Winter 2013

Winter 2013

Tuesday Afternoons at 4:00 PM
Natural Science Annex 101



January 15, 2013

Amy Draut, USGS

Sedimentary Deposits and Processes on the Elwha River, Washington, During Large-scale Dam Removal.

Hosts: Noah Finnegan/Allison Pfeiffer


January 22, 2013

Jon Warrick, USGS

Fire, Floods and Mudflows, Oh My...  Sediment Yields of the Coastal Watersheds of California

Hosts: Noah Finnegan/


January 29, 2013

Thorne Lay, University of California, Santa Cruz

Large Earthquakes Along the Queen Charlotte Fault Zone: Transpression on the Pacific-North America Plate Boundary

Hosts: Erin Todd


February 5, 2013

Kaj Johnson, Indiana University 

Using GPS Data in California Earthquake Hazard Analysis

Abstract: For the first time, GPS measurements of surface motions are being used formally in USGS earthquake hazard calculations. While GPS measurements help constrain where strain/stress is accumulating in the crust, it is not straightforward to map surface deformation onto faults at depth. This mapping requires a model that implements our partial understanding about the behavior of faults in the lower crust and deformation processes in the mantle. In this talk I will show how we are using deformation models and GPS data from California to obtain estimates of long-term slip rates on major faults and the potential for earthquakes off of the major faults for use in the next Unified California Earthquake Rupture Forecast (UCERF). I will show that the rate of storage of elastic energy in the crust is higher than the observed rate of release of seismic energy over the past 150 years. I will also show that a surprisingly large portion of the plate boundary deformation in California (~30%) is not accommodated by slip along the major faults.

Host: Thorne Lay


SPECIAL TALK

WEDNESDAY FEBRUARY 6th, 2013, EMS A340 11:30 a.m. - 12:30 p.m.

Kaj Johnson, Indiana University 

From Kinematics to Mechanics: Using Geodesy to Infer the Mechanics of Mountain Building and Transform Faulting

Abstract: Geodetic data provide observational constraints on the kinematics of surface motions in deforming plate boundaries. From these kinematical observations we want to infer mechanical properties of faults and the lithosphere. In this talk I will show how my students and I use geodetic data, along with seismic and geologic observations, to infer physical properties of faults and lithospheric loading conditions in Taiwan and California. We show that active mountain building in Taiwan is likely driven both by faulting at the flanks of the mountain range and lower crustal thickening underneath the high ranges due to stacking or underplating.  Along the central section of the San Andreas Fault, we use GPS and InSAR data to infer where the fault is locked and accumulating stress and where the fault is relaxing stress by creep. Creep on the San Andreas is inferred to occur at low strength and near-stability-neutral friction conditions from inversions of geodetic and seismic tremor measurements. 

Host: Thorne Lay


February 12, 2013

Terry Blackburn, Carnegie Institution for Science

Sink to Survive: The Persistence of Ancient Mountain Belts Through Crustal Density Changes

Mountain belts form when collisions between continents thicken Earth’s crust, which buoyantly rises to remain in isostatic equilibrium with the denser underlying asthenosphere. Just as isostasy leads to the birth of mountains, it contributes to their destruction by uplifting them in response to erosion, maintaining high elevations and promoting further erosion – a process that, if left unchecked, would eventually destroy the crust. Yet the ancient roots of Earth’s oldest mountains have persisted for billions of years, maintaining total crustal thicknesses near 40 km. One explanation for this preservation is that a mountain belt’s isostatic buoyancy diminishes over time as lithospheric cooling makes the lower crust denser. Here we test this hypothesis using: 1) a measure of mountain belt erosion on billion year time scales derived from U-Pb thermochronologic measurements of lower crustal xenoliths and 2) a global compilation of geologically recent erosion rates and exhumation rates in mountain belts with formation ages ranging from approximately 0 to 2 billion years. We compare the rates from each technique with a model for the thermal, erosional and density history of an idealized mountain belt. Measured and modeled data indicate that erosion is fastest in young, hot, low-density, and topographically high mountain belts, but slows dramatically after 200-300 million years, consistent with the timing of metamorphic garnet growth and densification of the lower crust that accompany lithospheric cooling. Though erosion rates may differ among active mountain belts with different tectonic or climatic settings, our analysis suggests that it is the isostatically determined relief, driven by the thermal and density evolution of the crust, that dominantly controls the continents’ billion-year erosional histories and ultimately leads to the preservation of Earth’s ancient mountain roots.

