Geology

Archived Colloquia

 

 

John Wiesenfeld CSUN Master's Candidate
Talk Title: “Construction Of Continental Crust By Deep Crustal Fractional Crystallization and Garnet Pyroxenite Root Development: Geochemical Evidence From Fiordland, New Zealand.”
Date: Friday, May 6, 2016
Time: 10:00am
Room: LO1229

Abstract:

A key problem in understanding the growth of continental crust in magmatic arcs centers on the mechanisms that control geochemical diversification, particularly in deep crustal MASH (melting, assimilation, storage, and homogenization) zones where high heat flow facilitates differentiation processes. The Malaspina Pluton in Fiordland, New Zealand is a particularly well exposed suite of gabbro to diorite that was emplaced into the root of the Gondwana arc at 12–14 kbars during a brief 5 myr interval from ca. 120 to 115 Ma. Previous studies on related rocks in the Western Fiordland Orthogneiss (WFO) have hypothesized that partial melting of mafic source rocks controlled the geochemical characteristics of the WFO, particularly heavy rare earth element depletions and high Sr/Y values (>>40). Similarly, partial melting of lower crust has been implicated as a mechanism for producing the characteristic tonalite­trondhjemite-granodiorite (TTG) suites in the adjacent, mid crustal Separation Point Suite; however, the production of large volumes of mafic to intermediate magmas by partial melting alone is difficult to reconcile with thermal and mass balance considerations. The issue is compounded by the brief interval over which these large volumes of melts were emplaced into the crust (e.g., 5 Myr in the case of the Malaspina Pluton).

To better understand magma sources and processes of geochemical diversification in the Malaspina Pluton, we integrate literature data with 36 new whole rock x-ray fluorsecence analyses and 17 new ICP-MS solution whole-rock analyses. Our results reveal that primitive gabbros (49.2–52.6 wt.% SiO2) in the Malaspina Pluton are magnesian, metaluminous and calc-alkalic to alkali-calcic in composition. They are characterized by high Al2O3 (>18.3–20.6 wt.%) and Sr/Y values (>46–223), but low CaO (6.7–8.6 wt.%), Mg# (40–50), Y (7–24 ppm) and heavy rare element concentrations. Zircon d18O values for these rocks range from 5.67– 5.75‰, and overlap entirely with the mantle oxygen isotope field (5.3 ± 0.6 ‰; 2SD: Valley, 2003). These features indicate that Malaspina magmas were derived from hydrous partial melting of mantle peridotite resulting in a high Al2O3 parental composition. However, CaO concentrations and Mg# values are too low to be in equilibrium with direct mantle melts; therefore, Malaspina magmas underwent extensive fractional crystallization prior to and/or during emplacement in the deep crust. Geochemical trends are consistent with garnet + clinopyroxene (or omphacite) controlling liquid line of descent. Using average Malaspina diorite and a hypothetical high Al2O3 basalt as a starting composition (Rapp and Watson, 1995), major element mass balance models also predict a cumulate assemblage of garnet + clinopyroxene (or omphacite) ± apatite, rutile and ilmenite, with an approximately 1:1 ratio of melt-to­cumulate by volume. This predicted fractionating assemblage is consistent with field observations of garnet pyroxenite cumulates as rafts within the Malaspina Pluton and in the Breaksea orthogneiss which equilibrated at 1.8 GPa (De Paoli et al., 2009). Our results signify that the Cretaceous arc was >65 km thick and possessed a high-P ultramafic root that extended an additional 25 km below the base of the arc. These results agree with seismic velocity data for the Doubtful Sound region which demonstrate the existence of a high velocity body (Vp > 7.5 km s-1) centered on the Malaspina Pluton that extends to 40 km depth and are likely the remnants of the Cretaceous ultramafic root.


