John P. Platt - US grants

The Alboran Sea was created in the wake of the young subduction zones that swept across the Western Mediterranean in the Neogene during the convergence of Africa and Europe. The Gibraltar Arc and Alboran Sea are the westernmost part of the system and one of the most confusing. The mixture of westward rollback, extension, strike-slip, volcanism, uplift and subsidence has defied attempts to compose a consistent scenario that explains all of the obervations. Partly as a result of inadequate data, there are many models that have been proposed involving subduction, slab breakoff, delamination and drips. There is credible evidence that the lithospheric mantle of the overriding plate of a west-facing subduction zone has been thinned by both back-arc stretching and some type of convective removal (e.g. drips, delamination, etc.). The process of convective thinning is poorly understood but is believed to be important in driving uplift and subsidence of the Earth?s surface, influencing rates of deformation in active orogens, and contributing to recycling of continental materials back into the mantle.

This award provides for a multidisciplinary, international investigation of the Alboran Sea, Gibraltar arc, Atlas Mountains and surrounding areas in the western Mediterranean using passive and active seismology, magnetotellurics, geochemistry, petrology/structural geology, and geodynamic modeling. The overall goal of the project is to study the processes responsible for convective thinning in the Gibraltar-Alboran Sea region. The project, known as PICASSO, has now been funded in Spain and Ireland as a colloaborative EU-US program, with proposals from a number of other EU nations submitted or in process. The project was selected as the pilot experiment for TopoEurope, an EarthScope-like initiative recently approved by the European Science Foundation. A large part of the field deployments will be done by European scientists, including a 3D EarthScope-type rolling array (IberArray), with additional targeted field experiments by US investigators. The IberArray is already underway with ~60 stations deployed in the PICASSO field area.

At about 20 km depth crustal rocks show a transition from the brittle behavior characteristic of the upper crust to ductile behavior characteristic of most of the Earths interior. The stress required to deform the ductile rock immediately below the brittle-ductile transition (BDT) is likely to approximate the bulk strength of the brittle crust immediately above the BDT. Rocks that have deformed by ductile processes can preserve microscale features in their crystalline structure that record the stress level during deformation. If these rocks have subsequently been brought to the surface (exhumed), we can use microstructural measurements to determine the strength of the crust around the BDT. The goal of this work is to obtain better estimates of the strength of the upper crust. Our measurements will allow us to reconstruct strength profiles of the continental crust. These will provide reliable data for assessing the strength of the tectonic plates in areas of active deformation. In particular, our results will improve mechanical models of the way continental crust responds to plate motions, with implications for mountain-building processes, the formation of sedimentary basins that host economically valuable reserves of petroleum and mineral resources, and the generation of earthquakes.

The upper 20-25 km of the continental crust is the coldest and strongest part of the tectonic plates, and because cold rock is brittle, it generates most of the earthquakes. The strength and mechanical properties of the upper crust are fundamental to understanding the way the plates respond to forces acting on their boundaries and to forces generated internally by gravity. The bulk strength of the crust is difficult to measure directly, however; estimates vary by about a factor of ten, because of uncertainties about the strength of faults, the effect of fluids within the crust, and the temperature gradient through the crust. In areas of continental rifting and extension, such as the Basin & Range Province of the western USA, rocks have been exhumed from the ductile middle crust, cooling through the BDT as they rise to the surface. During exhumation and cooling, deformation becomes increasingly localized into narrow shear zones. As a result, different parts of the rock body record stress levels from different depths. We plan to make measurements of microstructural features, mineral chemistry, and isotopic composition, to determine the stress-temperature-time history of rocks from areas affected by recent extensional tectonics in SE California and in southern Spain. Electron backscatter diffraction will be used to determine the grain size, grain misorientation, and crystallographic preferred orientation of dynamically recrystallized quartz-bearing rocks for stress measurements. Element exchange between different minerals will be used to calculate chemical equilibria that are functions of pressure and temperature. Several different radiogenic isotopic systems will be used to constrain the temperature during exhumation and cooling. Earthscope has provided generous support for the geochronology component of the work.

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Platt

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Granted funds will support acquisition of an electron backscattered diffraction (EBSD) camera and an energy dispersive spectrometer (EDS) for a recently acquired variable pressure field emission source scanning electron microscope (SEM) in the Earth Sciences Department at the University of Southern California. The EBSD camera will permit determination of the crystallographic preferred orientation patterns (CPOs) in mantle and crustal rocks that record lattice dislocations caused by physical conditions under which deformation occurred (e.g., flow stress, strain-rate, vorticity, and temperature). The EDS will permit simultaneous characterization of phase composition in the sample. PI and student research with a focus on the determination of the tectonic history of exhumed rocks will immediately benefit from the EBSD as will materials research on synthetically produced ultrafine grained nanostructured materials that have emerging commercial applications where extremely high strength materials are required.

