Stephan V. Sobolev

Subduction and collision processes in the Central Andes constrained by converted seismic phases

The Central Andes are the Earths highest mountain belt formed by ocean–continent collision. Most of this uplift is thought to have occurred in the past 20u2009Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovičić discontinuity—generally thought to separate crust from mantle) ranges from a depth of 75u2009km under the Altiplano plateau to 50u2009km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120u2009km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro–eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120u2009km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10–20u2009km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

Moho topography in the central Andes and its geodynamic implications

P-to-S converted waves at the continental Moho together with waves multiply reflected between the Earth’s surface and the Moho have been used to estimate the Moho depth and average crustal Vp/Vs variations in the central Andes. Our analysis confirms and significantly complements the Moho depth estimates previously obtained from wide-angle seismic studies and receiver functions. The resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins (30 km) further to the east in the Chaco plains. Beneath the Andean plateau, the Moho is deeper in the north (Altiplano) and shallower in the south (Puna), where the plateau attains its maximum elevation. A non-linear relation exists between crustal thickness and elevation (and Bouguer gravity), suggesting that the crust shallower than 50^55 km is predominately felsic in contrast to a predominately mafic crust below. Such a relation also implies a 100 km thick thermal lithosphere beneath the Altiplano and with a lithospheric thinning of a few tens of kilometers beneath the Puna. Absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau. In the Subandean ranges at 19^20‡S, the relation between altitude and crustal thickness indicates a thick lithosphere (up to 130^150 km) and lithospheric flexure. Beneath a relative topographic low at the Salar de Atacama, a thick crust (67 km) suggests that the lithosphere in this region is abnormally cold and dynamically subsided, possibly due to coupling with the subducting plate. This may be related to the strongest (Ms = 8.0) known intra-slab earthquake in the central Andes that happened very close to this region in 1950. The average crustal Vp/Vs ratio is about 1.77 for the Altiplano^Puna and it reaches the highest values (1.80^1.85) beneath the volcanic arc, indicating high ambient crustal temperatures and wide-spread intra-crustal melting. ? 2002 Elsevier Science B.V. All rights reserved.

Mapping the Hawaiian plume conduit with converted seismic waves

The volcanic edifice of the Hawaiian islands and seamounts, as well as the surrounding area of shallow sea floor known as the Hawaiian swell, are believed to result from the passage of the oceanic lithosphere over a mantle hotspot. Although geochemical and gravity observations indicate the existence of a mantle thermal plume beneath Hawaii, no direct seismic evidence for such a plume in the upper mantle has yet been found. Here we present an analysis of compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations on the Hawaiian islands, that indicate a zone of very low shear-wave velocity (< 4u2009kmu2009s -1) starting at 130–140u2009km depth beneath the central part of the island of Hawaii and extending deeper into the upper mantle. We also find that the upper-mantle transition zone (410–660u2009km depth) appears to be thinned by up to 40–50u2009km to the south-southwest of the island of Hawaii. We interpret these observations as localized effects of the Hawaiian plume conduit in the asthenosphere and mantle transition zone with excess temperature of ∼300u2009°C. Large variations in the transition-zone thickness suggest a lower-mantle origin of the Hawaiian plume similar to the Iceland plume, but our results indicate a 100u2009°C higher temperature for the Hawaiian plume.

Seismic imaging of a convergent continental margin and plateau in the central Andes (Andean Continental Research Project 1996 (ANCORP'96))

[1]xa0A 400-km-long seismic reflection profile (Andean Continental Research Project 1996 (ANCORP96)) and integrated geophysical experiments (wide-angle seismology, passive seismology, gravity, and magnetotelluric depth sounding) across the central Andes (21°S) observed subduction of the Nazca plate under the South American continent. An east dipping reflector (Nazca Reflector) is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km. We interpret parts of the Nazca Reflector as a fluid trap located at the front of recent hydration and shearing of the mantle, the fluids being supplied by dehydration of the oceanic plate. Patches of bright (Quebrada Blanca Bright Spot) to more diffuse reflectivity underlie the plateau domain at 15–30 km depth. This reflectivity is associated with a low-velocity zone, P to S wave conversions, the upper limits of high conductivity and high Vp/Vs ratios, and to the occurrence of Neogene volcanic rocks at surface. We interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during Neogene shortening and plateau growth. The imaging properties of the continental Moho beneath the Andes indicate a broad transitional character of the crust-mantle boundary owing to active processes like hydration of mantle rocks (in the cooler parts of the plate margin system), magmatic underplating and intraplating under and into the lowermost crust, mechanical instability at Moho, etc. Hence all first-order features appear to be related to fluid-assisted processes in a subduction setting.

Rift migration explains continental margin asymmetry and crustal hyper-extension.

When continents break apart, continental crust and lithosphere are thinned until break-up is achieved and an oceanic basin is formed. The most remarkable and least understood structures associated with this process are up to 200u2009km wide areas of hyper-extended continental crust, which are partitioned between conjugate margins with pronounced asymmetry. Here we show, using high-resolution thermo-mechanical modelling, that hyper-extended crust and margin asymmetry are produced by steady state rift migration. We demonstrate that rift migration is accomplished by sequential, oceanward-younging, upper crustal faults, and is balanced through lower crustal flow. Constraining our model with a new South Atlantic plate reconstruction, we demonstrate that larger extension velocities may account for southward increasing width and asymmetry of these conjugate magma-poor margins. Our model challenges conventional ideas of rifted margin evolution, as it implies that during rift migration large amounts of material are transferred from one side of the rift zone to the other.