 

 

Samantha Gebauer CSUN Masters's Candidate
Talk Title: “Spatio-Temporal Patterns of Lower Arc Cooling and Metamorphism, Northern Fiordland, New Zealand”
Date: Wednesday, May 4, 2016
Time: 9:00am
Room: LO1212

Abstract:

The exhumed root of the Triassic to Early Cretaceous continental arc in Fiordland, New Zealand preserves a record of deep crustal arc processes during and following high-flux magmatism from c. 124-115 Ma. We present new LASS-ICP-MS and SHRIMP-RG 206Pb/238U dates and temperatures for metamorphic zircon and titanite in order to (1) reconstruct the thermal history of northern Fiordland, and (2) illuminate spatio-temporal patterns in arc root metamorphism. Samples were collected over ~50 km along an arc-parallel transect from George Sound ( ~8 kbar) to Milford Sound (~16 kbar) in order to understand the thermal evolution of the arc as a function of paleocrustal depth.

Zircon rims that developed on samples from the Arthur River Complex indicate that Early Cretaceous metamorphism initiated at 134.9 ± 3.1 Ma (T = 720°C) and lasted until 126.5 ± 2.7 Ma (T = 700 °C). Subsequent granulite-facies metamorphic zircon growth occurred from 121.3 ± 1.6 to 116.0 ± 2.3 Ma in Bligh and George Sounds at 830-700°C. These dates and temperatures overlap with existing garnet Sm-Nd dates from the Pembroke granulite (Milford Sound) and likely reflect heating associated with deep crustal emplacement of the Western Fiordland Orthogneiss from c. 125-115 Ma during the final stages of Median Batholith magmatism.

Titanite LASS chronology of metasedimentary rocks shows complex spatio-temporal patterns that reveal multiple pulses of titanite growth and/or recrystallization. East of Sutherland Sound, titanites yield a date of 120.6 ± 4.4 Ma and a Zr-in-titanite temperature of 765°C, which is similar to metamorphic zircon results in the region. At George Sound, titanites yield a date of 112.0 ± 2.4 Ma and a temperature of 760°C which overlaps with values reported in central Fiordland from Caswell to Breaksea Sounds. Younger dates of 104.8 ± 1.6 and 94.0 ± 2.3 Ma also occur in Bligh and George Sounds, and yield temperatures of 930 and 840°C, respectively. These dates and temperatures indicate that titanite growth and/or recrystallization occurred during multiple pulses of lower crustal heating which we speculate may have resulted from either lithospheric thinning related to extension and/or episodic foundering of a high-density arc root produced during the arc flare-up event.


 

 

Meghann Decker CSUN Master's Candidate
Talk Title: “Triggering Mechanisms For A Magmatic Flare-Up Of The Lower Crust In Fiordland, New Zealand, From U-Pb Zircon Geochronology And O-Hf Zircon Geochemistry.”
Date: Monday, May 2, 2016
Time: 9:00am
Room: LO1212

Abstract:

The exhumed root of a Triassic to Cretaceous continental arc in Fiordland, New Zealand records a history of
episodic, subduction-related magmatism that culminated in a high magma addition rate (MAR) event during which
much of the plutonic arc root was emplaced from 125-115 Ma. To evaluate the triggering mechanism(s) associated
with this high MAR event samples were collected for paired oxygen and lutetium-hafnium analyses. Sampling spans
both the Inboard and Outboard Median Batholith and covers 150 km parallel and 60 km perpendicular to the paleoarcaxis.

We report over 200 SIMS zircon O analyses and over 500 LA-MC-ICP-MS Lu-Hf analyses from 27 samples.
In addition, we report, 6 new LA-SF-ICP-MS 206Pb/238U dates. Inboard magmatism took place between 134 to 115
Ma with the high-flux event occurring from 125-115 Ma during emplacement of the Western Fiordland Orthogneiss
(WFO). Zircon δ18O SIMS values for the Inboard Median Batholith range from 5.2-6.3‰ with >90% of analyses
overlapping mantle-like values (5.3 ± 0.6‰; 2SD). LA-MC-ICP-MS results yield initial εHf (Zrn) values ranging
from -2 to + 11 with a mean of +4.

