Crustal Root Beneath the Urals: Wide-Angle Seismic Evidence

Abstract

Wide-angle reflection and refraction data acquired as part of the URSEIS '95 geophysical experiment across the southern Uralide orogen provide evidence for a 12- to 15-kilometer-thick crustal root, yielding a total crustal thickness of 55 to 58 kilometers. Strong reflections from the Mohorovičić discontinuity (Moho) at relatively small precritical distances suggest that the crust-mantle transition beneath the crustal root is a sharp feature. The derived P - and S -wave velocity models constrain key physical properties of the crust, including the depth of the mafic rocks of the Magnitogorsk volcanic arc and the existence of a lower crustal zone of possible basic rock enrichment beneath the East Uralian zone.

A moderate exhumation rate for the high‐pressure Maksyutov Complex, southern Urals, Russia

“…Results of an integrated seismic experiment (URSEIS '95) revealed a 55 km thick crust in the internal parts of the southern Urals and suggests that the collisional architecture is well-preserved and essentially undisturbed by late-or post-orogenic re-equilibration processes Carbonell et al 1996;Echtler et al 1996;Knapp et al 1996). Petrophysical modelling indicates that the lithospheric mantle of the Archaean East European platform did not delaminate beneath the southern Urals and screened the crust from the asthenospheric heat¯ow (Sobolev and Echtler 1997).…”

Role of a phase: Change Moho in stabilization and preservation of the Southern Uralide orogen, Russia

“…This wcll-defined Moho reflection dies out toward the central part of the orogen that is dominated by a zone of diffuse reflectivity (175-300 km distance in Figure 2a). However, the Moho was previously projected to a depth of 60 km and interpreted to represent the base of the crustal root from initial processing of the widc angle data and the downward diminution of the zone of diffuse reflectivity beneath the axis of the orogen [Carbonell et al, 1996;Knapp et al, 1996;Steer et aI., 1998]. …”

“…A recently acquired ~500-km dynamite and vibroseis near-vertical and wide-angle incidence deep seismic reflection profile (U RSEIS) across the Southern U ralides provides an excellent means for investigating the crustal architecture and composition of this orogen through use of crustal-scale balanced sections Carbonell et al, 1996;Echtler et a1., 1996;Knapp et al, 1996] (Figure 1). The Southern Uralides, as shown by the URSEIS profile, constitute a bivergent orogen with highly reflective structurcs within the crust, both in the foreland basin and hinterland ( Figure 1) [e.g., Berzin et al, 1996].…”

“…Earlier geophysical investigations of the Southern Uralides indicated that a pronounced crustal root (10-15 km thick) underlies the orogen [Druzhinin et aI., 1988[Druzhinin et aI., , 1990Thouvenot et aI., 1995;Berzin et aI., 1996;Carbonell et at, 1996;Echtler et aI., 1996;Juhlin et al, 1996;Knapp et al, 1996]. The presence of a Uralian crustal root has long been a subject of controversy since the crust appears to be much thicker than required for the compensation of the subdued topography [Kruise and McNutt, 1988;Doring and GOtze, 1999].…”

“…While the subhorizontal Moho on both sides of the orogen deepens gently toward the central part of the orogen, it loses the pronounced reflective character and cannot be clearly identified on the seismic reflection profile. Previous interpretations of this relationship involved projection of the Moho bound ary to depths of -60 km Carbonell et al, 1996Carbonell et al, , 1998]. More recent analysis of the velocity structure of the crustal root suggests it is characterized by high P-wave velocity (7.7-8.0 km/s) [Druzhinin et al, 1988;Thouvenotet al, 1995;Carbonell et aI., 1998Carbonell et aI., ,2000, and it was interpreted as either remnant of the Paleozoic collision [Kruise and McNutt, 1988] or interlayered sequences of eclogites and peridotites [Carbonell et al, 2000].…”