Pobeda massif, in the eastern Kyrgyz Tien Shan, located at the boundary between the Tien Shan range and the Tarim Craton, which exhibits a high-temperature unit. In this unit, Permian magmatism resulted in the emplacement of alkaline charnockites at mid-crustal levels. The primary mineralogical assemblage is nominally anhydrous and made of ortho- and clino-pyroxenes, fayalite, K-feldspar, plagioclase and quartz. These charnockites are associated with partially-molten paragneisses and marbles. Thermobarometry on these rocks indicates that the charnockites emplaced following the intrusion of a melt at a temperature > 1000 °C and pressure of around 6 kbar, corresponding to depth of ~20 km. The resulting thermal anomaly triggered the partial melting of paragneisses. Bulk geochemistry including Sr, Nd, Pb and Hf isotopes suggests that charnockites fit into the Tarim Large Igneous Province magmatic series, with minor crustal assimilation. U-Pb ages on zircons of charnockites and surrounding paragneisses indicate that charnockites intruded and triggered partial melting of the gneisses at c. 287, 275 and 265 Ma. ⁴⁰Ar/³⁹Ar dating on amphibole gives a similar age as the U-Pb age at 276.2 ± 2.0 Ma. ⁴⁰Ar/³⁹Ar dating on biotite from the Charnockite unit marbles gives ages at ca. 256–265 Ma, which shows that exhumation onset directly follows the HT history, and is tentatively correlated to top-to-the-North thrusting of the Charnockite unit in a transpressive context. Additional ⁴⁰Ar/³⁹Ar dating on syn-kinematic white micas from an adjacent transpressive shear-zone indicates continuation of the strike-slip tectonics at shallow crustal levels, after the exhumation of the Charnockite unit, at 248–257 Ma. These results demonstrate that Tien Shan Permian magmatism is linked to the Tarim mantle plume activity. Lithosphere-scale shear zones in the Tien Shan range, could have been responsible for lateral flow focusing of the Tarim mantle plume up to the boundary with the Tien Shan range and subsequent decompression melting resulting in the Permian magmatism observed in the Pobeda area.
The deformation pattern and slip partitioning related to oblique underthrusting of the Tarim Basin in the eastern Tian Shan orogenic belt are not well understood because interior deformation images are lacking. The Baoertu fault is an E-W−striking, ∼350-km-long reactivated basement structure within the eastern Tian Shan. In this study, we quantify its late Quaternary activity based on interpretations of detailed high-resolution remote sensing images and field investigations. Three field observation sites along an ∼80-km-long fault segment indicate that the Baoertu fault is characterized by sinistral thrust faulting. Based on surveying of the displaced geomorphic surfaces with an unmanned drone and dating of the late Quaternary sediments using radiocarbon and optically stimulated luminescence (OSL) methods, we estimate a late Quaternary left-lateral, strike-slip rate of 1.87 ± 0.29 mm/yr and a N−S shortening rate of 0.26 ± 0.04 mm/yr for this fault. The lithospheric Baoertu fault acts as a decoupling zone and accommodates the left-lateral shearing caused by the oblique underthrusting of the Tarim Basin. In the eastern Tian Shan orogenic belt, the oblique convergence is partitioned into thrust faulting across the entire range and sinistral slip faulting on the high-dip basement structure within the orogen. This active faulting pattern in the eastern Tian Shan of sinistral shearing in the center and thrust faulting on both sides can be viewed as giant, crustal-scale positive flower structures.
The Eastern Tianshan and East Junggar orogenic belts are major constituents of the southwestern Central Asian Orogenic Belt. This study applies low-temperature thermochronology to constrain the thermo-tectonic history of these two domains. Apatite fission track (AFT) dating of Paleozoic basement samples from the Eastern Tianshan and East Junggar dominantly yield Cretaceous (∼126-70 Ma) AFT ages, except for two granitic samples from the East Junggar with older ages of ∼239 and ∼157 Ma, respectively. Thermal history modeling reveals that the Eastern Tianshan and southern part of the East Junggar experienced moderate to rapid basement cooling throughout the Cretaceous. We interpret it as a far-field effect of accretion and collision along the south Eurasia margin since the Early Cretaceous. Major faults were reactivated and thus may have played an important role in controlling localized fast uplift and cooling. We also dated seven Mesozoic sandstone samples collected from the eastern margin of the Junggar Basin. The detrital AFT age peaks, together with inverse thermal history modeling of the basement, reveal that the East Junggar underwent late Permian to Early Jurassic cooling episodes. These cooling events are thought to be related to post-orogenic transpression along major faults and distal effect of Qiangtang-Eurasia collision. Combined with already published evidence, our new data suggest that the Eastern Tianshan and East Junggar did not undergo significant exhumation during the Cenozoic.
Central Asia hosts the Tianshan, the largest intracontinental mountain belt in the world, which experienced major reactivation and uplift since the Oligocene in response to the collision of India with Asia. This reactivation was focused around pre-existing structures inherited from the Paleozoic tectonic history of the region. The significant Cenozoic tectonic reworking of Central Asia complicates efforts to understand earlier phases of intracontinental tectonics during the late Paleozoic and Mesozoic. The Tarbagatai Mountains of eastern Kazakhstan record a thermotectonic history that provides insight into the timing and distribution of intracontinental tectonic activity in Central Asia prior to the India-Eurasia collision. Apatite fission track and (U-Th-Sm)/He analysis of igneous samples from the Tarbagatai Mountains reveals two episodes of cooling as a result of exhumation following Paleozoic amalgamation. Initial intracontinental deformation during the Late Permian drove exhumation synchronous with activity along newly formed strike-slip faults spanning the Central Asian Orogenic Belt. The major Chingiz-Tarbagatai Fault was reactivated during the Early Cretaceous, driving localised exhumation along the fault. The relative lack of Cenozoic tectonic activity in the Tarbagatai Mountains means they provide unique insight into the broader thermotectonic evolution of Central Asia during the late Paleozoic and Mesozoic.
Supplementary material: Detailed thermochronological data, including plots and tables can be found in the supplementary data https://doi.org/10.6084/m9.figshare.c.5414555 .
