Beatrice de Voogd

Spatial distribution of hotspot material added to the lithosphere under La Réunion, from wide-angle seismic data

Wide-angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three-dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of La Reunion. The submarine part of the edifice has fairly low seismic velocities, without evidence for intrusives. An upper unit with a velocity-depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope. Between this unit and the top of the oceanic plate, imaged by normal incidence seismic reflection, a more homogeneous unit indicated by shadow zones on several wide-angle sections may correspond to lavas of a different nature, extruded underwater in the earlier phase of volcanism. Coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part, with limited evidence for some intracrustal intrusion. Deeper in the lithosphere, the presence of a layer of intermediate velocity between the crust and mantle is firmly established. It is interpreted as resulting from the advection of hotspot magmatic products, possibly partially molten, and of a composition for which the crust is a density barrier. The extensive wide-angle coverage constrains the extent of this body. It does not show the elongated shape expected from plate drift above a steady hotspot supply. Alternative propositions can hence be considered, for example, that La Reunion is caused by a solitary wave of hotspot material or by a young hotspot. The size of the underplate, 140 km wide and up to 3 km thick, corresponds to less than half the volume of the edifice on top of the plate.

Perturbation to the lithosphere along the hotspot track of La Réunion from an offshore-onshore seismic transect

A 250 km long NE-SW lithospheric transect spanning the 40 km wide island of La Reunion and its submarine edifice is derived from lines of air gun shots at sea on either side, along the assumed hotspot trace. Seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers, providing a system of reversed and overlapping observations. Low seismic velocity, and hence density, is found on average for the whole edifice above the oceanic plate. We attribute high- velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island-crossing segment. Unexpectedly, the main seismic interfaces, top and bottom of the prevolcanic crust, do not show significant flexural downwarping under the island. In addition, clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle. This lithospheric structure provides the first example where underplating occurs beneath an active volcanic island, suggesting a genetic relationship. The underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts. Evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity. In the plate/hotspot model commonly assumed, the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material.

Crustal extension and magmatic processes: COCORP profiles from Death Valley and the Rio Grande rift

New crustal-scale information on the interaction between normal faulting and magmatic activity is provided by recent COCORP deep seismic reflection profiles in Death Valley, California, and by reprocessing of the COCORP data from the Socorro area of the Rio Grande rift, New Mexico. The most striking feature on the seismic sections from these areas is a prominent, subhorizontal series of reflectors at mid-crustal depth. Previous studies have suggested that thin tabular magma bodies lie within the mid-crustal reflective zones. In addition, because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the COCORP data, these mid-crustal horizons are here inferred to be detachments or zones of tectonic decoupling. Upper-crustal Cenozoic faults do not appear to penetrate deeper than 15 km in Death Valley and 13 km in the Rio Grande rift. In Death Valley, these faults are relatively planar, with moderate dips (20° to 35°), and appear to bound large basement rocks. One such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central Death Valley to the surface location of a 690,000-yr-old basaltic cinder cone. Listric and low- to moderate-angle normal faults are evident on the reprocessed New Mexico data and constitute the structural component of upper-crustal extension. In particular, a listric master fault traceable to depths as great as 13 km is inferred to underlie the Albuquerque basin. Unlike the Death Valley data, no faults are observed to merge with the Socorro magma body per se. Rather, subhorizontal to moderately west-dipping packages of reflections are imaged between the base of the faulted upper crust (13 km depth) and the mid-crustal magma body (about 20 km depth). The middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion. Events seen in the middle and lower crust are generally subhorizontal, and prominent layering is observed. A band of reflections attributed to the crust-mantle boundary is evident on most seismic sections. The upper mantle appears seismically transparent. On some of the profiles, the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections, at least one of which is a layer of magma. This association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes.

Modes of raising northeastern Tibet probed by explosion seismology

Abstract New wide-angle reflection and refraction seismic data provide constraints on the structure of the upper lithosphere, and test models of its evolution to raise the northeastern part of the Tibetan Plateau. Amplitudes observed for reflections from the crust–mantle boundary are sufficiently large to suggest that there is no significant partial melt in the deep crust. The data show an increase of the crustal thickness between terranes from north of the Kun Lun Fault into the Qang Tang of central Tibet, and a contrast among their intracrustal images and compositions. In the north, P and S velocities are consistent with a dominantly felsic composition and show that only the upper crust thickened. South of the Kun Lun Fault a thicker crust made of two layers could result from the superposition of the originally thin crust of the Bayan Har terrane on the lower part of the crust of the domain to the north, which upper crust it shoved and thickened. Different modes of crustal thickening, either by thickening of individual layers or superpositions and imbrication among them appear to work jointly to raise the topography.