We investigate possible relative contributions of supra-crustal material during the arc flare-up through mass
balance calculations assuming the following mean zircon-normalized δ18O values: mantle (5.3 ‰), supra-crustal
rocks of the Deep Cove Gneiss (DCG) (8.9 ‰), and putative underthrusted Darran Suite (4.2‰). Results from
binary mass balance mixing of DCG with mantle magma yield values ranging from 0-21% supra-crustal input
during the high MAR event. Together, the homogeneity in δ18O (Zrn) and their mantle-like values, the mafic major
element geochemistry, and lack of zircon inheritance observed in the WFO suggests that the high MAR event was
triggered by mantle melting, likely resulting from changes in subduction zone dynamics, with only minor input from
supra-crustal sources (<<20%).


 

 

Jose Cardona CSUN Master's Candidate
Talk Title: “Constraining the most recent surface rupture on the Garnet Hill Fault, Coachella Valley, California”
Date: Friday, April 29, 2016
Time: 9:00am
Room: LO1229

Abstract:

The slip rate and rupture history of the Garnet Hill Fault (GHF) remain the least understood for Holocene-active strands of the San Andreas Fault system in the northern Coachella Valley. Here we report new trenching data that constrain the timing of the most recent surface rupture on the GHF from the East Whitewater Hill Paleoseismic site located near the Hwy 62/I-10 interchange. The map trace of the GHF is not well located here due to burial by young sediment but it is mapped in Qfaults at the base of the hillslope as it is inferred to cause uplift of the hills. Our trench revealed no definitive evidence of surface faulting. The trench exposed a charcoal-rich colluvial wedge-shaped deposit (CW) at the north end of the trench, with underlying coarse-grained cobble-gravel and overlying finer-grained fluvial material. The CW is light orangish­brown, matrix supported, with sub-rounded to sub-angular clasts and abundant ~1–2 cm-long pieces of detrital charcoal. The color suggests that the CW is derived from the steep, orangish-brown weathered gravels from the hillslope just north of the trench. We interpret that the Whitewater River sourced the underlying cobble gravel and smaller drainages north of the site provided the overlying finer fluvial material. The youngest charcoal ages from the base (A.D. 1404–1435) and top (A.D. 1474–1635) of the CW provide a maximum age for its formation of ~600 years ago. Given our inference that the fault lies beneath the trench, two possible hypotheses can explain the absence of observed faulting: (1) the colluvial wedge formed after an earthquake with a thrust tip that is undetectable (or blind) in the cobble gravels, or (2) the Whitewater River eroded and/or buried the near-surface expression of the fault. Both explanations provide a minimum age for the timing of the last earthquake at ~600 years ago. These results argue against the possibility that the ~A.D. 1690 event observed at sites on the Coachella section ruptured through the trench location. However, the data permit that the most recent rupture (~A.D.1400) documented to the northwest in San Gorgonio Pass (SGP) did rupture at this site and may be continuous with paleoearthquakes on the Coachella section.  This interpretation would support a model where ruptures that link SGP, the GHF and the Coachella section are less frequent than earthquakes that rupture only the Coachella section. 


 

 

Uchitha Nissanka CSUN Geophysics Master's Candidate
Talk Title: “Formation of Intraplate Seamount Chains by Viscous Fingering Instabilities in the Asthenosphere using low Reynolds Number Miscible Fluids with a Moving Surface Boundary. ”
Date: Tuesday, April 26,2016
Time: 1:00pm
Room: LO1229

Abstract:

