Thin crème brûlée rheological structure for the Eastern California Shear Zone

Since the occurrence of the 1992 CE Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes in the Mojave Desert (California, USA), postseismic deformation following both earthquakes has been intensively studied, and models with a strong crust overlying a low-viscosity mantle asthenosphere have been favored. However, we recently found that the near-field postseismic transients after the two earthquakes have lasted longer than previously thought, which requires a revision of the postseismic modeling. Our new modeling results based on the revised postseismic transients show that: (1) the effective viscosity of the lower crust beneath the Mojave region at the decadal time scale is ~2 × 1020 Pa·s (transient viscosity ~2 × 1019 Pa·s), i.e., only ~5 times that of the underlying mantle asthenosphere, and (2) the transient viscosity of the upper mantle exhibits a time-dependent increase, providing fresh geodetic evidence for frequency-dependent rheology (e.g., Andrade or extended Burgers rheology). The inferred transient rheology for the first year agrees well with that obtained for the July 2019 Mw 6.4 and Mw 7.1 Ridgecrest earthquakes ~180 km north of the two Mojave events. Our modeling results support a thin crème brûlée model for the Eastern California Shear Zone (part of the Pacific-North America plate boundary) in which both the lower crust and the upper mantle exhibit ductility at decadal time scales.

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Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

Solid Earth ◽

10.5194/se-8-1211-2017 ◽

2017 ◽

Vol 8 (6) ◽

pp. 1211-1239 ◽

Cited By ~ 3

Author(s):

Thomas van der Werf ◽

Vasileios Chatzaras ◽

Leo Marcel Kriegsman ◽

Andreas Kronenberg ◽

Basil Tikoff ◽

...

Keyword(s):

Grain Size ◽

Upper Mantle ◽

Lower Crust ◽

Baja California ◽

Plate Boundary ◽

Volcanic Field ◽

Strike Slip ◽

Low Viscosity ◽

The Pacific ◽

Flow Laws

Abstract. The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ⋅ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific–Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

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Inception of the eastern California shear zone and evolution of the Pacific-North American plate boundary: From kinematics to geodynamics

Journal of Geophysical Research Atmospheres ◽

10.1029/2009jb007055 ◽

2010 ◽

Vol 115 (B7) ◽

Cited By ~ 10

Author(s):

Mian Liu ◽

Hui Wang ◽

Qingsong Li

Keyword(s):

Shear Zone ◽

North American ◽

Plate Boundary ◽

North American Plate ◽

Pacific North American ◽

Eastern California Shear Zone ◽

The Pacific

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Supplemental Material: Highly localized upper mantle deformation during plate boundary initiation near the Alpine fault, New Zealand

10.1130/geol.s.14605269 ◽

2021 ◽

Author(s):

Steven Kidder ◽

et al.

Keyword(s):

New Zealand ◽

Shear Zone ◽

Upper Mantle ◽

Plate Boundary ◽

Zone Width ◽

Alpine Fault

Supplemental figures, data, and code related to shear zone width estimates.

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Constraints on the rheology of the mid- to lower continental crust from geodetic studies of the earthquake deformation cycle

10.5194/egusphere-egu2020-11951 ◽

2020 ◽

Author(s):

Tim Wright ◽

Tom Ingleby ◽

Ekbal Hussain

Keyword(s):

