We present a teleseismic P and S receiver function study using data from a temporary passivesourc... more We present a teleseismic P and S receiver function study using data from a temporary passivesource seismic array in the southernmost Puna Plateau and adjacent regions. The P receiver function images show the distribution of crustal thickness and Vp/Vs ratio for this area. Over much of the southern Puna plateau, the crustal thickness is 50-55 km, whereas to the west a thicker crust (~60 to 75 km) is observed beneath much of the Andean volcanic arc region. From the Puna *Manuscript Click here to view linked References southward, there is little obvious change in the crustal thickness across the border of the plateau (south of 28°S). The crust is seen to progressively thin towards the east in the Pampean Ranges where it is 35-40 km thick. The southern Puna plateau is characterized overall by a low crustal Vp/Vs ratio (less than 1.70), implying a felsic crustal composition. An anomalously high Vp/Vs ratio of 1.87 is observed beneath the Cerro Galan volcanic center, in the region where a prominent crustal low-velocity zone identified below ~10 km depth probably extends into the lower crust. The crustal thickness determined under the Cerro Galan area (59 km) is close to that of the rest of the southern Puna. The prominent high Vp/Vs ratio and low velocity zone beneath the Galan region implies the presence of a zone of partial melt or a magma chamber is consistent with hypothesis calling for lithospheric delamination beneath the Galan caldera. A widespread crustal low-velocity layer observed beneath much of the Southern Puna, correlates well with crustal low-velocity anomalies observed by teleseismic tomography. The lithosphere-asthenosphere boundary beneath the array can be clearly observed by both P and S receiver functions at depths of 70-90 km in agreement with previous studies suggesting a thin lithosphere beneath the high elevated plateau. The mantle transition zone discontinuities appear at expected depths.
Journal of Geophysical Research: Solid Earth, 2000
A number of different geodynamic models have been proposed to explain the extension that occurred... more A number of different geodynamic models have been proposed to explain the extension that occurred during the Miocene in the Alboran Sea region of the western Mediterranean despite the continued convergence and shortening of northern Africa and southern Iberia. In an effort to provide additional geophysical constraints on these models, we performed a local, regional, and teleseismic tomographic travel time inversion for the lithospheric and upper mantle velocity structure and earthquake locations beneath the Alboran region in an area of 800×800 km2. We picked P and S arrival times from digital and analog seismograms recorded by 96 seismic stations in Morocco and Spain between 1989 and 1996 and combined them with arrivals carefully selected from local and global catalogs (1964–1998) to generate a starting data set containing over 100,000 arrival times. Our results indicate that a N‐S line of intermediate‐depth earthquakes extending from crustal depths significantly inland from the sou...
We studied the crustal structure and tectonics in the north Tibetan Plateau from the Songpan-Ganz... more We studied the crustal structure and tectonics in the north Tibetan Plateau from the Songpan-Ganzi terrane to the Qaidam Basin using teleseismic receiver-function imaging, across a major lithospheric boundary, the Kunlun-Qaidam boundary, where previous studies suggest a ~15-20-km change in crustal thickness from thicker crust in the Kunlun Mountains to thinner crust in the Qaidam Basin. We report P receiver functions for 70 stations, largely the International Deep Profiling of Tibet and the Himalaya (INDEPTH), phase IV, experiment. Our most dense station coverage is located along the roughly north-south INDEPTH-IV active-source seismic profile at approximately 95° E longitude. Azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and V p /V s from across the study area. Receiver functions show strong converters that we interpret as the Moho at ~70 km depth beneath the Qiangtang, Songpan-Ganzi terranes and Kunlun Mountains and at ~50 km depth beneath the central Qaidam Basin. This large change in crustal thickness occurs more than 50 km north of the North Kunlun strike-slip fault, on which the 2001 M8.1 Kunlun earthquake occurred. Receiver functions for some of the stations north of the thickness change at the Kunlun-Qaidam boundary also show a deeper ~70-km bright converter in addition to the 50-km converter. The two converters appear to overlap by up to ~30 km in some locations along the south Qaidam Basin. We combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north Tibetan Plateau including crustal flow and crustal injection. At depths imaged here, shallower than ~100 km, we see no evidence of southward subduction of Eurasian lithosphere.