Regional seismic tomography studies in the Pacific ocean and continental western U.S show linear bands of low velocity anomalies that are aligned with absolute plate motion and occur beneath volcanic lineaments located within the interior of plates far from plate boundaries. But their origin and the formation are still unclear. Small-scale convection provides one possible explanation for these lineations but does not predict age progressive seafloor volcanism nor progressive enrichment trends that oppose plate motion. I propose a new hypothesis where viscous fingering instabilities form due to hot and wet mantle plumes which rise and discharge into the upper mantle asthenosphere and displace higher viscosity depleted mantle. Here I consider a physical fluid model which studies the viscous fingering in a Hele-Shaw cell using low Reynolds number miscible fluids. I perform laboratory fluid experiments scaled to the Earth's mantle, with stationary and moving surface plates that use glucose-water solutions with viscosities (µ) from 0.3 to 326 Pas and viscosity ratios (µ2/µ1) from 3 to 300. I test the effect of several physical properties including the viscosity ratio, absolute viscosity, Gamma (G), plate spacing, density difference and ultra-slow fluid injection rate. Viscous fingers are observed to form for all viscosity ratios I consider and after an initial growth period, exhibit a constant wavelength that depends on several parameters. Fingering wavelength is strongly dependent on plate spacing (and therefore asthenospheric layer thickness) but shows weak dependence on viscosity ratio and injection rate. For the case with a mobile upper plate, I define the flux ratio, G, as plume flux to plate velocity which varies from 0.005 to 12700 in our experiments and  considers the range expected in the Earth (0.0006 – 56). My laboratory results indicate that fingers align with plate motion both upstream and downstream and indicate longer wavelengths in the downstream direction. Experiments scaled to the Earth’s upper mantle show fingers form in the presence of surface plate motion for G = 0.5 if asthenospheric thickness is less than 386 km.  The initiation radius, Ro, where fingers first form, increases with increasing plate spacing. Scaling to study of the south Pacific seafloor shows fingers should develop for Ro = 350 km from the plume source. This new geodynamic model for viscous fingering in the asthenosphere links off-axis and rising mantle plumes indirectly to mantle return flow to the spreading centers where they contribute to melting, surface volcanism and the growth and formation of new lithosphere. 

 


 

 

Dr. Sora Kim from University Chicago
Talk Title: “Ecological and oceanographic dynamics through the geochemistry of shark teeth”
Date: Monday, March 16, 2015
Time: 10:00am
Room: LO1229

Abstract:

Sharks are one of the most persistent marine vertebrate predators, surviving mass extinction and climate change events. I delve into the environmental conditions and ecological history of sharks using the form and geochemistry of their teeth. For example, stable isotope analysis of shark teeth indicate a freshwater dominated Eocene Artic Ocean, which has implications for past ocean circulation. The small size of this population suggests possible growth limitations at high latitude or habitat use as a juvenile nursery. My work also includes modern sharks in controlled captive experiments and satellite tagging studies to establish how stable isotopes are incorporated from the environment and diet into tissues.

My research program explores ecosystem response to climate change. Sharks are one of my favorite (but not my only!) study system where I combine modern analogues, quantitative paleontological methods, and geochemical techniques to understand the interplay between environment and ecology.


 

Dr. Jennifer Cotton from University Utah
Talk Title: “Climate, CO2 and the history of C3 and C4 grasses in the western hemisphere”
Date: Wednesday, March 11, 2015
Time: 10:00am
Room: LO1229

Abstract:

The global expansion of C4 grasses in the late Miocene through Pliocene is one of the most dramatic ecosystem changes of the Cenozoic. The balance between C3 and C4 vegetation is sensitive to temperature, precipitation and atmospheric CO2 concentrations, but given the observations that global average temperature and atmospheric CO2 were relatively stable during this event 8–3 million years ago, the primary driver of this expansion is not well understood. Here, I use two methods to reconstruct the distribution of C4 grasses from the Miocene to the present to understand the climatic and environmental constraints on the distribution of C4 grasses. The first, applied to the Miocene through Pliocene, uses the carbon isotopic composition of preserved soil organic matter (SOM) and phytoliths to determine past C4 grass abundances. The second, applied to the Pleistocene through the present, uses the carbon isotopic composition of bison and mammoth tissues as well as climate model outputs and advanced statistical methods to model the distribution of C4 grasses across North America through time. In the late Miocene of Montana, paleosol SOM shows that C4 grasses were present in small abundances prior to the global expansion and were seemingly waiting for the right conditions to expand just a few million years later. In the late Miocene through the Pliocene of northwest Argentina, paleosol SOM and phytoliths show that high elevations and locally cool climates prevented C4 grasses from spreading into the region while at the same time C4 grasses were expanding in many other locations globally. From the Last Glacial Maximum through the present, the model based on bison and mammoth carbon isotopes shows large increases in the abundance of C4 grasses in the central and northern Great Plains, while C4 grasses remained dominant in the southern Great Plains. Additionally, this is the first study to directly infer the effect of atmospheric CO2 on the abundance of C4 grasses, and suggests that atmospheric CO2 concentrations only become an important below 270 ppm (preindustrial values). However, at atmospheric CO2 concentrations above 270 ppm, as have occurred for most of Earth’s history, growing season temperature and precipitation are dominant. Taken as a whole, this work supports the hypothesis that climate rather than CO2, and precipitation over temperature in particular drove the global expansion of C4 grasses in the late Miocene through Pliocene.


