Rheology and Deformation of the Lithosphere at Continental Margins - Summaries

Consequences of Asthenospheric Variability on Continental Rifting - W. Roger Buck

Models of continental rift evolution have typically tended to focus on the effects of lithospheric structure on rifting. Such models have had considerable success in explaining many rift features. As a possible solution to outstanding tectonic extension puzzles, Buck examines the less commonly considered role of the asthenosphere. In particular, he addresses three "paradoxes"�: 1) The "Tectonic Force" paradox, whereby the force required to extend thick lithosphere is apparently greater than is tectonically available. 2) The "Extra Subsidence"� (or "Upper Plate"�) paradox, which arises from a failure of simple dynamic and kinematic lithosphere-controlled models to explain the amount of subsidence observed in some rift locations. 3) The "No Magma"� paradox is an absence of significant magmatism associated with highly attenuated crust at some margins. He postulates that massive melting events associated with upwelling of anomalously hot mantle asthenosphere and its subsequent cooling may be the cause. Intriguingly, this approach offers a possible explanation for "No Magma"�, as well as the other paradoxes. While Buck acknowledges the speculative nature of his calculations, his approach encourages development of models that further investigate the joint role of asthenosphere and lithosphere in controlling continental rifting.

Velocity Fields, Faulting, and Strength on the Continents - James Jackson

In most continental areas, large-scale deformation can practically be described by a velocity field (e.g., as derived from GPS measurements). However, such studies fail to resolve local detail and deformation due to discontinuous seismic slip events, which may be too rare to be reliably sampled over a limited time-frame. Jackson discusses the difficulties inherent in extrapolating long-term average velocity fields from continuous deformation and discontinuous fault slip, and questions the assumptions made in translating surface vectors to deformation of the lithosphere through its whole thickness. In so doing, he raises arguments relating to shifts in the velocity field and/or active fault distribution as crustal blocks rotate out of favorable orientations, and to the ongoing debate regarding effective elastic thickness, seismogenic thickness, and the strength of continental lithosphere.

Mechanics of Low-Angle Normal Faults - Gary J. Axen

Whether lithospheric extension can progress by detachment faulting at low angles is controversial. This is because standard fault mechanical theory has so far been unable to explain such faulting, and diagnostic seismic events are rare. However, inactive low-angle faults have been observed in many locations worldwide; the crucial question remains, were these faults active at these low dips or were the faults rotated after the faulting became inactive? Axen details examples of low-angle faults for which observations indicate that the dip at the time of formation and/or during frictional slip was less than 30º. Examination of the field evidence in the context of predictions that can be made using mechanical models shows that a large gap remains between field observations and predictions that can be made using mechanical models. The implication is that our understanding of crustal mechanics is significantly lacking in some areas.

Depth-Dependent Lithospheric Stretching at Rifted Continental Margins - Mark Davis and Nick Kusznir

Rifting models in which extension is uniform with depth have enjoyed enduring success in explaining postrift subsidence of intracontinental rifts. However, observations of depth-dependent extension, including evidence of laterally extensive exhumation of mantle rocks at high extension factors in some margins, suggest that uniform extension models fail to address a major component of rift development. Davis and Kusznir explore new evidence for depth-dependent extension in the Goban Spur, Galicia Bank, Voring, and South China Sea continental margins. The two authors apply simple analytical and finite element fluid-flow models to investigate possible mechanisms for depth-dependent extension and mantle exhumation. Although more constraint on the timing of differential extension is needed, Davis and Kusznir find that continental lower crust and mantle could potentially be drawn out from under the extended upper crust by early seafloor spreading.

Limits of the Seismogenic Zone - Larry J. Ruff

Typically, continental seismogenesis is confined to the upper 15km of crust, but there are many exceptions. These include subduction zone interplate seismicity (<~40km depth), subducted lithosphere intraplate seismicity (<~700km depth), and, less commonly, collision zone events, intraplate oceanic events and rarer events in certain continental settings. Ruff provides an overview of the practical and conceptual problems affecting determination of the upper and lower limits of the seismogenic zone. He combines contributions from seismology and thermal modeling to test whether the maximum depth of the seismogenic zone is temperature-limited, finding that evidence from subduction zone seismicity and "exceptional" deep continental earthquakes conflicts with apparent critical temperature limits elsewhere (350°C in crustal rocks and most plate boundaries; 700-750°C in oceanic lithosphere mantle). His observations suggest a limiting mechanism that is not yet fully understood.

Controls on Subduction Thrust Earthquakes: Downdip Changes in Composition and State - R. D. Hyndman

The potential human impact and seismic magnitude of subduction zone earthquakes depends on the depths and locations of the updip and downdip limits of the seismogenic zone. In particular, shallow updip limits tend to be associated with larger than average tsunamis for a given earthquake magnitude, while maximum earthquake amplitudes are partly controlled by the extent of the source area bounded by the updip and downdip limits. The landward extent of the seismogenic zone, which is governed by the downdip limit, affects the potential proximity of large earthquakes to coastal cities. In this paper, Hyndman focuses on the factors controlling the updip and downdip limits to the seismogenic zone, with consideration of factors including compositional boundaries in the overlying plate, temperature, stress regime, pressure, slip rates, mineralogy, and subduction zone inputs. Predictions made by a range of models are compared against observed seismogenic zone limits from a number of subduction zones worldwide. Hyndman's conclusions find consistency between model and observed behavior, but with large uncertainties on both the modeling parameters and the translation of observations to source area boundaries that may extend beyond historically observed activity.