Abstract : The objective of this project is to construct seismic attenuation and velocity models ... more Abstract : The objective of this project is to construct seismic attenuation and velocity models for the crust and upper mantle in western China. This research will increase the coverage of critical areas throughout western China. This work used data collected by the 90-station ASCENT and 25-station NETS arrays (a total of 116 broadband stations) and integrated waveform data from existing broadband stations in western China, such as the MIT and Namche Barwa temporary arrays. These new data sets allow the application of new techniques designed to create robust models of seismic velocity and attenuation in which we reliably estimate the absolute amplitude of the velocity and Q variations across western China. We have created an anisotropic component to our attenuation models. We have extended the work on Sn propagation in eastern Tibet to regional phase Sn wave propagation across northeastern China where high frequency Sn is more prevalent. The work on Northeastern China was not in our original proposal; however, we were able to leverage existing data and human resources to deliver this new research.
The crustal thickness H and average crustal velocity ratio k (Vp/Vs) beneath south central Mongol... more The crustal thickness H and average crustal velocity ratio k (Vp/Vs) beneath south central Mongolia are investigated using the H‐k stacking method based on teleseismic radial receiver functions. Our primary results reveal that the local crustal thickness varies from 38 to 46 km with an average value of 43 km. Thicker crust is found beneath the western Hentey Mountains, while thinner crust is located in the southern area of the Zuunbayan fault zone. The Bouguer gravity anomalies exhibit a strong correlation with the overall crustal thickness pattern throughout most of our study regime. Moreover, a new approach which integrates the Bouguer anomaly gradient and the receiver function‐derived crustal thickness is adopted to calculate the density of the lower crust underneath central Mongolia. Fairly dense lower crust of approximately 3000 kg/m3 is found in the Middle Gobi Desert. The measured crustal Vp/Vs ratio ranges from 1.68 to 1.83 with an average value of 1.74. Low Vp/Vs ratio is found beneath the western Hentey Mountains. In general, low Vp/Vs ratios correlate well with regions of quartz‐rich crust and high heat flow. High Vp/Vs ratios occur in the Middle Gobi volcanic regions and the Mesozoic Southern Gobi Basin.
ABSTRACT Data from recent INDEPTH IV and other broadband deployments in Tibet are used to invert ... more ABSTRACT Data from recent INDEPTH IV and other broadband deployments in Tibet are used to invert for crustal delays and Pn velocities beneath the eastern Tibetan Plateau and surrounding regions. The station delays show the thickest crust is beneath Tanggula Shan in central Tibet, a significant decrease of crustal thickness in the northeast plateau, and a thin crust beneath the Ho-Xi corridor that lie north of the Qilian Shan. Crustal thinning in the northeast plateau slightly exceeds that expected from isostacy given the lower elevations there. The average Pn velocity for the region is 8.1 km/s but varies from 7.8 to over 8.3 km/s. Generally low Pn velocities are found in the northeastern Tibet in the Qiangtang and Songpan-Ganzi terranes as well as beneath the Qilian Shan. This includes a zone of very low Pn velocity just west of the Longmen Shan thrust, along the eastern Kunlun fault, and beneath the eastern Qilian Shan. A region of high Pn velocity underlies the eastern end of the Bangong-Nujiang Suture (bounded by 31°-33° N; 90°-98°E). It could represent part of the underthrust Indian shield, but has higher velocity than regions south of it. The region between the Qilian Shan and Kunlun Shan is also characterized by high Pn velocity with several zones of extremely high velocity. This includes two high velocity features beneath Qaidam Basin and a high velocity feature extending southeast of Qinghai Lake. These features may correspond to cratonic fragments that accreted during the closure of the Tethys Ocean and have impeded, but not stopped, the northward growth of the plateau.