 

Dr. Victoria Petryshyn from University of California, Los Angeles
Talk Title: “Reading stromatolites: Unlocking a 3.5 billion year history of life and climate”
Date: Monday, March 9, 2015
Time: 10:00am
Room: LO1229

Abstract:

Stromatolites, commonly defined as laminated, lithified organo-sedimentary structures built by microbial mats, constitute some of the oldest putative fossils on Earth, with a record going back 3.5 billion years. However, the processes that control the different aspects of stromatolite growth are poorly understood. Typically, laminations are interpreted to record the daily, seasonal, or yearly response of a microbial community to some environmental forcing. Unfortunately, morphology can be deceiving; abiogenic structures that mimic “real” stromatolites are known. Furthermore, numerical stromatolite growth models imply that microbial involvement may not be a prerequisite to form such morphologies at all. Still, as layered, accretionary structures that form subaqueously—and therefore record chemical information about their formation environment—stromatolites are potentially useful tools for finetimescale environmental and biological reconstructions, if they can be properly interpreted.

In order to highlight the potential of stromatolites as biologic and climatic indicators, I will present evidence from 3 different stromatolite localities and time periods: Holocene forms from Walker Lake in western Nevada; End-Triassic stromatolites that formed on the shallow margins of the Tethys Sea; and Mesoproterozoic fluvial/lacustrine stromatolites from the upper peninsula of Michigan. In all cases, detailed studies of isotopes (stable and clumped), texture, and morphology reveal that major environmental change can be recorded in stromatolites, with or without obvious changes in morphology.


 

Dr. Sarah Stamps from the University of California, Los Angeles
Talk Title: “Continental rift-parallel surface motions in East Africa”
Date: 3/2/15
Time: 10:00am
Room: LO1229

Abstract:

The East African Rift System (EARS) is the Earth’s most spectacular active continental rift. It spans N-S ~5000 km and currently experiences ~E-W extension. Previous kinematic studies of the EARS delineate 3 relatively rigid sub- plates (Victoria, Rovuma, and Lwandle) between the Nubian and Somalian plates. GPS observations of these block interiors confirm the rigid plate model, but new observations within individual rifts show rift-parallel surface motions that do not conform to large-scale E-W extension. Here we present (1) a new velocity field that quantifies along-rift surface motions within the Main Ethiopian Rift and along the Western Branch of the EARS and (2) results from ongoing numerical modeling experiments aimed at testing the role of mantle flow at the rift-scale. Our results suggest sub-crustal mantle lithosphere that is governed by dislocation creep flow law inhibits upper mantle flow from driving surface motions. This work indicates the mechanism(s) driving rift-parallel surface motions along the EARS reside in the crustal domain.


 

Dr. Betsy Madden from University of Massachusetts, Amherst
Talk Title: “The impact of fault shape and fault segmentation on earthquake behavior”
Date: 2/25/15
Time: 10:00am
Room: LO1229

Abstract:

