Biographies of MARGINS Post-Doctoral Fellows

Since 2003, the MARGINS program has funded a number of post-doctoral fellows, both within the special MARGINS Post-Doctoral Fellowship and within the regular NSF-MARGINS programs. The MARGINS Office would like to announce the current post-doctoral fellows: Benjamin Hotlzman and Jeremy Boyce, with research projects started in 2007 and 2006, respectively. Below, the two fellows introduce themselves and their research. Included are also the biographies of the former post-doctoral fellows (Jennifer Garrison, James Conder, Alison Shaw and Glenn Spinnelli) written at the time of their appointments since the fellowship program's inception (2003).

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CURRENT FELLOWS

Heather Savage
University of California, Santa Cruz

MARGINS Postdoctoral Fellowship: The effect of frictional properties in subduction zones on earthquake triggerability (NSF Award 0742242)

I recently began work on my NSF-MARGINS postdoc at the University of California, Santa Cruz with Emily Brodsky comparing the triggering potential of subduction zones and relating differences in triggerability to fault zone architecture. To date, my research has generally focused on the complex behavior of faults using experiments, fieldwork, and theory. During my Master’s with Michele Cooke at the University of Massachusetts, Amherst, I was very interested in questions of fault interactions over long time periods and how these interactions could affect structures such as overlying folds. I became interested in shorter time scale interactions and fault friction behavior during my Ph.D. with Chris Marone at Penn State. Using the biaxial deformation apparatus in the Penn State Rock and Sediment Mechanics lab, I conducted stick-slip friction experiments on bare rock and granular material to assess how oscillating stresses such as seismic waves and tides trigger seismicity. To do this, I applied a small oscillating shear velocity on a constant shear loading rate to mimic seismic stresses superimposed on tectonic rates. These experiments indicated that the presence or absence of a fault gouge layer (or any granular material in a shear zone) changed the sensitivity of a fault to earthquake triggering. In bare rock surface experiments the roughness of the surfaces determined sensitivity to triggering, whereas in the granular layer experiments, layer thickness determines triggerability. The bare rock experiments can be interpreted as immature faults where asperity contact determines fault strength, while the granular layers model mature faults with well-devel- oped gouge zones. If the analogy holds, I predict that fault maturity should influence triggerability. For my MARGINS postdoc, I will test this hypothesis on the largest scale possible: subduction zones. Beneath Cost Rica, the seafloor being subducted is dotted with seamounts and ridges that act as asperity contacts with the overriding plate. Presumably, we should see a difference in triggering potential between this subduction zone and others. By looking at triggering events of similar sizes in different subduction zones, I aim to assess which areas are more sensitive to changes in stress. To do this, we will search for locally triggered events during the passage of seismic waves from large, remote events, as well as counting the number of aftershocks from local events. I am excited to have this opportunity to learn some observational seismology and apply my ideas to real world observations.

Benjamin Holtzman
Columbia University, NY

MARGINS Postdoctoral Fellowship: Deciphering the Role of Melt Segregation and Strain Partitioning in Rifting Continents (NSF Award 0646696)

My current research, including that funded by the MARGINS postdoctoral fellowship, lies somewhere between rock mechanics, seismology and geo-dynamics. I want to understand how to interpret seismic anisotropy measured at plate boundaries in terms of both rock fabrics and melt distribution, creeping towards a more detailed image of the structure of rheological properties of plate boundaries. While still involved with experimental rock deformation studies, I am also working with models of anisotropic elastic and viscous properties of partially molten rocks developed by Yasuko Takei at the University of Tokyo. At Lamont, I am collaborating with Jim Gaherty’s group on the measurement, inversion and interpretation of surface wave anisotropy from the East African Rift in Ethiopia and the Gulf of California (the MARGINS site). But more generally, or in more detail (I’m not sure which), my research focuses on the interactions of melt migration and deformation, or how melt influences rheological properties of rocks at high temperature. As research often proceeds as a series of accidents, I have migrated from field geology to geophysics. My education in Earth Science began at Brown University. I did a field-based structural geology project on the nearest ophiolite (in Maine), mapping the structure and constraining emplacement with U/Pb dating of zircons. Then I went to the University of Minnesota to work with Christian Teyssier to study ophiolites as tectonic markers in the Himalayas, but I broke my knee playing soccer, so he concocted a modeling project (in Montpellier, France, where he was on sabbatical at the time) to study the phenomenon of fracturing pebbles in soft sediment (asking whether it was caused by seismic waves or by slow deformation of the matrix). My two month visit grew to seven months. The lab next door, headed by Adolphe Nicolas, focused on the Oman Ophiolite. He saw an analogy in my pebble problem to chromite grains that fracture while olivine creeps around them, as observed in Oman. So I had the great fortune of working with their group for three field seasons. I wrote my master’s thesis on these analogical problems. In studying the “chromite pods”, it seemed that the chromite was reducing the permeability of the rock, and modifying the flow of melt as they deformed. After a cheap sabbatical in Boston, I went back to Minnesota and joined David Kohlstedt’s lab, to do experiments on the deformation of partially molten olivine rocks. I added a little chromite to reduce the permeability and the melt segregated dramatically into networks of melt-rich shear zones. This phenomenon became the subject of my PhD thesis. In applying to Lamont, I proposed to study the seismic expression and rheological consequences of these melt rich networks, to explore their geodynamical role in the Earth. After several years, these ideas are taking a practical form. I am very grateful to the MARGINS program for granting me this support to pursue them further.