The northeastern margin of Tibet is characterised, from south to north, by the high plateau of th... more The northeastern margin of Tibet is characterised, from south to north, by the high plateau of the Qiangtang and Songpan-Ganzi terranes, large strike-slip structures of the Kunlun fault, the Kunlun mountain range, which drops dramatically from elevations exceeding 5000 m into the Qaidam basin at 3500 m elevation, which is covered by thick sedimentary layers. This area, crucial for our
The northeastern boundary of the Tibetan Plateau is a new focus of contemporary debate concerning... more The northeastern boundary of the Tibetan Plateau is a new focus of contemporary debate concerning continental plateau formation as an continent-continent collision. Recent geological studies and limited geophysical measurements in this region have been cited to argue that Asian continental lithosphere is being detached and 'subducted' into the Tibetan mantle. Upper mantle geophysical properties beneath the northern plateau have been
EvidencefromEarthquakeDataforaPartially o f K i n d e t a i (3) r o p r~) d~l c e o n e-d i m e n... more EvidencefromEarthquakeDataforaPartially o f K i n d e t a i (3) r o p r~) d~l c e o n e-d i m e nsional shear-wave velocity models for the Molten Crustal Layer in Southern Tibet averaged receiver function. T h e independent parameter in the inversion was the
Journal of Geophysical Research: Solid Earth, 1997
Shear wave splitting parameters of local and teleseismic S waves from intermediate and deep earth... more Shear wave splitting parameters of local and teleseismic S waves from intermediate and deep earthquakes in the southern Kurile and Japan subduction zones are combined with splitting parameters obtained from SKS and SKKS waves to determine depth variation in shear wave splitting both above and below the earthquake. Local S wave splitting results measured at station MAJO (Matsushiro, Japan) show fast directions, from NNE to NE, parallel to the extension directions measured from Quaternary fault and geodetic data. The inferred finite strain field from shear wave splitting is consistent with the closing of the Sea of Japan. At YSS (Yuzhno Sakhalinsk, Russia) the shear wave splitting parameters obtained from local S waves are approximately oriented N‐S and appear to be controlled by deformation of the upper plate rather than the subducted slab. SKS phases recorded at YSS, located on the southern tip of Sakhalin Island, show an approximately N‐S oriented fast direction and lag time of 1.3...
Journal of Geophysical Research: Solid Earth, 1997
Shear wave phases ScS, SKS, and SKKS, collected from the Project International Deep Seismic Sound... more Shear wave phases ScS, SKS, and SKKS, collected from the Project International Deep Seismic Sounding of Tibet and Himalayas II (INDEPTH‐II) array in the eastern Himalayas and Tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern Himalayas‐Tibet collision zone. No evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the Indus‐Tsangpo Suture. The lack of shear wave splitting beneath the stations in the vicinity of suture and the Tethyan Himalayas indicates that there is no appreciable large‐scale mantle polarization seismic anisotropy beneath this part of the collision zone. One explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened Indian lower continental lithosphere. North of the Gandese belt, the fast directions of the ...
Fast directions and lag times associated with SKS shear-wave splitting are determined for six dig... more Fast directions and lag times associated with SKS shear-wave splitting are determined for six digital stations in the Rio Grande Rift (RGR). The mean fast direction for three stations in the central RGR is about 45 ø oblique to the axis of the RGR. In contrast, fast directions approximately parallel to the axis of the RGR are found for two stations in the southern RGR (-10ø), and one station in the northern RGR (~26ø). The fast directions for southern stations are not aligned with the NE-SW extension direction of the Oligocene to early Miocene stage of extension. The significant N-S lateral variation in the SKS shear-wave splitting parameters is difficult to explain by fossil anisotropy because the manfie structure beneath the RGR is underlain by hot material with low seismic-wave velocities. Hence the fast direction of azimuthal anisotropy is probably re!ated to the preferred orientation of olivine created by the present deformation in the upper manfie. Because none of the fast directions are parallel to the present E-W extension direction of the RGR the dominant upper manfie flow direction is not perpendicular to the axis of the RGR. The pattern of the fast directions along the RGR provides evidence for a rising asthenosphere beneath the central to southern RGR and suggests that thermally driven small-scale convection may be faster for flow parallel to the rift than perpendicular to it. with lag times of approximately 0.05 to 0.25 seconds [Ando,
Teleseismic S, SKS and SKKS data, collected structural trend of the western Himalayas extends nor... more Teleseismic S, SKS and SKKS data, collected structural trend of the western Himalayas extends northwestfrom a temporary broadband array across the Himalayan southeast and contrasts sharply with the nearly east-west front in Pakistan, are analyzed for shear-wave splitting trending foreland structures of the Hazara Arc (Figure 1)[e.g. parameters. The S KS and S KKS phases have ray paths Ni et al., 1991]. This paradox in structural trends in this part originating from both the South Pacific and Colombia of the Himalayas led Seebet and Armbruster [1979] to suggest which have azimuths approximately 40 ø apart with respect to the Pakistan array. If significant seismic azimuthal anisotropy is present we should observe splitting associated with one of these ray paths. No evidence was seen for any shear-wave splitting beneath any of the stations in the array. Teleseismic S waves were also used in order to provide better azimuthal coverage for the shearwave splitting measurements. We were able to correct for any source-side anisotropy when needed. No receiver-side splitting was observed in any of the S wave data. The lack of shear-wave splitting beneath the Pakistan array indicates that there is no appreciable largescale azimuthal anisotropy beneath this part of the Himalayas. Therefore, if there is any significant strain in the upper mantle beneath this area, it must either be vertically oriented, or, if horizontal, vertically vary in such a way that the integrated effect on S wave splitting is null.
