Lab

Structural Geology & Tectonics Group @ ETH Zürich (formerly UT Austin)

About the lab

Research in the Structural Geology & Tectonics group in the Geological Institute at ETH Zürich focuses on the rapidly deforming zones that define Earth’s tectonic plate boundaries and generate many of the planet’s geohazards. We are 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.

Featured projects (4)

Project
The scope of this project aims to better understand the importance of pre‐orogenic sedimentary and structural features for the evolution of a tectonically inverted mountain belt. The Kopet Dagh, NE Iran, is an ideal example of a inverted basement‐involved fold‐and‐thrust belt. The tectonic history of northeast Iran underwent several stages of extension and compression until the Kopet Dagh Mountains were uplifted from Mid‐Cenocoic times on. Both the extensional and compressional phases of NE Iran exhibit an important involvement of the crustal basement.
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

Featured research (19)

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 subduction. Despite decades of research, the metamorphic and deformation history (P‐T‐D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn‐subduction return flow along similar P‐T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top‐to‐the‐S‐SW shear (prograde‐to‐peak subduction), top‐to‐the‐NE shear (blueschist facies exhumation), and then E‐W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi‐mineral Rb‐Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ∼50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ∼50–40 and ∼43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene‐Oligocene subduction channel proceeded at rates of ∼1.5–5 mm/yr and accommodated ∼80% of the total exhumation of this HP/LT complex.
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 sensitive to the composition of subducting crustal material-for example, sediments versus mafic igneous oceanic crust. Here we use 2-D numerical models of dynamically consistent subduction to systematically investigate how subduction interface viscosity influences large-scale subduction kinematics and dynamics. Our model consists of an oceanic slab subducting beneath an overriding continental plate. The slab includes an oceanic crustal/weak layer that controls the rheology of the interface. We implement a range of slab and interface strengths and explore how the kinematics respond for an initial upper mantle slab stage, and subsequent quasi-steady-state ponding near a viscosity jump at the 660-km-discontinuity. If material properties are suitably averaged, our results confirm the effect of interface strength on plate motions as based on simplified viscous dissipation analysis: a ∼2 order of magnitude increase in interface viscosity can decrease convergence speeds by ∼1 order of magnitude. However, the full dynamic solutions show a range of interesting behaviour including an interplay between interface strength and overriding plate topography and an end-member weak interface-weak slab case that results in slab break-of f/tearing. Additionall y, for models with a spatially limited, weak sediment strip embedded in regular interface material, as might be expected for the subduction of different types of oceanic materials through Earth's history, the transient response of enhanced rollback and subduction velocity is different for strong and weak slabs. Our work substantiates earlier suggestions as to the importance of the plate interface, and expands the range of quantifiable links between plate reorganizations, the nature of the incoming and overriding plate and the potential geological record.
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 and olivine–orthopyroxene, which commonly coexist in deformed crustal and mantle rocks, respectively. To analyze mineral pairs, we measured dynamically recrystallized grain sizes in both phases that deformed under approximately isoviscous conditions. Using the experimentally calibrated piezometers for quartz and olivine, we develop piezometric relationships for feldspar and orthopyroxene. These new feldspar and orthopyroxene piezometers reasonably predict grain size relationships observed for mineral pairs from naturally deformed lithospheric shear zones. Combining our naturally constrained datasets with the existing experimental datasets for feldspar and orthopyroxene, we also derive piezometers that are consistent with the wattmeter model and with observations from other studies of mylonitic rocks. Our results provide an alternative for estimating stress in rocks such as granulites and pyroxenites, for which the quartz and olivine piezometers are unsuitable.
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, as well as the rock types that define them, remain unclear. We characterize the seismic signature of a fossil subduction interface shear zone in northern California that is sourced from the same depth range as modern LVZs. Deformation was distributed across 3 km of dominantly metasedimentary rocks, with periodic strain localization to km-scale ultramafic lenses. We estimate seismic velocities accounting for mineral and fracture anisotropy, constrained by microstructural observations and field measurements, resulting in a Vp/Vs of 2.0. Comparable thicknesses and velocities suggest that LVZs represent, at least in part, the subduction interface shear zone.
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 sensitive to the composition of subducting crustal material-for example, sediments versus mafic igneous oceanic crust. Here we use 2-D numerical models of dynamically consistent subduction to systematically investigate how subduction interface viscosity influences large-scale subduction kinematics and dynamics. Our model consists of an oceanic slab subducting beneath an overriding continental plate. The slab includes an oceanic crustal layer that controls the rheology of the interface. We implement a range of slab and interface strengths and explore how the kinematics respond for an initial upper mantle slab stage, and subsequent quasi-steady-state ponding near a viscosity jump at the 660-km-discontinuity. If material properties are suitably averaged, our results confirm the effect of interface strength on plate motions as based on simplified viscous dissipation analysis: a ∼ 2 order of magnitude increase in interface viscosity can decrease convergence speeds by ∼ 1 order of magnitude. However, the full dynamic solutions show a range of interesting behavior including an interplay between interface strength and overriding plate topography and an end-member weak interface-weak slab case that results in slab breakoff/tearing. Additionally, for models with a spatially limited, weak sediment strip embedded in regular interface material, as might be expected for the subduction of different types of oceanic crust through Earth's history, the transient response of enhanced rollback and subduction velocity is different for strong and weak slabs. Our work substantiates earlier suggestions as to the importance of the plate interface, and expands the range of quantifiable links between plate reorganizations, the nature of the incoming and overriding plate, and the potential geological record.

Lab head

Whitney Maria Behr
Department
  • Department of Earth Sciences
About Whitney Maria Behr
  • 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.

Members (6)

Jonas B Ruh
  • ETH Zurich
Alberto Ceccato
  • ETH Zurich
Carolyn Tewksbury-Christle
  • Fort Lewis College
Leif Tokle
  • ETH Zurich
Miguel Cisneros
  • Lawrence Livermore National Laboratory
Zoe Braden
  • University of British Columbia - Vancouver

Alumni (7)

Kyle T. Ashley
  • University of Pittsburgh
Nick Dygert
  • University of Tennessee
Peter Owen Gold
  • University of Texas at Austin
Alissa Kotowski
  • Utrecht University