Hosts: Jeremy Hourigan/Jon Perkins


SPECIAL TALK

WEDNESDAY FEBRUARY 13th, 2013, EMS A340 11:30 a.m. - 12:30 p.m.

Terry Blackburn, Carnegie Institution for Science

Reconstructing the Chondrite Meteorite Parent Body Using U-Pb Thermochronology

Ordinary chondrites contain some of the oldest fragments of material in our solar system and provide a record of the earliest stages of planetary formation through the accretion of dust and chondrules at time scales between ~1-4 My after the formation of our solar system’s earliest forming solids or CAIs. The antiquity and large abundance of chondrites has raised questions concerning their origin and the conditions leading to the preservation of their primordial geochemical signatures and early forming accretion textures. In short, if these bodies accreted early in the history of the solar system, how has this material avoided melting in the presence of the short-lived and highly radioactive nuclides such as 26Al? Three models have emerged as possible explanations for their preservation: 1) chondrites avoid melting by cooling rapidly within a small, undifferentiated planetesimal; 2) chondrite accretion occurs millions of years after CAI formation, when heat production by the decay of 26Al has decreased; 3) chondrites make up the unmelted outermost lid of a large, differentiated protoplanet. The discernable difference between these models is the thermal history experienced by preserved chondrites. A reconstruction of this thermal history can be made through the use of a temperature-sensitive radiometric dating technique known as thermochronology. I’ll present preliminary work demonstrating how U-Pb thermochronology combined with thermal models describing heat transfer in a chondrite parent body can ascertain the planetary radius and timescale of planetary assembly.

Hosts: Jeremy Hourigan


February 19, 2013

Christy Till, USGS

Experimental Insights into Melt Generation at Convergent Plate Margins

Evidence preserved in the petrology and chemical composition of erupted arc lavas provides the basis for understanding the processes that give rise to arc magmas.  Work over the past 30 years has resulted in a preponderance of evidence to suggest arc parental magmas commonly contain up to 4–6 wt% H2O and some arc andesites contain up to 8–10 wt% H2O.  However, considerable uncertainty remains about the physically and compositionally complex processes that lead to the generation of hydrous arc magmas with these observed water contents.  In this talk, I will present new experimental evidence regarding the systematics of melting H2O-saturated and chlorite-bearing undepleted peridotite from 3 to 6 GPa.  These experiments are then used to understand the temperatures and chemical reactions of mantle wedge melting that constitute the primary controls on (1) the location of arc volcanoes and (2) the width of the volcanic arc.

Hosts: Jeremy Hourigan/Erinna Chen


SPECIAL TALK

WEDNESDAY FEBRUARY 20th, 2013, EMS A340 11:30 a.m. - 12:30 p.m.

Christy Till, USGS

Timescales & Mechanisms of Magma Genesis

An integrated understanding of the mechanisms and timescales of magma genesis is fundamental to understanding the chemical evolution of the Earth and other terrestrial planets.  This talk will examine two research approaches with this aim.  The first is a new melting model and thermo-barometer for continental basalts, which yields first order observations regarding the tectono-magmatic forces driving mantle melting or the thickness of the mechanical lithosphere for example.  The second is diffusion modeling (or “geospeedometry”), which yields precise timescales for volcanic and magmatic processes.  

