George Zandt - US grants

This research is to study the crust and upper mantle beneath central California using a new and independent teleseismic data set collected in the region by Pacific Gas and Electric Co. First, the receiver function technique will be used to study the orientation and possible imbrication of the oceanic crust beneath the continental margin in addition to looking for variations in crustal thickness along the margin. Second, the data set will be merged with that of the USGS to do a 3-D seismic travel time residual study of the crust and mantle beneath the margin. Understanding the tectonic history is essential for better understanding and characterization of the potential seismic hazards present today. This research is a component of the National Earthquake Hazard Reduction Program.

9505816 ZANDT This research is a study of the lithospheric structure of the Altiplano-Puna volcanic complex at the borders of Argentina, Bolivia, and Chile, in the central volcanic zone of south America. This zone has been the site of voluminous ignimbrite eruptions within the past 10 Ma and is one of the largest active caldera complexes on Earth. In addition to analyzing existing data for the region, the project will install PASCALL portable broadband instrumentation for one year in the area for recording local and teleseismic events. The aim of this work is to answer important questions about the deep structure of a major silicic volcanic field with implications for lithospheric magmatic processes in continental subduction zones, crustal thickening and melting, and the evolution of continental crust. *** ??

9806229 Zandt This grant provides partial funding to upgrade the University of Arizona seismology research computer facility to accommodate increasing demands in network loads, mass storage requirements, and computational speed. In the past five years the computational needs in seismology at the University of Arizona have expanded greatly with our involvement in four major field experiments and the addition of a new faculty member in 1997. The upgraded facility will be used in the research programs of the seismology group that includes four faculty members (Beck, Johnson, Wallace, and Zandt), a computer systems administrator, and currently one postdoc and 12 graduate students. Each faculty member has at least one active NSF research grant, and are involved in research that spans a broad range of topics including the source properties of intermediate-depth and deep earthquakes, the earthquake cycle of large and great circum-Pacific earthquakes, the lithospheric structure and regional tectonics of the central Andes and Tibet, the crustal structure and seismic stratigraphy of terranes in the western U.S., and monitoring of the comprehensive test ban treaty. ***

9805149
Zandt

This research involves the study of the crust and upper mantle of the region encompassing southeastern California and western Arizona, which covers the southwest quarter of the Colorado Plateau and much of the southern Basin and Range. An existing data set from a 75-station, 200km by 450km array will be analyzed to complete a seismic tomography study, incorporating modeling of available digital Bouguer gravity data, and to model existing broadband waveforms to characterize the upper mantle structure in the study region. This important data set fills a large gap in high-resolution P-wave tomography coverage for the western U.S. The region's Cenozoic tectonic history of transition from subduction related compression and crustal thickening to orogenic collapse with core complex formation, volcanism, and extension is unusually well documented in the geologic record. This study will constrain the present day composition and physical state of the upper mantle beneath the area to evaluate the ways it has retained, or lost, the imprint of the Cenozoic tectonic events.

This research involves the study, by means of PASSCAL portable
broadband seismometers in Argentina and Chile, of the structure
beneath the south central Andes where a major change in subducting
slab geometry occurs. The research will be done in conjunction
with research institutes from Chile, Argentina, France, and
Germany. At approximately 33.3 degrees South, the subducting Nazca
slab changes from a horizontal slab geometry without any active
volcanism to the north, to a 30-degree dipping slab with an active
volcanic arc to the south. This north-south change in slab dip is
one of the most dramatic bends or warps in a Wadati-Benioff zone
geometry anywhere globally. At 31 degrees South, the flattened
slab extends 300 km to the east before resuming its descent into
the mantle. This segment has high elevations and large amounts of
tectonic shortening. Across the dipping slab segment, the
elevation decreases, as does the amount of tectonic shortening.
Two PASSCAL transects will be deployed: one near 30.5 degrees South
above the horizontal slab segment and one at 36 degrees South above
the dipping slab segment. The seismic data will be gathered from
natural earthquakes and inverted to answer questions involving the
structure of the subducting slab across the transition, surrounding
mantle structure, mountain building, and earthquake generation
processes.

The largest mountains and highest plateaus on Earth are built when continents collide. Seismic converted phases (receiver functions) are used to investigate two key questions in continental collisions: does the lower crust flow beneath high plateaus, and does continental mantle subduct beneath mountain belts? If crustal flow occurs in a relatively thin channel, as some modeling predicts, it would develop a strong structural or mineral fabric that may be identified by seismic anisotropy. Unlike shear-wave splitting studies that provide only bulk anisotropy information between the source and receiver, azimuthal variations of converted phases can constrain anisotropy parameters for layers within the crust. Previous studies have generally found crustal anisotropies <5%, but highly anisotropic (>10%) layers within the crust have recently been identified in New Zealand and the Central Andes using the receiver function method. This new technique is being applied to receiver functions recorded across the highest elevations on Earth, the Tibetan Plateau, in order to search for evidence of large-scale crustal flow.

