Scott E. Johnson

University of Maine | UM · School of Earth and Climate Sciences

The 370–377 Ma Mooselookmeguntic igneous complex was emplaced at ~14 km depth into steeply dipping metaturbidites that were folded and metamorphosed ca. 400–405 Ma during the Acadian orog-eny. Gravity and drill-hole data, isograd geometry, thermal modeling, and struc-tural measurements all indicate that the eastern aureole of the complex represents...

Coseismic off-fault damage and pulverization significantly influence the mechanical and transport properties, and in turn the rupture dynamics of faults. Although field-based, laboratory, and numerical studies help elucidate the structure of damage zones adjacent to modern strike-slip faults, the vertical extent of these zones remains an open quest...

Off-fault damage zones comprise highly fractured rocks surrounding the dynamic slip surface of faults. These damage zones modify fault-zone rheology and rupture dynamics by changing the bulk elastic properties and modulating fluid flow. Damage zones in the brittle upper crust, reaching widths >100 m, are commonly characterized by measuring fracture...

The α ↔ β phase transition in quartz is accompanied by relatively large, anisotropic changes in cell volume and elastic stiffness properties. Because quartz is among the most abundant minerals in Earth's continental crust, these changes have the potential to profoundly influence the state of stress, mechanical properties, metamorphism, and rheology...

Seismological fracture or breakdown energy represents energy expended in a volume surrounding the advancing rupture front and the slipping fault surface. Estimates are commonly obtained by inverting ground motions and using the results to model slip on the fault surface. However, this practice cannot identify contributions from different energy‐con...

Strain localization occurs across the crust in both brittle and viscous regimes, but the exact causes remain debated. Natural rock observations suggest that changes in phase properties (such as physical properties, phase distribution, and grain geometry) are more influential in weakening than variations in stress and temperature. Investigating the...

The mechanical heterogeneity of Earth's lithosphere leads to significant amplification of stresses across spatial scales ranging from mineral grains to tectonic plates. These stress amplifications play a key role in mechanical and chemical processes within the rock that affect bulk rock strength. Identifying the most effective causes of stress ampl...

Mature faults with large cumulative slip often separate rocks with dissimilar elastic properties and show asymmetric damage distribution. Elastic contrast across such bimaterial faults can significantly modify various aspects of earthquake rupture dynamics, including normal stress variations, rupture propagation direction, distribution of ground mo...

Recognizing that exhumed shear zones had a seismogenic history is an important prerequisite for microstructural studies of energy partitioning in the earthquake source region. Using optical measurements of kinked micas along the Sandhill Corner shear zone, a seismogenic fault/shear zone exhumed from the base of the frictional-to-viscous transition,...

A macroscopic geological structure can geometrically map a local rock material anisotropy into a larger volume that may have different net anisotropic properties on a scale to which seismic waves respond. The bulk structure’s anisotropy intensity, symmetry type and orientation of symmetry axes will generally be different from the local rock; a typi...

A macroscopic geological structure can geometrically map a local rock material anisotropy into a larger volume that may have different net anisotropic properties on a scale to which seismic waves respond. The bulk structure's anisotropy intensity, symmetry type and orientation of symmetry axes will generally be different from the local rock; a typi...

Material anisotropy may significantly influence the behavior of fluid transport in sedimentary basins and other environments with laminated or foliated rocks. In this paper, we present a fracture mechanics model to investigate primary migration of petroleum (oil and gas) through propagation of a vertical, buoyancy-driven blade crack in a transverse...

A numerical methodology is presented for the plane stress analysis of pervasive cracking in heterogeneous materials. The smeared crack band concept is used in conjunction with the multi-directional crack model to objectively model cracking in a finite element analysis while allowing cracks to form at different orientations. The multi-directional cr...

Shear zones have different rheological properties than the surrounding rocks, indicating that the bulk strength of regions containing shear zone networks cannot be determined by considering the the host rock rheology alone. We demonstrate the value of this concept at the microscale. We first consider the phase arrangements in naturally deformed roc...