Hosts: Jeremy Hourigan/Claire Masteller


February 26, 2013

Esteban Gazel, Virginia Tech

Life Cycles of Mantle Plumes

The massive basaltic production responsible for the emplacement of Large Igneous Provinces (LIPS) during the Permian-Paleocene represents in some cases the initial phases of some of the mantle plumes that feed the current ocean island basalts (OIB). In some cases this magmatism was so voluminous that it produced serious global environmental impacts. Recent petrological, geochemical and geophysical studies of some of these localities like Samoa, Hawaii, Galapagos provide evidence that melting is related to a true mantle plume that originates from a boundary layer beneath the upper mantle. Thus, plume-related magmas produced in OIB and LIPS and their connecting plume tracks provide evidence on mantle temperature, size and composition of heterogeneities, and deep geochemical cycles. Although a lot of work has been done on LIPS and OIB, no complete record of the evolution of a mantle plume is available to this point. Galapagos-related lavas provide a complete record of the evolution of a mantle plume since the plume's initial stages ~95 Ma. Our temperature evolution reconstruction suggests a decrease from mantle potential temperature TP(max) of 1650 °C in the Cretaceous to 1500 °C in the present day in the Galapagos Plume (1 °C/Ma). New results from high precision olivine chemistry from lavas of the Galapagos Islands and older Galapagos-related lavas suggest that this secular cooling is related with increasing amounts of recycled crust in the plume.

Hosts: Emily Brodsky/Jon Perkins


SPECIAL TALK

WEDNESDAY FEBRUARY 27th, 2013, EMS A340 11:30 a.m. - 12:30 p.m.

Esteban Gazel, Virginia Tech

Lithosphere-Asthenosphere Boundary at the Basin and Range, Western USA: Results from Big Pine Volcanic Field

The lithosphere is the strong lid at the surface of planet Earth that defines the different tectonic plates, and consists of the crust and rigid uppermost mantle that moves on top of the viscous asthenospheric mantle. These two mechanical layers are separated by some kind of rheological or thermal lithosphere-asthenosphere boundary (LAB). This boundary has been correlated with a seismic low velocity zone below, that can be produced by high temperature, partial melt, and the presence of volatiles that reduce the velocity through anelastic effects. To further understand the role of these different possibilities in the creation, evolution and seismic properties of the lithosphere-asthenosphere boundary (LAB), it is necessary to integrate seismological observations with petrological and geochemical interpretations. The Basin and Range Province (B&R) in the western USA is a region where the lithosphere is actively evolving and where both lithospheric and asthenospheric mantle sources have been invoked to explain the geochemistry of mafic volcanism in the last 10 Ma. The Big Pine Volcanic Field (BPVF) is an ideal location for constraining mantle melting conditions in the western region of the B&R because previous studies confirmed that crustal contamination was not a dominant processes in magma generation and rapid magma ascent. This work is the first to report H2O and CO2 concentrations in Big Pine magmas and oxidation state (fO2), which are essential for accurate estimates of the melting conditions. Melt inclusions trapped in primitive olivines (Fo82-90) record surprisingly high H2O contents (1.5 to 3.0 wt.%) for a location not currently above an active subduction zone. Estimates of the oxidation state of BPVF magmas are also surprisingly high (FMQ +1.0 to +1.5), based on constraints from V partitioning between melt and olivine and melt inclusion S contents (up to 5000 ppm), yielding Fe3+/FeT ratios of 20 - 30%. Lithospheric mantle xenoliths from BPVF record extremely low H2O concentrations (whole rock <75 ppm). Pressures and temperatures of melt equilibration of the BPVF magmas indicate a shift over time, from higher melting temperatures (~1320 °C) and pressures (~2 GPa) for magmas that are >500 ka, to cooler (~1220 °C) and shallower melting (~1 GPa) conditions in younger magmas.  The depth of melting also correlates with trace element ratios in the magmas, with deeper melts having trace-element compositions closer to asthenosphere values and shallower melts with compositions more typical of subduction zone magmas, and within the range of the available lithospheric mantle xenolith data from BPVF. The correlated melting conditions and geochemical stratification of the mantle melts are consistent with seismic observations of a shallow lithosphere-asthenosphere boundary (~55 km depth). Combined trace element and cryoscopic melting models yield self-consistent estimates for the degree of melting (~5%) and source H2O concentration (~1000 ppm). Two possible geodynamic scenarios can explain the small convection necessary for magma generation. The first related to the Isabella seismic anomaly, either as a remnant of the Farrallon Plate or lithospheric foundering.  The second scenario is related to slow extension of the lithosphere at the B&R province.