Western Mexico, where the young and small Rivera Plate and the adjacent large Cocos Plate are sub... more Western Mexico, where the young and small Rivera Plate and the adjacent large Cocos Plate are subducting beneath the North American Plate, is a unique region on Earth where tearing of subducting oceanic plates, as well as fragmentation of the overriding continental plate, is occurring today. Characterizing the mantle flow field that accompanies the subduction of the Rivera and adjacent Cocos plates can help to clarify the tectonics and magma genesis of this young plate boundary. Here we report observations of seismic anisotropy, as manifested by shear wave splitting derived from local S and teleseismic SKS data collected by the Mapping Rivera Subduction zone array that was deployed from 2006 January to 2007 June, in southwestern Mexico, and from data collected by two of Mexico's Servicio Sismológico Nacional stations. SKS and local S-wave splitting parameters indicate that the fast directions of the split SKS waves for stations that lie on the central and southern Jalisco Block are approximately trench-normal, following the convergence direction between the Rivera Plate and Jalisco Block. S-wave splitting from slab events show a small averaged delay time of ∼0.2 s for the upper 60 km of the crust and mantle. Therefore, the main source of anisotropy must reside in the entrained mantle below the young and thin Rivera Plate. Trench-oblique fast SKS split directions are observed in the western edge of the Rivera Plate and the western parts of the Cocos slab. The curved pattern of fast SKS split directions in the western Jalisco block and beneath the Rivera-Cocos slab gap indicates 3-D toroidal mantle flow, around the northwestern edge of the Rivera slab and the Rivera-Cocos gap, which profoundly affect the finite strain field in the northwestern edge of the Rivera slab and the mantle wedge. Both the tomographic images and shear wave splitting results support the idea that the Rivera and western Cocos plates not only moved in a downdip direction but also have recently rolled back towards the trench and that the Colima rift is intimately related to the tearing between the Rivera and Cocos plates.
Crustal and upper-mantle seismic discontinuities beneath eastern Turkey are imaged using teleseis... more Crustal and upper-mantle seismic discontinuities beneath eastern Turkey are imaged using teleseismic S-toP converted phases. Three crustal phases are observed: the Moho with depth ranging between 30 and 55 km, indicating variable tectonic regimes within this continental collision zone; an upper-crustal discontinuity at approximately 10 km depth; and various crustal low-velocity zones, possibly associated with recent Quaternary volcanism. Imaging of the upper mantle is complicated by the three-dimensional geometry of the region, in particular due to the Bitlis-Zagros suture zone. However, several upper-mantle S-toP converted phase are identified as being the signature of the lithosphere-asthenosphere boundary (LAB). The inferred LAB for the Eastern Anatolian Accretionary Complex indicates that eastern Turkey has an anomalously thin (between ∼ 60 and 80 km) lithosphere which is consistent with an oceanic slab detachment model. The observed LAB phases for the Arabian shield and Iranian plateau indicate that lithospheric thickness for these stable regions is on the order of 100 to 125 km thick, which is typical of continental margins.
