Whitney Maria Behr

Whitney Maria Behr
ETH Zurich | ETH Zürich · Department of Earth Sciences

Ph.D. University of Southern California

About

55
Publications
24,148
Reads
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1,764
Citations
Introduction
My research focuses on the rapidly deforming zones that define Earth’s tectonic plate boundaries and generate many of the planet’s geohazards. I am interested in the rates and directions in which faults and shear zones move; their geometries, widths and mechanical behaviors at depth; and the processes that shape them over geologic time.
Additional affiliations
July 2012 - present
University of Texas at Austin
Position
  • Professor (Assistant)
July 2011 - June 2012
Brown University
Position
  • PostDoc Position
August 2006 - June 2011
University of Southern California
Position
  • PhD Student

Publications

Publications (55)
Article
Full-text available
In this study, we investigated the geomorphic and incision history for a ~5 km reach of the northern Rio Grande gorge in New Mexico using field and LiDAR-based geomorphic mapping and cosmogenic 3He surface exposure dating. This wide (>1.5 km) and deep (~240 m) section of the gorge exhibits Toreva blocks, incoherent landslides, rock falls, and slump...
Article
Full-text available
We use structural and microstructural observations from exhumed subduction-related rocks exposed on Syros Island (Cyclades, Greece) to provide constraints on the length scales and types of heterogeneities that occupy the deep subduction interface, with possible implications for episodic tremor and slow slip. We selected three Syros localities that...
Preprint
Full-text available
Deep-seated slow slip and tremor (SST), including slow slip events, episodic tremor and slip, and low-frequency earthquakes, occur downdip of the seismogenic zone of numerous subduction megathrusts and plate boundary strike-slip faults. These events represent a fascinating and perplexing mode of fault failure that has greatly broadened our view of...
Article
Full-text available
Fault slip rates inform models of strain accumulation and release, which over geologic time may vary or remain constant depending on factors like structural complexity, fault strength, deformation rates, and proximity to other faults. In this study, we present a Late Pleistocene–Holocene slip history based on four new geologic slip rates for the Ag...
Article
Full-text available
Variation in the effective strength of the lithosphere allows for active plate tectonics and is permitted by different deformation mechanisms operating in the crust and upper mantle. The dominant mechanisms are debated, but geodynamic models often employ grain-size-independent mechanisms or evaluate a single grain size. However, observations from n...
Article
Full-text available
Exhumed high‐pressure/low‐temperature (HP/LT) metamorphic rocks provide insights into deep (∼20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist‐to‐eclogite facies oceanic‐ and continental‐affinity rocks that record the structural and thermal evolution linked to Eocene subdu...
Article
Full-text available
Tectonic plate motions predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere and slab-upper plate interface. A wide range of observations from active subduction zones and exhumed rocks suggest that subduction interface shear zone rheology is sensitiv...
Article
Full-text available
Determining pressure and temperature variations between high-pressure/low-temperature (HP–LT) eclogite blocks is crucial for constraining end-member exhumation models; however, it has historically been challenging to constrain eclogite pressures due to the high variance associated with this bulk-rock composition. In this work, we utilize quartz-in-...
Article
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Differential stress magnitude is a fundamental parameter in the study of geodynamic processes in the continental lithosphere, and is typically estimated in quartz- and olivine-dominated lithologies using recrystallized grain size piezometers. Here we evaluate the piezometric relationships in natural mylonites with mineral pairs of quartz–feldspar a...
Article
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Plain Language Summary At subduction zones, an oceanic plate dives into the mantle below another plate. The downgoing plate is usually covered by sediments. These sediments can be carried down to depth along the interface and/or scraped off the top of the downgoing plate and appended to the edge of the upper plate, forming an accretionary wedge. Se...
Article