Geological study of the Khan Tengri and Pobedi massifs highlights two phases of compressional fault displacements. A Late Permian/Triassic displacement phase is highlighted (1) by biotite ⁴⁰Ar/³⁹Ar ages of 265–256 Ma, suggestive of cooling of Pobedi mid-crustal granulites during >8 km of top-to-the-north motion of the Pobedi Thrust; (2) by ⁴⁰Ar/³⁹Ar dating of syn-kinematic phengite at 249–248 Ma, which suggests a crystallization age during top-to-the-south motion of the Khan Tengri Thrust shear zone; (3) by Apatite Helium (AHe) ages of 280-240 Ma on the crystalline basement below the Mesozoic peneplain, which gives insight into the final exhumation stage. Cenozoic reactivation of the Pobedi Thrust is indicated by AHe thermochronology with a mean age of 8.3 ± 2.5 Ma, in agreement with ∼3 km exhumation and ∼4.2 km of top-to-the-north motion in the Late Miocene. Along a north-south transect of the west Tian Shan range from Issyk-Kul Lake to Tarim Basin, compiled thermochronological data outline out-of-sequence deformation beginning with the activation of north-directed crustal-scale faults at 22-15 Ma along the northern margin of the north Tian Shan, followed by 20-10 Ma motion at the boundary between the Middle and the South Tian Shan, and ending with the <10 Ma reactivation of the Pobedi Thrust in the South Tian Shan. This latter coincided with south-directed motion on the South Tian Shan Front (the Maidan Fault) and fold and thrust belt propagation towards the Tarim Basin.
Slipping behavior of active thrust faults are not well explored because of their variable vertical displacements governed by dip-angle changes along the fault striking. The range-front Maidan fault (MDF) is a thrust fault in the southern Tian Shan, and exhibits clear paleoearthquake rupturing during the late Quaternary. In this study, based on field investigation, trench exposing, detailed terrace surveying with differential GPS, and optically stimulated luminescence (OSL) dating, we revealed that the latest paleoearthquake on the MDF occurred at ~1.76 ka, and generated a vertical displacement of ~3.0 m. Then, we analyzed the vertical displacements on different terraces of the Yushanguxi River versus the corresponding terrace ¹⁰Be exposure ages. We suggest that the large-earthquake on the MDF is an irregular recurrence. The MDF still has a high seismic risk in spite of only ~1.76 ka elapsed time for the latest paleoearthquake. We propose that the growth and uplift of the western Tian Shan orogen is caused by fault thrusting and crustal shortening of a series of block-like patterns.
In Central Asia, numerous fragments of planation surfaces are visible within the present day topography. However, their precise timing of formations is still poorly constrained and it is not clear if they are remnants of a single extensive planation surface or if they represent different planation episodes. By reconstructing the landscape evolution of the Tian Shan region and by analyzing the relations between the planation surfaces preserved within the eastern Tarim Basin and the sedimentary record, we demonstrate that the numerous erosional surfaces preserved within the Tian Shan Range represent different episodes of surfaces genesis. These erosion events span from the late Paleozoic to the Early Cenozoic. The widespread preservation of large fragments of these surfaces within the Tian Shan Range implies that this region did not undergo strong relief building during most of its Mesozoic evolution but was dominated by plains associated to small hills along episodically active discrete tectonic structures. Finally, the preservation of these surfaces within the active Tian 2 Shan Range implies a long-term, strong non-equilibrium state of the topography during its Cenozoic evolution. This was probably promoted by the arid to semi-arid climate prevailing since the Late Paleogene onset of relief building.
The model of Southern Tian-Shan collisional belt is presented and the thrust structure of its northern (Bukantau-Kokshaal) branch described in details, supported with stratigraphic and lithologic data. Paleozoic geodynamic events as forming of the Turkestan (Southern Tian-Shan) ocean margins, island arcs and postcolligional development also characterized.
Some tables are in the separate file.
The EMCA (Earthquake Model Central Asia) catalogue (Mikhailova et al., 2015) includes information for 33620 earthquakes that occurred in Central Asia (Kazakhstan, Kyrgyzstan, Tajikistan, Uzbekistan and Turkmenistan). The catalogue provides for each event the estimated magnitude in terms of MLH (surface wave magnitude) scale, widely used in former USSR countries.MLH magnitudes range from 1.5 to 8.3. Although the catalogue spans the period from 2000 BC to 2009 AD, most of the entries (i.e. 33378) describe earthquakes that occurred after 1900. The catalogue includes the standard parametric information required for seismic hazard studies (i.e., time, location and magnitude values). The catalogue has been composed by integrating different sources (using different magnitude scales) and harmonised in terms of MLH scale.
download link at epsl : // https://authors.elsevier.com/a/1Ujlo_,1tQ0xx7
The similarity between the geometry of the West African and South American coastlines is among one of the strongest natural observations supporting the plate tectonic paradigm. However, using classical plate tectonic approaches to model these conjugate transform margins results in a high degree of variability in palaeogeographic reconstructions. Using state-of-the-art 3D coupled thermo-mechanical numerical models, we simulate for the first time, crustal deformation at the onset of oceanisation along large offset oblique margins. Our models show that obliquity causes oceanic rift propagation to stall, resulting in an apparent polyphased tectonic evolution, and in some circumstances leads to the formation of hyper-extended margins. As a result, conjugate margins located at the edge of future fracture zones are highly asymmetric from rifting to spreading, with their lengths differing by a factor of 5 to 10, before the final phase of break-up occurs. Accounting for this discrepancy should ameliorate future palaeogeographic reconstructions.
The tectonic evolution of the Central Asian Orogenic Belt (CAOB) is characterized by the successive accretion of lithospheric blocks, leading to different interpretations about the polarity of subductions during the Paleozoic, the number of microplates and oceanic basins and the timing of tectonic events. This is especially the case in the Tien Shan area. In this paper, we propose new structural maps and cross-sections of Middle and South Kyrgyz Tien Shan (MTS and STS respectively). These cross-sections highlight an overall dextral strike-slip shear zone in the MTS at the crustal scale and a North verging structure in the STS. These structures are Carboniferous in age and sealed by a late Carboniferous conglomerate, later overlain by Mesozoic and Cenozoic deposits. The STS exhibits two deformation phases: (1) a top-to-the-South normal shearing that can be related to subduction or exhumation dynamics and (2) a top to the North nappe stacking that we link to the late Paleozoic collisional events between the MTS and the Tarim block. We propose a new interpretation of the tectonic evolution of the Kyrgyz Tien Shan during the Late Paleozoic collision. This model involves a partitioned collisional deformation in Late Carboniferous times, with an orthogonal collision to the south, between the Tarim and MTS, and a strike-slip regime to the north along a dextral E-W zone located between the MTS and the North Tien-Shan/Kazakh platform, the so-called Nikolaev Line. The docking of the large Tarim Craton against the CAOB corresponds to a collision phase, which ended the long-lived Paleozoic subduction history in the CAOB and was followed in the TS region by intense strike-slip deformation during the Permian.
