Imbrication

Abstract: This paper describes the orogenic evolution of the southeast Anatolian orogenic belt based mostly on new geologic data collected from its constituent tectonic units in the course of new mapping programs of the past decade. The southeast Anatolian orogenic segment of the Alpides may be divided into three approximately east-west-trending zones. From south to north, they are the Arabian Platform, followed by a zone of imbrication, and then a zone of nappes. The Arabian Platform includes a mostly marine, sedimentary succession deposited from early Cambrian to middle Miocene time. The zone of imbrication is a narrow belt sandwiched between the Arabian Platform and the zone of nappes. It consists of imbricated thrust slices emplaced onto a Late Cretaceous to early Miocene sequence. The zone of nappes is the highest tectonic unit, consisting of two stacks of nappes simply designated the lower and upper nappes. The lower nappe is represented by the slices of a polyphase metamorpbic ophiolitic assemblage and the Maden Group. The upper nappe rests on the lower nappe and is represented by the Bitlis and Poturge metamorphic massifs. Southeastern Anatolia underwent two major episodes of Alpide deformation. The first occurred during the Late Cretaceous period, when ophiolite was emplaced on the Arabian Platform. This event was not the consequence of a continental collision. The ophiolite obduction onto the Arabian Platform was followed by a regionwide extension and a new marine transgression over the platform immediately after the ophiolite obduction. The second episode of deformation occurred during middle Eocene-Miocene time in two distinct stages as a result of the progressive elimination and complete closure of the ocean(s) which led to the collision between the zone of nappes located to the north (being a part of Eurasia) and the Arabian plate. The Maden marginal basin formed north of the subduction zone that eliminated the ocean between Eurasia and Arabia, along the southern margin of the Taurus belt, and abutted the Arabian Plate much later, during the final collision. The second episode of deformation formed the present orogenic segment leading to the amalgamation of different tectonic units. These amalgamated nappes collided with the Arabian plate and welded onto it at the latest stage of the orogenic evolution during the Miocene epoch.

Abstract: Modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent, dilute stream flow or viscous debris flow. The characteristics of these poorly sorted sediments suggest rapid deposition from high-concentration dispersions but not en masse . Sedimentation thus appears related to high-discharge flows intermediate in sediment/water ratio between stream flow and debris flow. The term “hyperconcentrated flood flow” is proposed for describing this intermediate condition. Hyperconcentrated flood-flow deposits are distinguished from debris-flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification. These deposits are distinguished from normal, dilute stream-flow deposits by lack of cross-stratification in sand facies and by very poor sorting, poor imbrication, and numerous clasts with long axes oriented parallel to flow direction in gravel facies. The horizontal bedding that dominates sandy hyperconcentrated flood-flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper-flow regime and should not be confused with the more familiar thin, graded laminae of fine- to medium-grained sand often associated with parting lineation. Hyperconcentrated flood-flow deposits are not unique to volcanic settings; they also occur in arid, alluvial-fan sequences. Debris-flow and hyperconcentrated flood-flow deposits, however, are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings. In volcanic regions, therefore, these deposits have greater preservation potential, show greater lateral variability, and are more voluminous. Transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water, recently observed at Mount St. Helens, is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow.

Abstract: Field and radiometric data are used to describe and date strain and stress states in southern (longitude 88 o to 91oE, latitude 28 o to 30oN) and western Tibet (longitude 79 o to 82oE, latitude 30 o to 34oN). We factorize deformation into syncollisional and postcollisional, and we present stretching lineation and displacement orientation maps, two sections across the Indian shelf sequence, and stress orientations calculated from mesoscale fault slip data. In southern Tibet, syncollisional stretching and displacement directions trend 90+46 o and displacement is top to south. Synkinematic, low-grade metamorphism is dated at 50 Ma at one locality in the Indian shelf sequence underlying the main mantle thrust of the Indus-Yarlung suture. This implies Paleocene onset of continental collision for the investigated section. Postcollisional structures comprise a "backthrust" group, which includes foreland- and hinterland-directed thrusts, reverse and strike-slip faults, and folds. It dominates postcollisional deformation, is concentrated along the Indus-Yarlung suture, and portrays N-S compression (otrend of 8o+17 o, o2 of 97o+17o). A "strike-slip" group consists of conjugate strike-slip faults, is concentrated in east trending, narrow, highly deformed zones, and indicates that N-S compression is locally compensated by E-W extension (oof 15o+29 o, o3 of 103o+30o). Synkinematic muscovite dates postcollisional deformation as late early Miocene (17.5 Ma) at one locality at the suture. Strike-slip and oblique normal (03 of 600+23 o, oof 144o+21 o) and normal (o3:114o+16 o) faulting, dated between late Miocene and Recent and including active deformation, represents (dominant) E-W and minor N-S extension due to E-W stretching of southern Tibet and oroclinal bending along the Himalayan arc. Restoring syncollisional and postcollisional deformation yields a minimum of 67% (258 km) shortening across the Indian shelf sequence. Incorporating recently published contraction estimates across the eastern Himalaya yields minimum shortening between undeformed India and the Indus-Yarlung suture of 66% (536 km). The Himalaya-Tibet orogenic system south of the Indus-Yarlung suture had an initial width of  811 km in the southern Tibetan section. In western Tibet, imbrication of an ophiolite sequence of the Bangong-Nujiang suture is top to south (stretching lineation trend of 15o+18o), and o3 of active deformation trends ESE. Faulting along the Shiquanhe fault zone, which transfers displacement from the northern part of the Karakorum fault to a system of rifts in western central Tibet, indicates dextral strike-slip alternating with sinistral-oblique normal faulting and block rotations around vertical axes during a prolonged shearing history. The Indian Shelf sequence south of Mount Kailas shows top to south imbrication (stretching lineation trend of 52o+60o). Both Indian shelf rocks and (?Oligocene-Miocene) Kailas conglo- merates record backthrusting and backfolding (oof 33 o) and Recent E-W extension (03 of 85o+28o).

Abstract: Extension in the eastern Alps encompasses the following: (1) Extension accompanying simple shear crustal stacking. Subhorizontal stretching occurs along flat-lying foliations and regionally consistent stretching lineations with associated high strains. On a local scale, stretching results from extrusion of ductile sediments squeezed between rigid basement blocks basement blocks during thrust-sheet imbrication. The overall geometric effect is crustal thickening. (2) Classical extensional tectonics. Extension first occurs as a consequence of accretion and underthrusting of continental and oceanic material and represents gravitational adjustment of an unstable orogenic wedge. Extension then occurs as a consequence of terminal continental collision and is accommodated by lateral extrusion of crustal of crustal blocks. The overall geometric effect is crustal thinning. Extension is generally oriented subparallel to the strike of the orogen.