The San Andreas Fault is the most well-known fault in California. However, California is host to hundreds of faults that interact with one another over geologic time and during earthquakes. Fault segmentation and irregular fault shapes challenge assessments of seismic hazard. Identifying the key characteristics affecting the coseismic behavior of complex fault systems has the potential to turn these unexpected events into those for which we can identify the seismic potential. How the separation distance between faults corresponds to behavior has been the focus of much previous study. I will present modeling results that show that the shape of the major faults involved, and smaller structures near to and within steps, have significant impacts on earthquake behavior. These smaller structures include coseismic tensile damage and discrete strike-slip faults, which may go unmapped prior to failure, but play the critical role of connector faults during an earthquake. This occurred during the 1992 M 7.3 Landers earthquake, which ruptured segments of five different faults. This work underscores the need for additional investigation of the types of structures present at fault steps, both at those that arrest earthquake ruptures and those that permit the continuation of slip. Motivated by this, I also am investigating fault development near steps over multiple earthquake cycles using the new modeling tool, GROW, and will present this new tool briefly at the end of the seminar.


 

Dr. Julian Lozos from Stanford University – USGS, Menlo Park
Talk Title: “Dynamic Modeling of Ruptures on the San Jacinto Fault: Evaluating Historic Earthquakes and Developing Scenarios”
Date: 2/23/15
Time: 10:00am
Room: LO1229

Abstract:

The San Jacinto Fault is one of the most active faults in southern California; together with the San Andreas Fault, it accommodates as much as 80% of the plate boundary strain south of Cajon Pass. In addition to regular active microseismicity, the San Jacinto is responsible for as many as eight ~M6 earthquakes since 1890, and there is paleoseismic evidence for multiple M7+ ruptures in the past 2000 years. The San Jacinto is also a fault that is characterized by complexity, from small- and large-scale geometrical irregularity to heterogeneity in on-fault stresses. This complexity has certainly influenced the past behavior of the San Jacinto Fault, and will continue to influence its future ruptures. Thus, understanding how this complexity influences the physics of rupture on the San Jacinto Fault is helpful for assessing the hazard that it poses, both individually and in interaction with other faults of the southern San Andreas system. As most of the larger earthquakes on the San Jacinto Fault were pre-instrumental, their rupture length and shaking intensity only be inferred from surface observations and scant historical records. However, physics-based dynamic rupture modeling can be used, in combination with observational data, to construct realistic earthquake scenarios that can provide insight into the processes that produce those observable features. In this presentation, I address several applications of dynamic rupture modeling on the San Jacinto Fault: determining the locations of major barriers to rupture in past and potential future events, construction of realistic ground motion scenarios, and evaluating the physics behind major ruptures in the paleoseismic record.


 

Dr. Scott Samson from Syracuse University
Title: Do you tell the truth about your age? When sediments lie… A play told in three acts
Date: Tuesday, February 24, 2015
Time: 4:00pm
Room: LO1229

Abstract:

Considerable progress has been made in developing methods to determine the provenance of sedimentary rocks.
We have moved from point-counting of sandstones, to heavy mineral analysis, to the geochemistry of clastic
igneous rocks, and now to determining the age of individual detrital zircon crystals. Detrital zircon
geochronology has become so popular that there are now well over 200,000 published detrital zircon ages.
However, sometimes sediments lie… The age distribution of zircon in many sandstones and much modern
alluvium is dominated by ages of 1.3 – 1.0 Ga, a so-called Grenville peak. However, this age peak is not
represented by the abundance of exposed Grenville crust as in some areas it makes up only a few percent of the
watershed where the alluvium was collected. This is the result of the extreme zircon fertility of Grenville rocks –
unusually large amounts of zircon, and those zircon crystals are often unusually large, in Grenville-age plutons.
Thus there is a considerable bias in the detrital zircon sedimentary record. We have been investigating the ages
of detrital monazite in alluvium and Pennsylvanian-Permian sandstones and demonstrate that monazite ages
reflect much more accurately the real areal extent of exposed crust. We conclude that in tectonic terms,
monazite ‘plays in high fidelity’.

