3rd TTU Tectonics & Geodynamics Seminar, 20 & 22 April, 11:00 AM -12:30 PM

Tuesday

11 AM Ryan Krueger, THE ANDEAN SUBDUCTION ZONE: HOW SUBDUCTED PLATE GEOMETRY, CONVERGENCEANGHLE AND VELOCITY, THICKNESS AND TEMPERATURE AFFECT THE RESULTING UPPER PLATE DEFORMATION AND ASSOCIATED VOLCANISM

Throughout the Andean Cordilleran, the subducted oceanic crust varies significantly in its coupling with the South American plate (SAP). Most significant is the variation observed in the dip of the Nazca plate eastward beneath the SAP. This variation however is attributed to the subduction of thicker oceanic plateaus or aseismic ridges in regions of shallow subduction (5-10o) and cooler, thinner subducted oceanic crust in steeper subduction (25-30o) segments. Shallow subduction occurs along the Andean margin where the Carnegie, Nazca and Juan Fernandez ridges are actively being subducted. Along these segments, increased interplate coupling results in increased seismicity, block thrust faulting and extension in an oblique convergence setting. Segments of the Andean margin that are characterized by steep slab subduction deform elastically in the forearc, contain a volcanic arc, a high plateau and shorten significantly (fold and thrust belts) to the east of the plateau in the eastern cordilleran and sub Andean zone. The great kinematic variability observed in the Andes can also be attributed to differences in plate velocities (oceanic and continental) and geometry.
The Andean margin can be divided into three distinct volcanic zones, northern (NVZ), central (CVZ) and southern (SVZ) segments. The NVZ (50N-40S) is characterized by shallow subduction and oblique convergence between the Nazca and SAP. Displacement along the dextral Dolores-Guayaquil Megashear (DGM) results in regional extension and incipient volcanism. Widespread volcanisn within the CVZ (150S-280S) is more complex. Variations in the convergence angle and plate geometry create areas of increased extension via both dextral and sinistral fault systems. Oblique convergence, the dextral Liquine-Ofqui fault system, and the subduction of the Chilean Rise provide the extension and temperature anomaly respectively responsible for the volcanism with the SVZ (330S-470S).

11:25 AM Wayne Marko, STEADY STATE CONDITIONS & THE CASCADIA OROGEN

Two distinct types of steady state conditions may be defined for accretionary wedges and orogenic terrains. Coulomb wedge theory defines a geometric equilibrium relationship between the upper slope angle (_) and the lower decollement dip (_) below the orogen. The upper slope surface or critical taper is maintained as long as the over riding wedge is able to overcome the basal friction between itself and the under thrust plate. Variables such as lithology, isostatic recovery and erosion (if the orogen is emergent) may affect steady-state conditions forcing the wedge into a super critical taper and causing internal deformation or tectonic thinning. A second steady-state condition is defined when accretionary flux (Fa) and erosional flux (Fe) are equal. Evidence for this condition may be approximated by comparing estimates of erosional down-cutting in stream channels and exhumation rates estimated from fission track dating of apatite and zircon and (U-Th)/He dating methods. Steady-state topographic conditions are also used to infer orogen stability.
The Cascadia accretionary wedge is commonly cited as having achieved steady state flux conditions as well as critical taper. The complex is continuous with the coast of North America and is under-thrust by the Juan de Fuca plate, which creates a basal decollement for the orogen. Plate convergence rates are estimated at 30 km/m.y. to the northeast with respect to North America. The accretionary complex became emergent at 18 Ma and is interpreted to have achieved a balance between erosion and accretion at 14 Ma. Tectonic models predict that this rate and geometry should be relatively consistent, thus variations in _ are not anticipated to affect current critical taper conditions. Lateral variations in plate geometry may account for the early emergence of the Olympic Mountains as opposed to Vancouver Island (emergent at 4 Ma). This is an effect predicted by Coulomb wedge theory if _ is variable laterally along the Juan de Fuca plate.