Orogens resulting from the closure of narrow oceans, such as the Alps or the Pyrenees, usually lack voluminous synsubduction and synorogenic magmatism. Such orogenies are essentially controlled by mechanical processes in which the initial architecture of the original rifted margins strongly controls the architecture of the orogen. In this paper we first provide a synthesis of the structure, dimensions, and lithology of hyperextended rift systems and oceans, based on recent seismic and petrologic data. We then investigate how rift-related inheritance influences crustal characteristics and mantle geochemistry of orogens related to the closure of narrow oceans, and compare them to orogens resulting from the closure of wide and/or mature oceans. Our results show that narrow oceans usually lack a mature spreading system forming Penrose-type oceanic crust (i.e., 6-7-km-thick basaltic oceanic crust typical of steady-state spreading systems; see Anonymous, 1972), in contrast to wide oceans. However, there is statistically no difference in the structural and lithological architecture of their passive continental margins. Thus, the main difference between narrow and wide oceans is whether the margins are separated by a significant amount of oceanic crust and underlying depleted mantle. In addition, due to the lack of significant magmatism during the closure of narrow oceans, the mantle wedge is likely to remain relatively fertile compared to the wedge above long-lasting subduction of wide oceans. This difference in mantle composition may dictate the magmatic budget of subsequent orogenic collapse or rifting events.
Mid-ocean ridges, transform faults, subduction and continental collisions form theconventional theory of plate tectonics to explain non-rigid behaviour at plate boundaries.However, the theory does not explain directly the processes involved in intraplatedeformation and seismicity. Recently, damage structures in the lithosphere have been linkedto the origin of plate tectonics. Despite seismological imaging suggesting that inheritedmantle lithosphere heterogeneities are ubiquitous, their plate tectonic role is rarelyconsidered. Here we show that deep lithospheric anomalies can dominate shallow geologicalfeatures in activating tectonics in plate interiors. In numerical experiments, we found thatstructures frozen into the mantle lithosphere through plate tectonic processes can behave asquasi-plate boundaries reactivated under far-field compressional forcing. Intraplate locationswhere proto-lithospheric plates have been scarred by earlier suturing could be regions wherelatent plate boundaries remain, and where plate tectonics processes are expressed as a‘perennial’ phenomenon.
The style of inversion of inherited extensional basins in the Western Alps is investigated through thermo-mechanical modeling. 2D models consist of a half-graben embedded in a relatively strong crust (basement) and filled with weak syn-rift sediments (cover). We investigate the relative influence of the internal friction (µ) of the basin-bounding normal fault, the tectonic burial (h) under an overlying nappe, and the geothermal gradient. We use a visco-plastic model with a symmetrical shortening. The inherited normal fault is implemented as a curved thin body with a variable friction coefficient (µ) ranging between 0.1 and 0.6. The style of basin inversion is controlled at shallow depth by the internal friction coefficient whose influence decreases with the increase of both burial depth and geothermal gradient. With increasing burial and/or geothermal gradient, the fault reactivation is inhibited and a distributed deformation in the basement induces the vertical extrusion of the cover. The basin inversion is accompanied by distributed deformation in the cover and by the shearing of the basin and basement interface. Results are consistent with the style of inversion of inherited half-grabens in the external Western Alps, where no significant fault reactivation occurred due to tectonic burial underneath the Alpine internal units during the early Alpine collision.
Metamorphic phase changes have a strong impact on the physical and mechanical properties of rocks including buoyancy (body forces) and rheology (interface forces). As such, they exert important dynamic control on tectonic processes. It is generally assumed that phase changes are mainly controlled by pressure (P) and temperature (T) conditions. Yet, in reality, whatever the PT conditions are, phase changes cannot take place without an adequate amount of the main reactant - water. In present day geodynamic models, the influence of water content is neglected. It is generally assumed that water is always available in quantities sufficient for thermodynamic reactions to take place at minimal Gibbs energy for given P, T conditions and a constant chemical composition. If this assumption was correct, no high-grade metamorphic rocks could to be found on the Earth’s surface, since they would be retro-morphed to low-grade state during their exhumation. Indeed, petrologic studies point out that water, as a limiting reactant, is responsible for the lack of retrograde metamorphic reactions observed in the rocks exhumed in typical MCC contexts. In order to study the impact of fluid content on the structure of metamorphic core complexes, we have coupled a geodynamic thermo-mechanical code Flamar with a fluid-transport and water-limited thermodynamic phase transition algorithm. We have introduced a new parameterisation of Darcy flow that is able to capture source/sink and transport aspects of fluid transport at the scale of the whole crust with a minimum of complexity. Within this model, phase transitions are controlled by pressure temperature and the local amount of free fluid that comes from both external (meteoric) and local (dehydration) sources. The numerical experiments suggest a strong positive feedback between the asymmetry of the tectonic structures and the depth of penetration of meteoric fluids. In particular, bending-stress distribution in asymmetric detachment zones drives the penetration of meteoric fluids to greater depths. However, thermal weakening and/or slow re-equilibration of the protolith rocks at depth tends to decrease the asymmetry of structure, changing the orientation of the bending stresses and to shut down the activity of asymmetric detachments in favor of spreading structures, which results in formation of double-domes
Regional strike-slip faults are widely distributed in continental interiors and play a major role in the distribution of far-field deformation due to continental collisions. Constraining the deformation history of the Talas–Fergana Fault (TFF), one of the largest of such faults in the Himalayan deformed interior, is vital to comprehend the hinterland kinematics of the India–Asia collision. New apatite fission track results from the NW Tien Shan define a rapid exhumation event at c. 25 Ma. This event is correlated with a synchronous pulse in the South Tien Shan, implying that both ranges experienced a simultaneous onset of rapid exhumation. We suggest that strike-slip motion along the TFF commenced at c. 25 Ma, facilitating counter-clockwise rotation of the Fergana Basin and enabling exhumation of the linked horsetail splays. Pamir indentation, located south of the Western Tien Shan, is postulated to be underway by c. 20 Ma. Recently published results suggest synchronous strike-slip deformation in the western Tarim Basin and eastern flank of the Pamir. Based on our results and published data, we are able to connect Tarim and Pamir deformation to the onset of TFF slip. We suggest that this pre-existing regional structure was responsible for transferring Pamir-induced shortening to the NW Tien Shan.
Supplementary material: Supplementary material is available at http://www.geolsoc.org.uk/SUP18845
Based on new structural and petrological investigations, we present two crustal-scale
cross-sections of the Kyrgyz South Tien Shan, and correlations of main faults and units between
Kyrgyzstan and China. The overall structure corresponds to a doubly-vergent mountain belt.