We present a numerical methodology for the thermomechanical analysis of real polycrystalline material microstructures obtained using electron backscatter diffraction techniques. The asymptotic expansion homogenization method is used in conjunction with the finite element method to perform comprehensive micromechanical analyses and determine the eff...

Microstructural investigations were carried out on quartz veins in schist, protomylonite, and mylonite samples from an ancient seismogenic strike-slip shear zone (Sandhill Corner shear zone, Norumbega fault system, Maine, USA). We interpret complexities in the microstructural record to show that: (1) pre-existing crystallographic preferred orientat...

We investigate extension of a vertical crack filled by water and methane gas induced by dissociation of methane hydrate in low permeability muddy sediment. We model the sediment as an impermeable elastic medium and consider a thin sheet configuration of hydrate that initially occupies a vertical fracture. The crack development and the amount of dis...

The strength of a polyphase aggregate comprising power-law materials is a function of the constitutive laws of the phases present, the arrangement of those phases and environmental conditions such as temperature. Primarily for geological applications,we consider the degree to which the arrangement of the phases has a significant influence on bulk s...

The constitutive laws of polyphase aggregates dominantly depend on the operative deformation mechanisms, phase morphology and modes, and environmental conditions. Each of these factors has the potential to dramatically affect bulk mechanical properties as well as the local stress and strain rate distributions. To focus on the effects of phase morph...

The present work describes a multi-physics model to investigate subcritical propagation of initially oil-filled, sub-horizontal collinear microcracks driven by the excess pressure induced by the conversion of oil to gas in a petroleum source rock under continuous burial. The crack propagation distance, propagation duration, crack coalescence and ex...

The kinematic record and bulk viscous strength of polyphase rocks depend in part upon the relative strengths and distributions of rheologically distinct fabric elements. Here, we explore the effects of microstructural and rheological heterogeneity in porphyroblastic schists. Electron backscatter diffraction and petrographic analyses reveal asymmetr...

To investigate primary petroleum migration through microfracturing of source rocks, we develop a theoretical multiphysics model incorporating simultaneous generation of oil and gas from kerogen, elastic anisotropy of the source rock and propagation of microcracks filled with oil and gas. The variations of excess fluid pressure in the crack and crac...

This paper examines the development of a subvolcanic magmatic breccia located along the contact of a granitic intrusion using fractal analysis and thermal-elastic modeling. The breccia grades from clast-supported, angular clasts adjacent to unfractured host rock to isolated, rounded clasts supported by the granitic matrix adjacent to the intrusion....

In this paper, we investigate subcritical propagation of an initially oil-filled, sub-horizontal microcrack driven by the excess fluid pressure associated with the conversion of oil to gas in a petroleum source rock under continuous burial. The crack propagation distance and propagation duration (the time required for the crack to propagate during...

The integration of zircon U–Pb ages and trace element chemistry with structural and petrologic rela-tions from a range of sample types provides important temporal constraints on the tectono-metamorphic evolution of the southern Parry Sound domain (PSD), Ontario, Canada, and the processes attending devel-opment of the underlying Twelve Mile Bay shea...

On a strand of the Norumbega fault system, a Paleozoic, subvertical, seismogenic fault system in northeastern New England, USA, we document changes associated with the formation and deformation of pseudotachylyte to form ultramylonite/phyllonite layers. We consider how those textural and mineralogical changes affected the rheology of the layer and...

Structural, microstructural and petrological data have enabled determination of the mechanical and geochemical processes involved in dynamic weakening and fabric transposition along the margins of a granulite nappe [the Parry Sound domain (PSD)] during transport to mid-crustal levels of the Grenville Orogen. The data establish a genetic link betwee...

The anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks. Phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust. Crenulation cleavage is one of the most common microstructural fabrics found in mul...

We present a finite difference/perturbation method to investigate transient dike propagation from a magma chamber 1–2 km below the level of neutral buoyancy in oceanic crust and explore the role of density gradation in dike propagation arrest. The dike is modeled as a magma-filled blade crack and the host rock is assumed to be a linear elastic medi...

