First posted December 29, 2023 For additional information, contact: Volcano Science Center - Menlo ParkU.S. Geological Survey345 Middlefield Road, MS 910Menlo Park, CA 94025 As part of a joint Saudi Geological Survey (SGS) and U.S. Geological Survey (USGS) project, we analyzed P-wave receiver functions from seismic stations covering most of the Kingdom of Saudi Arabia to map the thickness of the crust across the Arabia Plate. We present an update of crustal-thickness estimates and fill in gaps for the western Arabian Shield and the rifted margin at the Red Sea (the coastal plain), as well as the eastern Arabian Platform. We applied a conventional H-k stacking algorithm and included careful attention to stacking weights, two forms of sedimentary corrections for stations located on the Arabian Platform, and additional processing for noisy stations. We obtained useful results at 154 stations from 898 teleseismic events over a 2-year period from 1995–1997 (for non-SGS stations) and a 6-year period from 2008–2014 (for SGS stations). Average crustal thickness (that is, depth to the Mohorovičić discontinuity [Moho] below the surface) beneath the Red Sea coastal plain (the rift margin) is 29 kilometers (km), beneath the volcanic fields (known in Arabic as harra [plural] or harrat [singular]) is 35 km, beneath the Arabian Shield (excluding harrats) is 37 km, and beneath the Arabian Platform is 38 km. Crustal thinning appears not to extend east of the rift escarpment, suggesting uniform extension that is no broader at depth than at the surface. In contrast to some previous claims that the Arabian Platform crust is thicker than that of the Arabian Shield, we find no statistically significant difference between their whole crustal thicknesses. However, the average subsedimentary crustal thickness (that is, the crystalline crust) for stations on the Arabian Platform is 34 km, 3 km thinner than the crust of the Arabian Shield. Individual station P-wave (pressure) velocity and S-wave (shear) velocity ratios (VP/VS) are highly variable for the Arabia Plate, ranging from 1.60 to 1.97 and averaging 1.75, with a standard deviation of 0.07. There are no statistically significant differences between VP/VS ratios of the different geologic regions of Saudi Arabia. Similar VP/VS ratios, coupled with similar crustal thicknesses for harrats and the Arabian Shield, indicate that Cenozoic magmatism has contributed negligibly to crustal growth.
The 127 station NorthEast China Extended SeiSmic Array (NECESSArray) provides large quantities of high quality seismic data in northeast China that allow us to resolve lateral variations of Lg Q or crustal attenuation at 1 Hz (Qo) to 2.0° or greater. Using the reverse two-station/event method with 11 642 Lg path-amplitudes from 78 crustal earthquakes, we obtain a 2-D tomographic image of Lg Qo with values ranging from ∼50 to 1400. A high degree of detail in the lateral variation of Lg attenuation is revealed in our tomographic image. High Qo regions are found in the Great Xing'an, Lesser Xing'an and Songen-Zhangguangcai Ranges. Low Qo regions are observed in the Songliao, Sanjiang and Erlian Basins. The lowest Qo is found near the Wudalianchi volcanic field and other Quaternary volcanic fields, the southern Songliao Basin, the western edge of the Erlian Basin and the Sanjiang Basin. Low Qo values are measured for paths that cross sedimentary basins with thick, unconsolidated sediments. Most of the high Lg attenuation in the Songliao Basin correlates reasonably well with low crustal Rayleigh wave phase velocity anomalies. The highest attenuating regions also correlate well with regions of Holocene volcanism.
We analyzed P-wave receiver functions from seismic stations covering most of Saudi Arabia to map the thickness of the crust across the Arabian plate. We present an update of crustal-thickness estimates and fill in data gaps for the western shield and the rifted margin at the Red Sea, as well as the eastern Arabian platform. Our application of a conventional H-k stacking algorithm included careful attention to stacking weights, two forms of sedimentary corrections for stations located on the Arabian platform, and additional processing for noisy stations. Average crustal thickness (i.e. depth to Moho below surface) beneath the Red Sea coastal plain (the rift margin) is 29 km, beneath the volcanic harrats is 35 km, the shield (excluding harrats) is 37 km, and the platform is 38 km. Crustal thinning appears not to extend east of the rift escarpment, suggesting uniform extension, that is no broader at depth than at the surface. In contrast to some previous claims that the platform crust is thicker than the shield, we find no statistically significant difference between whole crustal thickness of the Arabian shield and platform. However, the average sub-sedimentary crustal thickness (i.e., the crystalline crust) of stations on the platform is 34 km, 3 km thinner that the crust of the shield. Individual station Vp/Vs wavespeed ratios are highly variable for the Arabian plate, ranging from 1.60 to 1.97 and averaging 1.75, with a standard deviation of 0.07. There are no statistically significant differences between Vp/Vs ratios of the different geologic regions of Arabia. Similar Vp/Vs ratios, coupled with similar crustal thicknesses for harrats and shield, imply that Cenozoic magmatism has contributed negligibly to crustal growth.
