Christian Teyssier

Strain modeling of displacement-field partitioning in transpressional orogens

In regions of oblique plate convergence, deformation is generally partitioned into strike-slip and contractional components. We use a strain model of transpression, based upon a three-dimensional velocity gradient tensor, to address the question of why such partitioning occurs. An exact relationship between angle of plate convergence, instantaneous strain, and finite strain is calculated, providing a predictive tool to interpret the type and orientation of geological structures in zones of oblique convergence. The instantaneous and finite strain axes are not coincident, and thus there is no simple relationship between instantaneous strain (or stress) and structures that developed over a protracted deformation, such as crustal-scale faults. In order to simulate more realistic geological settings, we use a partitioning model of transpression which quantifies the effect of fault efficiency on the displacement field. This model is applied to two transpressional settings: Sumatra and central California. Sumatra shows a very low degree of partitioning of the displacement field and a relatively small offset on the Great Sumatran fault. In contrast, central California displays a large degree of displacement partitioning, and efficient slip on the San Andreas fault system. Since both systems show partitioning of deformation into strike-slip and thrust motion, it is unlikely that the San Andreas fault is rheologically ‘weaker’ than the Great Sumatran fault. We propose that the difference in fault efficiency results from kinematic partitioning of the displacement field controlled by relative plate motion, rather than mechanical decoupling generated by fault weakness.

Oblique plate motion and continental tectonics

Three-dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins. Kinematic models of strike-slip partitioning for transpression and transtension allow the exact relation among three critical parameters—plate motion, instantaneous strain axes, and degree of strike-slip partitioning—to be calculated. Application to two end-member tectonic environments characterized by a low (South Island, New Zealand) and high (central California) degree of strike-slip partitioning demonstrates a remarkable consistency among the three parameters, suggesting that strike-slip partitioned transpression is a valid model for deformation in these regions. The extreme degree of strike-slip partitioning in wrench-dominated systems, such as central California, is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes.

Gneiss domes and orogeny

Many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust. Exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression. The decreasing buoyancy associated with increasing melt fraction drives further decompression at near-isothermal conditions as the partially molten crust rises diapirically. This combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep-seated migmatite terranes and inferred for active orogens. In exhumed orogens, the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite ± granite. The large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis.

Partial melting and flow of orogens

Abstract A comparison of large orogenic belts formed at convergent plate boundaries indicates that orogenic evolution comprises a period of crustal thickening followed by the formation of a continental plateau and commonly syn- to post-convergence extension and thinning of the previously thickened crust. The instantaneous and finite strain patterns recorded in orogenic belts do not appear to be linked in a simple way to plate kinematics. These patterns suggest that deformation of continental crust is not solely controlled by horizontal compression due to plate convergence and that gravity-driven flow is likely to play a significant role on the deformation of thickened crust. Geophysical data in southern Tibet, an actively convergent orogen, and geologic data of the exhumed mid-crustal core of eroded orogens reveals that orogenic crust is affected by widespread partial melting. Partial melting of thermally mature zones of thickened crust results in the generation of a layer of low-viscosity rocks that may affect the behaviour of the orogenic belt. In particular, the presence of this layer favours mechanical decoupling between the subducting plate and the overlying thickened orogenic crust, resulting in flow of the crust driven by contrasts in gravitational potential energy.

Crustal-scale, en echelon "P-shear" tensional bridges: A possible solution to the batholithic room problem

The apparent paradox of voluminous granitoid emplacement in an overall compressional magmatic are has been somewhat alleviated by the idea of pluton emplacement in tension cracks and dilational jogs within arc-parallel, strike-slip fault systems. As an alternative hypothesis, we propose that an en echelon P-shear array provides a continuous, batholith-scale zone of dilation along the trend of active magmatism and that it is favored by transpressional tectonics. In the Sierra Nevada, California, the Late Cretaceous granitoids of the Cathedral Range intrusive epoch are consistent with emplacement into bridges between P shears because (1) they are elongated oblique to the magmatic trend, consistent with P-shear orientation within a dextral system; (2) they were all emplaced within 5-10 m.y. along a 300 km trend, resulting in a displacement rate of ∼1 cm/yr across the arc; and (3) syn- to late-magmatic, dextral shear zones bound and crosscut the plutons, as in a P-shear tensional bridge.

Continental and oceanic core complexes

Core-complex formation driven by lithospheric extension is a first-order process of heat and mass transfer in the Earth. Core-complex structures have been recognized in the continents, at slow- and ultraslow-spreading mid-ocean ridges, and at continental rifted margins; in each of these settings, extension has driven the exhumation of deep crust and/or upper mantle. The style of extension and the magnitude of core-complex exhumation are determined fundamentally by rheology: (1) Coupling between brittle and ductile layers regulates fault patterns in the brittle layer; and (2) viscosity of the flowing layer is controlled dominantly by the synextension geotherm and the presence or absence of melt. The pressure-temperature-time-fluid-deformation history of core complexes, investigated via field- and modeling-based studies, reveals the magnitude, rate, and mechanisms of advection of heat and material from deep to shallow levels, as well as the consequences for the chemical and physical evolution of the lithosphere, including the role of core-complex development in crustal differentiation, global element cycles, and ore formation. In this review, we provide a survey of ∼40 yr of core-complex literature, discuss processes and questions relevant to the formation and evolution of core complexes in continental and oceanic settings, highlight the significance of core complexes for lithosphere dynamics, and propose a few possible directions for future research.

