G&G Colloquium Spring 2019

Jan. 25

Joann Stock

Tectonic Evolution of the Pacific Ocean Basin: Knowns and Unknowns

The Pacific basin has been a major laboratory for the development of plate tectonic theory. The basin’s large size and diverse plate boundaries include prime examples of active spreading, subduction, and transform faulting. This talk reviews our current understanding of present and past plates in the Pacific Basin, and the recent new studies concerning kinematic histories, past plate motions, and boundary dynamics, based on a combination of new technology, ocean drilling, and additional surveys.  Fragmentation of the former Farallon plate into the modern Explorer, Juan de Fuca, Rivera, Cocos, and Nazca plates, and earlier fossil microplates, provides an excellent example of evolution and demise of subduction zones as well as a caution regarding linkage of subducted plates to seismic velocity anomalies in the mantle. The microplates along modern spreading centers (Galapagos, Juan Fernandez, and Easter) illustrate a style of complexity that may be preserved elsewhere in older seafloor and should be considered within models of subduction initiation. Major questions remain, regarding past plate boundaries in the western Pacific, particularly between the Ontong Java and Manihiki plateaus; the nature of plate motion changes during the Cretaceous Normal Superchron; and the age and origin of crust currently being subducted at the NW corner of the Pacific plate.

Feb. 15

Harriet Lau
Tidal Tomography: What an often-neglected phenomenon known as Earth tides can tell us about buoyancy in the deepest part of the mantle
Earth’s mantle is a key component of the Earth system: its circulation drives plate tectonics, the long-term recycling of Earth’s volatiles, and as such, holds fundamental implications for the Earth’s surface environment. In order to understand this evolution, a key parameter of the mantle must be known, namely its buoyancy. In this talk, I will discuss how Earth’s body tide can provide fresh and independent constraints on deep mantle buoyancy through a newly developed technique called Tidal Tomography. This comes at a time when other interesting and exciting data sets sensitive to deep mantle buoyancy, e.g., Stoneley modes, have been brought to bear, and we will explore our conclusions in the context of other recent finds.

Mar. 1

Zhiming Kuang

Some applications of linear response functions in atmospheric dynamics

For a number of problems in climate dynamics, it is useful to describe the system with a set of macroscopic state variables and consider statistics of the other variables, which may represent highly nonlinear processes, as smooth functions of the macroscopic variables. The utility of a linear approximation to these functions, namely the linear response functions, is illustrated with two examples in atmospheric dynamics. The first is the stability of a moist convecting atmosphere, where the linear response functions are used to elucidate the dynamics of convectively coupled tropical waves and convective self-aggregation. Extension to nonlinear waves and implications for machine learning of convective parameterization will be discussed. The second example is the dynamics of the annular mode, where the linear response functions are used to quantify the strength of the eddy-mean-flow feedback.

Mar. 8 Kathleen Ritterbush Title
Mar. 28 Donald Fisher

Modeling of Fault-Related Fold Kinematics and Marine Terrace Uplift to Estimate Active Fault Slip Rates in North Canterbury, New Zealand

Fold kinematic models provide a means to infer fault structure and slip rates, but they are nonunique and subject to uncertainties in the choice and parameters of any given model. Geomorphic markers of deformation provide not only a way to measure deformation rates but also an additional constraint on the model itself, as they respond to the same patterns of uplift and deformation that determine the fold geometry. Here we model fault-propagation folds in the North Canterbury fold and thrust belt on the South Island of New Zealand, where dated Quaternary marine terraces uplifted on the limbs of fault-propagation folds serve as markers of deformation. A Markov chain Monte Carlo algorithm is used to test fold kinematic models by cross section restoration. Terrace strath geometry is integrated into this algorithm and restored along with the geologic structure in order to identify models that produce an initially near-horizontal terrace at the appropriate paleo-sea level. This joint inversion of geologic and geomorphic data narrows the range of possible models compared to the geologic data alone, and the model results provide estimates of fault slip rates and their uncertainties. The North Canterbury fold and thrust belt, or the onland expression of the laterally propagating Hikurangi forearc system, is characterized by a regionally extensive flight of marine terraces, with multiple terrace levels commonly occurring on a single structure. This region lies south of the strike-slip Marlborough fault system and at the southern end of the Hikurangi subduction zone within the regional southward transition from Hikurangi subduction to Alpine Fault transpression. Newly reported terrace ages indicate ongoing fold growth, with uplift rates of ~1 to 3 mm/yr. Map patterns indicate a region of predominantly landward-vergent thrusts in the east-central part of the fold and thrust belt, with predominantly seaward-vergent thrusts in the rest of the region. These regional variations presumably reflect the influence of Late Cretaceous-Paleogene rift structures that are inverted as the Hikurangi system migrates southwestward through time.  Fault slip rates along the coast are mostly about 1 mm/yr but range as high as 4 mm/yr and vary substantially over time on one of the structures studied. Fault slip rates offshore, as determined from folded growth strata, are <1 mm/yr and mostly about 0.2 mm/yr. Fold ages estimated from slip rates are similar for both onshore and offshore regions, and a similar listric fault model can explain folding in both regions. While some uncertainty remains in fault geometry and slip rates, joint modeling of geologic and geomorphic (marine terrace) data allows us to quantify this uncertainty and to narrow the range of possible models compared to those that fit the geologic data alone.

