Surface wave tomography, using the fundamental Rayleigh wave velocities and those of higher modes between 1 and 4 and periods between 50 and 160 s, is used to image structures with a horizontal ...resolution of ∼250 km and a vertical resolution of ∼50 km to depths of ∼300 km in the mantle. A new model, PM_v2_2012, obtained from 3×106 seismograms, agrees well with earlier lower resolution models. It is combined with temperature estimates from oceanic plate models and with pressure and temperature estimates from the mineral compositions of garnet peridotite nodules to generate a number of estimates of SV(P,T) based on geophysical and petrological observations alone. These are then used to estimate the unrelaxed shear modulus and its derivatives with respect to pressure and temperature, which agree reasonably with values from laboratory experiments. At high temperatures relaxation occurs, causing the shear wave velocity to depend on frequency. This behaviour is parameterised using a viscosity to obtain a Maxwell relaxation time. The relaxation behaviour is described using a dimensionless frequency, which depends on an activation energy E and volume Va. The values of E and Va obtained from the geophysical models agree with those from laboratory experiments on high temperature creep. The resulting expressions are then used to determine the lithospheric thickness from the shear wave velocity variations. The resolution is improved by about a factor of two with respect to earlier models, and clearly resolves the thick lithosphere beneath active intracontinental belts that are now being shortened. The same expressions allow the three dimensional variations of the shear wave attenuation and viscosity to be estimated.
•A Vs model is obtained from Rayleigh waves from 3×106 earthquakes.•The model has resolutions of ∼300×∼30 km horizontally and vertically.•Temperature, viscosity and attenuation are calculated from Vs.•The activation energy and volume of the viscosity agree with laboratory values.•Active Asiatic mountain belts have lithospheric thicknesses of 200 km or more.
Surface wave tomography using Rayleigh waves has shown that Tibet and the surrounding mountain ranges that are now being shortened are underlain by thick lithosphere, of similar thickness to that ...beneath cratons. Both their elevation and lithospheric thickness can result from pure shear shortening of normal thickness continental lithosphere by about a factor of two. The resulting thermal evolution of the crust and lithosphere is dominated by radioactive decay in the crust. It raises the temperature of the lower part of the crust and of the upper part of the lithosphere to above their solidus temperatures, generating granites and small volumes of mafic alkaline rocks from beneath the Moho, as well as generating high temperature metamorphic assemblages in the crust. Thermal models of this process show that it can match the P, T estimates determined from metamorphic xenoliths from Tibet and the Pamirs, and can also match the compositions of the alkaline rocks. The seismological properties of the upper part of the lithosphere beneath northern Tibet suggest that it has already been heated by the blanketing effect and radioactivity of the thick crust on top. If the crustal thickness is reduced by erosion alone to its normal value at low elevations, without any tectonic extension, over a time scale that is short compared to the thermal time constant of thick lithosphere, of ∼250 Ma, thermal subsidence will produce a basin underlain by thick lithosphere. Though this simple model accounts for the relevant observations, there is not yet sufficient information available to be able to model in detail the resulting thermal evolution of the sediments deposited in such cratonic basins.
•How do cratonic basins form?•Their slow subsidence suggests by lithospheric cooling.•Thick lithosphere cannot be heated by extension.•Shortening can heat thick lithosphere.•Erosion then leads to cooling and subsidence.
Azimuthal anisotropy derived from multimode Rayleigh wave tomography in China exhibits depth‐dependent variations in Tibet, which can be explained as induced by the Cenozoic India‐Eurasian collision. ...In west Tibet, the E‐W fast polarization direction at depths <100 km is consistent with the accumulated shear strain in the Tibetan lithosphere, whereas the N‐S fast direction at greater depths is aligned with Indian Plate motion. In northeast Tibet, depth‐consistent NW‐SE directions imply coupled deformation throughout the whole lithosphere, possibly also involving the underlying asthenosphere. Significant anisotropy at depths of 225 km in southeast Tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the Himalaya and Burma, respectively. The multilayer anisotropic surface wave model can explain some features of SKS splitting measurements in Tibet, with differences probably attributable to the limited back azimuthal coverage of most SKS studies in Tibet and the limited horizontal resolution of the surface wave results.
