Steven W. Roecker

Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography

An edited version of this paper was published by the American Geophysical Union. Copyright 2000, AGU. See also: http://www.agu.org/pubs/crossref/2000/2000JB900024.shtml; http://atlas.geo.cornell.edu/morocco/publications/calvert2000.htm

Missing roots and mantle drips : regional Pn and teleseismic arrival times in the southern Sierra Nevada and vicinity, California

Previous seismological studies have placed the source of the uplift of the Sierra either within the crust, suggesting a Mesozoic age for the source of the uplift, or in the upper mantle, consistent with late Cenozoic creation of the buoyant material producing the uplift of the range. We deployed 16 temporary seismometers in the high part of the southern Sierra Nevada to augment the permanent Southern California Seismic Network and record arrivals from regional and teleseismic earthquakes. Arrival times of P waves from 54 teleseisms recorded at these stations are advanced by over a second by a high-velocity body in the upper mantle west and northwest of Lake Isabella. Inversion of the arrival times indicates that this “Isabella anomaly” is of limited north-south extent (about 40–60 km), has compressional velocities about 4–5% higher than its surroundings, and probably extends from about 100 to 200 km depth. The limited north-south extent of the “Isabella anomaly” indicates that it is unrelated to the Sierra; we speculate that it is the downgoing part of a small scale convection system similar to that inferred beneath Southern California. This inversion does not clearly reveal either a large crustal root or a substantial low-velocity body in the upper mantle beneath the Sierra. Although the presence of either degrades the fit to the arrival times and requires high-velocity material beneath the low-velocity material of either root, Bouguer gravity anomalies require low-density material under the Sierra. Assuming that arrival times from earthquakes 150–350 km north and south from the southern Sierra come from a common refractor (the one-layer structure), the upper mantle P wave velocity (P_n) beneath the High Sierra is about 7.6–7.65 km/s; if the arrivals from north and south are from different refractors (the two-layer structure), material with a P wave velocity greater than ∼7.2 km/s (the “7 J.x” layer) would lie under a nearly flat interface from more normal crustal velocities and be separated by a north-dipping interface from underlying mantle with velocities about 7.9–8.1 km/s. The P_n velocity beneath the region immediately to the east is significantly greater (7.9–8.0 km/s) than that of material at equal depths under the Sierra. For the one-layer structure, further assuming that mean crustal velocities are uniform along north-south lines, we find little dip on the Moho in the area; using the arrival times from earthquakes to the south, we infer a depth of 33±5 km for the Moho beneath the southern High Sierra. This structure of a thin to normal crust over a low-velocity mantle can be reconciled with earlier observations that were used to infer a thick crust under the Sierra. By considering the Bouguer gravity anomaly, the surface geology, refraction profiles in this region, and our own observations, we suggest that 1/3 to 1/2 of the modern elevation in the range is supported by lateral (east-west) density contrasts in the crust; the remainder is supported by density contrasts in the uppermost mantle or lateral variations in the thickness of the “7.x” layer. Our interpretation is that the southern Sierra overlies mantle lithosphere that has been thinned and warmed in response to regional lithospheric extension in Neogene time. This part of the upper mantle might have provided the melt that migrated to the east and produced volcanics in the southwestern Great Basin; depletion of the upper mantle might have increased the seismic velocity and decreased the density of material about 60–100 km beneath the southern Sierra.

Deep structure of an arc‐continent collision: Earthquake relocation and inversion for upper mantle P and S wave velocities beneath Papua New Guinea

We examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction, by analyzing the seismicity and upper-mantle velocities in the New Guinea arc-continent collision. Earthquakes in the Papua New Guinea (PNG) region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities, using P and S wave arrival times recorded by seismographs in PNG. A total of 957 well-recorded earthquakes were chosen for use in three-dimensional velocity inversions from an initial catalog of 12,960 events, based on the stability of initial hypocenter relocations. Constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters, and to exploit a priori knowledge of the geometry of velocity anomalies. Spherical geometry was used to accurately calculate long ray paths (up to 1500 km in length). Most of the ∼700 best located hypocenters from the PNG data set are in the upper mantle beneath the Finisterre-Huon (FH) ranges (the newly accreted island arc terrane), beneath the Papuan Peninsula, and in the New Britain seismic zone (an oceanic subduction system). These hypocenters show a well-defined seismic zone dipping vertically or steeply to the north beneath the northern FH ranges from 125- to 250-km depth, continuous along strike with the New Britain seismic zone to the east (which shows earthquakes to 600 km depth). The steeply dipping intermediate-depth seismicity flattens near 100 km depth beneath the FH ranges and forms a subhorizontal seismic zone beneath the FH ranges. By contrast, seismicity continues upward to the surface beneath New Britain where the arc has not yet collided with New Guinea. The 100-km-deep flat zone does not continue south of the FH ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane, and no clear evidence for a south-dipping seismic zone could be found. The relationship of the flat part of the slab 100 km beneath the FH ranges and surface faulting is difficult to discern; the apparent surface suture bounding the south side of the FH ranges is 100 km directly above the southern termination of intermediate-depth seismicity. Nowhere in PNG could clear evidence for arc polarity reversal be found from seismicity. All of the well-located subcrustal earthquakes beneath the Papuan peninsula lie in a narrow horizontal line between 125- and 175-km depth that follows the center of the peninsula, and are distinctly separated from the FH seismic zone to the north. These events do not require a southward subducting plate beneath the Papuan peninsula. An alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere. Information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals. Allowing for only crustal heterogeneity reduces travel time residual variances by 40% from a simple one-dimensional structure, while allowing for additional mantle heterogeneity results in ≤ 10% further reduction for the various parameterization s attempted here. Inversions with several different block geometries show low velocities at 35- to 171-km km depth beneath the Bismarck Sea back-arc basin and high velocities just south of the intermediate depth FH seismic zone. The pattern of lateral heterogeneity supports the inference from seismicity of north-dipping slabs beneath both northeast New Guinea and New Britain.

Two-dimensional seismic image of the San Andreas Fault in the Northern Gabilan Range, central California: Evidence for fluids in the fault zone

A joint inversion for two-dimensional P-wave velocity (Vp), P-to-S velocity ratio (Vp/Vs), and earthquake locations along the San Andreas fault (SAF) in central California reveals a complex relationship among seismicity, fault zone structure, and the surface fault trace. A zone of low Vp and high Vp/Vs lies beneath the SAF surface trace (SAFST), extending to a depth of about 6 km. Most of the seismic activity along the SAF occurs at depths of 3 to 7 km in a southwest-dipping zone that roughly intersects the SAFST, and lies near the southwest edge of the low Vp and high Vp/Vs zones. Tests indicate that models in which this seismic zone is significantly closer to vertical can be confidently rejected. A second high Vp/Vs zone extends to the northeast, apparently dipping beneath the Diablo Range. Another zone of seismicity underlies the northeast portion of this Vp/Vs high. The high Vp/Vs zones cut across areas of very different Vp values, indicating that the high Vp/Vs values are due to the presence of fluids, not just lithology. The close association between the zones of high Vp/Vs and seismicity suggests a direct involvement of fluids in the faulting process.

Elevation of the 410 km discontinuity beneath the central Tien Shan: Evidence for a detached lithospheric root

We analyzed P-SV converted phases recorded by the Kyrgyzstan Broadband Network (KNET) to investigate the nature of phase transitions in the upper mantle beneath the central Tien Shan and the Kazakh shield. We find that P-SV phases from the 410 km discontinuity recorded by several stations located along the range front are about 2 s earlier from beneath the Tien Shan than they are from beneath the Kazach shield. Because the delay times at stations located on the Kazakh shield are normal and seismic velocities in the upper mantle beneath this part of the Tien Shan apparently are low (about 4% less than the Kazakh shield to the north), the early arrival of the conversion from 410 km implies that this discontinuity is elevated by about 20 km. Because the 410 km discontinuity can be elevated by the introduction of cold material, our results are consistent with a model in which the lithosphere in this area developed a root during collision that later detached and is now residing near 410 km depth.