Mar. 29 Donald Fisher

Geochemical and mechanical models of the subduction interface based on field observations of the Shimanto Belt and Kodiak Island

Observations of ancient subduction fault zones are used to constrain a new model for the kinetics of silica redistribution in the seismogenic zone and to characterize the role that geochemical processes play in the mechanical and hydrologic behavior of the subduction interface.  The fault zones of this study, from the Kodiak Accretionary Complex and Shimanto belt of Japan, are regionally extensive (100’s to 1000’s of km along strike) mélange belts, with sandstone blocks in a pervasively sheared mudstone matrix.  Typically, the mélange belt is 10’s-100’s of meters thick and is capped by discrete faults that are characterized by a narrow gouge layer.  Simple shear is accommodated in mudstones by development of scaly fabric that shows evidence for pressure solution and concentration of fluid-immobile elements.  Sandstone blocks embedded in the shearing mudstones experience hydrofracturing and mode I failure, with vein textures that record repeated cracking and sealing.   We construct a kinetic model to investigate the rate of mass transfer from the mudstone scaly slip surfaces to the sandstone fractures based on dissolution, diffusion, and precipitation of silica in an aqueous medium.  Our model is based on the driving force for silica redistribution coming from the difference in strength between mudstone scaly shears and the sandstone blocks, a value that is largely constant and equivalent to 10’s of MPa difference in mean stress. Cracks heal at rates relevant to the seismic cycle and with a temperature dependence on healing that should vary with subduction thermal structure from margin to margin.   This kinetic model is used to inform a 2-D numerical block-slider model for the subduction interface with stochastic nucleation, growth, and failure of chemically healed “asperities” at rates determined by Arrhenius-equation silica kinetics, leading to 1) supercycles of buildup and release of elastic strain, 2) a temperature-based up-dip limit to genesis of large earthquakes, and 3) a power law size distribution of earthquakes that varies as a function of temperature.  This chemical-mechanical model is then coupled with a model for a fluid flow system along the plate interface where geochemical healing coincides with reduction in fracture porosity, and failure of the interface allows fault valve behavior during slip events. The models of this study demonstrate that many of the characteristics of subduction zone earthquakes, particularly along sedimented margins such as Sumatra and Cascadia, can be explained in the context of nucleation and growth of asperities as a thermally activated geochemical process.  Over a temperature range typical of natural seismogenic zones, variations in effective mean stress and chemical potential along the plate interface lead to heterogenous frictional characteristics over the interseismic period that could control the location, recurrence time, and magnitude of earthquakes.



Adina Paytan

Insights into global carbon cycling from a 35 Ma record of Sr isotopes in seawater 

The radiogenic Sr-isotopic signature (87Sr/86Sr) of seawater fluctuates primarily in response to changes in the inputs of Sr from weathering and hydrothermal activity, which have distinct 87Sr/86Sr values. Changes in the isotopic ratio of the weathered terrain also contribute to observed changes in seawater 87Sr/86Sr. The stable Sr-isotope ratios in seawater (mass dependent isotopic fractionation; d88/86Sr) fluctuate primarily in response to the rate of calcium carbonate (CaCO3) accumulation at the seafloor. Together radiogenic and stable Sr can constrain the coupling between weathering and sedimentation and shed light on the relation between weathering, CaCO3 deposition, the global carbon (C) cycle and climate. Reconstruction of the coupled stable and radiogenic Sr seawater curves over the past 35 Ma of Earth history suggests that the Srflux in and out of the ocean, and thus the seawater dissolved Sr concentration have fluctuated considerably over this time interval. The location and rates of CaCO3 burial in the ocean have also fluctuated. Between 35 to 18 Ma an increase in the net Sr input results from a reduction in neritic CaCO3 burial and increased burial in pelagic settings. The trend is reversed between 18 and 5 Ma and finally over the last ~5 million years a rapid increase in the net Sr input to the ocean and change from neritic to pelagic burial is seen. The lack of a continuous increase in pelagic CaCO3 burial, inferred from the stable Sr data, indicates that silicate weathering rates have not increased monotonically over the past 35 Ma and hence associated atmospheric carbon-dioxide consumption rates were also not unidirectional. Modeling of the combined 87Sr/86Srand d88/86Sr data suggest a relatively modest increase of about 20% in continental weathering since 35 Ma.