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The style of convective force transmission to plates and strain-localization within and underneath plate boundaries remain debated. To address some of the related issues, we analyze a range of deformation indicators in southern California from the surface to the asthenosphere. Present-day surface strain rates can be inferred from geodesy. At seismo...
Article
Full-text available
Low Velocity Zones (LVZs) with anomalously high Vp-Vs ratios occur along the downdip extents of subduction megathrusts in most modern subduction zones and are collocated with complex seismic and transient deformation patterns. LVZs are attributed to high pore fluid pressures, but the spatial correlation between the LVZ and the subduction interface,...
Preprint
Full-text available
Tectonic plate motions predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere, and slab-upper plate interface. A wide range of observations from active subduction zones and exhumed rocks suggest that subduction interface shear zone rheology is sensiti...
Article
Full-text available
Basaltic and gabbroic rocks that define the seafloor have been suggested to act as sources of rheological heterogeneity during subduction, with the capacity to enhance or dampen seismicity. Despite this, relatively little is known from the rock record regarding the progression and conditions of mafic oceanic crust deformation during subduction, par...
Article
Full-text available
The deep roots of subduction megathrusts exhibit aseismic slow slip events, commonly accompanied by tectonic tremor. Observations from exhumed rocks suggest this region of the subduction interface is a shear zone with frictional lenses embedded in a viscous matrix. Here, we use numerical models to explore the transient slip characteristics of finit...
Preprint
Full-text available
Geodynamic numerical models often employ solely grain-size-independent dislocation creep to describe upper mantle dynamics. However, observations from nature and rock deformation experiments suggest that shear zones can transition to a grain-size-dependent creep mechanism due to dynamic grain size evolution, with important implications for the over...
Article
Full-text available
Retrograde metamorphic rocks provide key insights into the pressure-temperature (P-T) evolution of ex-humed material, and resultant P-T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P-T conditions of retrograde metamorphic rocks has historically been challenging and ha...
Article
Full-text available
Orthopyroxene is the second most abundant mineral in the upper mantle and, like olivine, can form crystallographic preferred orientations (CPO) during mantle deformation that may influence mantle seismic anisotropy. Here we use a compilation of 21 peridotites in which both olivine and orthopyroxene CPOs are preserved to document the range of orthop...
Preprint
Full-text available
Subduction megathrusts represent the largest and most hazardous seismogenic faults on Earth and exhibit a wide range of earthquake slip patterns. An especially perplexing form of slip on subduction megathrusts are `slow earthquakes', which are slip events that release similar amounts of energy as regular earthquakes, but do so over months to years,...
Article
Full-text available
The architecture and mechanical properties of the subduction interface impact large-scale subduction processes, including mass and volatile recycling, upper-plate orogenesis, and seismic behavior. The nature of the deep subduction interface, where a dominantly frictional megathrust likely transitions to a distributed ductile shear zone, is poorly u...
Article
Full-text available
Deep-seated slow slip and tremor (SST), including slow slip events, episodic tremor and slip, and low-frequency earthquakes, occur downdip of the seismogenic zone of numerous subduction megathrusts and plate boundary strike-slip faults. These events represent a fascinating and perplexing mode of fault failure that has greatly broadened our view of...
Article
Full-text available
We performed deformation and grain growth experiments on natural olivine aggregates with olivine water contents (COH = 600 ± 300 H/106 Si) similar to upper mantle olivine, at 1000–1200°C and 1,400 ± 100 MPa confining pressure. Our experiments differ from published grain growth studies in that most were (1) conducted on natural olivine cores rather...
Preprint
Full-text available
Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide insights into deep (~20-70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit (CBU) preserves blueschist-to-eclogite facies oceanic- and continental-affinity rocks that record the structural and thermal evolution associated with...
Preprint
Full-text available
We combine elastic thermobarometry with oxygen isotope thermometry to quantify the pressure-temperature (P-T) evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote...