MARGINS Postdoctoral Fellowship: Exploring the Record of Magmatic Volatiles in a Volcanic Arc via H, C, F, S, and Cl in Apatite (NSF Award 0549082)

When I approached Rick Hervig with the idea for our MARGINS proposal I had never met him, I had zero secondary ion mass spectrometer (SIMS) experience, and I had not worked on igneous processes in more than 5 years.  Yet here I am, a MARGINS Postdoctoral Fellow in the School of Earth and Space Exploration at Arizona State University, using the SIMS to study volatiles in magmatic systems.

As an undergraduate at UCLA, I was fortunate to be surrounded by energetic faculty and graduate students who challenged me, broadened my horizons, and introduced me to geology in the field at exotic locations all over the world.  I stayed at UCLA for an M.S. degree with Mary Reid and Marty Grove, evaluating the hypothesis that 40Ar/39Ar ages from melt-inclusion bearing quartz crystals represent magma residence time. For my Ph.D, I knew I wanted to work on the (U-Th)/He system, and Mary suggested that I apply to her alma mater (MIT) to work with Kip Hodges. My visit with the faculty and graduate students there quickly convinced me that MIT was the place I wanted to continue my education.  After we built the (U-Th)/He lab at MIT, we tackled a variety of problems related to (U-Th)/He, including a critical evaluation of the Durango apatite (U-Th)/He standard, determination of He diffusion rates in monazite, and most importantly, the development and application of laser microprobe (U-Th)/He geochronology.

I stumbled upon the idea of using apatite to constrain H2O and CO2 in magmas during my first year at MIT. At the time, one of my committee members suggested (as it did not fit into my thesis plan) that I keep the idea to myself until I had a chance to write a proposal. Years later, as I wrapped up my Ph.D, I recognized the need to diversify my research portfolio beyond noble gas geochronology.  The long-dormant apatite project seemed like a perfect change of pace, and I decided to try to make it happen.  I wrote to Rick, introducing myself and the idea I wanted to pursue, and the rest (as they say) is history.

Since my arrival in Tempe last Spring, my collaboration with Rick and interaction with other ASU researchers and visitors has opened up to me many new frontiers in the geosciences, stimulating a tremendous number of ideas for future projects. Our MARGINS study, along with the work of colleagues, collaborators, and competitors worldwide, is helping to develop apatite as a powerful indicator of volatile processes. Perhaps even more exciting is our recent discovery that apatite crystals may not only yield information about pre-eruptive magmatic water content, but also about the timescales of volatile processes in magmas.

Every year Ph.D programs produce far more graduates who want faculty positions than can be placed in them.  For those who want or need more time to develop their skills before starting the tenure clock, postdoctoral research experience is essential.  Yet few NSF programs set aside money for post-docs. I know how lucky I am that the MARGINS program has the vision to promote postdoctoral research, and I hope other NSF programs follow this example.

PAST FELLOWS

MARGINS Postdoctoral Fellowship: Time-scales and mechanisms of differentiation of mafic parents to rhyodacite in Central America (NSF Award 0405262)

In August of 2004, I packed my belongings and headed out to the Midwest to begin two years of research for my MARGINS post-doc at the University of Iowa with Mark Reagan.  This was the beginning of a series of challenging opportunities that will benefit me the rest of my career.

