Hematite (U-Th)/He thermochronometry constrains intraplate strike-slip faulting on the Kuh-e-Faghan Fault, central Iran
Introduction
Strike-slip faulting is the primary process by which horizontal crustal movements are accommodated and stresses are transferred away from a continental collision front into the intraplate domain (Allen et al., 2003; Molnar and Tapponnier, 1975; Nilforoushan et al., 2003; Vernant et al., 2004; Walpersdorf et al., 2014). The development and evolution of intraplate strike-slip fault systems can be punctuated in time and space and, commonly, linked to stress-state changes at the collisional plate boundaries (Calzolari et al., 2016a; Ellis, 1996; Glorie and De Grave, 2016; Spotila et al., 2007; van Hinsbergen et al., 2015; Webb and Johnson, 2006). Constraining the timing, rate, and kinematics of intraplate strike-slip faulting is fundamental to (i) assess the spatio-temporal distribution and modes of accommodation of far-field stresses away from plate boundaries; (ii) gain first-order insight into the dynamic response and stress propagation pathway from plate margins to intraplate domains; and (iii) reconstruct regional-scale tectonic events that may have been partially or totally obliterated along the plate margin domains by intense deformation.
Documenting the timing of brittle faulting in strike slip systems is critical for addressing these issues. This is classically achieved through relative dating approaches such as comparing the stratigraphic age of the faulted units and or by constraining strike-slip fault-related exhumation through bedrock low-temperature thermochronology (e.g., Ehlers and Farley, 2003). Limited geochronological methods exist for direct dating of fault activity. These include 40Ar/39Ar dating of authigenic illite in fault gouge (e.g., Duvall et al., 2011; van der Pluijm et al., 2001) and pseudotachylyte glasses (Di Vincenzo et al., 2012; Magloughlin et al., 2001; Sherlock et al., 2004), zircon fission-track dating of pseudotachylytes (Seward and Sibson, 1985; Tagami and O'Sullivan, 2005), and U-Pb (Nuriel et al., 2017, Nuriel et al., 2012) and U-Th-Pb dating of syntectonic calcite (Serpelloni et al., 2013; Williams et al., 2017). Hematite (U-Th)/He (hematite He) dating of hematite-coated fault surfaces (e.g., Ault et al., 2015; McDermott et al., 2017; Moser et al., 2017) provides an attractive target to resolve the timing and mechanisms of deformation in the brittle upper crust owing to the ubiquity of hematite in these settings, He retention over geologic time scales, and connections between hematite texture and hematite He dates that reflect different deformation processes. For example, hematite He thermochronometry may provide timing constraints on synkinematic hematite mineralization (e.g., Moser et al., 2017) or a record of the post hematite formation thermal history of the evolving fault system.
Owing to its extensive exposure and comparatively young geologic history, the Kuh-e-Faghan fault (KFF) is an ideal fault system to apply hematite He thermochronometry to document the timing and deformation processes associated with intraplate strike-slip faulting. The KFF is a major dextral fault at the northern boundary of the Lut block in central Iran (Fig. 1). The long-term tectonic evolution of the KFF has been interpreted as the intra-plate response to the evolution of the Arabia-Eurasia collision since the Early Miocene (Calzolari et al., 2016a). Prior work implies a punctuated deformation history and the eastward propagation of the KFF (Calzolari et al., 2016a; Calzolari et al., 2016b); however, the timing of this history is poorly resolved.
In this study, we present new hematite He thermochronometry data from a suite of striated, hematite-coated fault surfaces in the fault core and damage zone of the KFF to constrain episodes of fault slip on this major intraplate strike-slip fault system in central Iran. To accurately interpret the hematite He dates, we assess the structurally-controlled paleo-fluid conditions of hematite mineralization; the morphology, grain-size distribution, and textural fabrics of the hematite crystals; and the time-temperature (t-T) history of the host rock from conventional low-temperature thermochronometry. This approach allows us to identify different fault-related processes by discriminating between hematite mineralization and fault system related exhumation. These data refine the timing of faulting linked to Late Miocene-early Pliocene exhumation and regional tectonic reorganization of central Iran. Our results show that hematite He thermochronometry is an effective tool in investigating the spatio-temporal evolution of intracontinental strike-slip fault systems.
Section snippets
Tectonic setting of the Kuh-e-Faghan fault system
The Arabia-Eurasia collision zone is located along the Mesozoic-Cenozoic Alpine-Himalayan convergence zone (Fig. 1). Arabia-Eurasia convergence initiated in the mid-Jurassic (Agard et al., 2011, Agard et al., 2005) and culminated with the closure of the Neotethys ocean and polyphase continental collision (Agard et al., 2005; Bagheri and Stampfli, 2008; Jolivet and Faccenna, 2000; McCall, 1997; Mouthereau et al., 2012; Rossetti et al., 2010, Rossetti et al., 2014; Şengör, 1990; Stampfli and
Materials and methods
This study focuses on the eastern fault strand (EFS) of the KFF, where diffuse hematite mineralization, including hematite-coated fault surfaces, are preserved and the youngest (<5 Ma) fault-related exhumation occurred (Calzolari et al., 2016a). To characterize the structurally controlled paleofluid circulation and hematite mineralization conditions during faulting, we combined field structural analysis of the EFS structure and associated slip surfaces with description of the hematite-bearing
Fault zone architecture, structural analysis, and hematite occurrence
The EFS is a ~40-km-long, sub-vertical fault system comprising several synthetic faults that coalesce to form a curvilinear slip zone and prominent range front. The strike of this system changes from NW-SE to E-W at its eastward termination (Fig. 1). Faults form a decameter-to-hectometer-thick damage zone that consists of numerous mesoscale fault strands, which cut across the pre-Neogene basement units that are now tectonically juxtaposed against the Neogene successions (Fig. 3A and B;
Discussion
The multidisciplinary approach applied in this study refines the spatio-temporal burial, exhumation, and faulting history associated with the propagation of the KFF in intraplate, central Iran. New AFT results provide insight into basin deposition and thermal history. AFT dates from Neogene samples IR13 and IR18 are older than their stratigraphic age, indicating that ambient basin temperatures were insufficient (<100 °C) to fully reset the AFT system during the Neogene evolution of the KFF
Conclusions
We integrate structural and textural analysis with hematite He dating of brittle faults, AFT and apatite He thermochronometry, and thermal history modeling to constrain the timing and style of fault activity along a stike-slip fault system that cannot be readily dated with traditional geochronologic tools. Our new AFT thermochronometry data reveal that detrital apatite grains contained within the basal Neogene units where sourced from the Paleozoic-Mesozoic basement adjacent to Kuh-e-Faghan
Acknowledgements
Special thanks to M.R. Mazinani for assistance during fieldwork. The manager and staff of Khanyeh-e-Moallem of Kashmar are warmly thanked for their kind hospitality. We also thank Ali Rastpour and Hassan Faraji for driving to the field and logistic support. T. Theye is thanked for his advice during chlorite thermometry analysis. We gratefully acknowledge P. Reiners, U. Chowdhury, and E. Abel of the Arizona Radiogenic Helium Dating Laboratory for analytical assistance with the hematite He
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