The Kyrgyz and Chinese areas exhibit identical structural and metamorphic histories. To the
west, the Atbashi Range comprises high-pressure oceanic and continental units stacked by
north-verging thrusts above a low metamorphic accretionary prism. High-pressure (HP) gneisses
are bound to their south by a south-dipping detachment exhibiting mantle relicts. The high-pressure
oceanic and continental units underwent similar pressure–temperature (P–T ) paths with peak
conditions of around 500 8C–20 kbar, followed by rapid exhumation. The overall south-dipping
structure and kinematics indicate a south-dipping subduction of the Central Tien Shan Ocean at
320–310 Ma, ending with the docking of the Tarim block to the Kazakh continent. To the east,
the Pobeda Massif shows a narrow push-up structure. A major north-vergent thrust exhumes
deep-crustal-level granulites, constituting the highest summits, which were thrust towards the
north onto low-grade Devonian–Carboniferous schists. The southern part of South Tien Shan is
made up of a south-verging thrust stack that formed later during ongoing convergence, reactivated
throughout post-30 Ma phases.
The initiation and subsequent evolution of intra-plate orogens, resulting from continental plate interior deformation due to transmission of stresses over large distances from the active plate boundaries, is controlled by lateral and vertical strength contrasts in the lithosphere. We present lithospheric-scale analogue models combining 1) lateral strength variations in the continental lithosphere, and 2) different vertical rheological stratifications. The experimental continental lithosphere has a four-layer brittle–ductile rheological stratification. Lateral heterogeneity is implemented in all models by increased crustal strength in a central narrow block. The main investigated parameters are strain rate and strength of the lithospheric mantle, both playing an important role in crust–mantle coupling. The experiments show that the presence of a strong crustal domain is effective in localizing deformation along its boundaries. After deformation is localized, the evolution of the orogenic system is governed by the mechanical properties of the lithosphere such that the final geometry of the intra-plate mountain depends on the interplay between crust–mantle coupling and folding versus fracturing of the lithospheric mantle. Underthrusting is the main deformation mode in case of high convergence velocity and/or thick brittle mantle with a final asymmetric architecture of the deep lithosphere. In contrast, lithospheric folding is dominant in case of low convergence velocity and low strength brittle mantle, leading to the development of a symmetric lithospheric root. The presented analogue modelling results provide novel insights for 1) strain localization and 2) the development of the asymmetric architecture of the Pyrenees.
Thermochronological datasets for the Kyrgyz Tianshan and Siberian Altai-Sayan within Central Asia reveal a punctuated exhumation history during the Meso–Cenozoic. In this paper, the datasets for both regions are collectively reviewed in order to speculate on the links between the Meso–Cenozoic exhumation of the continental Eurasian interior and the prevailing tectonic processes at the plate margins. Whereas most of the thermochronological data across both regions document late Jurassic–Cretaceous regional basement cooling, older landscape relics and dissecting fault zones throughout both regions preserve Triassic and Cenozoic events of rapid cooling respectively. Triassic cooling is thought to reflect the Qiangtang–Eurasia collision and/or rifting/subsidence in the West Siberian basin. Alternatively, this cooling signal could be related with the terminal terrane-amalgamation of the Central Asian Orogenic Belt. For the Kygyz Tianshan, late Jurassic–Cretaceous regional exhumation and Cenozoic fault reactivations can be linked with specific tectonic events during the closure of the Palaeo-Tethys and Neo-Tethys oceans respectively. The effect of the progressive consumption of these oceans and the associated collisions of Cimmeria and India with Eurasia probably only had a minor effect on the exhumation of the Siberian Altai-Sayan. More likely, tectonic forces from the east (present-day coordinates) as a result of the building and collapse of the Mongol-Okhotsk orogen and rifting in the Baikal region shaped the current Siberian Altai-Sayan topography. Although many of these hypothesised links need to be tested further, they allow a first-order insight into the dynamic response and the stress propagation pathways from the Eurasian margin into the continental interior.
In this work, we present the seismic catalogue compiled for Central Asia (Kazakhstan, Kyrgyzstan, Tajikistan, Uzbekistan and Turkmenistan) in the framework of the Earthquake Model Central Asia (EMCA) project. The catalogue from 2000 B.C. to 2009 A.D. is composed by 33,034 earthquakes in the MLH magnitude (magnitude by surface waves on horizontal components widely used in practice of the former USSR countries) range from 1.5 to 8.3. The catalogue includes both macroseimic and instrumental constrained data, with about 32,793 earthquake after 1900 A.D. The main sources and procedure used to compile the catalogues are discussed, and the comparison with the ISC-GEM catalogue presented. Magnitude of completeness analysis shows that the catalogue is complete down to magnitude 4 from 1959 and to magnitude 7 from 1873, whereas the obtained regional b value is 0.805.
Chinese East Tianshan is a key area for understanding the Paleozoic accretion of the southern Central Asian Orogenic Belt. A first accretion-collision stage, before the Visean, developed the Eo-Tianshan range, which exhibits north-verging structures. The geodynamic evolution included: i) Ordovician-Early Devonian southward subduction of a Central Tianshan ocean beneath a Central Tianshan arc; ii) Devonian oceanic closure and collision between Central Tianshan arc and Yili-North Tianshan block, along the Central Tianshan Suture Zone; iii) Late Devonian-earliest Carboniferous closure of a South Tianshan back-arc basin, and subsequent Central Tianshan-Tarim active margin collision along the South Tianshan Suture Zone. A second stage involved: i) Late Devonian-Carboniferous south-ward subduction of North Tianshan ocean beneath the Eo-Tianshan active margin (Yili-North Tianshan arc); ii) Late Carboniferous-Early Permian North Tianshan-Junggar collision. The Harlike range, unit of Mongolian Fold Belt, collided with Junggar at Mid-Carboniferous, ending a north-dipping subduction. The last CAOB oceanic suture is likely the North Tianshan Suture Zone, between Yili-North Tianshan and Junggar. During the Permian, all the already welded units suffered from a major wrenching, dextral in Tianshan, sinistral in Mongolian Fold Belt, due to opposite motion of Siberia and Tarim.
The lateral variation of the mechanical properties of continental lithosphere is an important factor controlling the localization of deformation and thus the deformation history and geometry of intra-plate mountain belts. A series of three-layer lithospheric-scale analog models, with a strong domain (SD) embedded at various depths, are presented to investigate the development of topography and deformation patterns by having laterally heterogeneities within a weak continental lithosphere. The experiments, performed at a constant velocity and under normal gravity, indicate that the presence or absence of the SD controls whether deformation is localized or distributed at a lithospheric-scale. Deformation and topography localizes above the edges of the SD while the SD region itself is characterized by minor amounts of surficial deformation and topography. The depth of the SD (within the ductile crust, ductile mantle lithosphere, or both) controls the pattern of deformation and thus the topography. The presence of a SD in the ductile crust or in the mantle results in limited surficial topographic effects but large variations in the Moho topography. Strong Moho deflection occurs when the SD is in the ductile crust while the Moho remains almost flat when the SD is in the mantle. When the SD occupies the ductile lithosphere, the SD is tilted. These analog experiments provide insights into intra-plate strain localization and could in particular explain the topography around the Tarim Basin, a lithospheric-scale heterogeneity north of the India-Asia collision zone.