Three-dimensional elastic wave velocity structure of the Hualien region of Taiwan: Evidence of active crustal exhumation

The Hualien region of Taiwan is located at a complex transition of the boundary between the Eurasian and Philippine Sea Plates. To the southwest, the mountains of Taiwan are uplifting rapidly as a consequence of an ongoing arc-continent collision, while to the east the oceanic Philippine Sea Plate is subducting northward beneath Eurasia. We investigated the structure and dynamics of this region by analyzing seismograms of local earthquakes recorded during a deployment of the Portable Array for Numerical Data Acquisition II network. P and S wave velocity structures deduced from travel time tomography analysis show that the collisional suture to the south of Hualien is characterized by a narrow (< 10 km width), near vertically dipping zone of low velocities that extends to depths in excess of 20 km. Velocities in the Eastern Central Range west of the suture zone are significantly higher and define a feature 10–15 km wide that appears to be continuous from the near surface to depths as great as 40 km. Farther to the west beneath the Western Central Range, the velocities again decrease. Focal mechanisms of local earthquakes show that while thrust faulting is the predominate mode of deformation throughout the region, normal faulting occurs as well beneath the Eastern Central Range. Thus the rapid uplift of the mountains of Taiwan may be a result not only of compressional shortening but also of an excess of positive buoyancy. We suggest that the higher velocities and extensional mechanisms in the Eastern Central Range are caused by the ongoing exhumation of previously subducted continental crust, while the lower velocities to the west reflect continued underthrusting of the crust beneath the Eastern Central Range.

Surface wave dispersion across Tibet: Direct evidence for radial anisotropy in the crust

Recordings in western Tibet of Rayleigh and Love waves at periods less than 70 s from aftershocks of the 2008 Sichuan earthquake cannot be matched by an isotropic velocity model beneath Tibet. These intermediate‐period Rayleigh and Love waves require marked radial anisotropy in the middle crust of Tibet, with the vertically polarized S‐waves propagating more slowly than S‐waves with horizontal polarization. The magnitude of anisotropy inferred using paths entirely within Tibet is slightly greater than that obtained previously from a tomographic inversion of a dataset covering a larger region. Anisotropy in the middle crust likely reflects deformation of the middle crust, and is consistent with the notion of mid‐crustal flow and thinning of the crust.

P-wave residuals at stations in Nepal - Evidence for a high velocity region beneath the Karakorum

P-wave residuals recorded at stations in Nepal from events to the northwest and closer than about 20 deg are consistently earlier than those from other directions by about 2.5 sec. These early arrivals are associated with paths confined to the upper 300 km of the earth and suggest that cold material occupies the uppermost mantle beneath the Karakorum, northwest Himalaya, and western Kunlun. Thus, these data suggest that convective downwelling occurs more vigorously in this region than beneath the rest of the Himalaya, Tibet, and their surroundings.

Fine crustal structure beneath the junction of the southwest Tian Shan and Tarim Basin, NW China

The geometry of the entire crust from the northern part of the Tarim Basin to the southwestern Tian Shan east of Kashi is imaged on a N-S–directed explosive-source deep seismic-reflection profile. The profile reflects the sedimentary formations in the northern part of the Tarim Basin and the fold-and-thrust belt of the southern Tian Shan. N-dipping reflectors of the lower crust, as well as fluctuations in Moho depth, below which several mantle reflectors were observed, reveal the fine crustal structure beneath the junction of the southwest Tian Shan and the Tarim Basin. Mesozoic–Cenozoic shortening of the southwestern Tian Shan occurred at a crustal scale involving detachment-related folding in the basin directed northward toward the mountains and reverse faulting in the mountains directed toward the basin. In addition, a crocodile fabric developed within the lower crust beneath the basin area. The lithospheric structure revealed by the seismic-reflection section between the Tarim Basin and the Tian Shan Mountains reflects a process of intracontinental collision.

Deep earthquakes beneath central Taiwan: Mantle shearing in an arc‐continent collision

The island of Taiwan owes its existence to the collision of an island arc on the Philippine Sea Plate with the Eurasian continental shelf. Most of the earthquakes in the mantle beneath Taiwan clearly are related to one of these plates subducting beneath the other, but beneath the central part of the island there is a zone of subcrustal seismicity that is not obviously related to any subduction zone. We investigated the uncertainties in the locations of these anomalous earthquakes by rereading P and S wave arrival times and computing information density functions for several of these events. The seismic zone itself is then defined through a joint probability function incorporating the entire data set. Individual information density functions show that several events are likely to be located at depths exceeding 60 km, with two events exceeding 80 km depth. The joint probability function shows a narrow zone of seismicity, about 20 km wide along strike, plunging to the east. Combining these results with evidence from surface geology, focal mechanisms and subsurface tomography, we suggest that these events are caused by an eastward displacement of the Eurasian mantle north of 24°N which juxtaposes it against colder subducted crust to the south. Moreover, the heat produced by this type of shearing in the mantle must be insignificant for brittle failure to exist at these depths.