- NSF-PAR ID:
- 10275825
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 223
- Issue:
- 3
- ISSN:
- 0956-540X
- Page Range / eLocation ID:
- 1644 to 1657
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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With the ongoing discussion of Earth structure under West Antarctica and how it relates to the extension and volcanism of the area, we explore the possibility of a hydrated or thermally perturbed mantle underneath the region. Using P-wave receiver functions, we focus on the Mantle Transition Zone (MTZ) and how its thickness fluctuates from the global average (240-260 km). Prior studies have explored the West Antarctic regions of Marie Byrd Land and the West Antarctic Rift, but we expand this to include ~3-5 years of recent, additional seismic data from the Amundsen Sea and Pine Island Bay regions. Several years of additional data from the Ronne-Fichtner Ice Shelf, Ellsworth Land, and Marie Byrd Land regions will help provide a more complete picture of the mantle transition zone. Data for this study was obtained from IRIS for earthquakes of a 5.5 magnitude or greater. We use an iterative, time domain deconvolution method, filtered with Gaussian widths of 0.5, 0.75, and 1.0. All events within their respective Gaussian filter have undergone quality check by removing waveforms that have lower than 85% fit and visually checking for clear outliers. We migrate the receiver functions to depth and stack, using both single station stacking and common conversion point (CCP) stacking. We migrate the CCP stacks assuming both 1D (AK-135) and 3D velocity models throughout the region. Preliminary results from single-station stacks beneath the Thurston Island and Amundsen Sea regions suggest that the MTZ thickness is similar to the global average and the depth to the transition zone appears to be depressed, with average transition zone boundaries appearing around 430 and 680 km. If the MTZ is thinner than the global average, it may be an indication for high temperature thermal anomalies or a plume under West Antarctica that may help explain the history of extension and uplift there. These results could be useful for glacial isostatic adjustment and/or geothermal heat flux models that attempt to understand ice sheet history and stability.more » « less
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SUMMARY To evaluate the plate flexure model for the formation of the Appalachian Basin, we investigate the extent to which crustal structure beneath and surrounding the basin was modified by the Palaeozoic orogenic events that created the basin. We jointly invert receiver functions and surface wave dispersion measurements to obtain 1-D crustal Vs profiles for 261 seismic stations located within and around the basin. The average crustal thickness for the region is 44 km, and the crust gradually thins to the east, consistent with previous studies. Four areas of anomalous crust are identified with respect to the eastward thinning of the crust. An area of thick crust is found along the Grenville Front on the western side of the Appalachian Basin where the crust thickens by ∼5–10 km. Moho depths of up to 54 km in this region likely result from suture-thickening. The crust is thinner beneath the Neoproterozoic Scranton rift by ∼5–7 km, coincident with a ∼40 mGal Bouguer gravity high. Across the Neoproterozoic Rome Trough, the crust thins by ∼4–5 km, coincident with a ∼10 mGal Bouguer gravity high. Density models for these rifts show that the rift-related crustal thinning is sufficient to explain the gravity anomalies. The Vs models obtained for stations in the rifts indicate little, if any, mafic layering in the mid-crust and only a modest amount of mafic layering in the lower crust. In the northwestern portion of the Appalachian Basin in northeastern Ohio and northwestern Pennsylvania within the Elzevir block, another area of anomalously thick crust (50–52 km) is found. This region is not associated with any known tectonic structures or boundaries or a gravity anomaly. The lower ∼5–10 km of the crust in this region is characterized by high (>3.9 km s−1) shear wave velocities and thus appears to be mafic. The origin of anomalous crustal structure in all four areas is best attributed to Precambrian tectonic events that predate the formation of the Appalachian Basin, indicating that the crystalline crust beneath and surrounding the basin was not significantly affected by the Palaeozoic basin-forming orogenic events, a finding which supports the use of plate flexure models for understanding basin formation.
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Abstract Large-scale geological structures have controlled the long-term development of the bed and thus the flow of the West Antarctic Ice Sheet (WAIS). However, complete ice cover has obscured the age and exact positions of faults and geological boundaries beneath Thwaites Glacier and Pine Island Glacier, two major WAIS outlets in the Amundsen Sea sector. Here, we characterize the only rock outcrop between these two glaciers, which was exposed by the retreat of slow-flowing coastal ice in the early 2010s to form the new Sif Island. The island comprises granite, zircon U-Pb dated to ~177–174 Ma and characterized by initial ɛNd,87Sr/86Sr and ɛHfisotope compositions of -2.3, 0.7061 and -1.3, respectively. These characteristics resemble Thurston Island/Antarctic Peninsula crustal block rocks, strongly suggesting that the Sif Island granite belongs to this province and placing the crustal block's boundary with the Marie Byrd Land province under Thwaites Glacier or its eastern shear margin. Low-temperature thermochronological data reveal that the granite underwent rapid cooling following emplacement, rapidly cooled again at ~100–90 Ma and then remained close to the Earth's surface until present. These data help date vertical displacement across the major tectonic structure beneath Pine Island Glacier to the Late Cretaceous.
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Abstract We developed 3‐D isotropic crustal seismic velocity models of central Idaho and eastern Oregon from the IDOR (western IDaho and eastern ORegon) Passive seismic data. Ambient noise tomography yielded crustal velocity structure from vertical component Rayleigh wave group and phase velocity measurements. Results include a strong shear wave velocity contrast—faster in accreted Blue Mountains terranes west of the western Idaho shear zone (WISZ), slower in the Idaho batholith, emplaced within the Archean Grouse Creek block east of the WISZ—restricted to the upper‐to‐middle crust. In deeper crust not affected by mafic underplating during Columbia River Flood Basalt magmatism, the shear wave velocity of the Mesozoic Olds Ferry continental arc terrane is indistinguishable from that of the Archean Grouse Creek block basement. Crustal columns of the Olds Ferry terrane and the Permian‐Jurassic Wallowa intraoceanic arc terrane are characterized by low seismic velocities, consistent with felsic lithologies down to ∼20 km. West of the WISZ, the Bourne and Greenhorn subterranes of the Baker terrane, an accretionary complex between the arc terranes, have distinct shallow crustal seismic velocities. The Greenhorn subterrane to midcrustal depths is in an overthrust geometry relative to the Bourne subterrane. Lack of mafic lower crust in our results of the Wallowa or Olds Ferry arcs may be due to imbrication of upper crustal felsic plutonic complexes of these arcs. Shortening and thickening of the Blue Mountains arc terranes crust to >30 km, and subduction or delamination of their mafic lower crustal sections is a viable mechanism for growth of a felsic continental crust.
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