Subduction complexes such as the Franciscan Complex of California contain large volumes of rock that have been exhumed from up to 40 km depth, and which preserve a record of the ambient physical conditions and deformational processes. In particular, the South Fork Mountain Schist (SFMS) in the Northern Coast Ranges is a 240 km long and up to 5 km thick body of intensely deformed sedimentary and volcanic rock that occupied the subduction zone interface at around 123 Ma, at a depth and temperature corresponding to the source area of present-day episodic slow-slip and seismic tremor (ETS) events. The SFMS preserves an extraordinary high-strain deformational microstructure that indicates the activity of a combination of microcracking, pressure-solution and dislocation creep at relatively high stress, burial depths of around 40 km, and temperatures of around 350 degrees C. The SFMS offers an easily accessible window into the physical conditions and deformational mechanics during ETS events. This project quantifies (a) deviatoric stress using recrystallized grain-size piezometry on rocks affected by dislocation creep, (b) strain and strain-rate using the estimated subduction rate for this period, the thickness of the deformation zone, and microstructural features including microfold geometry and deformed clastic quartz and radiolarians, (c) temperature using laser Raman spectroscopy on carbon and the Ti content of quartz, (d) pressure (and hence depth) using phase assemblages in the blueschist-facies rocks, and multi-equilibrium thermobarometry on coexisting chlorite and white mica, and (e) water activity using cathodo-luminescence and FTIR analyses on quartz. These measurements will allow thermomechanical modeling of the effect of dissipative heating, and calculation of the rate of moment release, which can be compared with the seismic record during ETS events. Elastic modeling of the rate of crack propagation will allow calculation of the rate of fluid transport and the rate of propagation of slip events.

The most powerful and damaging earthquakes known occur between 15 and 40 km depth in regions known as subduction zones where oceanic plates are being carried down into the Earth's mantle. Below that, there is a transition downwards from the abrupt movements that cause earthquakes to steady sliding without earthquakes. This transition produces several types of ground motion, including "slow earthquakes", in which slip continues for up to two weeks, but without ground shaking. The transition may play a role in building up the stress that produces normal earthquakes at shallower depths. This project investigates rocks in northern California that were formed in a subduction zone at about 40 km depth, and which preserve a record of the physical conditions (pressure, temperature, stress, and water content) and processes that lead to slow earthquakes and related phenomena in present-day subduction zones.

This project will investigate how fault zones, of the type and scale that form plate boundaries, change character with depth in the lithosphere below the seismogenic upper crust. This project aims to reconstruct the characteristics of a well-studied example of a major fault zone at the time of motion, with the aim of determining if there is a scaling relationship between important factors that control fault zone behavior: shear zone width, rheology, stress, strain-rate and temperature. The project will focus on the Simplon fault in the Swiss Alps, a fault with a well-constrained rate of displacement, so that strain-rate at different structural levels can be calculated and the shear zone rheology determined. The results of this research will be of value for geodynamic modeling of the lithospheric response to plate motions, the interpretation of geophysical data across plate boundary shear zones, the interpretation of geodetic measurements of post-seismic deformation, and for understanding how seismogenic faults are loaded. The proposed activity will advance desired societal outcomes by full participation of women in STEM, public engagement with STEM through virtual field trips, and development of a diverse, globally competitive STEM workforce through undergraduate and graduate student training plus an undergraduate field course.

Faults that accommodate relative plate motion must transect much of the lithosphere, yet their width, internal structure, and mechanical properties as a function of time and depth are poorly understood. This proposal aims to reconstruct the characteristics of a well-studied example of a major fault zone at the time of motion, with the aim of determining if there is a scaling relationship between shear zone width, rheology, stress, strain-rate and temperature. The Simplon fault in the Swiss Alps, will be investigated using field and laboratory techniques to determine its original width, flow stress, strain-rate, and rheology as a function of temperature and depth. The fault is a normal sense shear zone, active between 25 and 3 Ma at a rate of 3-5 mm/year, which exhumed its footwall during motion, exposing highly deformed rocks from depths of up to 30 km. Analytical methods will include: (a) microstructural analysis using optical and SEM methods; (b) measurement of the dynamically recrystallized grain size of quartz for determination of stress during deformation; (c) measurement of the crystallographic preferred orientation of quartz, using electron backscatter diffraction methods, to determine the active slip systems; (d) determination of temperature and pressure during deformation using the Ti content of quartz, laser Raman spectrometry of carbonaceous material, and multi-equilibrium thermobarometry of coexisting minerals; (e) thermochronology using 39Ar/40Ar analysis on hornblende, muscovite and biotite plus U-Th/He analysis on zircon and apatite; and (f) numerical analysis of the thermal evolution of the exhumed shear zone. Strain-rate during deformation will be calculated from the width and displacement rate; together with the stress, this can be used to calibrate the shear zone rheology.