Neogene, syn‐collisional extensional exhumation of Asian lower–middle crust produced the Shakhdara–Alichur gneiss‐dome complex in the South Pamir. The <1 km‐thick, mylonitic–brittle, top‐NNE, normal‐sense Alichur shear zone (ASZ) bounds the 125 × 25 km Alichur dome to the north. The Shakhdara dome is bounded by the <4 km‐thick, mylonitic–brittle, top‐SSE South Pamir normal‐sense shear zone (SPSZ) to the south, and the dextral Gunt wrench zone to its north. The Alichur dome comprises Cretaceous granitoids/gneisses cut by early Miocene leucogranites; its hanging wall contains non/weakly metamorphosed rocks. The 22–17 Ma Alichur‐dome‐injection‐complex leucogranites transition from foliation‐parallel, centimeter‐ to meter‐thick sheets within the ASZ into discordant intrusions that may comprise half the volume of the dome core. Secondary fluid inclusions in mylonites and mylonitization‐temperature constraints suggest Alichur‐dome exhumation from 10–15 km depth. Thermochronologic dates bracket footwall cooling between ~410–130 °C from ~16–4 Ma; tectonic cooling/exhumation rates (~42 °C/Myr, ~1.1 km/Myr) contrast with erosion‐dominated rates in the hanging wall (~2 °C/Myr, <0.1 km/Myr). Dome‐scale boudinage, oblique divergence of the ASZ and SPSZ hanging walls, and dextral wrenching reflect minor approximately E–W material flow out of the orogen. We attribute broadly southward younging extensional exhumation across the central South Pamir between ~20–4 Ma to: (i) Mostly northward, foreland‐directed flow of hot crust into a cold foreland during the growth of the Pamir orocline; and (ii) Contrasting effects of basal shear related to underthrusting Indian lithosphere, enhancing extension in the underthrust South Pamir and inhibiting extension in the non‐underthrust Central Pamir.
Metamorphic core complexes in the western North American Cordillera are commonly interpreted as the result of a single phase of large‐magnitude extension during the middle to late Cenozoic. We present evidence that mylonitic shear zones in the Harcuvar and Buckskin‐Rawhide core complexes in west‐central Arizona also accommodated an earlier phase of extension during the Late Cretaceous to early Paleocene. Microstructural data indicate substantial top‐NE mylonitization occurred at amphibolite‐facies, and40Ar/39Ar thermochronology documents post‐tectonic footwall cooling to <500°C by the Paleocene to mid‐Eocene. Amphibolite‐facies mylonites are spatially associated with voluminous and variably deformed footwall leucogranites that were emplaced from ca. 74–64 Ma, and a late kinematic ca. 63 Ma dike indicates this phase of mylonitization had waned by the early Paleocene. Reconstruction of the footwall architecture indicates that this latest Cretaceous—early Paleocene deformation occurred within a NE‐dipping extensional shear zone. The leucogranites were likely the result of crustal melting due to orogenic thickening, consistent with a model whereby crustal heating triggered gravitational collapse of overthickened crust. Other tectonic processes, such as the Laramide underplating of Orocopia Schist or mantle delamination, may have also contributed to this episode of orogenic extension. Miocene large‐magnitude extension was superimposed on this older shear zone and had similar kinematics, suggesting that the location and geometry of Miocene extension was strongly influenced by tectonic inheritance. We speculate that other Cordilleran core complexes also experienced a more complex and polyphase extensional history than previously recognized, but in many cases the evidence may be obscured by later Miocene overprinting.
more » « less- NSF-PAR ID:
- 10398179
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Tectonics
- Volume:
- 42
- Issue:
- 2
- ISSN:
- 0278-7407
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
A microstructural and thermochronometric analysis of the Coyote Mountains detachment shear zone provides new insight into the collapse of the southern North American Cordillera. The Coyote Mountains is a metamorphic core complex that makes up the northern end of the Baboquivari Mountains in southern Arizona. The Baboquivari Mountains records several episodes of crustal shortening and thickening, and regional metamorphism, including the Late Cretaceous-early Paleogene Laramide orogeny which is locally expressed by the Baboquivari thrust fault. Thrusting and shortening were accompanied by magmatic activity recorded by intrusion of Paleocene muscovite-biotite-garnet peraluminous granites such as the ~58 Ma Pan Tak Granite, interpreted as anatectic melts representing the culmination of the Laramide orogeny. Following Laramide crustal shortening, the northern end of the Baboquivari Mountains was exhumed along a top-to-the-north detachment shear zone, which resulted in the formation of the Coyote Mountains metamorphic core complex. Structural and microstructural analysis show that the detachment shear zone evolved under a strong component of non-coaxial (simple shear) deformation, at deformation conditions of ~450 ± 50°C, under a differential stress of ~60 MPa, and a strain rate of 1.5 ×10-11 s-1 to 5.0 × 10-13 s-1 at depth of ~11–14 km. Detailed 40Ar/39Ar geochronology of biotite and muscovite, in the context of the deformation conditions determined by quartz microstructures, suggests that the mylonitization associated with the formation of the Coyote Mountains metamorphic core complex started at ~29 Ma (early Oligocene). Apatite fission track ages indicate that the footwall of the Coyote Mountains metamorphic core complex experienced rapid exhumation to the upper crust by ~24 Ma. The fact that mylonitization and rapid extensional exhumation post-dates Laramide thickening by ~30 Myr indicates that crustal thickness alone was insufficient to initiate extensional tectonic and required an additional driving force. The timing of mylonitization and rapid exhumation documented here and in other MCCs are consistent with the hypothesis that slab rollback and the effect of a slab window trailing the Mendocino Triple Junction have been critical in driving the development of the MCCs of the southwest.more » « less