We present a teleseismic P and S receiver function study using data from a temporary passivesourc... more We present a teleseismic P and S receiver function study using data from a temporary passivesource seismic array in the southernmost Puna Plateau and adjacent regions. The P receiver function images show the distribution of crustal thickness and Vp/Vs ratio for this area. Over much of the southern Puna plateau, the crustal thickness is 50-55 km, whereas to the west a thicker crust (~60 to 75 km) is observed beneath much of the Andean volcanic arc region. From the Puna *Manuscript Click here to view linked References southward, there is little obvious change in the crustal thickness across the border of the plateau (south of 28°S). The crust is seen to progressively thin towards the east in the Pampean Ranges where it is 35-40 km thick. The southern Puna plateau is characterized overall by a low crustal Vp/Vs ratio (less than 1.70), implying a felsic crustal composition. An anomalously high Vp/Vs ratio of 1.87 is observed beneath the Cerro Galan volcanic center, in the region where a prominent crustal low-velocity zone identified below ~10 km depth probably extends into the lower crust. The crustal thickness determined under the Cerro Galan area (59 km) is close to that of the rest of the southern Puna. The prominent high Vp/Vs ratio and low velocity zone beneath the Galan region implies the presence of a zone of partial melt or a magma chamber is consistent with hypothesis calling for lithospheric delamination beneath the Galan caldera. A widespread crustal low-velocity layer observed beneath much of the Southern Puna, correlates well with crustal low-velocity anomalies observed by teleseismic tomography. The lithosphere-asthenosphere boundary beneath the array can be clearly observed by both P and S receiver functions at depths of 70-90 km in agreement with previous studies suggesting a thin lithosphere beneath the high elevated plateau. The mantle transition zone discontinuities appear at expected depths.
Journal of Geophysical Research: Solid Earth, 2000
A number of different geodynamic models have been proposed to explain the extension that occurred... more A number of different geodynamic models have been proposed to explain the extension that occurred during the Miocene in the Alboran Sea region of the western Mediterranean despite the continued convergence and shortening of northern Africa and southern Iberia. In an effort to provide additional geophysical constraints on these models, we performed a local, regional, and teleseismic tomographic travel time inversion for the lithospheric and upper mantle velocity structure and earthquake locations beneath the Alboran region in an area of 800×800 km2. We picked P and S arrival times from digital and analog seismograms recorded by 96 seismic stations in Morocco and Spain between 1989 and 1996 and combined them with arrivals carefully selected from local and global catalogs (1964–1998) to generate a starting data set containing over 100,000 arrival times. Our results indicate that a N‐S line of intermediate‐depth earthquakes extending from crustal depths significantly inland from the sou...
We studied the crustal structure and tectonics in the north Tibetan Plateau from the Songpan-Ganz... more We studied the crustal structure and tectonics in the north Tibetan Plateau from the Songpan-Ganzi terrane to the Qaidam Basin using teleseismic receiver-function imaging, across a major lithospheric boundary, the Kunlun-Qaidam boundary, where previous studies suggest a ~15-20-km change in crustal thickness from thicker crust in the Kunlun Mountains to thinner crust in the Qaidam Basin. We report P receiver functions for 70 stations, largely the International Deep Profiling of Tibet and the Himalaya (INDEPTH), phase IV, experiment. Our most dense station coverage is located along the roughly north-south INDEPTH-IV active-source seismic profile at approximately 95° E longitude. Azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and V p /V s from across the study area. Receiver functions show strong converters that we interpret as the Moho at ~70 km depth beneath the Qiangtang, Songpan-Ganzi terranes and Kunlun Mountains and at ~50 km depth beneath the central Qaidam Basin. This large change in crustal thickness occurs more than 50 km north of the North Kunlun strike-slip fault, on which the 2001 M8.1 Kunlun earthquake occurred. Receiver functions for some of the stations north of the thickness change at the Kunlun-Qaidam boundary also show a deeper ~70-km bright converter in addition to the 50-km converter. The two converters appear to overlap by up to ~30 km in some locations along the south Qaidam Basin. We combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north Tibetan Plateau including crustal flow and crustal injection. At depths imaged here, shallower than ~100 km, we see no evidence of southward subduction of Eurasian lithosphere.