Host: Emily Brodsky


SPECIAL TALK

THURSDAY FEBRUARY 28th, 2013, EMS A340 4:00 p.m. - 5:00 p.m.

Ake Fagereng, University of Cape Town

Fault Rock Heterogeneity and the Spectrum of Fault Slip Styles

Shear displacements in active crustal fault zones are accommodated by a range of slip styles, including standard earthquakes, tectonic tremor, and transient and continuous aseismic creep. Subduction megathrusts tend to exhibit a particularly wide range of fault slip styles, with a variation in seismic style both across and along strike. Exhumed subduction-related rock assemblages typically contain melanges comprising variably sheared rocks of a range of compositions. Melanges contain more competent lenses in a relatively incompetent matrix; localized shear surfaces occur within or along competent domains, while matrix flow accommodates shearing by distributed strain. Competent lenses are typically composed of chert, sandstone, or basalt, whereas the matrix is dominantly mudstone. In active subduction zones, these rock types are typically intermingled in the trench, and their distribution can vary both down-dip and along strike. If the style of deformation in a melange reflects partitioning between seismic and aseismic slip, then the ratio of competent to incompetent material seems likely to be a critical factor affecting seismic style along a megathrust interface. Fault rock heterogeneity is not limited to subduction-related faults. Faults in a range of tectonic settings and at depths throughout the crust, that are tabular features containing a variety of materials, can also develop an internal structure where bulk rheology varies in space, likely affecting the distribution of fault slip styles. 

Host: Thorne Lay


SPECIAL TALK

FRIDAY MARCH 1st, 2013, EMS A340 10:00 a.m. - 11:00 a.m.

Ake Fagereng, University of Cape Town

Geological Processes and Tectonic Tremor

Tectonic, or non-volcanic, tremor is a recently discovered seismic style that generally occurs near the seismic-aseismic transition on subduction interfaces, but is also observed on the deep extension of the San Andreas fault. Tremor is commonly interpreted as shear slip under low effective stress, and tends to repeat in approximately the same location at regular intervals. In subduction margins, tremor has been found to correlate with zones of inferred fluid overpressure, which may correlate with fluid release from the downing slab. Along the San Andreas fault, however, the metamorphic conditions are not compatible with a local fluid source. 
Considering the variety in depth, temperature, and tectonic setting of the locations where tremor has been observed, the physical process causing tremor must occur at a range of crustal conditions. Deformation within a fault-fracture mesh as magma moves through the crust has been postulated as a mechanism leading to volcanic tremor. An analogous mechanism for non-volcanic tremor would be the movement of water through the crust. Hydrothermal veins provide a record of fracturing and precipitation from an aqueous fluid within a fracture system. Slickenfibre-coated veins represent shear slip at low effective stress. The microstructure of slickenfibre shear veins suggest repeated, incremental slip at length-scales typically in the 10 - 100 micrometer range, comparable with that inferred for single events within the tremor signal. Moreover, slickenfibre shear veins form in a variety of conditions, which have in common the presence of weak, pre-existing planes, and relatively high local fluid pressure. Based on these similarities, slickenfibre veins may be a product of tectonic tremor, and preserve a record of the processes behind tremor activity, if this hypothesis is true, it implies that tremor can occur wherever there are localized zones of shear displacement along pre-existing weak planes under low effective stress. 