Modeling suggests that oblique extension facilitates rifting and continental break‐up

[1]xa0In many cases the initial stage of continental break-up was and is associated with oblique rifting. That includes break-up in the Southern and Equatorial Atlantic, separation from eastern and western Gondwana as well as many recent rift systems, like Gulf of California, Ethiopia Rift and Dead Sea fault. Using a simple analytic mechanical model and advanced numerical, thermomechanical modeling techniques we investigate the influence of oblique extension on the required tectonic force in a three-dimensional setting. While magmatic processes have been already suggested to affect rift evolution, we show that additional mechanisms emerge due to the three-dimensionality of an extensional system. Focusing on non-magmatic rift settings, we find that oblique extension significantly facilitates the rift process. This is due to the fact that oblique deformation requires less force in order to reach the plastic yield limit than rift-perpendicular extension. The model shows that in the case of two competing non-magmatic rifts, with one perpendicular and one oblique to the direction of extension but otherwise having identical properties, the oblique rift zone is mechanically preferred and thus attracts more strain.

Anatomy of the Dead Sea Transform from lithospheric to microscopic scale

Fault zones are the locations where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades, our knowledge of their geometry, petrophysical properties, and controlling processes remains incomplete. The central questions addressed here in our study of the Dead Sea Transform (DST) in the Middle East are as follows: (1) What are the structure and kinematics of a large fault zone? (2) What controls its structure and kinematics? (3) How does the DST compare to other plate boundary fault zones? The DST has accommodated a total of 105 km of left-lateral transform motion between the African and Arabian plates since early Miocene (similar to 20 Ma). The DST segment between the Dead Sea and the Red Sea, called the Arava/Araba Fault (AF), is studied here using a multidisciplinary and multiscale approach from the mu m to the plate tectonic scale. We observe that under the DST a narrow, subvertical zone cuts through crust and lithosphere. First, from west to east the crustal thickness increases smoothly from 26 to 39 km, and a subhorizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none have kilometer-size zones of decreased seismic velocities or zones of high electrical conductivities in the upper crust expected for large damage zones. Third, the AF is the main branch of the DST system, even though it has accommodated only a part (up to 60 km) of the overall 105 km of sinistral plate motion. Fourth, the AF acts as a barrier to fluids to a depth of 4 km, and the lithology changes abruptly across it. Fifth, in the top few hundred meters of the AF a locally transpressional regime is observed in a 100-300 m wide zone of deformed and displaced material, bordered by subparallel faults forming a positive flower structure. Other segments of the AF have a transtensional character with small pull-aparts along them. The damage zones of the individual faults are only 5-20 m wide at this depth range. Sixth, two areas on the AF show mesoscale to microscale faulting and veining in limestone sequences with faulting depths between 2 and 5 km. Seventh, fluids in the AF are carried downward into the fault zone. Only a minor fraction of fluids is derived from ascending hydrothermal fluids. However, we found that on the kilometer scale the AF does not act as an important fluid conduit. Most of these findings are corroborated using thermomechanical modeling where shear deformation in the upper crust is localized in one or two major faults; at larger depth, shear deformation occurs in a 20-40 km wide zone with a mechanically weak decoupling zone extending subvertically through the entire lithosphere.

Major crustal features between the Harz Mountains and the Baltic Shield derived from receiver functions.

Major crustal features between the Harz Mountains and the Baltic Shield derived from receiver functions.

Viscosity in transition zone and lower mantle: Implications for slab penetration

[1]xa0The evolution of a subducting slab is strongly influenced by the viscosity of the mantle which it overlies. However, there is still no consensus about the viscosity in the transition zone and shallower lower mantle. We use a numerical self-consistent subduction model and run a set of experiments in order to find critical patterns of viscosity that would allow the evolution of the different subduction styles that can actually be found in nature. Our results show that a jump in viscosity of ∼5 times from transition zone to lower mantle gives the most reasonable results. Optimal values of viscosity in the transition zone are in the range of 3 * 1020–1021 Pa s. Higher values would produce piling up of the slab and later sinking or even slab flattening. Lower viscosities result in velocities (>30 cm/yr) that are too high, while the new slab subducts through the upper mantle and transition zone, a phenomenon that is rarely seen in nature. Reduction of the Clapeyron slope, related to the spinel-perovskite transition, variations in oceanic crustal thickness and in the age of the slab do not influence much the style of subduction. When overriding velocity is applied to the upper plate, the previously penetrating slab tends to lay at the 660 km boundary but does not substantially change the subduction velocity.

Seismic observation of narrow plumes in the oceanic upper mantle

[1]xa0We have found seismic evidence for the existence of narrow (about 200 km diameter) and hot plumes in the upper mantle above the 410 km discontinuity. They are located mainly under ocean islands, not however, under the central parts of major continents. These results are obtained from comparing globally the observed travel time differences between Ps conversions and SS precursors from the discontinuity at 410 km depth, using published data and new observations from ocean island stations. Comparison of SS precursors and Ps conversions, however, leads also to indications of significantly fewer narrow plumes in the upper mantle transition zone between 410 and 660 km depth, than in the upper mantle above 410 km depth.