-
The late Mesozoic Era was a time of widespread crustal extension in eastern Asia resulting in both rift basin and metamorphic core complex formation. Two of the more recently documented examples of this extensional phase are the Ereendavaa and Buteel metamorphic core complexes (EMCC, BMCC). Both are located in northern Mongolia proximal to the Mongol Okhotsk Suture Zone (MOSZ). The MOSZ is a profound, yet enigmatic structure that formed due to closure of the Mongol-Okhotsk Ocean, a basin that separated the Siberian and North China cratons and intervening terranes of the Central Asian Orogenic Belt. Based on published work by others, the core complexes record NW-SE extension, cooling and deformation from c. 135 to 120 Ma. We present new data as part of a collaborative research project that aims to constrain the evolution of the MOSZ more broadly and its relationship to intracontinental deformation after suturing. Our methods include analysis of satellite imagery and digital elevation models with synthesis of field, (micro)structural, and geochronologic data with published maps and studies. Based on our findings, the EMCC likely extends several 10's of km to the NE. Satellite imagery and DEMs suggest large-scale corrugations along the N-flank consistent with NW-SE extension. To the SW of the EMCC, Early Cretaceous rift basins are associated with strong NE-SW oriented lineaments. We examined the BMCC along its SW mapped extent, an area for which no data were presented in prior publications; we confirmed the presence of a top-to-the-SE detachment fault. The EMCC and BMCC, like the Yagan-Onch Hayrhan MCC in southern Mongolia, have footwall rocks previously mapped as Precambrian that are, in large part, metamorphosed Paleozoic and Mesozoic igneous and sedimentary rocks. All three MCCs exhibit evidence for structural complexity, such as NE-SW trending lineations orthogonal to the NW-SE extension direction. As in S Mongolia, we hypothesize that the NE-SW lineations in the EMCC and BMCC formed during an earlier phase of shortening. The expression of the Early Cretaceous extension (rift basin vs. MCC) appears to be controlled by the inherited structure.more » « less
-
more » « less
Abstract New phase equilibrium modelling, combined with U–Th/Pb petrochronology on monazite and xenotime, and40Ar/39Ar geochronology on white mica, reveal the style of deformation and metamorphism near the southern tip of the extruded Himalayan metamorphic core (HMC). In the Jajarkot klippe, west Nepal foreland, greenschist to lower amphibolite facies metamorphism is entirely constrained to the Cenozoic Himalayan orogeny, in contrast with findings from other foreland klippen in the central Himalaya. HMC rocks exposed in the Jajarkot klippe yield short‐lived, hairpin pressure–temperature–time–deformation paths that peaked at 550–600°C and 750–1,200 MPa at 25 Ma. The Main Central thrust (MCT) and the South Tibetan detachment (STD) bound the base and the top of the HMC, respectively, and were active simultaneously for at least part of their deformation history. The STD was active at
c . 27–26 Ma and possibly as late asc . 19 Ma, while the MCT may have been active as early as 27 Ma and was still active atc . 22 Ma. The tectonometamorphic conditions in the Jajarkot klippe are characteristic of crustal thickening and footwall accretion of new material at the tip of the extruding metamorphic orogenic core. Our new results reveal that collisional processes active in the middle to late Miocene at the base of the HMC now exposed in the hinterland were also active earlier, during the Oligocene, at the tip of the southward‐extruding middle crust. -
more » « less
Abstract The timing of crustal shortening and strike‐slip faulting along the East Kunlun Shan provides insight into the history of surface uplift and may constrain the time at which the Tibetan Plateau reached high elevations. We investigate a series of extensional basins and restraining bends along the Xidatan strand of the Kunlun strike‐slip fault, which provide an ideal setting to unravel the tectonic history of the northern plateau margin. We present new apatite (U‐Th)/He, apatite fission track, and zircon (U‐Th)/He ages and QTQt thermal modeling,40Ar/39Ar fault gouge dating, and structural mapping from the central East Kunlun Shan. Our data suggest that the East Kunlun Shan experienced slow to negligible exhumation until late Cretaceous time, followed by an increase in rate by 65–50 Ma. Along with a ~47 Ma fault gouge age, we posit that the Paleocene–early Eocene was a time of crustal shortening along the northern plateau. Rapid exhumation along transpressional portions of the Xidatan fault initiated by 23–20 Ma, which we interpret as the local onset of strike‐slip faulting. An early Miocene transition from north‐south crustal shortening to left‐lateral shear along the East Kunlun Shan, the onset of normal and strike‐slip faulting in central and southern Tibet by 18 Ma, and lower crustal flow in eastern Tibet by 13 Ma suggest the establishment of orogen‐wide east‐west oriented extension and extrusion by the middle Miocene. The plateau‐wide shift in stress accommodation implies that high gravitational potential energy, and likely high elevation, was attained by the middle Miocene.
