Patrick J. Shore

Crust and upper mantle structure of the Transantarctic Mountains and surrounding regions from receiver functions, surface waves, and gravity: Implications for uplift models

[1] This study uses seismic receiver functions, surface wave phase velocities, and airborne gravity measurements to investigate the structure of the Transantarctic Mountains (TAM) and adjacent regions of the Ross Sea (RS) and East Antarctica (EA). Forty-one broadband seismometers deployed during the Transantarctic Mountain Seismic Experiment provide new insight into the differences between the TAM, RS, and EA crust and mantle. Combined receiver function and phase velocity inversion with niching genetic algorithms produces accurate crustal and upper mantle seismic velocity models. The crustal thickness increases from 20 ± 2 km in the RS to a maximum of 40 ± 2 km beneath the crest of the TAM at 110 ± 10 km inland. Farther inland, the crust of EA is uniformly 35 ± 3 km thick over a lateral distance greater than 1300 km. Upper mantle shear wave velocities vary from 4.5 km s � 1 beneath EA to 4.2 km s � 1 beneath RS, with a transition between the two at 100 ± 50 km inland near the crest of the TAM. The � 5k m thick crustal root beneath the TAM has an insufficient buoyant load to explain the entire TAM uplift, suggesting some portion of the uplift may result from flexure associated with a buoyant thermal load in the mantle beneath the edge of the TAM lithosphere.

P and S velocity structure of the upper mantle beneath the Transantarctic Mountains, East Antarctic craton, and Ross Sea from travel time tomography

P and S wave travel times from teleseismic earthquakes recorded by the Transantarctic Mountains Seismic Experiment (TAMSEIS) have been used to tomographically image upper mantle structure beneath portions of the Transantarctic Mountains (TAM), the East Antarctic (EA) craton, and the West Antarctic rift system (WARS) in the vicinity of Ross Island, Antarctica. The TAM form a major tectonic boundary that divides the stable EA craton and the tectonically active WARS. Relative arrival times were determined using a multichannel cross-correlation technique on teleseismic P and S phases from earthquakes with mb ≥ 5.5. 3934 P waves were used from 322 events, and 2244 S waves were used from 168 events. Relative travel time residuals were inverted for upper mantle structure using VanDecars method. The P wave tomography model reveals a low-velocity anomaly in the upper mantle of approximately δVp = −1 to −1.5% in the vicinity of Ross Island extending laterally 50 to 100 km beneath the TAM from the coast, placing the contact between regions of fast and slow velocities well inland from the coast beneath the TAM. The magnitude of the low-velocity anomaly in the P wave model appears to diminish beneath the TAM to the north and south of Ross Island. The depth extent of the low-velocity anomaly is not well constrained, but it probably is confined to depths above ∼200 km. The S wave model, within resolution limits, is consistent with the P wave model. The low-velocity anomaly within the upper mantle can be attributed to a 200–300 K thermal anomaly, consistent with estimates obtained from seismic attenuation measurements. The presence of a thermal anomaly of this magnitude supports models invoking a thermal buoyancy contribution to flexurally driven TAM uplift, at least in the Ross Island region of the TAM. Because the magnitude of the anomaly to the north and south of Ross Island may diminish, the thermal contribution to the uplift of the TAM could be variable along strike, with the largest contribution in the Ross Island region. The tomography results reveal faster than average velocities beneath East Antarctica, as expected for cratonic upper mantle.

Upper mantle structure beneath Cameroon from body wave tomography and the origin of the Cameroon Volcanic Line

The origin of the Cameroon Volcanic Line (CVL), a 1600 km long linear volcanic chain without age progression that crosses the ocean-continent boundary in west-central Africa, is investigated using body wave tomography. Relative arrival times from teleseismic P and S waves recorded on 32 temporary seismic stations over a 2-year period were obtained using a multichannel cross-correlation technique and then inverted for mantle velocity perturbations. The P and S wave models show a tabular low-velocity anomaly directly beneath the CVL extending to at least 300 km depth, with perturbations of −1.0 to −2.0% for P and −2.0 to −3.0% for S. The S wave velocity variation can be attributed to a 280 K or possibly higher thermal perturbation, if composition and other effects on seismic velocity are negligible. The near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the CVL that invoke plumes or decompression melting under reactivated shear zones, but are possibly consistent with a model invoking edge-flow convection along the northern boundary of the Congo Craton lithosphere. If edge-flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the CVL, then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness.