My Ph.D. research at UCLA with Jon Davidson had taken me to Ecuador, where I studied the geochemistry of lava flows and pyroclastic deposits related to Cotopaxi volcano in the Northern volcanic zone (NVZ) of South America. Cotopaxi is one of the few composite volcanoes in Ecuador that has erupted a bimodal suite of rhyolite and andesite lavas, and we used trace element and isotopic compositions of whole rocks and mineral separates to study the timescales and processes of magma differentiation operating in the NVZ.  In addition to valuable research and travel opportunities, my Ph.D. project provided me with a solid background in the analytical techniques that I would soon put to use during my postdoc. 

I initially met with Mark Reagan at the 2003 AGU/EGS meeting in Nice, France where we discussed possibilities and funding options for a project in Central America.  In spring 2004, I was excited to receive notification that our MARGINS project had been funded, and we started working in El Salvador in February of 2005.  Our MARGINS project focused on voluminous dacites erupted from three calderas in El Salvador and Nicaragua (Ilopango, Apoyo and Apoyeque) over the last 20 ka.  The youngest of these was the climactic eruption from Ilopango caldera in El Salvador about 1605 years ago.  We used trace element and isotope geochemistry to assess differences among these calderas.  While conducting our field research in El Salvador we were fortunate to be accompanied by Dolors Ferres from the local government office that monitors volcanic activity in El Salvador (Servicio Nacional de Estudios Territoriales, or SNET). After completing field work in El Salvador, I had the opportunity to participate in a field excursion to Santa Maria volcano in Guatelama, led by Dr. Bill Rose from MTU. The hike and three day stay in this very active crater was an experience I’ll never forget.  After returning to Iowa City, I began the process of separating minerals and preparing rocks for analysis.  Most of the sample preparation was done in the University of Iowa clean lab facility.  The analysis of the U-series isotopes (U, Th, Ra), and the long-lived radiogenic isotopes (Pb, Sr, Nd) were accomplished at Woods Hole Oceanographic Institute.

I spent a total of four months at WHOI working with Ken Sims to prepare and analyze samples.  On the basis of Th, Sr, Nd, and Pb isotopes, our preliminary data show that the rhyodacites in this system are produced by fractionation of local, more mafic magmas.  Samples from all three systems have 230Th excesses, which is significant because most of the more mafic rocks from this area have 238U excesses that result from fluid addition from the slab.  Positive correlations between (230Th/232Th), Ba/Th, and Ce/Pb, as well as large differences in Th isotope ratios from north to south suggest that these differences correspond to changes in the amount of subducted sediment involved in magma genesis.  This is important because relating geochemical variation to petrogenetic processes and subsequently relating these processes to timescales is critical to our understanding of magma genesis and volcanic evolution.  I am currently preparing these results for publication, and we will also present our research at the upcoming June MARGINS workshop in Costa Rica (Workshop to Integrate Subduction Factory and Seismogenic Zone Studies in Central America, June 18-22, 2007).

As a result of my time spent at WHOI, I have initiated important contacts and have planned future collaborations, including research in Ecuador on El Reventador volcano.  I am very thankful to have been afforded the opportunity for this research project.

A Numerical Investigation of the Relative Importance of Different Melting Mechanisms at Volcanic Arcs (NSF Award 0305292)

Working as a post-doc at Washington University has been a great opportunity to study geologic processes at volcanic arcs and back-arc basins and also to become involved with the MARGINS program.  Previous and ongoing WashU seismic experiments at the Tonga and Mariana subduction zones have helped stimulate my interest in magma production and migration at these geologically important regions.

Growing up in Salt Lake City helped give me an appreciation for the outdoors and the many diverse, but interlinked, aspects of the natural and physical sciences. I discovered Geology as an undergraduate at the University of Utah, and worked for a year at a mining exploration company before heading to the East Coast for graduate school. Although my snowboarding skills severely atrophied after leaving the Wasatch Mountains, it has been an immensely rewarding experience, particularly because of the interaction with the many great people and discussions of interesting research I have encountered within the geophysical community.

