Foreland thrusting and slab formation in the Pamir

<p>The western and northern sectors of the northward convex Pamir arc are underlain by a steep Benioff zone dipping east to south, traced by earthquakes to depths of 250 km in the southwest and 150 km in the northeast. This slab has been interpreted to indicate intracontinental subduction. However, the convergence accommodated in thrust belts around the western and northern Pamir margins seems to fall short of the values required to produce the observed slab lengths. Delamination models in which the slab only consists of Asian mantle lithosphere avoid that problem but predict shallow asthenosphere beneath the Pamir, conflicting with geophysical evidence. This contradiction is resolved in a forced delamination scenario (Kufner et al. 2016) where indenting/underplating Indian lithosphere forces down and immediately replaces the delaminating Asian lithosphere. In this scenario the formation of the slab would be largely accommodated by south-directed thrust imbrication at crustal level, unrelated to substantial north-vergent thrusting in the Pamir.</p><p>Based on published and our own analyses of foreland thrusting we propose that the formation of the slab does to some extent reflect shortening in the Pamir thrust belts. Thin-skinned shortening in the Tajik basin and the External Pamir further north and east decreases northeastward from 150 to 75 and 30(?) km. The slab lengths show a similar trend. Interpreted mimimum shortening values correspond to 60-50 (20?) percent of the slab length on the same transect. With crustal and lithospheric thicknesses taken from seismological data, 70 km of shortening on a translithospheric thrust fault are sufficient to subduct mafic lower crust to asthenospheric depth and probably induce eclogite formation. Rather than the comparison with slab lengths alone, which may be biased by low estimates of shortening, geometrical relations call for additional slab delamination and rollback towards the foreland. The sedimentary cover stacked in the thin-skinned belts restores to at least tens of km of across-strike (N-S) width, underlain by a subhorizontal to gently dipping basal décollement. Basement-involving faults on the internal borders of the thin-skinned belts such as the Darvaz fault and Main Pamir thrust (MPT) must merge with or flatten into this décollement and thus cannot directly connect to the present-day updip end of the slab via a steeply dipping fault. We hypothesize that the Pamir slab was initiated by a translithospheric thrust fault (MPT and equivalents) around 20 Ma and owes at least half of its length to displacement on these faults and imbrication of the sedimentary cover in their footwalls. Delamination and rollback lengthened the slab and displaced it north- and westward. Mantle lithosphere, not necessarily of Indian affinity, contemporaneously moved in from the southeast, preventing the opening of a lithospheric gap and upwelling of asthenosphere.</p><p> </p><p>Reference:</p><p>Kufner, S. K. et al. (2016). Deep India meets deep Asia: Lithospheric indentation, delamination and break-off under Pamir and Hindu Kush (Central Asia). Earth and Planetary Science Letters, 435, 171-184.</p>

Tectonic evolution of the southeastern part of the Pohorje Mountains (Eastern Alps, Slovenia)

Tectonic evolution of the southeastern part of the Pohorje Mountains (Eastern Alps, Slovenia)Field relations and deformation structures in the southeastern part of the Pohorje Mountains constrain the tectonic evolution of the Austroalpine high-pressure/ultrahigh pressure (HP/UHP) terrane. The Slovenska Bistrica Ultramafic Complex (SBUC) forms a large (ca. 8 × 1 km size) body of serpentinized harzburgite and dunite including minor garnet peridotite and is associated with partly amphibolitized eclogite bodies. The SBUC occurs in the core of an isoclinal, recumbent, northward closing antiform and is mantled by metasedimentary rocks, mostly gneisses and a few marbles, including isolated eclogite/amphibolite lenses. Before this folding, the SBUC formed the deepest part of the exposed terrane. We interpret the SBUC to be derived from near-MOHO, uppermost mantle which was intruded by gabbros in the subsurface of a Permian rift zone. During Cretaceous intracontinental subduction, the SBUC was most likely part of the footwall plate which experienced HP to UHP metamorphism and was folded during exhumation. In the Miocene, the Pohorje Pluton intruded and, subsequently, the metamorphic rocks together with the pluton were deformed probably due to east-west extension and contemporaneous north-south shortening, thus forming an antiformal metamorphic core complex.

Exhumation of high-pressure rocks under continuous compression: a working hypothesis for the southern Hellenides (central Crete, Greece)

Combined kinematic, structural and palaeostress (calcite twinning, fault-slip data) analyses are used to study the exhumation mechanism of the high-pressure rocks exposed on the island of Crete (southern Aegean, Greece). Our study shows that the evolution of windows in central Crete was controlled by two main contractional phases of deformation. The first phase (D1) was related to the ductile-stage of exhumation. NNW–SSE compression during D1 caused layer- and transport-parallel shortening in the upper thrust sheets, resulting in nappe stacking via low-angle thrusting. Synchronously, intracontinental subduction led to high-pressure metamorphism which, however, did not affect the most external parts of the southern Hellenides. Subsequent upward ductile extrusion of high-pressure rocks was characterized by both down-section increase of strain and up-section increase of the pure shear component. The second phase (D2) was associated with the brittle-stage of exhumation. D2 was governed by NNE–SSW compression and involved conspicuous thrust-related folding, considerable tectonic imbrication and formation of a Middle Miocene basin. The major D2-related Psiloritis Thrust cross-cuts the entire nappe pile, and its trajectory partially follows and reworks the D1-related contact between upper and lower (high-pressure) tectonic units. Eduction and doming of the Talea Window was accompanied by gravity sliding of the upper thrust sheets and by out-of-the-syncline thrusting. Late-orogenic collapse also contributed to the exhumation process. Therefore, it seems that the high-pressure rocks of central Crete were exhumed under continuous compression and that the role of extension was previously overestimated.