We study the influence of microstructural variables on seismic wave speed anisotropy in crustal rocks. The bulk elastic properties and corresponding wave velocities are calculated for synthetic rock samples with varying amounts of muscovite and quartz, different muscovite and quartz grain orientations and varying spatial distributions of the muscov...

We conduct a parametric study on the subcritical propagation of an oil-filled, penny-shaped microcrack induced by the pressure increase caused by transformation of kerogen to oil. The excess oil pressure on the crack surfaces, and the subcritical crack propagation distance and duration, are obtained using a coupled model of fracture mechanics and k...

The finite element method was used to investigate how the elastic interactions of quartz and muscovite minerals affect grain-scale stress and strain distributions at different stages of crenulation cleavage development. The polymineralic structure comprises individual grains that were each assigned their own 3D stiffness tensor and orientation. Gra...

We use a fracture mechanics model to study subcritical propagation and coalescence of single and collinear oil-filled cracks during conversion of kerogen to oil. The subcritical propagation distance, propagation duration, crack coalescence and excess oil pressure in the crack are determined using the fracture mechanics model together with the kinet...

The kinematic vorticity number is an important quantity in structural geology and tectonics, giving a nonlinear ratio of simple shear to pure shear deformation. We use natural observations and numerical models to show how rigid clast methods for determining the kinematic vorticity number (Wk) are compromised where strain localization occurs at the...

a b s t r a c t Clast-based methods for estimating the mean kinematic vorticity number W m are compromised by strain localization at the clast margins. Localization increases with modal matrix mica content as determined with samples from the Sandhill Corner mylonite zone – a crustal-scale, high-strain, strike-slip shear zone in Maine. Using these s...

A well preserved strain and reaction gradient records the progressive transformation of a megacrystic Kfs+Cpx+Opx+Bt1±Qtz syenitic pluton to a strongly sheared Kfs+Act+Bt2+Ab+Qtz tectonite within the exhumed Norumbega Fault System, Maine, USA. Detailed microstructural analysis indicates that fracturing and localized fluid infiltration initiated the...

We address under what conditions a magma generated by partial melting at 100 km depth in the mantle wedge above a subduction zone can reach the crust in dikes before stalling. We also address under what conditions primitive basaltic magma (Mg # >60) can be delivered from this depth to the crust. We employ linear elastic fracture mechanics with magm...

This contribution shows unequivocally that porphyroblasts rotate relative to one another during ductile deformation. The porphyroblasts described here have special significance because they are from the original "millipede" rocks that led to the nonrotation hypothesis. Thus, the debate that has lasted for more than 20 years is settled. Despite this...

We present a fracture-mechanics-based formulation to investigate primary oil migration through the propagation of an array of periodic, parallel fractures in a sedimentary rock with elevated pore fluid pressure. The rock is assumed to be a linearly elastic medium. The fracture propagation and hence oil migration velocity are determined using a frac...

Dike swarms consisting of tens to thousands of subparallel dikes are commonly observed at Earth's surface, raising the possibility of simultaneous propagation of two or more dikes at various stages of a swarm's development. The behavior of multiple propagating dikes differs from that of a single dike owing to the interacting stress fields associate...

1] We present a series of three-dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid-crustal section in the New England Appalachians. The natural mid-crustal section records evidence for spatially a...

The kinematic behaviour of rigid particles in flowing viscous fluids is a fundamental problem in various branches of science and mathematics. In the Earth sciences, there are several primary areas of interest. For example, we are interested in how the bulk viscosity of flowing materials, such as magma, changes when rigid particles are introduced. E...

4.8.1 Overview Changing mineralogy during metamorphism can significantly affect the strength of mid-crustal rocks, and by extension the strength of relatively large volumes of the middle crust. The growth of relatively strong porphy-roblasts during prograde metamorphism is a common phenomenon in metapelitic rocks and the increasing abundance of eff...