While a graduate student at Brown University, I learned the value of combining different seismic observations with other methods, particularly numerical modeling, to understand mantle flow and melt processes.  My research at Brown primarily focused on the South- east Indian Ridge and the East Pacific Rise.  One thesis chapter demonstrated with numerical models of mantle flow incorporating pressure- and temperature-dependent viscosity that asthenospheric flow driven from one side of the spreading axis to the other can account for the unexpected degree of asymmetry observed across the East Pacific Rise from the MELT experiment.  Applying these same modeling techniques to understand the observed asymmetric seismic structure of the Lau back-arc spreading cen- ter led to the find that decompression melting should be expected beneath some volcanic arcs as the subducting slab viscously erodes the base of the overlying lithosphere.  Models using temperature-dependent viscosity also predict slab temperatures >100 deg C warmer than isoviscous models.  These observations put volcanic arcs in the interesting position of potentially having three possible melting mechanisms for magma generation: wedge hydration, wedge decompression, and slab heating.  As a MARGINS post-doc, I plan to use numerical models along with seismic observations to map out how the importance of each melting mechanism varies from arc-to-arc as the thermal structure and other governing parameters at subduction zones change.

Collaborative Research: Constraining the Volatile and Slab Flux in the Izu-Bonin-Mariana MARGIN Using Geothermal Fluids, Phenocrysts and Melt Inclusions (NSF Award 0305052)

After completing my undergraduate degree in geosciences at McGill University, I left the Great White North to pursue graduate studies in sunny Southern California at Scripps Institution of Oceanography.  During the course of my Ph.D. research at Scripps with Dr. David Hilton, I had the opportunity to work on a MARGINS related project investigating the CO2 flux though the Central American Arc. Using He-C relationships in geothermal fluids (fumaroles, hot springs and geothermal well gases) collected from various volcanic centers along the arc, I was able to assess the provenance of the volcanic C output, allowing for an estimate of how much of the C input at the trench (as pelagic carbonates, organic C and altered oceanic crust carbonate) was recycled through the arc system. As a complement to my geothermal fluid arc studies, my postdoctoral research at the Carnegie Institution of Washington with Dr. Erik Hauri will focus on melt inclusions in recently erupted tephras from the Izu-Bonin-Marianas arc system. Melt inclusions are thought to represent preeruptive melts, and thus offer the exciting possibility to look at the composition of sub-arc partial melts. Since it is assumed that inclusions remain isolated after entrapment, they are thought to preserve the characteristics of primitive melts and are less influenced by magma chamber processes than erupted lavas. This study will be part of a collaborative volatile study of the arc using geothermal fluid data and remote sensing techniques, along with melt inclusions in mafic phenocrysts, to gain a better understanding of how volatiles are cycled through arc systems.

MARGINS Postdoctoral Fellowship: The Role of Sediment Diagenesis and Dewatering on Fluid and Heat Flow, Costa Rica Margin (NSF Award 0304946)

I am excited to be starting a MARGINS post-doctoral fellowship this year. After spending most of my life near the edges of this continent (including growing up in New Jersey and attending grad school at the University of California, Santa Cruz), it seems natural to head to the heartland (Missouri) to study continental margins. After getting a B.S. at Penn State, I went to UC Santa Cruz where my research encompassed hydrogeology and sedimentology — ranging from groundwater seepage into San Francisco Bay and fluid seepage through sediments on the Juan de Fuca Ridge flank, to sedimentation patterns on northern California’s Eel River margin. As a MARGINS post-doctoral fellow, I will further develop my interests in both hydrogeology and sediments by studying sediment dewatering within the Costa Rica margin subduction zone.

Sediments subducted off Costa Rica have high opal and smectite contents (both contain a large amount of water that is expelled at moderate temperatures and pressures within the shallow subduction zone). Smectite dewatering has been shown to significantly affect pore fluid pressure and chemistry in the subduction zones of Barbados and Nankai. On the Costa Rica margin, opal dewatering provides another source of water during sediment diagenesis. Additionally, in Costa Rica, large along-strike variations in heat flow may lead to differences in the depth within the subduction zone at which the dewatering reactions occur. This has the potential to generate along-strike fluid pressure gradients and therefore margin parallel fluid flow within the subduction zone. Fluid pressure and fluid flow patterns in response to sediment dewatering in the subduction zone, in turn have implications for fault strength and potentially the location of the updip limit of seismicity.

This study will consist of two phases: a characterization of the sediments offshore Costa Rica (in cooperation with Mike Underwood at the University of Missouri), and modeling of sediment dewatering and fluid flow within the Costa Rica margin subduction zone (in cooperation with Demian Saffer at the University of Wyoming).

Direct inquiries to the MARGINS Office