Thick-skinned and thin-skinned tectonics of the Zagros orogen, Iran: Constraints from structural, microstructural and kinematics analyses.

Graphical abstract Highlights • Locations and characteristics of five exhumed high-grade hinterland gneissic mylonite shear zone bounding basement nappes that are related to the Zagros thick-skinned tectonics. • Quantitative data on quartz c-axis preferred orientations (LPO), finite strain (R s), kinematic vorticity number (W k) and deformation temperatures. • Construction of the entire crustal-scale balanced and restored cross-sections to illustrate the kinematic evolution of the thick-skinned and thin-skinned tectonics. • The present study leads to a new crustal-scale structural model and subdivisions on both thin-skinned and thick-skinned tectonics of the whole Zagros orogen. Abstract Our understanding of the geological structures and kinematics of thick-skinned and thin-skinned tectonics during the evolution of an orogenic system is strongly related to field evidence of décollement-dominated tectonics or deformation involving the entire middle and lower continental crust. In this paper, we compare thin-skinned and thick-skinned deformation of the Zagros orogen from small to large-scales. In the foreland thin-skinned region sedimentary rocks with several décollement horizons have undergone in-sequence thrusting with dominant NW-striking, NE-dipping and SW-verging displacement. The gneissic basement of the foreland underwent Permo-Triassic rifting and the normal faults were reactivated as thrust or strike-slip faults during subduction-to-collision orogenic system. There is no field exposures of deformed basement windows in this external part of orogen. However, in the internal parts of the orogen, the newly recognized hinterland fold-and-thrust belt and slate belt, there are outcrops of gneissic basement slices, mylonitic shear zones, and granite-gneissic flow. The Tutak granite-gneiss dome is associated with sole thrusts. Therefore, it contains deformed margins of both continents. Thus in deformation in this part of the orogen was thick-skinned. Measurements of quartz c-axis opening angles of the exhumed mylonite nappes document high temperature (∼642°±43° and ∼424°±31 °C). Relationship between kinematic vorticity number (Wk = Wm = 0.76 ± 0.06) of the mylonite nappes which are related to the hinterland fold-and-thrust belt indicate 55 ± 7.8% simple shear and 45 ± 7.4% pure shear components for instantaneous 2D flow which indicate sub-simple shear deformation. On the more thickened side of the Zagros deformed basement-involved contains continuous to discontinuous shear-band boudins of eclogite and slices of blue schist. This provide kinematic indicators suggesting simultaneous dip-slip and strike-slip and shear zones accelerating inclined lateral exhumation of the high-pressure metamorphic rocks of oceanic and continental crustal origin. Both the thin and thick-skinned portions of the Zagros on three major cross sections indicate the intensity of deformation in the NW and SE parts of the belt that display the opposite senses of shear on the two sides. We infer that both senses of shear were developed coevally under a bulk sub-simple shear, regional-scale deformation along a curved inclined transpression. Study of crustal-scale cross sections of the Paleozoic and Mesozoic sequences shows that asymmetrical to symmetrical folding an d thrusting in the central part of the Zagros foreland fold-and-and-thrust belts in the Fars area are gentle in comparison with those in the SE and NW of the orogen. The highest intensity of deformation occurs in the rotated asymmetrical folding and thrusting near the Bandar Abbas Syntaxis. [ABSTRACT FROM AUTHOR]