Microstructural and chemical processes interacted in the transformation of a hornblende-biotite tonalite to a mylonite in an approximately 28-m wide shear zone on the margins of the Cerro de Costilla complex, Baja California, México. Deformation in the tonalite at the edge of the shear zone was initiated by cleavage slip in biotite, leading to wisp...

We present a perturbation method to investigate the steady-state propagation of a dyke from an over pressured source (e.g. a magma chamber) into a semi-infinite elastic solid with graded mass density. The non-linear dyke propagation/magma transport problem is reduced to a series of linear problems using a perturbation technique with the small non-d...

In the low-pressure, high-temperature metamorphic rocks of western Maine, USA, staurolite porphyroblasts grew at c. 400 Ma, very late during the regional orogenesis. These porphyroblasts, which preserve straight inclusion trails with small thin-section-scale variation in pitch, were subsequently involved in the strain and metamorphic aureole of the...

Foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite-facies metaturbidites in the Presidential Range of the White Mountains, in eastern New Hampshire. An early-formed foliation, developed during km-scale nappe folding, consistently displays larger bedding-foliatio...

An estimate for the vorticity of a mid-Paleozoic deformation was made using deformed and/or rotated calcite veins as natural vorticity and shear-strain gauges in a northern part of the U.S. Appalachians. The spatial distribution of extended, shortened, and shortened-then-extended elongate fibrous calcite veins in limestone layers were used to const...

Numerical models are used to examine the effects of porphyroblast growth on the rheology of compositionally layered rocks (metapelites and metapsammites) and by extension the middle crust during prograde metamorphism. As porphyroblast abundance increases during prograde metamorphism, metapelitic layers will strengthen relative to porphyroblast-free...

Pre-Silurian bedrock units played key roles in the early Paleozoic history of the Maine-Quebec Appalachians. These units represent peri-Laurentian material whose collision with the craton deformed the Neoproteozoic passive margin and initiated the Appalachian mountain-building cycle. We present new field, petrological, geochronological, and geochem...

The Chain Lakes massif has long been an enigmatic component of the Appalachian orogen, but new structural, microstructural, and geochronological information provides the basis for the following new interpretation of the massif and its history. In the early Paleozoic, sediments and volcanic rocks from Laurentia or a Laurentian-derived microcontinent...

In this paper we model coupled spatial and temporal changes in stress and temperature to calculate dislocation creep strain rates in host rock associated with spherical magma chamber expansion with and without material removal by stoping and/or assimilation. Given a constant magma-chamber pressure of 100 MPa, we show that stress and temperatures in...

Low-pressure anatexis, whereby rocks melt in place after passing through the andalusite stability field, develops under more restricted conditions than does low-pressure metamorphism. Our thermal modelling and review of published work indicate that the following mechanisms, operating alone, may induce anatexis in typical pelitic rocks without induc...

The San José pluton in Baja California, México, comprises at least two well-defined, texturally distinct units. The northern unit was intruded by the central unit after the former had extensively crystallized at its margins. During intrusion of the central unit, the margin of the northern unit underwent brittle and crystal–plastic deformation, at l...

In the San José tonalite pluton, Baja California, México, interior magmatic fabrics give way progressively to subparallel, solid-state fabrics with S–C structures near the margins. The outer rim (‘marginal northern unit’), approximately 1 km thick, is inferred to have been deformed during the emplacement of the inner parts of the magma chamber. The...

Mount Desert Island, Maine, comprises a shallow level, Siluro-Devonian igneous complex surrounded by a distinctive breccia zone ("shatter zone" of Gilman and Chapman, 1988). The zone is very well exposed on the southern and eastern shores of the island and provides a unique opportunity to examine subvolcanic processes. The breccia of the Shatter Zo...

Dikes must maintain a critical width and flow velocity in order to propagate several kilometers from source to sink without freezing; consequently, they must support a high volumetric magma flow rate. This in turn implies that plutons fed by dikes must fill rapidly. In an effort to predict host-rock strain rates required by dike-fed growth, we empl...