Preprint
Full-text available
We performed deformation and grain growth experiments on natural olivine aggregates with moderate olivine water contents (COH = 600±300 ppm H/Si) at 1000-1200°C and a confining pressure of 1400±100 MPa. Our experiments differ from published grain growth studies in that most were: 1) conducted on natural olivine cores rather than hot-pressed aggrega...
Article
Earth is the only planet in the Solar System that shows a form of planetary evolution known as plate tectonics. In this mode of planetary cooling, a convecting mantle conducts heat through a relatively rigid outer shell called the lithosphere. This shell is produced at spreading centres (boundaries between two separating plates), and is recycled ba...
Article
Full-text available
Removal of mantle lithosphere by Rayleigh‐Taylor (R‐T) instabilities is invoked to explain the formation of high plateaus and mountain ranges. Here we report geochemical and microstructural observations from mantle xenoliths from Lunar Crater volcanic field, central Nevada, which we interpret to directly sample a R‐T instability beneath the Basin a...
Article
Tectonic plate velocities predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere, and slab–upper plate interface. A range of observations suggest that slabs may be weak, implying a more prominent role for plate interface dissipation than previously th...
Article
We investigated quartzite flow laws based on data from 21 different experimental studies on samples deformed with added water at temperatures from 700 to 1200C. Two clear trends emerge: one with a power-law stress exponent of n = 4 +/- 0.3 and the other, at higher stresses and lower temperatures, with a stress exponent of n = 2.7 +/- 0.3. The chang...
Preprint
Full-text available
Tectonic plate velocities predominantly result from a balance between the potential energy change of the subducting slab and viscous dissipation in the mantle, bending lithosphere, and slab-upper plate interface. A range of observations suggest that slabs may be weak, implying a more prominent role for plate interface dissipation than previously th...
Article
Full-text available
We present observations from an exhumed subduction complex that resembles the environment of modern deep episodic tremor and slow slip (ETS). We focus on the Cycladic Blueschist Unit on Syros Island in Greece. Syros metabasites consist of blueschists and eclogites that record prograde deformation, with peak metamorphism of 1200–1600 MPa and 450–550...
Article
We use xenoliths from young cinder cones in the eastern Mojave region of southern California to investigate deformation fabrics and their implications for strain localization, lithospheric viscosity, and controls on mineral lattice preferred orientations (LPO) and seismic anisotropy at Moho depths. Lower crustal gabbros and upper mantle peridotites...
Article
Full-text available
High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core comple...
Article
Full-text available
High strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core comple...
Article
Laramide crustal deformation in the Rocky Mountains of the west-central United States is often considered to relate to a narrow segment of shallow subduction of the Farallon slab, but there is no consensus as to how deformation along the slab-mantle lithosphere interface was accommodated. Here we investigate deformation in mantle rocks associated w...
Article
The Betic Cordillera of southern Spain is a complex orogen formed in the context of convergence between Africa and Iberia from the Mesozoic to the present. The internal zone of the orogen includes three tectonic complexes, two of which have been subducted to high pressure conditions, then exhumed back to the surface during subsequent extension. Sub...
Article
Deformation of middle crustal shear zones likely varies with time as a result of the stress build-up and release associated with earthquakes and post-seismic deformation, but the processes involved and their microstructural signature in the rock record are poorly understood. We conducted a series of experiments on quartzite at 900 °C to characteriz...
Article
X-ray computed tomographic scanning of a 44 g Murchison stone (USNM 5487) reveals a preferred alignment of deformed, partially altered chondrules, which define a prominent foliation and weak lineation in 3D. The presence of a lineation and evidence for a component of rotational, noncoaxial shear suggest that the deformation was caused by impact. Ol...
Article
Northwest directed slip from the southern San Andreas Fault is transferred to the Mission Creek, Banning, and Garnet Hill fault strands in the northwestern Coachella Valley. How slip is partitioned between these three faults is critical to southern California seismic hazard estimates, but is poorly understood. In this paper, we report the first sli...