The March 9, 1994 (M w 7.6), deep Tonga earthquake: Rupture outside the seismically active slab

We investigate the rupture process of the March 9, 1994, M w 7.6 deep Tonga earthquake and its relationship to the background seismicity of the subducted Tonga slab. Variations in observed P and S wave pulse duration indicate that the rupture propagated to the NNE and extended well beyond the background seismicity. We inverted 47 P and SH waveforms, including regional broadband waveforms from the Southwest Pacific Seismic Experiment, using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane. The results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10-20 km beyond the bounds of the previous seismic activity. A significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region. Rupture along the near-vertical NNE striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near-horizontal nodal plane. This result, combined with the planar alignment of aftershocks and the general NNE directivity of the waveforms, provides strong evidence that the rupture occurred on the near-vertical plane. Thermal modeling of the Tonga slab indicates that the rupture terminated in material about 200°C warmer than the temperature that normally limits the occurrence of smaller earthquakes. Additionally, aftershocks seem to be suppressed in the outer regions of the rupture, which contain about half of the moment release but only 1 of the 15 well-located aftershocks. We suggest that slabs may be composed of an inner cold core, where seismic rupture initiates and small earthquakes occur, and a thermal halo of warmer material, which can sustain rupture and only a few aftershocks. The mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core; nucleation may occur through a process limited to the cold core such as transformational faulting, and propagation through the warmer material may occur through ductile faulting or plastic instabilities. Isolated deep earthquakes in other subduction zones, such as the 1994 Bolivia event, may occur almost completely within the warmer zone, accounting for the lack of background seismicity and the dearth of aftershocks.

New constraints on Red Sea rifting from correlations of Arabian and Nubian Neoproterozoic outcrops

New constraints on the mechanics of Red Sea opening were obtained by correlating Neoproterozoic outcrops of the Arabian and Nubian Shields along two thirds of the Red Sea coastlines. Using a mosaic of 23 Landsat thematic mapper scenes (5×105 km²) together with field, geochemical, and geochronological data, we identified and mapped lithologic units, mobile belts, and terranes within the Arabian and Nubian Shields. Features best align if Arabia is rotated by 6.7° around a pole at latitude 34.6°N, longitude 18.1°E. Implications of our reconstruction include (1) the amount of continental crust underlying the Red Sea is small because the restored Red Sea coasts are typically juxtaposed, (2) only a single pole is needed, implying that the Arabian and Nubian Shields were rigid plates during Red Sea rifting, (3) coastlines reorient to align with preexisting structures, suggesting the rift propagated in part along pre-existing zones of weakness, (4) large sinistral displacements of up to 350 km along the Red Sea are not supported, (5) the pole is inconsistent with the Pliocene-Pleistocene motion along the Dead Sea transform (pole: 32.8°N, 22.6°E +/− 0.5° [Joffe and Garfunkel, 1987]), indicating that more than one phase of motion is required to account for the Red Sea opening. However, our pole is similar to that for the total motion along the Dead Sea transform (pole: 32.7°N, 19.8°E +/− 2° [Joffe and Garfunkel, 1987]), suggesting that the motion between Arabia and Nubia was parallel to the total motion along the Dead Sea transform.

Nature of the Red Sea crust: A controversy revisited

Whether the Red Sea floor is underlain mostly by oceanic or extended continental crustal material is a controversial topic. To test between the two hypotheses, we used a digital color mosaic of 23 Landsat thematic mapper (TM) scenes with field, geochemical, and geochronological data to identify and correlate crosscutting geologic features on the African and Arabian sides. Faults, shear zones, sutures, granitic complexes, volcano-sedimentary units, and dike swarms align if Arabia is rotated relative to Africa by 6.7° around a pole at lat 34.6°N, long 18.1°E. This solution implies that the amount of continental crust underlying the Red Sea is small because the restored Red Sea coasts are typically juxtaposed.

Using MOMA Broadband Array ScS‐S data to image smaller‐scale structures at the base of the mantle

ScS-S residuals obtained at stations of the Missouri-to-Massachusetts (MOMA) temporary broadband seismic array are used to delineate variations in seismic velocity structure above the core-mantle boundary (CMB) at scales smaller than observable with tomographic models. South American earthquakes recorded at MOMA reveal a slow-velocity anomaly that is at least as small as the limit of the resolution of ScS waves, about 300 km across. This is modeled as being within a region of fast velocities in whole-mantle models. The slow ScS-S residuals correlate well with a peak in ScS/S relative amplitudes. The small region of slow shear velocity at the CMB could be a pocket of lower mantle rock trapped beneath the descending Farallon slab, or evidence of chemical boundary layer variations.