The 108 km2 San José pluton forms part of the Jurassic to Cretaceous Peninsular Ranges batholith of northern Baja California, México. The pluton was formed by three nested, southward-migrating intrusive pulses, and the internal contacts between them indicate juxtaposition while the adjoining pulses were magmas. SHRIMP U–Pb zircon data indicate that...

In metapelitic rocks of western Maine, a pluton-related M3 metamorphic gradient ranging in grade from garnet to upper sillimanite zone was superposed on a fairly uniform M2 regional metamorphic terrain characterized by the assemblage andalusite+staurolite+biotite+/−garnet. As a result, M2 assemblages re-equilibrated to the P, T, and aH2O conditions...

Subvolcanic ring complexes are unusual in that they preserve a rapidly frozen record of intrusive events. This sequential history is generally lost or complicated in plutons owing to mixing and mingling in a dynamic state. Thus, subvolcanic ring complexes are more like erupted rocks in their preservation of instantaneous events, but the self-contai...

The Jura-Cretaceous Peninsular Ranges batholith (PRb) of southern and Baja California is a remarkable example of a zoned batholith containing distinct oceanic (western) and continental (eastern) basements. The transition between these basements is marked by a crustal-scale boundary along which distinct volcano-sedimentary, structural, and metamorph...

Diapirism and dike-fed expansion are two popular models for the ascent and emplacement of large batches of granitoid magma into the middle and upper crust. Many structural and petrologic relationships in and around plutons can be cited as permissive evidence for either model, and so it has been difficult to differentiate between them on these groun...

In this paper we provide the following three examples of how the software system Mathematica can be used to reconstruct or model the three-dimensional shapes of folded surfaces. (1) First, we revisit the reconstruction of the central inclusion surface within a garnet porphyroblast that contains spiral-shaped inclusion trails. (2) Next, we revisit t...

Cretaceous (115-103 Ma) tonalites throughout the western Peninsular Ranges Batholith of Baja California can be explained as hybrids between andesitic fractionates (similar to 65%) and dehydration melts of amphibolite (similar to 35%). Mixing occurred between these two magmas at shallow (<0.23 GPa) crustal levels, where heterogeneous tonalite-trondh...

In the Cooma Complex, SE Australia, leucosomes first appear as small patches and veinlets in high-grade, muscovite-free gneisses containing cordierite, andalusite and K-feldspar. Simultaneously, fibrous sillimanite appears in discontinuous folia. The leucosomes consist of quartz, microperthitic K-feldspar and cordierite, rarely with minor andalusit...

Many recent papers show how porphyroblast microstructures play an important role in a wide range of structural and metamorphic studies. This paper reviews ten current applications of these microstructures: (1) porphyroblast growth-timing criteria; (2) tracking progressive foliation development relative to changing metamorphic conditions; (3) timing...

Geologic and SHRIMP U-Pb zircon studies in the Sierra San Pedro Mártir area of northern Baja California, Mexico, suggest that the western and eastern parts of the Peninsular Ranges batholith originated as separate arcs. They are now juxtaposed along a well-exposed ductile thrust (Main Mártir thrust) marked by an age discontinuity of at least 10 to...

The western half of the Cooma Complex, New South Wales, consists of three thrust-bound blocks that contain the same structural fabrics, but with different orientations and intensities, owing largely to heterogeneous strain late in the deformation history. Correlation of these fabrics with those found regionally outside the complex shows that a well...

Orogenic belts are geometrically complex owing to repeated deformation. Within this complexity, there is evidence that may suggest a common pattern of sequential steeply dipping and gently dipping foliations. Seven possible explanations are presented for the sequential development of these foliations, which can probably be reduced to four of genera...

The Early Cretaceous (∼115 Ma) Zarza Intrusive Complex is a small (<10 km2), bimodal ring complex that may represent a magmatic microcosm of the western Peninsular Ranges batholith. Its tholeiitic gabbro bosses (25% by area; Al2O3 > 17 wt %, Sr < 463 ppm) formed at subvolcanic depths <0.2 GPa (8 km) by >30% plagioclase accumulation from andesitic m...