Transpressional kinematics and magmatic arcs

Abstract Most continental magmatic arcs occur in obliquely convergent settings and display strike-slip movement within, or adjacent to, the magmatic arc, and contractional structures in the forearc and backarc regions. Thus, three-dimensional transpressional kinematics typifies many arc settings, both modern and ancient. Intrusions cause magma-facilitated strike-slip partitioning, even in cases where the relative angle of plate convergence is almost normal to the plate boundary. Transpressional systems are preferentially intruded by magmas because of the steep pressure gradients in vertical strike-slip shear zones and their ability to force magma upward. Both buoyancy and transpressional dynamics cause a component of magma overpressuring, which in turn expels granitic magma upward following the vertical pressure gradient. The tectonic and magmatic processes are linked in a positive feedback loop which facilitates the upward movement of magma. We propose a lithospheric-scale, three-dimensional model of transpressional arc settings. Strike-slip motion is partitioned into the magmatic arc settings because of the linear and margin-parallel trend of the vertical, lithospheric-scale weakness caused by ascending magma. The parallelism of contraction structures in the forearc and backarc regions is caused by mechanical coupling through the lower crust and upper lithospheric mantle. The displacement field of the basal layer of the arc system provides the boundary condition for the upper-crustal, strike-slip partitioned deformation.

Partial convective overturn of Archaean crust in the east Pilbara Craton, Western Australia: driving mechanisms and tectonic implications

Strain varies systematically from weakly-developed, outward-dipping, S-tectonites in the ∼3320–3310±10 Ma Mount Edgar Batholith to intensely deformed, subvertical, L-tectonites in greenstones of the Warrawoona syncline. A consistent ‘greenstone-down/batholith-up’ sense of shear is recorded in batholithic domal margins and adjacent high-grade supracrustal rims: lineations converge to a central zone of subvertical extension (zone of sinking) along the synclinal axis. At domal margins, early kinematic granitoid sheets and ‘intrusive diatexites’ are subconcordant to a well-developed, dome-parallel schistosity, but late- to post-kinematic intrusives are discordant, high-level plutons. All granitoids are the same age, within analytical error. These syn-doming features conform with structural tests for diapirs, and differ from those expected during metamorphic core complex formation and cross-folding. Diapirism is part of a larger process involving partial convective overturn of the crust. Based on strain patterns and kinematic criteria, we argue that deformation was initiated by sinking of greenstones, which rapidly subsided; the domes then rose passively as accommodation structures. Ongoing doming (D4) partitioned strain to the southern rim of the batholith and uplifted a wedge of the Warrawoona Syncline keel. We consider that crustal overturn occurred in response to mantle plume activity: Eruption of a 5–10 km thick, ∼3325 Ma, mafic–ultramafic greenstone pile onto an older granite–greenstone terrain created a negatively buoyant crust, but convective overturn occurred some 5–20 Ma later, triggered by widespread anatexis (thermal softening). Convective overturn may have been a common Archaean process, occurring in response to a hotter mantle, and represents an end-member deformation mechanism that includes thrust-accretion of lithotectonic assemblages in other Archaean terranes, such as the Superior Province of Canada.

Thermomechanical evolution of a ductile duplex

The thermomechanical evolution of a midcrustal ductile duplex in central Australia has been reconstructed through space and time using 40Ar/39Ar thermochronology, flow stress estimates, cross-sectional restoration of dislocation creep microstructures, and microstructural and structural analysis. A critical aspect of this analysis is the identification of populations of white micas in quartzite mylonites that have neocrystallized below their closure temperature and which record the time when ductile deformation ceased. In dating these micas the mylonitic microstructures have effectively been dated. The time-temperature history of the duplex has been constrained through multidomain thermal modeling of K-feldspar 40Ar/39Ar data. The modeling demonstrates that a temperature gradient existed across the duplex during its formation. The concept of microstructural continuity during ductile deformation has great potential for elucidating the kinematic evolution of ductile duplexes. Mapping of the deformation mechanisms and recrystallized grain sizes of quartzites deformed under greenschist facies conditions has been used to evaluate tectonic offsets that occurred after microstructural freezing. This analysis shows that the duplex formed as a forward propagating thrust system accommodating ∼60 km of convergence between the upper and lower plates of the megathrust, with a significant fraction of the displacement occurring after microstructural freezing. Finally, using the data as input to published flow laws for quartz aggregates provides a strain rate history for the duplex. Although uncertainties are clearly large, the timing of highest-estimated strain rates during duplex evolution does, indeed, correlate with the highest rates of convergence between the upper and lower plates of the megathrust system (according to regional cooling history studies) and with coeval sedimentation in adjoining molasse basins.

Extension rates, crustal melting, and core complex dynamics

Two-dimensional thermomechanical experiments reveal that the crystallization versus exhumation histories of migmatite cores in metamorphic core complexes give insights into the driving far-field extensional strain rates. At high strain rates, migmatite cores crystallize and cool along a hot geothermal gradient (35–65 °C km−1) after the bulk of their exhumation. At low strain rates, migmatite cores crystallize at higher pressure before the bulk of their exhumation, which is accommodated by solid-state deformation along a cooler geothermal gradient (20–35 °C km−1). In the cases of boundary-driven extension, space is provided for the domes, and therefore the buoyancy of migmatite cores contributes little to the dynamics of metamorphic core complexes. The presence of melt favors heterogeneous bulk pure shear of the dome, as opposed to bulk simple shear, which dominates in melt-absent experiments. The position of migmatite cores in their domes reveals the initial dip direction of detachment faults. The migmatitic Shuswap core complex (British Columbia, Canada) and the Ruby–East Humboldt Range (Nevada, United States) possibly exemplify metamorphic core complexes driven by faster and slower extension, respectively.