Key Points
Surface wave tomography revealed depth‐dependent azimuthal anisotropy in Tibet
The dominant fast direction is EW at shallower depths and NS at greater depths
The surface wave model can predict some features of SKS splitting measurements
The existence of subcrustal continental earthquakes beneath the Alpine-Himalayan Belt was recognised more than 60 years ago. There is general agreement that most of those beneath the western part of ...the belt in the Mediterranean result from the subduction of oceanic lithosphere. There is less agreement about the origin of those beneath Vrancea in Romania, the Hindu Kush, and the Pamir. Because there is little evidence for the former existence of oceanic lithosphere beneath these regions, many authors have argued that these seismic zones result from the separation of the mantle part of the continental lithosphere from the crust before it sinks into the mantle. However, this model has become steadily less satisfactory. Detailed studies of the depth of earthquakes beneath all stable regions of continents have shown that substantial subcrustal earthquakes, with magnitudes greater than 5.5, are rare. We show that this distribution is controlled by temperature, with material hotter than ∼600 °C being aseismic. This simple rule accounts for the distribution of almost all earthquakes in oceanic and continental lithosphere, including those in subduction zones. We argue that the subcrustal continental earthquakes must also result from the subduction of oceanic lithosphere. This proposal is not new but has generally been dismissed because of the lack of surface geological evidence that suitable pieces of oceanic lithosphere existed. However, the depth distribution of continental earthquakes makes it steadily harder to avoid.
The shear wave velocity
V
s
as a function of depth
z can be obtained from surface wave tomography, using the phase velocities of fundamental and higher mode Rayleigh waves. Since
V
s
is principally ...controlled by temperature, rather than by composition, it can be used to map the lithospheric thickness. Extensive regions of thick lithosphere underlie some, but not all, cratons. Conversely, thick lithosphere underlies some platforms and belts of active deformation. Because of this lack of correspondence, and because their age cannot be determined from seismology, we refer to regions of thick lithosphere as ‘cores’ rather than ‘cratons’. The shape of such cores has controlled the geometry of continental deformation and the distribution of diamond-bearing kimberlites. The strength of the cores resides in the dry crust, which is insulated from the hot convecting mantle by the thick buoyant lithosphere. The most surprising feature is the presence of thick lithosphere beneath Tibet and Iran, whose velocity structure closely resembles that of the cores beneath cratons, though they have a thicker hotter crust. Tibet and Iran appear to be places where cratons are now being formed.
SUMMARY
This paper is concerned with the implications of earthquake depth distributions in the Himalayan–Tibetan collision zone for the general understanding of lithosphere rheology. In particular, ...recent studies have argued that microearthquakes in the uppermost mantle beneath Nepal and some earthquakes at 80–90 km depth, close to the Moho in SE and NW Tibet, reinforce the conventional view of the last 25 yr that the continental lithosphere is well represented by strong seismogenic layers in the upper crust and uppermost mantle, separated by a weak and aseismic lower crust. That view was recently challenged by an alternative one suggesting that the continental lithosphere contained a single strong seismogenic layer, that was either the upper crust or the whole crust, but did not involve the mantle. We re‐examine the seismic structure and seismicity of the Himalayan–Tibetan collision zone, recalculating earthquake depths in velocity structures that are consistent with seismic receiver functions and surface wave dispersion studies, and calculating a geotherm for the Indian Shield consistent with kimberlite nodule geochemistry. Earthquakes occur throughout the crustal thickness of the Indian Shield, where the lower crust is thought to consist of dry granulite, responsible for its seismogenic behaviour and strength as manifested by its relatively large effective elastic thickness. The crust of the Indian Shield is thin (∼35 km) for an Archean shield, and this, in turn, leads to a steady‐state Moho temperature that could be as low as ∼500 °C. When this shield is thrust beneath the Himalaya in Nepal, the relatively low mantle temperature, together with the high strain rates associated with it adopting a ‘ramp‐and‐flat’ geometry, may be responsible for the mantle microearthquakes that accompany other earthquakes in the lower crust. Further north, the upper crust of India south of the Indus Suture Zone has been removed, the uppermost lower crust of India has heated up, and seismicity is restricted to a few earthquakes very close to the Moho at 80–90 km, where errors in Moho and earthquake depth determinations make it unclear whether these events are in the crust or mantle. A similar situation exists in NW Tibet beneath the Kunlun, where earthquakes at 80–90 km depth occur very close to the Moho. Both places are about 400 km north of the Himalayan front, and we suspect both represent the minimum distance India has underthrust Tibet, so that India underlies most of the SE and nearly all of the NW Tibetan plateau. The distribution of earthquake depths throughout the region is consistent with a generic global view of seismicity in which earthquakes occur in (1) ‘wet’ upper crustal material to a temperature of ∼350 °C, or (2) higher temperatures in dry granulite‐facies lower crust or (3) mantle that is colder than ∼600 °C.
Over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength, structure and evolution that produce dramatic contrasts ...between the geological histories of oceans, ancient shields and young orogenic belts. Those developments involve a wide range of observations from a great diversity of geological, geophysical and geochemical disciplines that, none the less, provide a mutually consistent and coherent overall picture. This paper summarizes, in one place, the essential stages in the evolution of the relevant ideas and observations that have led to this situation.
