Browsing by Author "James, D. E."
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- Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratonsNguuri, T. K.; Gore, J.; James, D. E.; Webb, S. J.; Wright, C.; Zengeni, T. G.; Gwavava, O.; Snoke, J. A.; Kaapvaal Seismic Group (American Geophysical Union, 2001-07-01)The formation of Archean crust appears to involve processes unique to early earth history. Initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern Africa re veal significant differences in the nature of the crust and the crust-mantle boundary between Archean and post-Archean geologic terranes. With the notable exception of the collisional Limpopo belt, where the crust is thick and the Moho complex, the crust beneath undisturbed Archean craton is typically thin (similar to 35-40 km), unlayered, and characterized by a strong velocity contrast across a relatively sharp. Moho. This crustal structure contrasts markedly with that beneath post-Archean terranes and beneath Archean regions affected by large-scale Proterozoic events (the Bushveld complex and the Okwa/Magondi belts), where the crust tends to be relatively thick (similar to 46-50 km) and the Moho is complex.
- Lithospheric structure of the Chaco and Parana Basins of South America from surface-wave inversionSnoke, J. A.; James, D. E. (American Geophysical Union, 1997-02)Surface-wave data from a portable broadband array have been used to invert for the velocity structure of the crust and upper mantle beneath the Chaco and Parana Basins of central South America. The upper-mantle velocity structure beneath the Parana Basin is cratonic in character, whereas that beneath the Chaco Basin is tectonic or asthenospheric in character. The surface-wave analysis used ;broadband recordings from a subset of a 14-station array deployed in a roughly east-west sawtooth arrangement along 20 degrees S latitude, with a total EW aperture of similar to 1,400 km. Results from receiver-function analysis, as well as direct P-wave regional travel-time data, were used in the inversions to help constrain Moho depths and crust and upper-mantle velocities. S-wave structure for the intracratonic Parana Basin was determined using interstation phase and group velocities for Rayleigh waves (fundamental and first higher mode) and Love waves (fundamental mode only) based on seven events with paths which traverse the eastern Parana Basin and one event with a path across the western Parana Basin. The average Moho depth in the eastern Parana Basin is similar to 42 km. The high-velocity upper-mantle lid has a maximum S-wave velocity of 4.7 km/s, with no resolvable low-velocity zone to at least 200 km depth. This cratonic velocity structure indicates the presence of a lithospheric root beneath the Parana Basin despite emplacement of the Parana plume. The limited data from the western Parana Basin are consistent with a homogeneous upper-mantle structure throughout the Parana Basin. Waveform inversion of fundamental-mode and first-higher-mode Rayleigh waves from a single subandean event was used to obtain estimates for pure-path dispersion along propagation paths through the Chaco Basin and the western half of the Parana Basin. The data were partitioned to isolate the partial-path contribution of the phase and group velocities for the Chaco Basin. The phase and group velocities from this somewhat sparse data set were inverted to obtain: a velocity-depth model for the Chaco Basin. The distinguishing features of the Chaco model consist of a rather shallow Moho depth, 32 km, and low (''asthenospheric'') upper-mantle S-wave velocities, about 4.2 km/s, with velocity increasing only slightly to about 4.3 km/s at 150 km depth.
- Structure of the subducting Nazca plate beneath PeruNorabuena, E. O.; Snoke, J. A.; James, D. E. (American Geophysical Union, 1994-05)Arrival times from intermediate-depth (110-150 km) earthquakes within the region of flat subduction beneath the subandean zone and foreland basins of east-central Peru provide constraints on the geometry and velocity structure of the subducting Nazca plate. Hypocentral locations and origin times for these events were determined using observations from a 15 station digitally recording locator array deployed in the epicentral region of eastern Peru. Observed P wave arrival times for coastal stations in Peru, some 3-6-degrees from the epicenters, are up to 4 s early relative to predicted arrival times based on the best fit velocity-depth model used for hypocenter locations. These large negative time residuals appear to be the result of propagation paths which have long segments in the colder, higher-velocity subducting plate. P wave travel times were modeled for the effects of the slab using three-dimensional (3-D) ray tracing. Computed ray paths show that travel times to coastal stations for the eastern Peru events can be satisfactorily modeled with average velocities relative to the surrounding mantle 6% lower within the uppermost slab (assumed on the basis of other studies to be unconverted basaltic oceanic crust 6 km thick) and 8% higher within the cold uppermost mantle of the slab. Ray tracing for this plate model shows that P wave ''shadow zones'' can occur if the source-slab-receiver geometry results in seismic rays passing through regions in which the clip angle of the slab changes significantly. Such geometries exist for seismic waves propagating to some coastal stations from intermediate-depth earthquakes located east of the Andes. Observed first-arrival times for such cases do in fact have smaller negative residuals than those for geometries which allow for ''direct'' paths at similar distances. Modeling such arrivals as internally reflected waves propagating through the high-velocity part of the plate produces a significant improvement in the travel time residuals. For the slab velocities given above, we obtain a model thickness of approximately 36 km for the cold slab interior and a slight northwest component of dip in the region of subhorizontal subduction.