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UTIG Seminar Series: Fellowship Talk

Start:	September 7, 2012 at 10:30 am	End:	September 7, 2012 at 11:30 am
Location:	PRC, 10100 Burnet Road, Bldg 196, Rm 1.603, Austin, TX 78758
Contact:	Gail Christeson, gail@ig.utexas.edu, 471-0463
URL:	Event Link

“Subglacial Channel Evolution Observed in Western Greenland from Moulin Water Level and GPS-Derived Surface Velocities” (Lauren Andrews)

Abstract:

Fluctuations in supraglacial melt input to the ice sheet bed, delivered by moulins, drive changes in surface velocities. Here we present contemporaneous local measurements of the supraglacial, englacial, and subglacial hydrological system of the Greenland Ice Sheet, with the goal of understanding how the subglacial hydrological system evolves over the course of the melt season. Between 18 July and 22 August 2011, we measured moulin water level, stream water level, and GPS-derived sub-daily surface velocity in the Pakitsoq region of Greenland. In addition, we drilled boreholes in the vicinity of the moulins and measured subglacial water pressure. These data demonstrate clearly that daily peaks in moulin water levels occur simultaneously with peak surface velocities throughout the measurement period, while peak borehole water levels consistently lag peak velocities by several hours. However, the magnitude of the surface-velocity relationship with moulin water levels varies on multiple timescales. On a daily timescale, the relationship between surface velocity and moulin water level displays hysteresis; moulin water levels are associated with higher velocities before the diurnal peak in meltwater input. During periods of prolonged surface melt, the moulin water level eventually drops below the sensor depth where it sometimes remains for several days. These periods are correlated with decreases in surface velocity to the winter background rate. In addition, we observe a general decrease in surface velocity magnitude over the observation period despite a relatively constant daily peak in moulin water level. We suggest that these observations are direct indicators of the evolution of the drainage efficiency of the subglacial hydrological system on seasonal and diurnal timescales.

“Correlation of the Dome C and Vostok Ice Cores Using Airborne Radar Sounding along the Great Byrd-Totten Glacier Ice Divide, East Antarctica” (Marie Cavitte)

Abstract:

Airborne radar sounding surveys are used across wide areas of the Antarctic ice sheet to trace internal layering. Radar isochrons from direct visual tracking of reflective layers provide an independent signal for directly correlating ice core stratigraphies that have high relative accuracy compared to indirect correlations based on paleoclimate proxies and ice flow models. However, these two approaches can be combined to provide horizontal continuity over basin-wide areas, therefore allowing the propagation of ice core age-depth data through laterally extensive areas of the ice sheet. Ice core communities strongly benefit from such studies as areas of deep old ice can be observationally constrained and so do not have to rely solely on model assumptions. Between Dome Concordia and Vostok, ICECAP (Internationally Collaborative Exploration of the Cryosphere through Airborne Profiling) radar isochron studies are characterized by an error of the order of ±800 years at 2km depth down the ice column in comparison to ±2kyr quoted for ice core timescales (Vostok ice core, Suwa and Bender, 2008) at equivalent depths.
Initially, a deep age-depth correlation was obtained between the Dome C and Vostok ice cores for the time period corresponding to depths covering a period from the Eemian interglacial back to the previous interglacial. To accomplish this, the Bender and Suwa (2007) Vostok O2/N2 age-depth chronology was used to propagate Vostok ages to EPICA Dome C layer depths of 1597m to 2216m. A complementary analysis for shallower horizons spanning the last glacial period is also presented to extend the deep correlation. Age-depth uncertainties obtained for this shallower addition are compared to the preceding glacial period; and ultimately spatial geometries of dated radar isochrones are contrasted over the Totten-Byrd Glacier ice divide for the last two glacial cycles using ICECAP airborne radar sounding data acquired with UK, US and French support. Transient ice sheet behavior is identified at the divide from observed layer disruption, and we give a first assessment of its temporal and spatial distribution.