Thermo-Mechanical Models of Convergent Orogenesis: Thermal and Rheologic Dependence of Crustal Deformation - Sean D. Willett and Daniel C. Pope

Deformation associated with orogenesis driven by plate convergence is diverse and complex, and can be distributed up to hundreds of km from the plate boundary. Willet and Pope present a historical overview of mechanical convergent orogenesis models, and review the importance of numerical (especially finite-element) models in investigation of the processes at work. They use their own finite-element models to investigate the role of crustal rheology in the evolution of two well-studied convergent orogen configurations. In the first case they consider formation of a "doubly vergent"� orogenic wedge, in which post-subduction continent-continent collision thrusts material over the two convergent plates, deforming and elevating the overlying material. In the second case they demonstrate an apparent role of temperature-dependent crustal flow in formation of orogenic plateaus such as the Tibetan plateau. Because their finite-element model allows constitutive behavior (viscous and/or plastic) to be controlled, along with physical parameters and state-variable dependencies thereof, it can aid investigation of rheological parameters beyond those that are observable. Willet and Pope believe that evolution of these models will allow more sophisticated investigation of such systems as understanding of the governing tectonic processes increases.

Structure of Large-Displacement, Strike-Slip Fault Zones in the Brittle Continental Crust - F. M. Chester, J. S. Chester, D. L. Kirschner, S. E. Schulz, and J. P. Evans

The exhumed Punchbowl and North Branch San Gabriel faults in southern California expose products of large-displacement brittle faulting in the upper continental crust. Though inactive, their location within the active Pacific-North America rifting segment of the San Andreas fault system, means that their study can yield insights into the products of brittle faulting in the upper continental crust in that region. Chester et al. present a synthesis of recent and published work on these faults, investigate particle grinding and slip distribution within the fault cores, and address the relationship between fault-zone structure and mechanical and fluid-flow properties. They find that a hypothesis that damage between paired fault cores can be predicted by linear superposition of damage from two single cores is inconsistent with observations of damage around the single core (North Branch San Gabriel) and paired core (Punchbowl) faults. The authors suggest that the excess damage between the paired Punchbowl fault cores is caused by mechanical interaction between the two faults. Overall structure and particle-size distributions are similar relative to structural position away from each fault core location and along-strike. Hence the two locations apparently share similar histories of high-shear strain and enhanced fluid-rock reactions within the fault cores and stress cycling throughout the fault-displacement. Questions remain as to whether processes such as extreme slip localization at 2-5 km depths are common to other large-displacement strike-slip faults.

The Strength of the San Andreas Fault: A Discussion - Christopher H. Scholz and Thomas C. Hanks

Scholz and Hanks extensively discuss the long-standing controversy - initially fuelled by apparent weakness of the San Andreas fault - over whether the Byerlee friction law actually applies to all faults. In the context of purported evidence for San Andreas weakness and low-angle faulting, they ask whether continental faults really can be weaker than laboratory experiments predict, and if so what conditions allow such weakness. The historical arguments are complex, but the authors make a case that the available evidence, including the widely believed “proof” from low observed heatflow, is either equivocal or negative in regard to the low-strength hypothesis.

Deformation Behavior of Partially Molten Mantle Rocks - Yaqin Xu, M. E. Zimmerman, and D. L. Kohlstedt

Small melt fractions can strongly influence the deformation and seismic properties (velocity, attenuation and anisotropy) of partially molten rocks. Xu et al. present new results from experiments on anelastic and viscous deformation of aggregates of olivine and mid-ocean ridge basalt melt under low stresses, low strains and low seismic frequencies. The results are discussed in the context of recently published studies of seismic attenuation in aggregates of olivine � melt under steady-state (large strain) deformation, and with reference to laboratory and theoretically derived creep flow laws. The final section of the paper focuses on experiments and theory on formation of melt-rich, high-permeability channels due to positive-feedback pressure gradients developed during deformation in partially molten rock. Because viscosity and permeability are sensitive to melt fraction, these channels should have strong anisotropic and spatially heterogeneous effects on seismic wave and shear zone properties in the mantle.

Relations Among Porosity, Permeability, and Deformation in Rocks at High Temperatures - Brian Evans, Yves Bernabé, and Greg Hirth

Permeability and porosity of partially molten rock are dynamic quantities that vary in time and space. These properties influence and are influenced by deformation. Hence, melt flow dynamics in environments such as spreading centers and subduction zones are dependent on rheological and transport properties of the partially molten rock, and on deformation and fluid flow interaction. Using experimental results, Evans et al. consider the problems in three parts: 1) Effects of dynamic pore space geometries on mechanical and transport properties. 2) Evolution of porosity and permeability under the influence of dynamic pore structure, driving forces and thermodynamic conditions. 3) Qualitative description of permeability evolution given an initial state in terms of geometry, driving forces, and mechanisms. Although focused on melt transport, their discussion is also relevant to fluid-flow in other environments, such as accretionary wedges.

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