11:50 AM Nate Zimermann, CALCULATING CRUSTAL STRAIN ACCUMULATION USING SAR EARTHQUAKE INTERFEROMETRY: A CASE STUDY FROM THE DENALI FAULT EARTHQUAKE, ALASKA 2002

Recent coupling of GPS ground receivers and a satellite remote sensing technique, Synthetic Aperture Radar Interferometry (InSAR), has proven a valuable tool to test the plate tectonic paradigm. Recognition of field and seismological relations has long shown that earthquakes episodically deform small regions along major faults. Accurate prediction of earthquake occurrence in the past has been inhibited by the ability to precisely measure and archive amounts of strain accumulation after major episodes of seismic activity. Some authors content to argue that large strike slip fault systems, in Japan, have been shown to accommodate constant amounts of strain through short intervals of time, as measured via dense networks of GPS ground receivers (Miura et al, 2002). InSAR methods, introduced in this paper, provide an alternate avenue to test such deformation events and associated rates.
InSAR measures the geomorphology of a region at specific points in time through satellite radar scanning. Backscatter values in the C-band are recorded for a scene and processed through algorithms to locate features to <3mm accuracy from differences in backscatter values. Interferograms plot changes in backscatter with respect their locations on Earth, which are processed to calculate Cartesian displacement values. Advantages over ground GPS stations include aerial coverage.
The Oct. 23, M6.7 Nenana earthquake and Nov. 3, M7.9 Denali Fault earthquake occurred within a 10 day window. The calculated hypocenters occurred in adjacent regions not less than 50 km apart in Southeastern Alaska. Topographical changes along the Denali Fault were recorded and processed via InSAR techniques along strike of the fault trace at recorded time intervals (Lu, Z. et al, 2003). The processed interferogram shows a sinusoidal pattern of offset, (5.66 cm max. offset), radiating parallel to the strike of the Denali fault trace south of the Nov. 3 earthquake epicenter.
Future studies using InSAR techniques should incorporate published supervised field classifications. Visual confirmation of the surface and topography changes from the two linked Denali, Alaska, quakes was provided through various pictures taken by organizations (USGS etc.). However, it is unknown if the pictures and field measurements were used with any type of statistical classification to the InSAR data. Such data, if recorded systematically, could be amassed into a GIS database placing ground truth structural constraints on the SARS data. The archived interferograms with classified field data would be used in future statistical regressions calculating surface deformation rates, opening a pathway for more accurate earthquake prediction.

12:15 PM Jeannette Wolak, TECTONIC EVOLUTION OF MARS

Recent missions to Mars (i.e. Pathfinder, 2004 Rovers) have resulted in significant contributions to the existing observational database and reignited interest in (ancient) Martian tectonics. Though it is generally agreed that Mars lacks a self-sustaining magnetic field and is no longer tectonically active, the surface of Mars is covered in evidence for an active tectonic history. Both extensional features (i.e. grabens and rifts) and contractional features (i.e. wrinkle ridges) are common on the Martian surface while evidence of strike-slip movement is rare. The southern hemisphere is densely cratered and likely represents an ancient impact-scarred surface that has not been weathered; however, the northern hemisphere has relatively few craters. This hemispherical dichotomy is paralleled by an overall global irregularity. The southern hemisphere is several kilometers higher than the northern plains regions. This observation is generally believed to be a first-order feature of the Martian crust but has been variously ascribed to long-wavelength mantle convection, post-accretional core formation, and giant impacts. Numerous volcanic features on the Martian surface span a wide range of geologic ages. In particular, studies of the Tharsis region, a volcanic plateau with several large shield volcanoes, and Olympus Mons, a shield volcano twice the size of Mauna Loa in Hawaii, can provide information on the petrology and structure of the Martian interior. Ultimately, surficial observations such as these can provide constraints for models of the origin and thermal evolution of Mars. Current (1980+) tectonic models vary between two end members: 1.) models of single-plate tectonics, and 2.) models of dynamic plate interactions similar to those observed on Earth.