Host: Thorne Lay


March 5, 2013

Victor Tsai, Caltech

Non-Traditional Seismology:From Sea Ice to the Earth's Core


March 12, 2013

Matthew Jackson, Boston University

Returning from the Deep: Archean Atmospheric Fingerprints in Modern Hotspot Lavas

The Earth’s mantle preserves a history of the long-term evolution and differentiation of the planet.  Oceanic crust and sediments are introduced to the mantle at subduction zones, but the fate of this subducted material within the mantle, as well as the antiquity of this process, is unknown.  Basaltic lavas erupted at some oceanic hotspot volcanoes have long been considered to be melts of ancient subducted lithosphere. However, compelling evidence for the return of subducted materials in mantle plumes is lacking. The Samoan hotspot erupts lavas with the clearest signature of recycled, continentally-derived sediment to-date, but clear evidence of recycled oceanic lithosphere has been more elusive.  Recently discovered mass independently fractionated (MIF) S-isotope signatures in olivine-hosted sulfides in young oceanic hotspot lavas provide powerful evidence for recycling of Archean oceanic lithosphere. Terrestrial MIF S-isotope signatures were generated exclusively through atmospheric photochemical reactions until ~2.45 billion years ago.  Therefore, the discovery of MIF-S in young OIBs indicates that sulfur—likely derived from hydrothermally-altered oceanic crust—was subducted into the mantle before 2.45 Ga and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with MIF 33S depletions, in the mantle source for Mangaia lavas. An Archean age for recycled oceanic crust provides key constraints on the length of time that subducted crustal material can survive in the mantle and on the timescales of mantle convection from subduction to melting and eruption at plume-fed hotspots.  The new S-isotope measurements confirm inferences about the cycling of sulfur between the major reservoirs from the Archean to the Phanerozoic, extending from the atmosphere and oceans to the crust and mantle, and ultimately through a return cycle to the surface that is completed in Mangaia lavas.  It remains to be seen whether hotspots lavas sampling different compositional mantle endmembers will exhibit evidence for recycling of Archean protoliths.

Host: Jim Zachos


SPECIAL TALK

WEDNESDAY MARCH 13th, 2013, EMS A340 11:30 a.m. - 12:30 p.m.

Matthew Jackson, Boston University

Opening Pandora’s Box:  What is the Earth’s Composition?

The bulk composition of the silicate portion of the Earth (BSE) has long been assumed to be tied to chondrites, in which refractory, lithophile elements like Sm and Nd exist in chondritic relative abundances.  The radioactive isotopes of Sm, 146Sm (half-life = 68 million years) and 147Sm (half life = 106 billion years), decay to 142Nd and 143Nd, respectively.  If the BSE is chondritic, then it must have chondritic 142Nd/144Nd and 143Nd/144Nd ratios.  However, a recent discovery challenges the traditional BSE model:  The 142Nd/144Nd ratios of modern terrestrial samples exhibit small, but measurably higher (18±5 ppm) ratios C and O-chondrites. If this terrestrial 142Nd excess is related to a Sm/Nd ratio 6% higher than chondritic, then the BSE must also have superchondritic 143Nd/144Nd. We develop a compositional model for BSE in which the elevated Sm/Nd requires a shift of 143Nd/144Nd from 0.51263 (chondritic) to 0.51300, a value that is closer to lavas erupted at mid-ocean ridges than chondrites.  A higher BSE 143Nd/144Nd demands, in turn, corresponding changes to 87Sr/86Sr and 176Hf/177Hf and the associated parent-daughter ratios—Rb/Sr, Lu/Hf.  These modified parent-daughter ratios define a normalized incompatible-element-depleted trace element pattern (spidergram) relative to the chondrite-based BSE.  The new BSE composition has lower K concentrations than a chondritic BSE, and offers a solution the “missing” 40Ar paradox.  The new BSE trace element model requires that >80% of the mantle (the upper and lower mantle) contributes to the formation of continental crust, instead of the long-standing estimate of ~25% (the upper mantle only). The new BSE compositional model also implies a ~30% reduction U, Th and K in the Earth, and therefore in the current rate of radiogenic heating, which requires a new family of thermal evolution models for the Earth. 

We identify an existing reservoir in the Earth’s mantle, called FOZO (Focus Zone), which closely matches the composition of the new BSE.  This reservoir is sampled by lavas erupted at some oceanic hotspots and flood basalts, and is characterized by the most primitive noble gas isotopic signatures in the Earth.  Careful geochemical characterization of these lavas offers an important way forward in constraining the bulk composition and the earliest history of the Earth.

Host: Jim Zachos