Abstract : The objective of this project is to construct seismic attenuation and velocity models ... more Abstract : The objective of this project is to construct seismic attenuation and velocity models for the crust and upper mantle in western China. This research will increase the coverage of critical areas throughout western China. This work used data collected by the 90-station ASCENT and 25-station NETS arrays (a total of 116 broadband stations) and integrated waveform data from existing broadband stations in western China, such as the MIT and Namche Barwa temporary arrays. These new data sets allow the application of new techniques designed to create robust models of seismic velocity and attenuation in which we reliably estimate the absolute amplitude of the velocity and Q variations across western China. We have created an anisotropic component to our attenuation models. We have extended the work on Sn propagation in eastern Tibet to regional phase Sn wave propagation across northeastern China where high frequency Sn is more prevalent. The work on Northeastern China was not in our original proposal; however, we were able to leverage existing data and human resources to deliver this new research.
The crustal thickness H and average crustal velocity ratio k (Vp/Vs) beneath south central Mongol... more The crustal thickness H and average crustal velocity ratio k (Vp/Vs) beneath south central Mongolia are investigated using the H‐k stacking method based on teleseismic radial receiver functions. Our primary results reveal that the local crustal thickness varies from 38 to 46 km with an average value of 43 km. Thicker crust is found beneath the western Hentey Mountains, while thinner crust is located in the southern area of the Zuunbayan fault zone. The Bouguer gravity anomalies exhibit a strong correlation with the overall crustal thickness pattern throughout most of our study regime. Moreover, a new approach which integrates the Bouguer anomaly gradient and the receiver function‐derived crustal thickness is adopted to calculate the density of the lower crust underneath central Mongolia. Fairly dense lower crust of approximately 3000 kg/m3 is found in the Middle Gobi Desert. The measured crustal Vp/Vs ratio ranges from 1.68 to 1.83 with an average value of 1.74. Low Vp/Vs ratio is found beneath the western Hentey Mountains. In general, low Vp/Vs ratios correlate well with regions of quartz‐rich crust and high heat flow. High Vp/Vs ratios occur in the Middle Gobi volcanic regions and the Mesozoic Southern Gobi Basin.
ABSTRACT Data from recent INDEPTH IV and other broadband deployments in Tibet are used to invert ... more ABSTRACT Data from recent INDEPTH IV and other broadband deployments in Tibet are used to invert for crustal delays and Pn velocities beneath the eastern Tibetan Plateau and surrounding regions. The station delays show the thickest crust is beneath Tanggula Shan in central Tibet, a significant decrease of crustal thickness in the northeast plateau, and a thin crust beneath the Ho-Xi corridor that lie north of the Qilian Shan. Crustal thinning in the northeast plateau slightly exceeds that expected from isostacy given the lower elevations there. The average Pn velocity for the region is 8.1 km/s but varies from 7.8 to over 8.3 km/s. Generally low Pn velocities are found in the northeastern Tibet in the Qiangtang and Songpan-Ganzi terranes as well as beneath the Qilian Shan. This includes a zone of very low Pn velocity just west of the Longmen Shan thrust, along the eastern Kunlun fault, and beneath the eastern Qilian Shan. A region of high Pn velocity underlies the eastern end of the Bangong-Nujiang Suture (bounded by 31°-33° N; 90°-98°E). It could represent part of the underthrust Indian shield, but has higher velocity than regions south of it. The region between the Qilian Shan and Kunlun Shan is also characterized by high Pn velocity with several zones of extremely high velocity. This includes two high velocity features beneath Qaidam Basin and a high velocity feature extending southeast of Qinghai Lake. These features may correspond to cratonic fragments that accreted during the closure of the Tethys Ocean and have impeded, but not stopped, the northward growth of the plateau.