Thursday

11 AM Li Yujia, D” LAYER: CHARACTERISTICS & INFLUENCE ON EARTH'S DYNAMICS

The Earth’s core-mantle boundary (D”-layer), at a depth of 2900 km beneath the surface, was revealed by seismic data to be a physically heterogeneous region with anomalously high densities. The seismic wave front in addition to the wave velocity have helped to map the thickness of D”, which varies from undetectable to 300 km. In order to reveal the chemical components of the inaccessible D” region, diamond-anvil experiments have been conducted to simulate the reaction at the core-mantle boundary. These experiments produced a mixture of metallic alloys (FeSi and FeO) and insulating oxides (MgSiO3 and SiO2) with dense crystal form. These reactions may explain the 10% density difference of outer core and calculated pure Fe at the estimated P-T condition. A hypothesis to explain the observed characteristics of the D” suggests that the products of the reaction between mantle and core materials were pulled up by the slow upward moving mantle driven by the thermal energy of the underlying core. As a result, the alloy-rich substances tend to pile up at the bottom of the mantle near regions of upwelling and be thinned by the downwelling mantle.
This D” layer is thought to influence many global scale geologic processes and thus knowledge of the layer is crucial to understanding the chemical and thermal evolution of the planet. The dynamics of the zone may affect the slight wobbling of the earth’s axis of rotation and characteristics of the geomagnetic field. It also may modulate the mantle convection, which influences lithospheric movement.

11:25 AM Tracy Hulsey, MANTLE PLUMES

A mantle plume is generally considered to be a blob of relatively hot, low-density mantle that rises because of its positive buoyancy. In the Earth, buoyancy differences in the mantle originate at two well-documented thermal boundary layers: at the base of the lithosphere and in the D’’ layer. In the mantle, convection is driven by a combination of three thermal processes: heating at the bottom by heat loss from the core, heating by internal radioactive sources; and cooling from the top, resulting in sinking of cool lithospheric slabs into the mantle. A fundamental question in large igneous province (LIP) and hotspot research involves their origin. This paper will discuss different origins throughout geological time along with unique examples such as Columbia River Basalt, the Bushveld Complex, the Azores, and the Galapagos. Morgan (1971) suggested that large igneous provinces (LIPs) were formed by melting of plume heads, whereas hotspot volcanic chains were derived from partial melting of plume tails. Mantle plumes are known to be responsible for the emplacement of continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, ocean flood basalts, and hotspots. While other models such as decompression melting in a rift setting, edge-driven convention, back-arc setting, and meteorite impact may also be responsible for the origin of LIP and hotspot. However, there is a need for the development and application of a set of tests to distinguish between different origins. Several tests include: seismic tomography: presence or absence of domal uplift: and relative timing of rifting and magmatism.

11:50 AM Jime Lehane, PLATE TECTONICS AND THE EFFECT ON CO2 CONCENTRAION IN THE ATMOSPHERE AND THE OVERALL EFFECT ON GLOBAL CLIMATE

Global climate is profoundly affected by different plate processes that are occurring at any given time. Periods of rapid spreading are accompanied by warmer climates while periods of less activity are relatively cooler. The decarbonation of subducted rocks produce the CO2 seen in arc magmas, although the amount of CO2 released is far less than the amount subducted. The source of the mantle CO2 is derived from these subducted slabs, serving as a major reservoir for CO2 taken from the atmosphere. Several plate tectonic factors affect the amount of CO2 being release and taken from the atmosphere including subduction of the oceanic slabs, release of magma from the mantle, oceanic seawater circulation through the spreading centers and spreading rates, which affect sea level and deposition of carbonates. There are several current and prehistoric reservoirs for CO2 including soils, carbonates, ocean water, and the mantle. Different processes have different effects and degrees of effect on the CO2 within the atmosphere causing different results, some of them compounded. Plate tectonics has the largest affect on global CO2, since the mantle is the largest of the reservoirs and the plate motion rates are unaffected by different atmospheric properties. The deposition of carbonates offers another reservoir for CO2, but this is affected by climate, where warmer climates induce more carbonate deposition. Plate tectonics has a profound affect on the paleoclimate as well as the modern climate and is thought to be the reason for global warming throughout the Cenozoic.