Spatio-temporal evolution of intraplate strike-slip faulting: the kuh-e-faghan fault, central Iran

Abstract

Central Iran provides an ideal test site to study the morphotectonic response to enucleation and propagation of intraplate faulting. In this study, a multidisciplinary approach that integrates structural and stratigraphic field investigations with geochronological (optically stimulated luminescence, OSL) and thermochronological (apatite fission track (AFT), (U-Th)/He su apatite (AHe) e ematite (HeHe)) analyses, is used to reconstruct the spatio-temporal evolution of the Kuh-e-Faghan Fault (KFF), in northeaster Central Iran. Structural mapping reveals that KFF defines a narrow, ca. 80 km long, deformation zone consisting of three main broadly left stepping, E-W trending, right-lateral fault strands which cut through the Mesozoic-Paleozoic substratum and the Neogene-Quaternary sedimentary covers. The AHe thermochronology results indicate that the intra-fault blocks along the KFF experienced two major episodes of fault-related exhumation at ~18 Ma and ~4 Ma. The first faulting/exhumation episode at ~18 Ma is chiefly recorded by the Neogene deposits architecture along the KFF. In particular, a source-to-sink scenario can be reconstructed for this time frame, when topographic growth caused the synchronous erosion/exhumation of the pre-Neogene units and deposition of the eroded material in the surrounding fault-bounded continental depocenters. Successively, the KFF gradually entered a period of relative tectonic quiescence and, probably, regional subsidence during which a thick pile of fine-grained onlapping sediments were deposited, caused the resetting of the (U-Th)/He system of the detritic apatite grains hosted both within the pre-Neogene and the basal Neogene successions. AFT dating indicates that the basement units, that are now exposed to the surface, where most probably below the closure temperature (100-120 °C) of the AFT system during the first exhumation event (at ~18 Ma) and, similarly, the AFT system of the pre-Neogene and Neogene deposits have not been reset by Miocene burial (>4 Ma). Together, the AHe and AFT data from the Neogene basin sediments imply burial temperatures in excess of ~60 °C and less than 100 °C. The second faulting episode at ~4 Ma caused the final fault exhumation event, resulting in the current fault zone and topographic architecture. The HeHe ages of hematite coated fault surfaces register both formation ages during the first exhumation event (>6-4Ma) and exhumation ages during the second exhumation event (<6-4 Ma), thus confirming the polyphase history of the KFF system. Collectively the stratigraphic, structural and thermochronological datasets shows that the KFF enucleated in the west and propagated eastward in two punctuated events. Topographic analysis of the KFF reveals that the two fault propagation and exhumation events that shaped the structural and stratigraphic architecture of the fault system can be recognized in the current topographic configuration of the KFF. In particular 10-km wavelength relief corresponding to the ~42 km long western topographic domain and the overall eastward narrowing and decreasing in elevation and relief,~23 km long eastern topographic domain, are identified as the remnant topographic signals of the first (~18 Ma) and second (~4 Ma) exhumation event, respectively. Geomorphological analysis and OSL dating of the KFF Quaternary alluvial deposits show that the generation and development of the deposits where primarily controlled by late Quaternary and Holocene, climate-driven aggradation phases recognized throughout the Iranian plateau by other studies. Nevertheless, their geometric configuration and style of deposition documented along the KFF indicated that they have been strongly influenced by the fault system tectonic activity and the consequent drainage network rearrangement. In particular OSL of quartz grains from faulted alluvial deposits indicates that the KFF is interested by active E-W oriented, Late Pleistocene to Holocene right-lateral tectonics. The morphometric slope analysis of the quaternary alluvial deposits indicates that the KFF is still producing fault related topographic growth. The major outcomes of this study have important implications on (i) understanding the propagation and spatio-temporal evolution of intraplate strike-slip fault systems; (ii) the intraplate response to tectonic reorganization at the collisional boundaries; and (iii) the Neogene-Quaternary kinematic and tectonic evolution of Central Iran. (i) A conceptual model of fault initiation and propagation is proposed. Deformation starts from diffuse deformation accomplished by distributed en-echelon shears, which control initial topographic growth and focused erosion/sedimentation. The strain hardening and velocity-strengthening properties of the syn-tectonic sedimentary deposits progressively inhibits the fault propagation and causes the overall inward migration of the fault zone. This results in an overall localization of the shear deformation along thoroughgoing fault strands and creation of mature fault zones. Strike-slip shear dissipation at eastern fault zone terminations is achieved through the development of trailing extensional imbricate fan, inhibiting further length-ward fault propagation. The extensional/transtensional faulting and the associated tectonically-controlled subsiding depocenter, strongly control the deposits distribution and thicknesses. As a result, the fault systems may then propagate by renewed diffuse shear deformation in undeformed regions at the fault tip, creating new fault strands. (ii) The two fault-related exhumation episodes, at ~18 and ~4 Ma, well fit with the regional early Miocene collision-enhanced uplift/exhumation and the late Miocene–early Pliocene widespread tectonic reorganization of the Turkish-Iranian collision zone, respectively. As suchthe study shows that the long term evolution history of the KFF system in intraplate Iran is characterized by punctuated events nearly coincident with major tectonic events at the plate boundaries. This strongly suggests that the intraplate response along strike-slip fault systems is particularly sensitive to major (far-field) tectonic changes occurring at the plate boundaries, and, as such, intraplate deformation zones can be regarded as a gage for plate-tectonics induced state of stress changes at the plate boundaries. (iii) Results from this study, together with the outcomes from recently published studies, impose a reappraisal of the spatio-temporal kinematic evolution of Central Iran in the aftermath of the Arabia-Eurasia collision history. It is proposed that the Neogene-Quaternary deformation in Iran has been primarily controlled by the spatial and temporal variations in the degree of coupling along the Arabia-Eurasia collision interface. The spatio-temporal changes in the degree of coupling, together with the capacity of accommodating convergence along the collision zone, controlled the amount of northward convergence that was transferred to the intraplate domain and therefore controlled the deformation distribution kinematic configuration in space and time. The revised tectonic/kinematic model needs to be improved and validated by further studies to be carried out along the major fault systems of south-central Iran, in order to verify their persistence, distribution, kinematics and overall space-time evolution.