First posted September 21, 2023 For additional information, contact: Earthquake Science CenterU.S. Geological Survey350 N. Akron Rd.Moffett Field, CA 94035 As part of a joint Saudi Geological Survey (SGS) and United States Geological Survey project, we analyzed P-wave receiver functions from seismic stations covering most of the Kingdom of Saudi Arabia to map the thickness of the crust across the Arabia Plate. We present an update of crustal-thickness estimates and fill in gaps for the western Arabian Shield and the rifted margin at the Red Sea (the coastal plain), as well as the eastern Arabian Platform. We applied a conventional H-k stacking algorithm and included careful attention to stacking weights, two forms of sedimentary corrections for stations located on the Arabian Platform, and additional processing for noisy stations. We obtained useful results at 154 stations from 898 teleseismic events over a 2-year period from 1995–1997 (for non-SGS stations) and a 6-year period from 2008–2014 (for SGS stations). Average crustal thickness (that is, depth to the Mohorovičić discontinuity [Moho] below the surface) beneath the Red Sea coastal plain (the rift margin) is 29 kilometers (km), beneath the volcanic fields (known in Arabic as harra [plural] or harrat [singular]) is 35 km, beneath the Arabian Shield (excluding harrats) is 37 km, and beneath the Arabian Platform is 38 km. Crustal thinning appears not to extend east of the rift escarpment, suggesting uniform extension that is no broader at depth than at the surface. In contrast to some previous interpretations that the Arabian Platform crust is thicker than that of the Arabian Shield, we find no statistically significant difference between their whole crustal thicknesses. However, the average sub-sedimentary crustal thickness (that is, the crystalline crust) for stations on the Arabian Platform is 34 km, 3 km thinner than the crust of the Arabian Shield. Individual station P-wave (pressure) velocity and S-wave (shear) velocity ratios (VP/VS) are highly variable for the Arabia Plate, ranging from 1.60 to 1.97 and averaging 1.75, with a standard deviation of 0.07. There are no statistically significant differences between VP/VS ratios of the different geologic regions of Saudi Arabia. Similar VP/ VS ratios, coupled with similar crustal thicknesses for harrats and the Arabian Shield, indicate that Cenozoic magmatism has contributed negligibly to crustal growth.
We study the Moho, the mid-lithospheric discontinuity (MLD), and the lithosphere-asthenosphere boundary (LAB) from southern Africa to northern Arabia, from Archean cratons to active rifts, at 1° resolution using our comprehensive new database of shear-wave receiver functions (SRFs). The good agreement between the Moho depth obtained from our SRFs and published P-wave receiver function (PRF) results provides confidence that our images of deeper lithospheric discontinuities are robust, including boundaries not normally visible on PRFs. We map the Moho and a deeper negative velocity gradient (NVG) almost everywhere we have data coverage. Our synthetic tests and comparisons of SRFs processed with and without deconvolution, and with varying filter parameters, indicate the observed NVG represents earth structure, not a processing artifact. Depth comparisons with seismic tomography and tectonothermal age studies suggest the NVG represents the MLD beneath Archean cratons but represents the LAB beneath non-cratonic regions. Both preserved crustal thickness and lithospheric thickness in the Nubia-Somalia-Arabia plates are statistically thinner for Phanerozoic and late Proterozoic terranes and older regions reactivated during these eras, than for cratons not reworked since the early Proterozoic or Archean. In contrast, NVG depth is uniform for all tectonothermal ages, though with a possible increase in amplitude with age. The equivalence of NVG depth and LAB depth in Phanerozoic lithosphere suggests that low-wavespeed compositions are frozen into the lithosphere as it thickens by cooling, forming our observed MLD at the present day.
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