SLICING INTO THE EARTH

Regional arrays of seismometers provide a powerful means of mapping the details of deep-Earth structure. Our understanding of the geological processes at work within our planet depends on our ability to examine them; seismic techniques remain the best tool available. However, spatial aliasing due to the less-than-optimal distribution of global seismometers has long made it difficult to determine deep-Earth structure from teleseismic waves. The temporary deployment of portable broadband seismometers can help by providing high-resolution windows into the Earth. Patterns of global mantle convection create seismically observable features such as anisotropy at the top and bottom of the mantle, topography of upper mantle discontinuities, and heterogeneous structure at the core-mantle boundary.

A Seismic Transect Across West Antarctica: Evidence for Mantle Thermal Anomalies Beneath the Bentley Subglacial Trench and the Marie Byrd Land Dome

West Antarctica consists of several tectonically diverse terranes, including the West Antarctic Rift System, a topographic low region of extended continental crust. In contrast, the adjacent Marie Byrd Land and Ellsworth-Whitmore mountains crustal blocks are on average over 1 km higher, with the former dominated by polygenetic shield and stratovolcanoes protruding through the West Antarctic ice sheet and the latter having a Precambrian basement. The upper mantle structure of these regions is important for inferring the geologic history and tectonic processes, as well as the influence of the solid earth on ice sheet dynamics. Yet this structure is poorly constrained due to a lack of seismological data. As part of the Polar Earth Observing Network, 13 temporary broadband seismic stations were deployed from January 2010 to January 2012 that extended from the Whitmore Mountains, across the West Antarctic Rift System, and into Marie Byrd Land with a mean station spacing of ~90 km. Relative P and S wave travel time residuals were obtained from these stations as well as five other nearby stations by cross correlation. The relative residuals, corrected for both ice and crustal structure using previously published receiver function models of crustal velocity, were inverted to image the relative P and S wave velocity structure of the West Antarctic upper mantle. Some of the fastest relative P and S wave velocities are observed beneath the Ellsworth-Whitmore mountains crustal block and extend to the southern flank of the Bentley Subglacial Trench. However, the velocities in this region are not fast enough to be compatible with a Precambrian lithospheric root, suggesting some combination of thermal, chemical, and structural modification of the lithosphere. The West Antarctic Rift System consists largely of relative fast uppermost mantle seismic velocities consistent with Late Cretaceous/early Cenozoic extension that at present likely has negligible rift related heat flow. In contrast, the Bentley Subglacial Trench, a narrow deep basin within the West Antarctic Rift System, has relative P and S wave velocities in the uppermost mantle that are ~1% and ~2% slower, respectively, and suggest a thermal anomaly of ~75 K. Models for the thermal evolution of a rift basin suggest that such a thermal anomaly is consistent with Neogene extension within the Bentley Subglacial Trench and may, at least in part, account for elevated heat flow reported at the nearby West Antarctic Ice Sheet Divide Ice Core and at Subglacial Lake Whillans. The slowest relative P and S wave velocity anomaly is observed extending to at least 200 km depth beneath the Executive Committee Range in Marie Byrd Land, which is consistent with warm possibly plume-related, upper mantle. The imaged low-velocity anomaly and inferred thermal perturbation (~150 K) are sufficient to support isostatically the anomalous long-wavelength topography of Marie Byrd Land, relative to the adjacent West Antarctic Rift System.

Aftershock sequences of moderate‐sized intermediate and deep earthquakes in the Tonga Subduction Zone

Intermediate and deep focus earthquakes are generally thought to produce few aftershocks. The two year operation of a network of broadband seismographs in the Tonga-Fiji region provides an opportunity to monitor the aftershock activity of moderate-sized earthquakes. In addition to the March 9, 1994 (Mw 7.6) event that shows 144 aftershocks, we found that all nine intermediate and deep earthquakes with Mw > 6.0 showed some aftershock activity, suggesting that aftershock sequences are ubiquitous. On average, deep Tonga earthquakes show an order of magnitude fewer aftershocks above a given magnitude than shallow California earthquakes of similar moment. The largest aftershocks of each sequence have an mb that is 2.0 units smaller on average than the Mw of the mainshock, whereas the California earthquakes show a mean difference of 1.1. Since b-values of the deeper sequences appear to be similar to shallow sequences, the difference in aftershock productivity results from the larger size difference between the mainshock and the larger aftershocks. The most prolific sequences show an activity level not far below shallow earthquakes of the same moment. This is in contrast with observations of little aftershock activity for deep earthquakes in South America, Japan, and Kuriles, and suggests that the Tonga subduction zone is unusually prolific in aftershock production, perhaps due to cold slab temperatures.