The topographic evolution of continents and especially the growth and dismembering of mountain ranges plays a major role in the tectonic evolution of orogenic systems, as well as in regional or global climate changes. A large number of studies have concentrated on the description, quantification and dating of relief building in active mountain ranges. However, deciphering the topographic evolution of a continental area submitted to recurrent tectonic deformation over several hundred millions of years remains a challenge. Here we present a synthesis of the tectonic, geochronological and sedimentological data available on the intracontinental Tian Shan Range to describe its general topographic evolution from Late Palaeozoic to Early Tertiary. We show that this evolution has occurred in two very distinct geodynamic settings, initiating during the Carboniferous in an ocean subduction - continent collision tectonic context before becoming, from Early Permian, purely intra-continental. We show that during most of the Mesozoic, the topography is mostly characterized by a progressive general decrease of the relief. Nonetheless localized, recurrent deformation induced the formation of small-scale reliefs during that period. These deformations were driven by far field effects of possibly several geodynamic processes in a way that still remains to be fully understood.
thermal inheritance detrital thermochronology margin inversion inverse modeling Pyrenees The extent to which heat recorded in orogens reflects thermal conditions inherited from previous rift-related processes is still debated and poorly documented. As a case study, we examine the Mauléon basin in the north-western Pyrenees that experienced both extreme crustal thinning and tectonic inversion within a period of ∼30 Myrs. To constrain the time–temperature history of the basin in such a scenario, we provide new detrital zircon fission-track and (U–Th–Sm)/He thermochronology data. The role of rift-related processes in subsequent collision is captured by inverse modeling of our thermochronological data, using relationships between zircon (U–Th–Sm)/He ages and uranium content, combined with thermo-kinematic models of a rift-orogen cycle. We show that the basin recorded significant heating at about 100 Ma characterized by high geothermal gradients (∼80 • C/km). Our thermo-kinematic modeling and geological constraints support the view that subcontinental lithospheric mantle was exhumed at that time below the Mauléon basin. Such a high geothermal gradient lasted 30 Myr after onset of convergence at ∼83 Ma and was relaxed during the collision phase from ∼50 Ma. This study suggests that heat needed for ductile shortening during convergence, is primarily inherited from extension rather than being only related to tectonic and/or sedimentary burial. This should have strong implications on tectonic reconstructions in many collision belts that resulted from inversion of hyper-extended rift basins.
The Precambrian evolution of the Tarim Craton in NE China, in particular during the early Precambrian stage, remains enigmatic. In this contribution, we report field observation, petrology, geochemistry, zircon Lu–Hf isotopes and U–Pb ages of the major rock formations of the Aketage area in the southeastern section of the Tarim Craton. The Milan Group in Aketage is dominantly composed of 2.7–2.5 Ga gneissic amphibolite–TTG complex with minor paragneiss. Both the mafic and silicic rocks exhibit geochemical features consistent with an arc affinity. The arc-signature of the 2.01–2.03 Ga gneissic granites and gabbros which intrude the Archean basement, as well as the major 2.0 Ga metamorphic event revealed by zircon U–Pb dating, suggest an important subduction-collision event possibly related to the assembly of the Paleoproterozoic Columbia supercontinent. The ca. 1848–1856 Ma massive potassic granites, 1867 Ma mafic dyke swarm and 1844 Ma massive leucogranite dykes reveal magmatism in a post-collisional extensional setting.
Simulations of long-term lithospheric deformation involve post-failure analysis of high-contrast brittle materials driven by buoyancy and processes at the free surface. Geody-namic phenomena such as subduction and continental rifting take place over millions year time scales, thus require efficient solution methods. We present pTatin3D, a geodynamics modeling package utilising the material-point-method for tracking material composition, combined with a multigrid finite-element method to solve heterogeneous, incompressible visco-plastic Stokes prob-lems. Here we analyze the performance and algorithmic tradeoffs of pTatin3D's multigrid preconditioner. Our matrix-free geomet-ric multigrid preconditioner trades flops for memory bandwidth to produce a time-to-solution > 2× faster than the best available methods utilising stored matrices (plagued by memory bandwidth limitations), exploits local element structure to achieve weak scaling at 30% of FPU peak on Cray XC-30, has improved dynamic range due to smaller memory footprint, and has more consistent timing and better intra-node scalability due to reduced memory-bus and cache pressure.
Crustal inheritance is often considered important in the tectonic evolution of the Wilson Cycle. However, the role of the mantle lithosphere is usually overlooked due to its difficulty to image and uncertainty in rheological make-up. Recently, increased resolution in lithosphere imaging has shown potential scarring in continental mantle lithosphere to be ubiquitous. In our study, we analyze intraplate deformation driven by mantle lithosphere heterogeneities from ancient Wilson Cycle processes and compare this to crustal inheritance deformation. We present 2-D numerical experiments of continental convergence to generate intraplate deformation, exploring the limits of continental rheology to understand the dominant lithosphere layer across a broad range of geological settings. By implementing a ‘jelly sandwich’ rheology, common in stable continental lithosphere, we find that during compression the strength of the mantle lithosphere is integral in generating deformation from a structural anomaly. We posit that if the continental mantle is the strongest layer within the lithosphere, then such inheritance may have important implications for the Wilson Cycle. Furthermore, our models show that deformation driven by mantle lithosphere scarring can produce tectonic patterns related to intraplate orogenesis originating from crustal sources, highlighting the need for a more formal discussion of the role of the mantle lithosphere in plate tectonics.
Continents have a rich tectonic history that have left lasting crustal impressions. In analyzing Central Australian intraplate orogenesis, complex continental features make it difficult to identify the controls of inherited structure. Here the tectonics of two types of inherited structures (e.g., a thermally enhanced or a rheologically strengthened region) are compared in numerical simulations of continental compression with and without “glacial buzzsaw” erosion. We find that although both inherited structures produce deformation in the upper crust that is confined to areas where material contrasts, patterns of deformation in the deep lithosphere differ significantly. Furthermore, our models infer that glacial buzzsaw erosion has little impact at depth. This tectonic isolation of the mantle lithosphere from glacial processes may further assist in the identification of a controlling inherited structure in intraplate orogenesis. Our models are interpreted in the context of Central Australian tectonics (specifically the Petermann and Alice Springs orogenies).