The upper mantle beneath the stable interiors of continents is layered on a large scale. Some layer boundaries are observed regionally and denoted by seismologists as "H", "X", and "L". The origin of the stratification of the continental upper mantle is still poorly understood. In some locations the layering has significant dip. Receiver functions have been used to image modern subductions zones as well as identify dipping layers in the upper mantle beneath ancient cratons. Some of these layers beneath cratons appear to have anisotropic properties and may identify relict continental subduction zones. Even if the origin of the stratification is unknown, it can be used as a "tracer" to follow the subduction or destruction of an impinging strong plate beneath a continental collision zone, for example, to track the Indian plate beneath the Himalayas and Tibet.

The primary goal of this project is to map out crustal anisotropy and upper mantle layering across the Tibetan Plateau, and use the results to improve our understanding of how high plateaus are built. This research will continue the development of the receiver function technique in the study of seismic anisotropy and mantle stratigraphy, and contribute to the technical education of two young scientists.

Continental Lithospheric Deformation Along a Major Strike-slip Fault:
The Central North Anatolian Fault Zone

The 1500 km long North Anatolian Fault Zone (NAFZ) is one of the world's largest active continental strike-slip faults and forms the northern margin of the Anatolian block or plate. All indications are that the NAFZ is a newly coalescing continental transform plate boundary. This gives us an opportunity to study a snapshot of lithospheric deformation in its evolution into a plate boundary at the surface and at depth. Despite much geological work at the surface, the deep structure of the NAFZ is relatively unknown. We propose to study the central portion of the NAFZ using data from a 2-year, 40 station portable broadband seismic deployment to address the following questions:
(1) Is the NAFZ a lithospheric-scale transform fault and if so, how does the lithosphere respond to the deformation? Is the NAFZ a thin vertical fault at depth or a broad zone of deformation? Are the crust and upper mantle deformation coupled or decoupled? (2) How does the deeper asthenospheric flow signature relate to the crustal velocity field (as defined by GPS studies) and westward tectonic escape model for the Anatolian plate? (3) Does the spatial distribution and composition of magmatism along the fault zone in the last 10 Ma relate to the deep structure of the fault? (4) How do the fault segments, rupture segments and major splays interact along strike and at depth? What is the depth of the seismogenic zone, and how does it relate to the segmentation defined by the major earthquakes this century? We will use seismic tomography (teleseismic, regional and local), receiver function analysis, modeling of regional waveforms, surface wave dispersion, crust and mantle anisotropy measurements, gravity modeling, and seismicity studies to address these questions.

0309885
Ducea

The goal of this project is to establish the existence or absence of mafic-ultramafic roots to batholiths and whether such roots can reconcile the fundamentally basaltic nature of mantle magmatism with the intermediate average composition of the crust. Two hypotheses are considered: 1) that the dense ultramafic residue founders into the mantle leaving the intermediate crustal compositions above, or 2) the residue exists below the Moho and is seismologically indistinct from mantle peridotite. The PIs will use a combination of petrology, geochemistry and geochronology on the plutons of the Central Coast Range Batholith to track changes in source, depth of origin through time and seismic and gravity data to image the present state of the lithosphere. The Central Coast Range Batholith (CCRB) in British Columbia is a very good location for this study. It is a very large batholith which is relatively young and has not had a protracted tectonic history. It's rugged topography gives access to depth profiles of 20 to 25 km.

Major advances have occurred in our understanding of thin-skinned thrust belts and plateau systems in both collisional and noncollisional mountain belts. However, our understanding of how thick-skinned deformation forms in a tectonic setting of flat subduction has lagged behind. Similarly much progress has been made in the study of subduction zone processes but we still lack a clear understanding of what controls flat slab subduction and the impact on mantle processes. One of the best places in the
world to study the link between flat subduction and continental deformation is in the Precordillera and Sierras Pampeanas of western Argentina. In this project, the investigators propose to use all existing seismic data and collect additional data with a 40-station broadband seismic deployment for high resolution imaging of the Sierras Pampeanas and the flat slab. They will build on results from their previous work in Argentina in the CHARGE project to answer a number of questions about flat subduction and the deformation of the foreland basement. Among the broader impacts of this proposal are improving the international collaboration among seismologists from the National University in San Juan, Argentina, Chile and France and the training of graduate and undergraduate students. The Sierras Pampeanas of Argentina have a very high rate of crustal seismicity and have a history of devastating magnitude 7 earthquakes (1944, 1977). This study will contribute to a better understanding of earthquakes hazards and directly improve earthquake locations and source studies in the region by providing better velocity models. The results will be important for the study of the Laramide deformation in the western U.S., one of the stated objectives for Earthscope science.

0651540
Beck

This grant supports upgrade of the seismology computing facilities in the Department of Geosciences at the University of Arizona. Funds will be used to: 1) upgrade networking infrastructure; 2) acquire a hard disk mass store, tape backup and file server; and 3) acquire 12 iMac workstations and related software. The computational upgrade will facilitate a range of NSF funded seismological research on lithospheric structure and orogenic processes in mountain belts, crustal and mantle anisotropy, earthquake source studies, and lithospheric structure of major strike slip fault zones. The computational equipment will support one postdoctoral researcher, eight Ph.D. students (1 female and 3 international students) and 1 undergraduate student. The upgraded facility willl enhance PI participation in collaborative, interdisciplinary, and international projects, including EarthScope and Continental Dynamics projects.