Crustal Structure of Ouachita Mountains, Arkansas: A Model Based on Integration of COCORP Reflection Profiles and Regional Geophysical Data

COCORP deep seismic reflection profiles across the Ouachita Mountains in western Arkansas suggest that a large fraction of the crust in this region is composed of tectonically thickened Paleozoic sediments (and metasediments). Reflections representing a southward-thickening wedge of layered rock on the northern portions of the survey are associated with approximately 12 km (39,000 ft) of Carboniferous flysch overlying thin, lower to middle Paleozoic shelf strata in the Frontal thrust zone. Toward the interior of the mountain belt, the Benton uplift is a broad antiform, apparently cored by crystalline basement at depths below 7 km (23,000 ft). Beneath the southern Ouachitas and the adjacent Gulf coastal plain, a zone of south-dipping reflections probably represents at leas 14 km (46,000 ft) of tectonically thickened, lower to middle Paleozoic off-shelf strata and Carboniferous flysch. Regional Bouguer gravity data show a minimum coincident with the thickest accumulation of flysch in the Frontal thrust zone. To the south, the Benton uplift lies on a steep gravity gradient which is continuous along most of the Ouachita trend and which may be analogous to a gradient observed along the Appalachian chain. The Ouachita gravity signature can be modeled as a southward shallowing of the Moho (from 40 km [131,000 ft] in northern Arkansas to about 30 km [98,000 ft] just south of the Ouachitas), coincident with the tectonic thickening of the Paleozoic strata interpreted from the COCORP data. The resulting crustal section can be interpreted as the remnants of an early Paleozoic passive margin which was subducted beneath a thick accretionary wedge in Carboniferous time. The Bent n uplift is viewed as a late-stage involvement of crystalline basement in foreland thrusting as the margin entered the south-dipping subduction zone.

Death Valley bright spot: A midcrustal magma body in the southern Great Basin, California?

A previously unrecognized midcrustal magma body may have been detected by COCORP deep seismic reflection profiles in the Death Valley region of the southern Great Basin. High-amplitude, relatively broad-band reflections at 6 s (15 km) are attributed to partially molten material within a subhorizontal intrusion. This “bright spot” extends laterally at least 15 km beneath central Death Valley. A moderately dipping normal fault can be traced from the inferred magma chamber upward to a 690 000-yr-old basaltic cinder cone. The fault zone is inferred to have been a magma conduit during the formation of the cinder cone. Vertical variations in crustal reflection character suggest that the Death Valley magma body may have been emplaced along a zone of decoupling that separates a faulted brittle upper crust from a more ductile and/or intruded lower crust. The Death Valley bright spot is similar to reflections recorded by COCORP in 1977 in the Rio Grande rift, where both geophysical and geodetic evidence support the inference of a tabular magma chamber at 20-km depth.

Vertical movements and material transport during hotspot activity: Seismic reflection profiling offshore La Réunion

The structure of the submerged part of the La Reunion hotspot island is determined by a grid of multichannel seismic reflection profiles. The submarine part of the edifice appears as a poorly stratified wedge of material lying above a significant thickness of preexisting sediments and the oceanic basement. The dense data coverage has allowed us to derive contour maps of the top of the basement and of the base of the volcaniclastic edifice, further constrained by coincident wide-angle profiles. The resulting isobath maps reveal new, unsuspected features that could not be deduced from observation along a single seismic line since the geometry of these horizons varies significantly from one radial profile to the next. Both maps show a large degree of heterogeneity in the topography, with no axial nor cylindrical symmetry, indicating that plate flexure is not dominant. A slight depression toward the island is observed only in the southern area, ahead of the hotspot trace. The lack of angular unconformity in the volcano-sedimentary pile that covers the oceanic basement firmly establishes the lack of significant vertical movement and flexure. The base of the edifice is roughly domed, centered on the island, with several topographic highs or lows superimposed. The submarine apron appears as a composite constructional body, spreading by slumping of its flanks. Superficial lenses of laterally transported material are observed on the seismic data south of the island, not only to the east of the active Piton de la Fournaise volcano. Oceanic sediments trapped beneath the apron seem undeformed.