Apr. 5 Adina Paytan
Corals on Acid - Ocean acidification impacts on coral reefs
Rising atmospheric CO2 and its equilibration with surface ocean seawater is lowering both the pH and  carbonate saturation state of the oceans. Numerous calcifying organisms, including reef-building corals, may be severely impacted by declining aragonite and calcite saturation, but the fate of coral reef ecosystems in response to ocean acidification remains largely unexplored. Naturally low saturation low pH groundwater has been discharging for millennia at localized submarine springs (called ‘‘ojos’’) at Puerto Morelos, Me´xico near the
Mesoamerican Reef. This ecosystem provides insights into potential long term responses of coral ecosystems to low saturation conditions. In-situ chemical and biological data indicate that both coral species richness and coral colony size decline with increasing proximity to low-saturation, low-pH waters at the ojo centers. Only three scleractinian coral species seem to grow in undersaturated waters at all ojos examined. Because these three species are rarely major contributors to Caribbean reef framework, these data may indicate that today’s more complex frame-building species may be replaced by smaller, possibly patchy, colonies of only a few species along the Mesoamerican Barrier Reef. Calcification rates also drop at the low pH springs. The growth of these scleractinian coral species at undersaturated conditions illustrates that the response to ocean acidification is likely to vary across species and environments; thus, our data emphasize the need to better understand the mechanisms of calcification to more accurately predict future impacts of ocean acidification. To that end we are investigating the genetic response to acidification using in situ transplantation experiments.
Apr. 12 Graham Slater  The Recent Evolution of Gigantism in Baleen Whales (or, why fossils are as important as phylogenies in modern macroevolutionary analyses)
Apr. 19 Rita Parai

Xenon and the history of volatile transport into the mantle

Xenon is a unique tracer of volatile transport. The modern deep Earth Xe budget reflects primordial volatiles delivered during accretion, radiogenic ingrowth, outgassing in association with mantle processing, and regassing as Xe is carried within hydrous phases in downwelling lithologies. The Xe isotopic composition of the mantle thus reflects the integrated long-term history of volatile transport between the deep Earth and surface reservoirs. We present a numerical model of concurrent mantle degassing, regassing and fissiogenic production. We test a wide variety of outgassing and regassing rates and take the sequestration of Pu and U into the continental crust and the evolution of the atmospheric Xe isotopic composition into account. Model realizations that satisfy Xe isotopic constraints from mantle-derived rocks indicate that significant recycling of atmospheric Xe into the deep Earth could not have occurred prior to 2.5 Ga. Because Xe is carried into Earth’s interior in hydrous mineral phases, our results indicate that downwellings were drier in the Archean era relative to the present. Our results indicate that the mantle experienced net degassing throughout the Archean and transitioned to net regassing at some point after 2.5 Ga. Progressive drying of the Archean mantle would allow for slower convection and decreased heat transport out of the mantle, suggesting non-monotonic thermal evolution of Earth’s interior. If plate tectonics and plate subduction were initiated before 2.5 Gyr ago, then early downwelling subducted material was either hydrated to a lesser extent at the surface than in the modern-day, or volatiles were more efficiently expelled from Archean downwellings slabs at shallow depths and returned to the surface.

Apr. 24 Slava Solomatov

Magma oceans and primordial mantle differentiation

A number of geophysical and geochemical arguments lead to the magma ocean hypothesis, that during early history Earth and other planets experienced such a high degree of melting that the silicate molten reservoir that was formed during that time can be termed a magma ocean. Fluid dynamical analysis suggests that crystallization of magma oceans is controlled by external as well as internal processes, some of which are stochastic, and can produce planets with different structures and dynamics.