“Crustal Accretion in the Manila Trench Wedge at the Transition from Subduction to Mountain-Building in Taiwan” (William Lester)

Abstract:

New marine seismic reflection and wide-angle OBS data from offshore south Taiwan provide new high-resolution constraints on the crustal structure of subducting crust and the Manila trench accretionary wedge at the earliest stages of arc-continent collision. These new data reveal transitional crust consisting of hyper-extended continental crust with interspersed volcanic bodies and high-velocity lower crust outboard of the trench and subducting beneath the accretionary prism. A sharp morphologic break marks the transition from the well-imaged imbricate thrusts and folded strata of the accretionary prism lower slope to the sparsely reflective accretionary prism upper slope. Although it has been speculated that the change from lower to upper slope is caused by out-of-sequence thrusting, as frequently observed in other prisms, no such fault is apparent in the reflection image. Instead, we identify a fast seismic velocity anomaly that may be evidence for underplated transitional crust beneath the accretionary prism upper slope that can account for the change in prism geometry. Similar crustal materials are exposed throughout the Central Range in Taiwan that may be examples of transitional crust that was subducted and underplated at the onset of collision and later exhumed to the surface during subsequent stages of collision.

“The Interplay of Climate and Seismicity in Creation of the Largest Known Submarine Slope Failure, Gulf of Alaska” (Robert Reece)

Abstract:

Recent seismic studies reveal an isolated, anomalously thick mass-transport deposit (MTD) with an unusually short runout in the Gulf of Alaska. The MTD is located on the Aleutian margin proximal to the deformation front for Yakutat terrane subduction. The MTD geometry, size and location on a convergent margin lend support to recent studies suggesting seismic strengthening and infrequent sediment failure on tectonically active margins. Study of this MTD may provide insight into the magnitude and scope of triggers required for events of this type, including the influence of climate and sea level change. The previously uninterpreted MTD is buried in the Surveyor Fan off the Kayak Trough slope in the Gulf of Alaska, and we refer to it as the Surveyor MTD. The MTD is buried beneath at least 1 km of sediment in water depths of 3.5 to 4.5 km in the Surveyor Fan. Preliminary calculations suggest this MTD is the largest by volume globally with an area of 7,950 km2, minimum thickness of 500 m, and volume of 4,470-6,705 km3. The deposit consists of debris flow with large rafted blocks 5-10 km in length that traveled as far as 50 km from the base of the slope. These blocks are present in the full vertical extent of the MTD, suggesting that the deposit represents one event at these locations, instead of several layered events. Distinct from other large MTDs, the Surveyor MTD has a short runout at ~80 km, and remains thick over a large area.

Due to Yakutat terrane and Pacific plate subduction zones, great earthquakes are expected in the Gulf of Alaska every few 100 years, yet no other MTDs are observed. The lack of additional MTDs, the large volume, and short runout distance all suggest high sediment strength of the MTD source material, which may be the result of seismic strengthening. Possible factors involved in overcoming the high shear stress to mobilize the high strength material are a significant increase in sediment flux and larger oscillations in sea level and glacial extent caused by the mid-Pleistocene transition since ~ 1 Ma. The combined effects of these processes may even be necessary prerequisites for a Surveyor MTD magnitude failure based on the lack of other MTDs associated with historical great earthquakes on this margin.

“Evidence for Ice-Flow-Coupled Subglacial Water Systems beneath West Antarctica’s Potentially Unstable Thwaites Glacier” (Dustin Schroeder)

Abstract:

Subglacial water in various forms has been observed and theorized to accelerate the flow of overlying ice. The acceleration depends on the flux through the subglacial water system and whether the dynamic state is hydrologically “distributed” or “concentrated” . Marine ice sheets with landward-sloping beds are in an unstable configuration for which such accelerations can initiate or modulate grounding line retreat and ice loss. Thwaites Glacier (TG) is one the largest, most rapidly changing glaciers on earth and its landward- sloping bed reaches the interior of the marine West Antarctic Ice Sheet (WAIS) which impounds enough ice to yield meters of sea level rise. Despite the potential instability of this configuration, the subglacial water systems beneath TG and their control on ice flow have not been characterized by geophysical analysis. Although, the size of TG makes airborne radar sounding the only practical means of observation, previous radar analysis approaches have proven inadequate to characterize the dynamic state and geographic extent of its subglacial water systems. We use advanced processing to focus radar- sounding data collected over TG and measure the angular distribution of energy returned from the bed. This allows us to characterize the meter-scale geometry and dynamic state of subglacial water systems across TG and validate our interpretations with meter-scale imaging. Our results show substantial water volumes ponding in a system of “distributed” canals upstream of a bedrock ridge that is breached and bordered by a system of “concentrated” channels. The transition between the “distributed” and “concentrated” systems is both co-located and physically consistent with increasing basal shear stress, surface slope, and water flux, indicating a strong feedback between the subglacial water and overlying ice. This feedback raises the possibility that variations in subglacial water flow could trigger a grounding line retreat in TG capable of spreading to the rest of the WAIS.