Shear Zone ◽

Continental Crust ◽

Power Law ◽

Lower Crust ◽

Postseismic Deformation ◽

Strain Accumulation ◽

Earthquake Cycle ◽

Accumulation Rates ◽

Deformation Cycle ◽

Time Invariant

In this presentation I will review geodetic constraints on the rheology of the mid- to lower continental crust from observations and models of all phases of the earthquake deformation cycle. I will focus on observations of slow interseismic strain accumulation and rapid postseismic strain transients, both of which result primarily from deformation in the mid- to lower crust. I will argue that, with a few exceptions, interseismic strain is focused in zones around faults with widths that are compatible with strain at depth being focused on a fault or distributed in a shear zone up to ~3 x the seismogenic layer thickness. I will show that for the North Anatolian Fault, the strain accumulation rate appears to be approximately constant for the entire earthquake cycle, once the postseismic transient has decayed. This is consistent with observations at other fault where geodetic measurements were made prior to major earthquakes; the broad agreement between geological and geodetic estimates of slip rate is also consistent with interseismic strain accumulation rates being relatively time invariant. Time-invariant interseismic strain accumulation rates require a relatively strong mid- to lower crust, where relaxation times are equal to or greater than the average earthquake revisit time. Postseismic deformation transients are commonly observed following most earthquakes, but they are interpreted using a variety of very different deformation mechanisms. By compiling all observations of postseismic deformation we show that the largest transient postseismic velocities decay following a simple t-1 power-law, analogous to Omori&#8217;s law for aftershock decay. This is consistent with frictional afterslip and/or power-law creep in a narrow shear zone. This model of a weak shear zone embedded within a stronger substrate can explain most observations of the earthquake deformation cycle. Exceptions to this simple model might occur in locations where the lower crust is weaker, perhaps due to the presence of partial melt. Geological constraints on rheology are critical for making further progress in understanding the earthquake deformation cycle &#8211; geological models for the mid- to lower crust can be tested by comparing geodetic observations with geologically-realistic earthquake cycle models.

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Supplemental Material: Highly localized upper mantle deformation during plate boundary initiation near the Alpine fault, New Zealand

10.1130/geol.s.14605269.v1 ◽

2021 ◽

Author(s):

Steven Kidder ◽

et al.

Keyword(s):

New Zealand ◽

Shear Zone ◽

Upper Mantle ◽

Plate Boundary ◽

Zone Width ◽

Alpine Fault

Supplemental figures, data, and code related to shear zone width estimates.

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Connecting subduction, extension, and shear localization across the Aegean Sea and Anatolia

Geophysical Journal International ◽

10.1093/gji/ggab078 ◽

2021 ◽

Author(s):

S Barbot ◽

J R Weiss

Keyword(s):

Shear Zone ◽

Upper Mantle ◽

Lower Crust ◽

Eastern Mediterranean ◽

Kinematic Model ◽

Aegean Sea ◽

Strike Slip ◽

Western Turkey ◽

The North ◽

Slip Deformation

Summary The Eastern Mediterranean is the most seismically active region in Europe due to the complex interactions of the Arabian, African and Eurasian tectonic plates. Deformation is achieved by faulting in the brittle crust, distributed flow in the viscoelastic lower-crust and mantle, and Hellenic subduction but the long-term partitioning of these mechanisms is still unknown. We exploit an extensive suite of geodetic observations to build a kinematic model connecting strike-slip deformation, extension, subduction, and shear localization across Anatolia and the Aegean Sea by mapping the distribution of slip and strain accumulation on major active geologic structures. We find that tectonic escape is facilitated by a plate-boundary-like, trans-lithospheric shear zone extending from the Gulf of Evia to the Turkish-Iranian Plateau that underlies the surface trace of the North Anatolian Fault. Additional deformation in Anatolia is taken up by a series of smaller-scale conjugate shear zones that reach the upper mantle, the largest of which is located beneath the East Anatolian Fault. Rapid north-south extension in the western part of the system, driven primarily by Hellenic Trench retreat, is accommodated by rotation and broadening of the North Anatolian mantle shear zone from the Sea of Marmara across the north Aegean Sea, and by a system of distributed transform faults and rifts including the rapidly extending Gulf of Corinth in central Greece and the active grabens of western Turkey. Africa-Eurasia convergence along the Hellenic Arc occurs at a median rate of 49.8 mm/yr in a largely trench-normal direction except near eastern Crete where variably-oriented slip on the megathrust coincides with mixed-mode and strike-slip deformation in the overlying accretionary wedge near the Ptolemy-Pliny-Strabo trenches. Our kinematic model illustrates the competing roles the North Anatolian mantle shear zone, Hellenic Trench, overlying mantle wedge, and active crustal faults play in accommodating tectonic indentation, slab rollback, and associated Aegean extension. Viscoelastic flow in the lower crust and upper mantle dominate the surface velocity field across much of Anatolia and a clear transition to megathrust-related slab pull occurs in western Turkey, the Aegean Sea, and Greece. Crustal scale faults and the Hellenic wedge contribute only a minor amount to the large-scale, regional pattern of Eastern Mediterranean interseismic surface deformation.

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