Proposal: 0745588 Collaborative Research: Using Mineral Physics to Interpret Seismic Anisotropy of the Basin and Range Crust


The tectonics-scale purpose of this research is to interpret the anisotropy of the lower crust in the Basin and Range using measurements of the anisotropy of local rocks. Understanding the anisotropy of the lower crust is important to geodynamicists, geodesists, petrologists, structural geologists and seismologists who seek to understand its role in processes as diverse as the rheology of the lithosphere, the generation of magmas, geochemical cycling between the crust and mantle, and the formation and genesis of the entire crust. The broader scale purpose of this research is to improve understanding of the causes of anisotropy of the lower crust in general by using mineral-physics based knowledge about the causes of anisotropy as a predictive tool.
Rock samples are being studied from two xenolith pipes and three crustal sections in the Basin and Range. Electron-backscatter diffraction is used to measure orientations of all crystals in representative samples. Single-crystal stiffness data are used to calculate the elasticities of the samples. These elasticities are then combined with rock abundance and structure data collected in the field to yield bulk km-scale elasticities for each of the five study sites. Armed with km-scale elasticities for the five study sites plus elasticities for the constituent lithologies we interpret the anisotropy of the Basin and Range lower crust measured from EarthScope's USArray. For the anisotropy studies, we search for anisotropy signals in the receiver functions from the stations closest to the localities of the field samples. Finally, we use what we learn from the site-specific studies to investigate province-wide anisotropy and its tectonic implications.

This is an ambitious project that has the potential to fill in important gaps in the overall picture of orogenesis in the central Andes, and of convergent-margin tectonism in general. The project is constructed around a well defined basic-science question, did the Andes rise in a rapid pulse, or did they rise gradually? Producing elevations and crustal thicknesses of the magnitude found in this study area remains a key problem in continental tectonics.

This question provides a foundation from which the PIs develop a variety of linked projects, including: 3-D structural analysis of fold-thrust belt shortening in the Andes, testing of new methods of paleo-elevation analysis, use of seismic studies to characterize the roots of the range (both in the deep crust and in the underlying mantle), creative use of petrologic and isotopic data to constrain thickened crust at times in the past. The project has the potential to address 3-D mass balance issues during orogeny, as well as the impact of a rising mountain belt on continent-scale weather systems. Of note, to put the analysis of orographic weather studies in context, the PIs will also undertake a broader paleo-climate study. All of the questions to be studied are current and important, and are of interest across traditional disciplinary boundaries and, the research strategy as outlined has a high potential to answer the questions that it poses.

Convergent margins, where an oceanic plate dives under the edge of a continental plate, are the sites of many fundamental Earth processes and features that include the world?s largest earthquakes, the longest volcanic chains with some of the youngest supervolcano eruptions, and one of the world?s longest mountain ranges, the Andes. The subduction of the (Pacific) oceanic plate beneath the western margin of South America constitutes the largest ocean-continent convergent margin system on Earth, and hence, is an ideal location to study these important processes and features. We propose to study a large portion of the South American convergent margin by taking advantage of existing seismic data from over 1000 permanent and portable seismic stations that span over 4000 km of the central Andes. We will use modern computational techniques to generate seismic images of the subsurface beneath the Andes to depths as great as 1000 km. By using these new seismic images, our specific scientific goals are to improve our understanding of how earthquakes and volcanoes are building and destroying the mountains, and whether and how water is transported to great depths in the Earth. This project will build on and synthesize two decades of work in the region by the seismology group at the University of Arizona and our scientific collaborators in Peru, Bolivia, Chile, Argentina, and Brazil. The broader impacts include training graduate students in modern seismic techniques and strengthening our international collaborations in South America.

Seismic methods potentially provide the highest resolution images of the Earth from the upper crust to the lower mantle. We will use Surface Wave Tomography, Finite-Frequency Teleseismic Travel Time Tomography (P- & S-wave and Vp/Vs), Receiver Functions and a joint inversion of Surface Wave Data and Receiver Functions to produce uniform seismic images of the western margin of South America. Using our newly developed seismic images, our specific scientific goals are to:
? Evaluate the along strike role of delamination in the building of the Andes.
? Investigate the along strike variability in lower crustal accommodation of crustal shortening, including pure shear thickening, lower crustal flow, and high-angle Moho-cutting faults.
? Revisit the question of the importance of magmatic addition in the thickening of the Andean crust.
? Investigate the deformation of the subducted Nazca slab as it penetrates the mantle transition zone, and the role of deeply subducted fluids in the mantle transition zone and below the slab.
This project will increase our knowledge about mountain building, crustal deformation and the subduction process by producing a uniform set of high-resolution seismic images of a large along-strike region of the western margin of South America. With uniform seismic images, we can better evaluate different mechanisms that create high elevations and the causes of along-strike variability. Our last goal is related to improving our understanding of the deeper structure of the Nazca slab and the surrounding mantle. Our improved seismic images will help us evaluate the presence of and pathways for water in the mantle transition zone.