We carry out a joint inversion of surface wave dispersion curves and teleseismic shear wave arrival times across the Tibetan collision zone, from just south of the Himalaya to the Qaidam Basin at the ...northeastern margin of the plateau, and from the surface to 600 km depth. The surface wave data consist of Rayleigh-wave group dispersion curves, mainly in the period range from 10 to 70 s, with a maximum of 2877 source–receiver pairs. The body wave data consist of more than 8000 S-wave arrival times recorded from 356 telesesmic events. The tomographic images show a ‘wedge’ of fast seismic velocities beneath central Tibet that starts underneath the Himalaya and reaches as far as the Bangong–Nujiang Suture (BNS). In our preferred interpretation, in central Tibet the Indian lithosphere underthrusts the plateau to approximately the BNS, and then subducts steeply. Further east, Indian lithosphere appears to be subducting at an angle of ∼45°. We see fast seismic velocities under much of the plateau, as far as the BNS in central Tibet, and as far as the Xiangshuihe–Xiaojiang Fault in the east. At 150 km depth, the fast region is broken by an area ∼300 km wide that stretches from the northern edge of central Tibet southeastwards as far as the Himalaya. We suggest that this gap, which has been observed previously by other investigators, represents the northernmost edge of the Indian lithosphere, and is a consequence of the steepening of the subduction zone from central to eastern Tibet. This also implies that the fast velocities in the northeast have a different origin, and are likely to be caused by lithospheric thickening or small-scale subduction of Asian lithosphere. Slow velocities observed to the south of the Qaidam suggest that the basin is not subducting. Finally, we interpret fast velocities below 400 km as subducted material from an earlier stage of the collision that has stalled in the transition zone. Its position to the south of the present subduction is likely to be due to the relative motion of India to the northeast.
The Zagros of Iran form one of the youngest collisional orogenic belts on Earth. At shallow depths, shortening across the Zagros is accommodated by folding in the sediments, high‐angle thrust ...faulting in the basement and thickening of the lower crust, but how shortening is accommodated by the lithospheric mantle has been uncertain largely because the upper mantle seismic structure has been poorly known. We map the lateral variations in upper mantle shear wave speed beneath this region using a large, multimode surface wave data set. The upper mantle is slow for most of the Middle East, but a high shear wave speed lid extending to ∼225 km depth exists beneath the Zagros. We use aT(Vs, z) relation to convert the shear wave speed profiles to temperature profiles and fit these with geotherms to identify the base of the lithosphere. The upper mantle temperatures from the seismic model are consistent with temperatures derived from geochemical modeling. The lithosphere is less than ∼120 km thick over the region except for a thick lithospheric root beneath the Zagros, implying that shortening in the mantle is accommodated by lithospheric thickening. The composition of the volcanic rocks from above the area of the thickened lithosphere has depleted magma source regions with densities ∼60 kg m−3 less than the MORB source. Elsewhere in the Middle East the volcanic source regions have compositions and densities similar to that of MORB. The shortening across the Zagros is accommodated by lithospheric thickening but the cool thickened lithosphere has been stabilized from delamination by depletion.
Key Points
Map lateral variations in upper mantle shear‐wavespeed beneath the Middle East
Convert velocity to temperature profiles to identify base of lithosphere
Cool thickened Zagros lithosphere stabilized from delamination by depletion
The Tien Shan is the largest active intracontinental orogenic belt on Earth. To better understand the processes causing mountains to form at great distances from a plate boundary, we analyse passive ...source seismic data collected on 40 broad-band stations of the MANAS project (2005–2007) and 12 stations of the permanent KRNET seismic network to determine variations in crustal thickness and shear wave speed across the range. We jointly invert P- and S-wave receiver functions with surface wave observations from both earthquakes and ambient noise to reduce the ambiguity inherent in the images obtained from the techniques applied individually. Inclusion of ambient noise data improves constraints on the upper crust by allowing dispersion measurements to be made at shorter periods. Joint inversion can also reduce the ambiguity in interpretation by revealing the extent to which various features in the receiver functions are amplified or eliminated by interference from multiples. The resulting wave speed model shows a variation in crustal thickness across the range. We find that crustal velocities extend to ∼75 km beneath the Kokshaal Range, which we attribute to underthrusting of the Tarim Basin beneath the southern Tien Shan. This result supports the plate model of intracontinental convergence. Crustal thickness elsewhere beneath the range is about 50 km, including beneath the Naryn Valley in the central Tien Shan where previous studies reported a shallow Moho. This difference apparently is the result of wave speed variations in the upper crust that were not previously taken into account. Finally, a high velocity lid appears in the upper mantle of the Central and Northern part of the Tien Shan, which we interpret as a remnant of material that may have delaminated elsewhere under the range.
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