The northeastern margin of Tibet is characterised, from south to north, by the high plateau of th... more The northeastern margin of Tibet is characterised, from south to north, by the high plateau of the Qiangtang and Songpan-Ganzi terranes, large strike-slip structures of the Kunlun fault, the Kunlun mountain range, which drops dramatically from elevations exceeding 5000 m into the Qaidam basin at 3500 m elevation, which is covered by thick sedimentary layers. This area, crucial for our
The northeastern boundary of the Tibetan Plateau is a new focus of contemporary debate concerning... more The northeastern boundary of the Tibetan Plateau is a new focus of contemporary debate concerning continental plateau formation as an continent-continent collision. Recent geological studies and limited geophysical measurements in this region have been cited to argue that Asian continental lithosphere is being detached and 'subducted' into the Tibetan mantle. Upper mantle geophysical properties beneath the northern plateau have been
EvidencefromEarthquakeDataforaPartially o f K i n d e t a i (3) r o p r~) d~l c e o n e-d i m e n... more EvidencefromEarthquakeDataforaPartially o f K i n d e t a i (3) r o p r~) d~l c e o n e-d i m e nsional shear-wave velocity models for the Molten Crustal Layer in Southern Tibet averaged receiver function. T h e independent parameter in the inversion was the
Journal of Geophysical Research: Solid Earth, 1997
Shear wave splitting parameters of local and teleseismic S waves from intermediate and deep earth... more Shear wave splitting parameters of local and teleseismic S waves from intermediate and deep earthquakes in the southern Kurile and Japan subduction zones are combined with splitting parameters obtained from SKS and SKKS waves to determine depth variation in shear wave splitting both above and below the earthquake. Local S wave splitting results measured at station MAJO (Matsushiro, Japan) show fast directions, from NNE to NE, parallel to the extension directions measured from Quaternary fault and geodetic data. The inferred finite strain field from shear wave splitting is consistent with the closing of the Sea of Japan. At YSS (Yuzhno Sakhalinsk, Russia) the shear wave splitting parameters obtained from local S waves are approximately oriented N‐S and appear to be controlled by deformation of the upper plate rather than the subducted slab. SKS phases recorded at YSS, located on the southern tip of Sakhalin Island, show an approximately N‐S oriented fast direction and lag time of 1.3...
Journal of Geophysical Research: Solid Earth, 1997
Shear wave phases ScS, SKS, and SKKS, collected from the Project International Deep Seismic Sound... more Shear wave phases ScS, SKS, and SKKS, collected from the Project International Deep Seismic Sounding of Tibet and Himalayas II (INDEPTH‐II) array in the eastern Himalayas and Tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern Himalayas‐Tibet collision zone. No evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the Indus‐Tsangpo Suture. The lack of shear wave splitting beneath the stations in the vicinity of suture and the Tethyan Himalayas indicates that there is no appreciable large‐scale mantle polarization seismic anisotropy beneath this part of the collision zone. One explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened Indian lower continental lithosphere. North of the Gandese belt, the fast directions of the ...
Fast directions and lag times associated with SKS shear-wave splitting are determined for six dig... more Fast directions and lag times associated with SKS shear-wave splitting are determined for six digital stations in the Rio Grande Rift (RGR). The mean fast direction for three stations in the central RGR is about 45 ø oblique to the axis of the RGR. In contrast, fast directions approximately parallel to the axis of the RGR are found for two stations in the southern RGR (-10ø), and one station in the northern RGR (~26ø). The fast directions for southern stations are not aligned with the NE-SW extension direction of the Oligocene to early Miocene stage of extension. The significant N-S lateral variation in the SKS shear-wave splitting parameters is difficult to explain by fossil anisotropy because the manfie structure beneath the RGR is underlain by hot material with low seismic-wave velocities. Hence the fast direction of azimuthal anisotropy is probably re!ated to the preferred orientation of olivine created by the present deformation in the upper manfie. Because none of the fast directions are parallel to the present E-W extension direction of the RGR the dominant upper manfie flow direction is not perpendicular to the axis of the RGR. The pattern of the fast directions along the RGR provides evidence for a rising asthenosphere beneath the central to southern RGR and suggests that thermally driven small-scale convection may be faster for flow parallel to the rift than perpendicular to it. with lag times of approximately 0.05 to 0.25 seconds [Ando,
Teleseismic S, SKS and SKKS data, collected structural trend of the western Himalayas extends nor... more Teleseismic S, SKS and SKKS data, collected structural trend of the western Himalayas extends northwestfrom a temporary broadband array across the Himalayan southeast and contrasts sharply with the nearly east-west front in Pakistan, are analyzed for shear-wave splitting trending foreland structures of the Hazara Arc (Figure 1)[e.g. parameters. The S KS and S KKS phases have ray paths Ni et al., 1991]. This paradox in structural trends in this part originating from both the South Pacific and Colombia of the Himalayas led Seebet and Armbruster [1979] to suggest which have azimuths approximately 40 ø apart with respect to the Pakistan array. If significant seismic azimuthal anisotropy is present we should observe splitting associated with one of these ray paths. No evidence was seen for any shear-wave splitting beneath any of the stations in the array. Teleseismic S waves were also used in order to provide better azimuthal coverage for the shearwave splitting measurements. We were able to correct for any source-side anisotropy when needed. No receiver-side splitting was observed in any of the S wave data. The lack of shear-wave splitting beneath the Pakistan array indicates that there is no appreciable largescale azimuthal anisotropy beneath this part of the Himalayas. Therefore, if there is any significant strain in the upper mantle beneath this area, it must either be vertically oriented, or, if horizontal, vertically vary in such a way that the integrated effect on S wave splitting is null.