Article
A global compilation of shear stress magnitude from mylonites developed along major fault zones suggests that maximum stresses between 80 and 120 MPa are reached at temperatures between 300 and 350 °C on normal, thrust, and strike-slip faults. These shear stresses are consistent with estimates of brittle rock strengths based on sliding friction (e....
Article
Metamorphic core complexes are products of normal-fault displacements sufficient to exhume rocks from below the brittle–ductile transition. These faults (detachments) may initiate within the brittle crust at steep angles, but they sole into the ductile middle crust, and during displacement rotate to gentler dips due to hanging-wall extension. The e...
Article
We use field and microstructural observations, coupled to previously published P-T-time histories, to track the rheological evolution of an intracontinental subduction complex exposed in the Betic Cordillera in the western Mediterranean region. The body of rock we focus on, known as the Nevado-Filabride Complex (NFC), was originally part of the upp...
Article
The Betic-Rif arc and the Alboran hinterland provide examples of tectonic processes that are not predictable from the rules of rigid-plate tectonics. The Alboran hinterland reveals a two-stage history of subduction and exhumation. The first stage built a thick accretionary orogen that subsequently underwent rapid extension associated with removal o...
Article
We examine the kinematic and thermal evolution of the Nevado-Filabride Complex (NFC), an Early to Middle Miocene subduction complex within the Betic Cordillera of southern Spain. Thermobarometry from the NFC in the Sierra Alhamilla reveals an inverted geothermal gradient, with peak temperatures and pressures of 560°C and 13 kbar reached in the uppe...
Article
Full-text available
We calculate the cumulative width w of ductile shear zones accommodating plate motion in continental lithosphere, based on the assumptions that (1) the flow stress is controlled by the yield strength of intact rock at any given depth; (2) the yield strength profile through the crust can be constrained from observations in exhumed shear zones; and (...
Article
Full-text available
The recently developed titanium-in-quartz (TitaniQ) geothermobarometer of Wark and Watson (2006) and Thomas et al. (2010) has the potential to be applied to a wide range of igneous and metamorphic rocks. For Ti concentrations > similar to 10 ppm, the concentrations can be measured using an electron microprobe, but lower concentrations are below det...
Article
At high stresses and low temperatures, grainsize reduction by dynamic recrystallization profoundly modifies rock rheology. Strain energy driven grain-boundary migration (ρGBM) is involved both in the nucleation of new grains by the grain-boundary bulging mechanism (BLG), and in the subsequent evolution of the microstructure. Above the Dmin line, wh...
Article
We present a method in which paleopiezometry, Ti-in-quartz thermobarometry (TitaniQ), and 2-D thermal modeling are used to construct a naturally constrained stress profile through the middle crust in an area of exhumed mid-crustal rocks. As an example, we examine the footwall of the Whipple Mountains metamorphic core complex (WMCC). Rocks in the WM...
Article
Full-text available
Below the seismogenic zone, plate boundaries are defined by lithospheric ductile shear zones. These localize strain, and are the main reason that convective motions in the Earth's interior are expressed at the surface as plate tectonics. We argue here that lithospheric shear zones operate at approximately constant stress, equal to the yield strengt...
Article
Full-text available
This study focuses on uncertainties in estimates of the geologic slip rate along the Mission Creek strand of the southern San Andreas fault where it offsets an alluvial fan (T2) at Biskra Palms Oasis in southern California. We provide new estimates of the amount of fault offset of the T2 fan based on trench excavations and new cosmogenic 10Be age d...
Article
Full-text available
U-series dating of pedogenic carbonate-clast coatings provides a reliable, precise minimum age of 45.1 ± 0.6 ka (2σ) for the T2 geomorphic surface of the Biskra Palms alluvial fan, Coachella Valley, California. Concordant ages for multiple subsamples from individual carbonate coatings provide evidence that the 238U-234U-230Th system has remained cl...

Projects

Projects (2)
Project
Combine structural and microstructural analysis, thermobarometry, and geochronology to decipher the polyphase tectonometamorphic history of the Cycladic Blueschist Unit exposed on Syros Island, Greece