UTIG Seminar Series: John Haines, GNS Science

Start:	September 21, 2012 at 10:30 am	End:	September 21, 2012 at 11:30 am
Location:	PRC, 10100 Burnet Road, Bldg 196, Mail Code R2200, Austin, TX 78758
Contact:	Laura Wallace, lwallace@ig.utexas.edu, 471-0324
URL:	Event Link

"The Darfield Mw 7.1 Earthquake of September 4, 2010 and the Christchurch Mw 6.3 Aftershock of February 22, 2011"

I will present a wide ranging summary of what happened in these very "costly" earthquakes, from the work of dozens of scientists at GNS Science. I will discuss the seismology of both of these earthquakes, and present an overview of surface rupture and slip on the Greendale Fault during the Darfield earthquake. In particular, seismology and geodesy reveal unusually complex source mechanisms in both of the events. These earthquakes have produced an extraordinary number of aftershocks that have heavily impacted the local community in Christchurch. I will discuss the implications that these aftershocks have for our understanding of this earthquake sequence, as well the outreach efforts geoscientists in New Zealand have made to help the public understand what has happened, and what they might expect to happen in the future. Intriguing hydrological transients in the Canterbury Plains have also occurred as a result of this earthquake sequence and I will discuss those. Some of the most notable damage in the Christchurch area has been the result of local site effects on the seismic shaking, liquefaction, and rock falls. The February event produced some of the largest ground motions ever recorded. These extreme ground motions appear to be largely the result of (a) unusually high energy release, (b) strong directivity towards Christchurch, and (c) energy released during a "slap down" or "trampoline" effect where different geological layers beneath Christchurch rose and fell at differing rates during the earthquake. Finally, I will overview the current thinking in New Zealand on where and how to rebuild Christchurch.

UTIG Seminar Series: Mladen Nedimovic, Columbia University

Start:	September 28, 2012 at 10:30 am	End:	September 28, 2012 at 11:30 am
Location:	PRC, 10100 Burnet Road, Bldg 196, Rm 1.163, Austin, TX 78758
Contact:	Gail Christeson, gail@ig.utexas.edu, 471-6156
URL:	Event Link

"Juan de Fuca Plate from Accretion to Subduction"

Abstract:

Cascadia margin and the immediate offshore region are a natural laboratory for studies of various tectonic and geologic processes of global importance. Crustal accretion, oceanic plate evolution, subduction earthquake cycle, slab dehydration and intraslab seismicity are some of the many processes that occur within a distance of only several hundred kilometers. I will focus on presenting deep ocean basin observations made by interpreting reflection and tomography images formed by analyzing multichannel seismic data collected across the Pacific and Juan de Fuca plates. In the direction of increasing crustal age these are:

a) Localized lower crustal and Moho transition zone reflections;

b) Differential rate of P-wave velocity increase for layers 2A and 2B with crustal aging

c) Direct and indirect images of extensional faulting seaward from the trench. Localized lower crustal and Moho transition zone reflections are interpreted as gabbro sills providing support for existence of complex magmatic plumbing systems at mid-ocean ridges and for crustal generation hypothesis from multiple magma bodies. Measured difernce in the velocity increase for layers 2A and 2B with crustal aging suggests that the layer 2A evolution, which has generally been considered to
be rapid, is rather tepid in comparison to layer 2B evolution. Images of extensional faulting seaward from the trench indicate that Juan de Fuca plate hydration becomes active at a great distance from the trench but that it has limited depth penetration. This contrasts numerous onshore observations that support abundant presence of water within the subduction zone, suggesting that the JdF plate is significantly hydrated prior to subduction at the trench.

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