J. Paul Winberry

Basal mechanics of ice streams: Insights from the stick‐slip motion of Whillans Ice Stream, West Antarctica

The downstream portion of Whillans Ice Stream, West Antarctica, moves primarily by stick-slip motion. The observation of stick-slip motion suggests that the bed is governed by velocity-weakening physics and that the basal physics is more unstable than suggested by laboratory studies. The stick-slip cycle of Whillaňs Ice Plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude. To understand this variability, we modeled the forces acting on the ice stream during the stick phase of the stick-slip cycle. The ocean tides introduce changes in the rate at which stress is applied to the ice plain. Increased loading rates promote earlier failure and vice versa. Results show that the bed of Whillans Ice Stream strengthens over time (healing) during the quiescent intervals in the stick-slip cycle, with the bed weakening during slip events. The time-dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0.35 kPa during the course of a single day. Copyright 2009 by the American Geophysical Union.

Simultaneous teleseismic and geodetic observations of the stick–slip motion of an Antarctic ice stream

Long-period seismic sources associated with glacier motion have been recently discovered, and an increase in ice flow over the past decade has been suggested on the basis of secular changes in such measurements. Their significance, however, remains uncertain, as a relationship to ice flow has not been confirmed by direct observation. Here we combine long-period surface-wave observations with simultaneous Global Positioning System measurements of ice displacement to study the tidally modulated stick–slip motion of the Whillans Ice Stream in West Antarctica. The seismic origin time corresponds to slip nucleation at a region of the bed of the Whillans Ice Stream that is likely stronger than in surrounding regions and, thus, acts like an ‘asperity’ in traditional fault models. In addition to the initial pulse, two seismic arrivals occurring 10–23 minutes later represent stopping phases as the slip terminates at the ice stream edge and the grounding line. Seismic amplitude and average rupture velocity are correlated with tidal amplitude for the different slip events during the spring-to-neap tidal cycle. Although the total seismic moment calculated from ice rigidity, slip displacement, and rupture area is equivalent to an earthquake of moment magnitude seven (Mw 7), seismic amplitudes are modest (Ms 3.6–4.2), owing to the source duration of 20–30 minutes. Seismic radiation from ice movement is proportional to the derivative of the moment rate function at periods of 25–100 seconds and very long-period radiation is not detected, owing to the source geometry. Long-period seismic waves are thus useful for detecting and studying sudden ice movements but are insensitive to the total amount of slip.

Crustal structure of the West Antarctic rift system and Marie Byrd Land hotspot

The West Antarctic rift system is one of the largest zones of continental extension on Earth. However, little is known of its crustal structure owing to the vast ice sheet that dominates the region. We report on new insights gained from a recent broadband seismic experiment. The 25-km-thick crust measured on the southern flank of the Marie Byrd Land dome suggests that the high topography there is partially supported by a low-density mantle, possibly a hotspot, whereas the interior of the rift appears to be underlain by average-density mantle, suggesting that active volcanism is not present beneath the interior of the West Antarctic Ice Sheet. The fact that a crustal thickness of only 21 km was measured in the Bentley subglacial trench suggests that the region has undergone locally extreme extension.

Subglacial Sediments as a Control on the Onset and Location of two Siple Coast Ice Streams, West Antarctica

Laterally continuous subglacial sediments are a necessary component for ice streaming in the modern onset regions of the ice streams draining the Siple Coast of West Antarctica on the basis of new seismic data combined with previous results. We present geophysical results from seismic reflection and refraction experiments in the upper reaches of ice streams C and D that highlight continuous sedimentary basins within and upstream of the current onset regions of both ice streams, with streaming ice overlying these sedimentary packages. The subglacial environment changes from no-sediment to discontinuous-sediment to continuous-sediment cover along a longitudinal profile from the ice sheet to tributary C1B. Along this same profile, we observe a speedup of ice flow and then full development of the ice stream tributary. Ice stream D flows above a thick sedimentary package with an uppermost low-seismic-velocity zone indicative of soft till, and the upglacier and lateral extensions of ice stream D are tightly constrained by the extent of continuous sediments. The inland termination of these sediments suggests that future migration of high-velocity, low-shear-stress ice flow in these regions appears unlikely.