Western Mexico, where the young and small Rivera Plate and the adjacent large Cocos Plate are sub... more Western Mexico, where the young and small Rivera Plate and the adjacent large Cocos Plate are subducting beneath the North American Plate, is a unique region on Earth where tearing of subducting oceanic plates, as well as fragmentation of the overriding continental plate, is occurring today. Characterizing the mantle flow field that accompanies the subduction of the Rivera and adjacent Cocos plates can help to clarify the tectonics and magma genesis of this young plate boundary. Here we report observations of seismic anisotropy, as manifested by shear wave splitting derived from local S and teleseismic SKS data collected by the Mapping Rivera Subduction zone array that was deployed from 2006 January to 2007 June, in southwestern Mexico, and from data collected by two of Mexico's Servicio Sismológico Nacional stations. SKS and local S-wave splitting parameters indicate that the fast directions of the split SKS waves for stations that lie on the central and southern Jalisco Block are approximately trench-normal, following the convergence direction between the Rivera Plate and Jalisco Block. S-wave splitting from slab events show a small averaged delay time of ∼0.2 s for the upper 60 km of the crust and mantle. Therefore, the main source of anisotropy must reside in the entrained mantle below the young and thin Rivera Plate. Trench-oblique fast SKS split directions are observed in the western edge of the Rivera Plate and the western parts of the Cocos slab. The curved pattern of fast SKS split directions in the western Jalisco block and beneath the Rivera-Cocos slab gap indicates 3-D toroidal mantle flow, around the northwestern edge of the Rivera slab and the Rivera-Cocos gap, which profoundly affect the finite strain field in the northwestern edge of the Rivera slab and the mantle wedge. Both the tomographic images and shear wave splitting results support the idea that the Rivera and western Cocos plates not only moved in a downdip direction but also have recently rolled back towards the trench and that the Colima rift is intimately related to the tearing between the Rivera and Cocos plates.
Crustal and upper-mantle seismic discontinuities beneath eastern Turkey are imaged using teleseis... more Crustal and upper-mantle seismic discontinuities beneath eastern Turkey are imaged using teleseismic S-toP converted phases. Three crustal phases are observed: the Moho with depth ranging between 30 and 55 km, indicating variable tectonic regimes within this continental collision zone; an upper-crustal discontinuity at approximately 10 km depth; and various crustal low-velocity zones, possibly associated with recent Quaternary volcanism. Imaging of the upper mantle is complicated by the three-dimensional geometry of the region, in particular due to the Bitlis-Zagros suture zone. However, several upper-mantle S-toP converted phase are identified as being the signature of the lithosphere-asthenosphere boundary (LAB). The inferred LAB for the Eastern Anatolian Accretionary Complex indicates that eastern Turkey has an anomalously thin (between ∼ 60 and 80 km) lithosphere which is consistent with an oceanic slab detachment model. The observed LAB phases for the Arabian shield and Iranian plateau indicate that lithospheric thickness for these stable regions is on the order of 100 to 125 km thick, which is typical of continental margins.
Uploads
Papers by Eric Sandvol