The Precambrian tectonic evolution and growth of continental crust in the southwestern Tarim Craton in NW China remain enigmatic. In this contribution, we report petrography, zircon U–Pb ages, geochemistry and Nd–Hf–O isotope data on the ca. 1.4 Ga granitic pluton in southwestern Tarim Terrane. We also present zircon U–Pb ages and Hf isotope data on the Paleoproterozoic intrusive complex from this area, and use the combined data to evaluate Precambrian tectonic evolution of the region in relation to the Columbia supercontinent. The ca. 1.4 Ga Azibailedi pluton is mainly composed of biotite monzogranite with contents SiO2 (72.8–75.1 wt.%). The major element chemistry of the pluton reveals alkali-calcic and ferroan affinities and metaluminous to peraluminous feature with A/CNK ranging from 0.86 to 1.15. The granite shows high total REE contents with variable enrichment in LREE ((La/Yb)N = 8.5–12.2). Both REE and other incompatible elements features define A-type affinities. The initial epsilon Nd values vary from −3.5 to −2.2 and zircon ɛHf(t) and δ18O values range from −4.1 to +6.1 and 7.1‰ to 8.6‰. Integrating geological, geochemical and Nd–Hf–O isotope data, we suggest that the Azibailedi A-type granites were derived from pre-existing early Paleoproterozoic mafic lower crust with involvement of variable amount of Paleoproterozoic juvenile crust, possibly trigged by basaltic magma upwelling, during the breakup of the Columbia supercontinent.
We investigate the dynamic evolution of fold and thrust structures which form by compression and inversion of a sequence of half-graben basins. The choice for a half-graben geometry is motivated by seismic studies and reconstructions of preinversion geometry of inverted regions, which show that rifting often leads to a series of half-grabens. To examine the deformational structures which result from basin inversion, we use a two-dimensional, viscous-plastic numerical model and start our experiments from a preexisting extensional geometry. We find that synrift and postrift sediments are uplifted in the initial stages of basin inversion. This uplift is accompanied by rotation of the basement blocks beneath the basins. In the postrift sequence new shear zones form which are a continuation of basin-bounding faults. With continuing shortening, further inversion is more difficult owing to relative strengthening of the half-graben region. Significant surface erosion facilitates inversion. Back thrusts mainly develop in association with listric basin-bounding faults and less for planar (domino array) faults. Weak sediments (such as salt or shales) at the base of the basins promote the development of basement shortcut faults. The presence of a postrift decollement layer tends to decouple deformation of the postrift sediments from the material below it. In our model, preexisting weak basin-bounding shear zones are a requirement for substantial amounts of basin inversion to occur. Our numerical model results display many first order characteristics of examples from nature and analogue studies.
In recent two decades, new data on the age of strata and their spatial arrangement and geodynamic formation conditions have appeared, which give new insight into the geologic history of the Tien Shan. This mountain system evolved in several large megastages and stages, from the Archean through the Quaternary, which are recognized according to the predominant oceanic or continental regime of development. The fold-nappe structure of the Tien Shan is the result of crushing of the edge of the ancient supercontinent into a series of small blocks, which were then localized on the periphery of the Paleoasian ocean and underwent accretion, completed in the late Late Paleozoic. The modern Eurasian plate within Central Asia is separated into several blocks, whose counter and rotational movements determine the recent geodynamics of the region. The Tien Shan lithosphere is layered as a result of the appearance of a migma layer in its lower part, which causes a relatively free transfer and deformation of its upper part.
In the South Tien Shan range (Kyrgyzstan), the Late Paleozoic geodynamic evolution remains debated especially to the west of the Talas-Fergana fault (TFF) fault where suture-related high-pressure (HP) rocks are scarce. We provide new petrological and geochronological data on garnet amphibolites from the Chatkal range, to the west of the TFF, northwest of the South Tien Shan suture. These rocks are retrogressed eclogites. We used a micro-mapping approach combined with forward modelling and empirical thermobarometry to decipher the P-T path of these amphibolitized eclogites. The metamorphic peak conditions culminated at 490 ± 50 °C and 18.5 ± 2 kbar and were followed by higher temperature retrogression (∼ 560 °C at 11 to 7 kbar). In order to constrain the age of the HP stage, we dated allanite crystals texturally coeval to the HP mineral assemblage. Allanite grains dated in-situ with a U-Pb LA-ICPMS methodology yield an age of 301 ± 15 Ma. Compared with previously published data for the east of the TFF, these P-T constraints allow improving the understanding of the Late Paleozoic geodynamic evolution of the South Tien Shan. To the east of TFF, the Turkestan Ocean closed around 320 Ma with the collision of the Tarim Craton with the Kazakh microcontinent. To the west of TFF, the Turkestan Ocean closed around 300 Ma, when the Alai block collided with the Kazakh microcontinent. This later collision involved nappe-stacking and intense subvertical folding in the western South Tien Shan. This complex folding explains the S-shape of the suture to the west of the TFF that cannot be observed in the eastern part. These new data allow us to propose a distinct tectonic evolution of the two sides of the TFF, which suggests that this fault was a major transform fault before being a strike-slip intra-continental fault.
Based on the geological mapping, the Gangou ophiolitic mélange (GOM) in Gangou, Mishigou and Wusutong area, Turkson County has been distinguished from the lower Paleozoic strata along the boundary fault between the Northern Tianshan Belt (NTB) and Central Tianshan Belt (CTB). It can be divided into two kinds of assemblages, which are the tectonic blocks and the mélange matrix. The tectonic blocks are mainly composed of some relics of the ophiolites and island-arc volcanic rocks, whereas the mélange matrix is composed of some volcanic-sedimentary rocks formed in a fore-arc setting. The ophiolites in the Gangou area are mainly composed of meta-peridotites, gabbros, diabases and meta-basalts (green schist), which are scattered and overthrusted into the mélange matrix of volcanic-sedimentary rocks. The geochemistry of the meta-peridotites is characterized by low SiO2, TiO2, Al2O3 and CaO contents, and high MgO contents, which are similar to that of the buchnerites in a mid-oceanic ridge. In addition to, the characteristics of low∑REE, depleted in LREE, enriched in Cr and Ni contents are similar to that of the ultra-mafic rocks in a typical ophiolite. The basalts possess middling TiO2, contents, high MgO contents. They also show geochemical characteristics of low contents and unfractionation in HFSE, without any enrichment in Th and depletion in Nb and Ta. All above evidences suggest that the peridotites and basalts from the Gangou area were derived from a depleted mantle source in a mid-oceanic ridge setting. The island-arc volcanic rocks are mainly distributed in the south part of the GOM belt. They are mainly composed mugearites which are characterized by high MgO, Al2O3, K2O and Na2O contents, low TiO2, and CaO contents. Additionally, they are enriched in LILE (Rb, Ba, Zr, Th, U and LREE), depleted in Nb and Ta. All these geochemical signatures show that the magma source were inputted by some subducted components, which indicated that they were formed in an island-arc setting in an active continental margin. It is the ophiolites and island-arc volcanic rocks distributing in the GOM that show there had existed a paleo-ocean and subduction between the CTB and NTB.