Seismic and geodetic evidence for grounding-line control of Whillans Ice Stream stick-slip events

The tidally modulated, stick-slip events of Whillans Ice Stream in West Antarctica produce seismic energy from three locations near the grounding line. Using ice velocity records obtained by combining time series from colocated broadband seismometers and GPS receivers installed on the ice stream during the 2010–2011 and 2011–2012 austral summers, along with far-field seismic recordings of elastic waves, we locate regions of high rupture velocity and stress drop. These regions, which are analogous to “asperities” in traditional seismic fault studies, are areas of elevated friction at the base of the ice stream. Slip events consistently initiate at one of two locations: near the center of the ice stream, where events associated with the Ross Sea high tide originate, or a grounding-line spot, where events associated with the Ross Sea low tide initiate, as well as occasional high-tide events following a skipped low-tide event. The grounding-line site, but not the central site, produces Rayleigh waves observable up to 1000 km away, through fast expansion of the slip area. Grounding-line initiation events also show strong directivity in the downstream direction, indicating initial rupture propagation at 1.5 km/s, compared to an average of 0.150 km/s for the entire slip event. Following slip initiation, additional seismic energy is produced from two sources located near the grounding line: first at the downstream end of Subglacial Lake Engelhardt and second toward the farthest downstream extent of the ice stream. This evidence suggests that the stronger, higher-friction material along the grounding line controls motion throughout the stick-slip region.

Upper mantle structure of central and West Antarctica from array analysis of Rayleigh wave phase velocities

The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two-plane-wave tomography method, and are inverted for shear velocity using a Monte-Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70-100 km thick lithosphere, underlain by a low velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome, and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity.

Glacier slip and seismicity induced by surface melt

Many of the key processes governing fast glacier flow involve interaction between a glacier and its basal hydrological system, which is hidden from direct observation. Passive seismic monitoring has shown promise as a tool for remotely monitoring basal processes, but lack of glacier-bed access prevents clear understanding of the relationships between subglacial processes and corresponding seismic emissions. Here we describe direct measurements of basal hydrology, sliding, and broadband seismicity made in a unique subglacial facility in Norway during the onset of two summer melt seasons. In the most pronounced of these episodes, rapid delivery of surface meltwater to the bed briefly enhanced basal slip following a period of elevated high-frequency seismic activity related to surface crevassing. Subsequent ground tilt derived from ultralong-period seismic signals was associated with subglacial bedrock deformation during transient pressurization of the basal hydraulic system. These signals are interpreted to represent hydraulic jacking as the supply of water to the bed exceeded the capacity of the hydraulic system. Enhanced slip terminated 2.5 h after it started, when ice-bed decoupling or increased connectivity in the basal cavity network relieved cavity overpressure. The results support theoretical models for hydraulic jacking and illustrate how melt-induced increases in speed can be short lived if cavity growth or ice-bed decoupling allows basal water more efficient drainage.

Seismic evidence for lithospheric foundering beneath the southern Transantarctic Mountains, Antarctica

The 3000-km-long Transantarctic Mountains (TAMs), which separate cratonic East Antarctica from tectonically active West Antarctica, remain one of the least understood of Earth’s major mountain ranges. The tectonic mechanism that generates the high elevation, as well as the processes that produce major differences between various sectors of the TAMs, are still uncertain. Here we present newly constructed seismic images of the crust and uppermost mantle beneath central Antarctica derived from recently acquired seismic data, indicating ongoing lithospheric foundering beneath the southern TAMs. These images reveal an absence of thick, cold cratonic lithosphere beneath the southern TAMs. Instead, an uppermost-mantle slow seismic anomaly extends across the mountain front and 350 km into East Antarctica, beneath a high plateau near the South Pole. Under the slow anomaly, a relatively high-wavespeed root is found at ~200 km depth, connected with the East Antarctic lithosphere, suggesting that sinking lithosphere has been replaced at shallow depths by warm, slow-velocity asthenosphere. A mantle lithosphere foundering model is proposed to interpret these images, which best explains the present large area of high elevation and the uplift of the TAMs, as well as Miocene-age volcanism in the Mount Early region.

Changes in speed near the onset of Bindschadler Ice Stream, West Antarctica

Abstract Ice-stream velocities can change rapidly. Understanding the spatial and temporal pattern of these changes and the forcings responsible is essential for predicting ice-sheet mass balance. Inland migration of the onset location will lead to more efficient drainage of inland ice. One way to monitor the stability of the onset location is to investigate changes in the velocity field. We report on the velocity near the onset of Bindschadler Ice Stream, West Antarctica, in 2002 and compare these data to the velocity measured in 1996. Mean annual velocities were determined by measuring the GPS position of markers during consecutive seasons. We compare our results with similar measurements from 1996 to investigate temporal changes in this ice-stream onset. Our results indicate that only minimal changes have occurred in the speed of the ice stream between 1996 and 2002.