 


 

Dr. John Jaeger from the University of Florida 
Title: The Mid-Pleistocene Climate Transition and Its Impact on the Rapidly Uplifting St. Elias Mountains, Alaska
Tuesday, December 9, 2014 - 5:00 pm; LO1229

Abstract:

A longstanding debate in earth sciences is the relative influence of climate change on mountain building, where it has been suggested that a change to a more non-steady and possibly more erosive climate ~1 million years ago has altered how some active mountain ranges form. Scientific ocean drilling in the Gulf of Alaska and mapping of sediment layers shows a remarkable increase in the amount of material eroding from the coastal St. Elias Mountains over the last 1 Myr compared to the previous few million years.  Some of the highest rates ever reported of sediment delivery to the deep sea appear to result from longer (100, 000 year) glacial cycles in a wet, actively growing mountain belt where glaciers that reach the sea erode and transport sediment away from the mountains. In coastal Alaska and for the last million years, erosion driven by climate change is outpacing plate tectonic construction of the mountains.


 

Dr. Isaac Larsen from Caltech
Title: Rapid soil production and weathering in the Southern Alps, New Zealand.
Wednesday, October 29, 2014 - 3:30pm; LO1229

Abstract:

Knowledge of hillslope denudation rates and processes is necessary for understanding landscape response to tectonic and climatic forcing and for determining the degree to which mountains regulate biogeochemical cycles and global climate. Landslide erosion and soil production are the principle denudation processes in high-relief terrain, but quantitative estimates of landslide erosion on spatial and temporal scales relevant to landscape evolution are lacking, and there have been no prior measurements of soil production and weathering rates in Earth's most tectonically-active landscapes. I will present landslide erosion rate estimates from the Tsangpo Gorge region of the eastern Himalaya, soil production and chemical weathering rate measurements from the western Southern Alps of New Zealand, and a model of global-scale denudation. In the Tsangpo Gorge region of the eastern Himalaya, landslide erosion rates are spatially coupled with stream power and long-term exhumation rates, but become decoupled from hillslope gradients when hillslope angles exceed 30°. These results indicate landslide erosion is coupled with bedrock river incision and rock uplift, but not topography, hence providing the first direct confirmation of a `threshold hillslope' model of landscape evolution that has emerged over the last two decades. Results from the field study in New Zealand indicate that soil production rates in the western Southern Alps can greatly exceed those measured elsewhere. Moreover, soil physical erosion rates are linearly coupled with chemical weathering rates. Using the relationship between physical and chemical denudation rates to model global weathering fluxes as a function of mean local slope, I demonstrate that the small, mountainous fraction of Earth's surface dominates the global chemical weathering flux. The weathering measurements and model results do not support the contention that erosion and weathering are decoupled in mountains, but instead support the hypothesis that mountain uplift influences global climate over geological timescales via links among topography, erosion, weathering, and carbon dioxide cycling.

 


 

Dr. Kathleen Marsaglia
Title: The record of Philippine sea plate tectonics and sedimentation at IODP Site U1438: Birth, life, and "death" of a magmatic arc.
Tuesday, September 23, 2014 - 12:30pm; LO1229

Abstract:

The Izu-Bonin-Mariana (IBM) system was the focus of several International Ocean Discovery Program (IODP) expeditions (350, 351 and 352) in 2014 which were designed to answer questions about the fundamental plate-tectonic processes of convergent-margin initiation and crustal development in intra-oceanic settings. The primary objectives of IODP Expedition 351 included discovering the nature and origin the pre-subduction basement (lithosphere) in the Amami Sankaku Basin, and the subsequent history of magmatic arc initiation and evolution as recorded in the overlying stratigraphic section. These objectives were achieved by drilling a single, deep hole where a 1.61 km volcanic/sedimentary section was cored. The Holocene to Eocene stratigraphy at Site U1438 was subdivided into five lithostratigraphic units: four sedimentary units (I, II, III, and IV) overlying basaltic volcanic rocks designated as igneous Unit 1 (1461.1-1611.1 mbsf). Sedimentary Unit I (0-160.3 mbsf) comprised mainly hemipelagic mud with minor ash beds, whereas Sedimentary Units II (160.3-309.6 mbsf), III (309.6-1361.4 mbsf) and IV (1361.4-1461.1 mbsf) were dominated by coarser (sand-gravel) marine volcaniclastic sediments. These rocks will be the focus of numerous post-cruise studies by shipboard scientists, including me, and their students.