We have determined slip rates on the most active reverse faults, reconstructed an extensive preorogenic erosion surface, constructed local and regional cross sections, and dated syntectonic Tertiary sedimentary rocks by magnetostratigraphy along a north-south transect that spans the Kyrgyz portion of the west-central Tien Shan. The cumulative Late Quaternary shortening rate along this transect is 10 mm/yr. The trabsect consists of five major fault zones, and the most active faults lie in the interior of the range. Using geometric models developed in other regions of basement-involved determination, we estimate shortening during the Late Cenozoic at 35-80 km. Apparent simultaneous onset of sedimentary basins (at least 3 major basins) about 12 Ma BP is interpreted to mark the onset of the current orogeny. Given the current shortening rate of about 10 mm/yr, measured across active faults and by GPS, we infer that the rate increased with time. We assumed accelerated shortening and have shown that it has always been of similar style, dominated by north-south shortening across east-west trending basement-involved reverse faults. Deformations were localized in five zones, which border the largest and deepest Tertiary basins, show the greatest structural relief, and contain the currently most active faults.
The Tien Shan is the best example of active intracontinental mountain building in the world. In order to determine how strain has accumulated in the range, a 28 station network was deployed in the Tien Shan during 1998-2000. Preliminary analysis of seismograms from this network showed two important results: first, P-SVwave conversions from the Moho reveal that the crust beneath the central Tien Shan is unusually thin, being no thicker than that beneath the Kazakh Shield to the north or the Tarim Basin to the south. Second, splitting of SKS-waves shows that the strain in the mantle is largely a passive response to north-south shortening and that this strain is imprinted on the mantle far to the north of the orogeny itself.
On the basis of synthetic studies of geology and geochemistry, an ophiolitic m lange in Wuwamen area was discovered that emplaced in the boundary fault separating the Southern Tianshan and Middle Tianshan belt. It is composed of different tectonic blocks with different lithological features and ages, and chaotic matrixes. The blocks contacting each other by a shear zone or chaotic matrix mainly include ophiolite slices, amphibolite and gneiss blocks of metamorphic basement of Middle Tianshan belt, and Devonian marble blocks from Southern Tianshan Belt. The ophiolite is composed of meta-peridotites, gabbros and basalts slices. Geochemical analysis suggests that the composition of these peridotites being similar to that of the lherzolite in a mid-ocean ridge with high SiO2, TiO2, Al2O3, CaO contents, and low MgO content. Additionally, it is characterized by low ∑ REE, and depleted in LREE intensively. Meanwhile, the basalts from the Wuwamen ophiolitic mélange show high TiO2 and MgO content, low Al2O3, K2O, P2O5 content, and Na2 O > K2 O compositional features. In addition to, the geochemical characteristic of low ∑ REE, depleted in LREE and unfractionated in most HSF elements (Zr, Hf, Sm, Y and Yb) indicates that the basalts are similar to a normal mid-ocean basalt (N-MORB). Above all, some samples are showing distinguishable enrichment in large-ion lithophile (LIL) elements, Th and Pb contents, and depletion in Nb and Ta contents. It is suggest that the magma source was influenced by input of the subduction components. Base on all above evidences and regional geological data, it is reasonable suggest that the Wuwamen ophiolite was formed in a back-arc basin setting which have been existed along the southern margin of the Middle Tianshan Belt.
Positive structural inversion involves the uplift of rocks on the hanging-walls of faults, by dip slip or oblique slip movements. Controlling factors include the strike and dip of the earlier normal faults, the type of normal faults — whether they were listric or rotated blocks, the time lapsed since extension and the amount of contraction relative to extension. Steeply dipping faults are difficult to invert by dip slip movements; they form buttresses to displacement on both cover detachments and on deeper level but gently inclined basement faults. The decrease in displacement on the hanging-walls of such steep buttresses leads to the generation of layer parallel shortening, gentle to tight folds — depending on the amount of contractional displacement, back-folds and back-thrust systems, and short-cut thrust geometries — where the contractional fault slices across the footwall of the earlier normal fault to enclose a “floating horse”. However, early steeply dipping normal faults readily form oblique to strike slip inversion structures and often tramline the subsequent shortening into particular directions.
Examples are given from the strongly inverted structures of the western Alps and the weakly inverted structures of the Alpine foreland. Extensional faulting developed during the Triassic to Jurassic, during the initial opening of the central Atlantic, while the main phases of inversion date from the end Cretaceous when spreading began in the north Atlantic and there was a change of relative motion between Europe and Africa. During the mid-Tertiary well over 100 km of Alpine shortening took place; Alpine thrusts, often detached along, or close to, the basement-cover interface, stacking the late Jurassic to Cretaceous sediments of the post-extensional subsidence phase. These high level detachments were joined and breached by lower level faults in the basement which, in the external zones of the western Alps, generally reactivated and rotated the earlier east dipping half-graben bounding faults. The external massifs are essentially uplifted half-graben blocks. There was more reactivation and stacking of basement sheets in the eastern part of this external zone, where the faults had been rotated into more gentle dips above a shallower extensional detachment than on the steeper faults to the west.
There is no direct relationship between the weaker inversion of the Alpine foreland and the major orogenic contraction of the western Alps; the inversion structures of southern Britain and the Channel were separated from the Alps by a zone of rifting from late Eocene to Miocene which affected the Rhone, Bresse and Rhine regions. Though they relate to the same plate movements which formed the Alps, the weaker inversion structures must have been generated by within plate stresses, or from those emanating from the Atlantic rather than the Tethyan margin.
In this paper we describe a computational methodology that is specifically designed for studying three-dimensional geodynamic processes governed by heterogeneous visco-plastic Stokes flow. The method employs a hybrid spatial discretization consisting of a Q 2 - P 1 disc mixed finite element formulation for the Stokes problem, coupled to a material-point formulation which is used for representing material state and history-dependent variables. The applicability and practicality of this methodology is realized through the development of an efficient, scalable and robust variable viscosity Stokes preconditioner. In this work, these objectives are achieved through exploiting matrix-free operators and a geometric multigrid preconditioner employing hybrid coarse level operators, Chebyshev smoothers and hybrid Krylov coarse level solvers. The robustness and parallel efficiency of this strategy is demonstrated using an idealized geodynamic model. Lastly, we apply the new methodology to study geodynamic models of continental rifting and break-up in order to understand the diverse range of passive continental margins we observe on Earth today.
The South Tianshan Accretionary Complex (STAC), forming the southern segment of the Central Asian Orogenic Belt, underwent a long-lived and subduction-related accretionary orogenic process. The high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks within this complex are traditionally considered to be metamorphic ophiolitic slices. In this paper, we report a detailed study of petrology and water content of nominally anhydrous minerals (NAMs) of granulites from the Yushugou HP massif occurring as a fault-bounded tectonic slab in the Paleozoic accretionary complex. The studied granulites consist of garnet, orthopyroxene, plagioclase, K-feldspar, quartz, biotite, ilmenite and rutile and show distinct mylonitic foliation. The augen garnet is dominated by almandine and pyrope components, and has compositional zoning with increasing grossular content from the core to rim of the grain. The augen orthopyroxene has high Al2O3 content (up to 7.91 wt.%), and shows compositional zoning characterized by a decreasing Al2O3 content from core to rim. Phase equilibria modeling indicates that the granulite underwent ultrahigh-temperature (UHT) (> 930 °C) and HP (10.5–14.5 kbar) metamorphism and partial melting under a high geothermal gradient of ca. 24 °C/km, and a possible prograde process characterized by heating and burial. Analyses of Fourier transform infrared spectroscopy indicate that hydrogen was incorporated in all NAMs of the granulites in the manner structural OH and sub-microscopic fluid inclusions and that the average water content (H2O weight) is in the range of 63–215 ppm in garnet, 1–54 ppm in orthopyroxene, 172–533 ppm in feldspar and 34–66 ppm in quartz. The present results show that the Yushugou massif probably derived from the deep root of hot continental magmatic arc. The trace amounts of water in NAMs obviously affected ductile deformation of the near-dry granulites. This study indicates that the thickened lower crust of the Paleozoic Tianshan accretionary orogen is characterized by high-thermal flow, UHT granulite-facies metamorphism, anatexis, ductile deformation and coeval magmatism and crustal growth.
Geochronological and geochemical data regarding eclogites and their metasedimentary host rocks exposed in two localities of the South Tianshan (U)HP-LT metamorphic belt are presented to reveal the protolith of the eclogites and the provenance of the metasediments. The rim domains of zircon grains from the eclogites contain omphacite, phengite and rutile inclusions and give a U-Pb Concordia age of 321.4 ± 2.4 Ma, representing the peak of eclogite-facies metamorphism. The core domains of zircon grains with magmatic oscillatory zoning yield a U-Pb Concordia age of 453.9 ± 9.4 Ma, suggesting a Late Ordovician age for the eclogites' protolith. Furthermore, the inherited cores of some zircon grains have apparent U-Pb ages between 609 Ma and 2305 Ma, implying the involvement of the Precambrian basement in the formation of the eclogites' protolith. The depletion of high field strength elements and the trace element ratios indicate the eclogite protolith's continental arc affinity. The zircon U-Pb age data of the high-pressure micaschists yield seven age groups ranging from 401 Ma to 3201 Ma and cluster at a pronounced peak of ~ 445 Ma. The major and trace element compositions of the micaschists overlap those of the average upper continental crust. The protolith of the micaschist seems to have formed at an accretionary wedge, which is predominantly composed of sediments derived from Ordovician-Silurian arc-type magmatic rocks and Precambrian basement rocks in an active continental margin. The basic blocks represented by the protoliths of the eclogites were most likely scraped from the basement of a continental arc by basal erosion during the subduction of the South Tianshan Ocean in Late Paleozoic. At the same time, the fragments composing the micaschists' protoliths are believed to have been dragged into the subduction channel by the frontal erosion of the accretionary prism. Both the basic blocks and the sediment fragments were forced into the subduction channel, mingled together, and then transported to the deeper parts of a cold subduction zone where they experienced high pressure metamorphism.
A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.
[1] Basement-cored ranges formed by reverse faulting within intracontinental mountain belts are often composed of poly-deformed lithologies. Geological data capable of constraining the timing, magnitude, and distribution of the most recent deformational phase are usually missing in such ranges. In this paper, we present new low temperature thermochronological and geological data from a transect through the basement-cored Terskey Range, located in the Kyrgyz Tien Shan. Using these data, we are able to investigate the range's late Cenozoic deformation for the first time. Displacements on reactivated faults are constrained and deformation of thermochronologically derived structural markers is assessed. These structural markers postdate the earlier deformational phases, providing the only record of Cenozoic deformation and of the reactivation of structures within the Terskey Range. Overall, these structural markers have a southern inclination, interpreted to reflect the decreasing inclination of the reverse fault bounding the Terskey Range. Our thermochronological data are also used to investigate spatial and temporal variations in the exhumation of the Terskey Range, identifying a three-stage Cenozoic exhumation history: (1) virtually no exhumation in the Paleogene, (2) increase to slightly higher exhumation rates at ~26–20 Ma, and (3) significant increase in exhumation starting at ~10 Ma.
Tarim Craton, located in the center of Asia, was involved in the assembly and breakup of the Rodinia supercontinent during the Neoproterozoic and the subduction-accretion of the Central Asian Orogenic Belt (CAOB) during the Paleozoic. However, its tectonic evolution during these events is controversial, and a link between the Neoproterozoic and Paleozoic tectonic processes is missing. Here we present zircon U-Pb ages, Hf isotopes, and whole-rock geochemical data for the extensive granitoids in the western Kuruktag area, northeastern Tarim Craton. Three distinct periods of granitoid magmatism are evident: circa 830-820 Ma, 660-630 Ma, and 420-400 Ma. The magma sources, melting conditions (pressure, temperature, and water availability), and tectonic settings of various granitoids from each period are determined. Based on our results and the geological, geochronological, geochemical, and isotopic data from adjacent areas, a long-lived accretionary orogenic model is proposed. This model involves an early phase (circa 950-780 Ma) of southward advancing accretion from the Tianshan to northern Tarim and a late phase (circa 780-600 Ma) of northward retreating accretion, followed by back-arc opening and subsequent bidirectional subduction (circa 460-400 Ma) of a composite back-arc basin (i.e., the South Tianshan Ocean). Our model highlights a long-lived accretionary history of the southwestern CAOB, which may have initiated as part of the circum-Rodinia subduction zone and was comparable with events occurring at the southern margin of the Siberian Craton, thus challenging the traditional southward migrating accretionary models for the CAOB.