Abstract New U-Pb zircon, monazite, 40 Ar/ 39 Ar, and apatite fission track ages provide constraints on the timing of formation and exhumation of the Mabja Dome, southern Tibet, shed light on how this gneiss dome formed, and provide important clues on the tectonic evolution of middle crustal rocks in southern Tibet. Zircons from a deformed leucocratic dyke swarm yield a U-Pb age of 23.1 ± 0.8 Ma, providing the first age constraint on the timing of middle crustal ductile horizontal extension in the North Himalayan gneiss domes. Zircons and monazite from a post-tectonic two-mica granite yield ages of 14.2 ± 0.2 Ma and 14.5 ± 0.1, respectively, indicating that vertical thinning and subhorizontal stretching had ceased by the middle Miocene. Mica 40 Ar/ 39 Ar ages from schists and orthogneisses increase structurally down-section from 12.85 ± 0.13 Ma to 17.0 ± 0.19 Ma and then decrease at the deepest structural levels to 13.29 ± 0.09 Ma. Micas from the leucocratic dyke swarm and post-tectonic two-mica granites yield similar 40 Ar/ 39 Ar cooling ages of 13.48 ± 0.13 to 12.84 ± 0.08 Ma. The low-temperature steps of potassium feldspar 40 Ar/ 39 Ar spectra yield ages of c. 11.0–12.5 Ma and apatite fission track analyses indicate the dome uniformly cooled below c. 115°C at 9.5 ± 0.6 Ma. Based on these data, calculated average cooling rates across the dome range from c. 40–60°C/million years in schist and orthogneiss and following emplacement of the leucocratic dyke swarm, to c. 350°C/million years following emplacement of the two-mica granites. The mylonitic foliation, peak metamorphic isograds, and mica 40 Ar/ 39 Ar chrontours are domed, whereas the low-temperature step potassium feldspar 40 Ar/ 39 Ar and apatite fission track chrontours are not, suggesting that doming occurred between 13.0 and 12.5 Ma and at temperatures between 370 and 200°C. Our new ages, along with field, structural and metamorphic data, indicate that the domal geometry observed at Mabja developed by middle-Miocene southward-directed thrust faulting upward and southward along a north-dipping ramp above cold Tethyan sediments. The structural, metamorphic and geochronologic histories documented at Mabja Dome are similar to those of Kangmar Dome, suggesting a common mode of occurrence of these events throughout southern Tibet. Vertical thinning and horizontal stretching, metamorphism, generation of migmatites, and emplacement of leucogranites in the domes of southern Tibet are synchronous with similar events in the Greater Himalayan Sequence that underlie the high Himalaya. These relations are consistent with previously proposed models for a ductile middle-crustal channel bounded above by the South Tibetan detachment system and below by the Main Central thrust in the High Himalaya that extended northward beneath southern Tibet.
Metamorphic core complexes within the western U.S. record a history of Cenozoic ductile and brittle extensional deformation, metamorphism, magmatism, and exhumation within the footwalls of high-angle Basin and Range normal faults. In models proposed for the formation of metamorphic core complexes there is a close temporal and spatial link between upper crustal normal faulting, lower crustal ductile extension and flow, and detachment faulting. To provide constraints on the timing of ductile extension in the northern Snake Range metamorphic core complex (Nevada) and thereby test these models, we present new 238 U- 206 Pb dates on zircons from both deformed and undeformed rhyolite dikes intruded into this core complex. The older age bracket is from the northern dike swarm, which was emplaced in the northwestern part of the range pretectonic to syntectonic with ductile extension. The younger age bracket is from the Silver Creek dike swarm, which was emplaced in the southern part of the range after ductile extensional deformation. The 238 U- 206 Pb zircon ages from these dikes provide tight bounds on the timing of ductile extension, between 37.806 ± 0.051 Ma and 22.49 ± 0.36 Ma. Our field observations, petrography, and 238 U- 206 Pb zircon ages on these dikes combined with published data on the geology and kinematics of extension, moderate- and low-temperature thermochronology on lower plate rocks, and age and faulting histories of Cenozoic sedimentary basins, are interpreted as recording an episode of localized upper crustal brittle extension during the late Eocene that drove upward ductile extensional flow of hot middle crustal rocks from beneath the northern Snake Range detachment soon after, or simultaneously with, emplacement of the older dike swarm. Exhumation of the lower plate continued in a rolling hinge–isostatic rebound style; the western part of the lower plate was exhumed first and the eastern part extended ductilely either episodically or continuously until the latest Oligocene–earliest Miocene, when the post-tectonic younger dike swarm was emplaced. Major brittle slip along the eastern part of the northern Snake Range detachment and along high-angle normal faults exhumed the lower plate during middle Miocene.
Research Article| October 01, 2013 Synchronous Oligocene–Miocene metamorphism of the Pamir and the north Himalaya driven by plate-scale dynamics Michael A. Stearns; Michael A. Stearns 1Earth Science Department, University of California–Santa Barbara, Santa Barbara, California 93106, USA Search for other works by this author on: GSW Google Scholar Bradley R. Hacker; Bradley R. Hacker 1Earth Science Department, University of California–Santa Barbara, Santa Barbara, California 93106, USA Search for other works by this author on: GSW Google Scholar Lothar Ratschbacher; Lothar Ratschbacher 2Geologie, Technische Universität Bergakademie Freiberg, 09599 Freiberg, Germany Search for other works by this author on: GSW Google Scholar Jeffrey Lee; Jeffrey Lee 3Geological Sciences, Central Washington University, Ellensburg, Washington 98926, USA Search for other works by this author on: GSW Google Scholar John M. Cottle; John M. Cottle 1Earth Science Department, University of California–Santa Barbara, Santa Barbara, California 93106, USA Search for other works by this author on: GSW Google Scholar Andrew Kylander-Clark Andrew Kylander-Clark 1Earth Science Department, University of California–Santa Barbara, Santa Barbara, California 93106, USA Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (10): 1071–1074. https://doi.org/10.1130/G34451.1 Article history received: 04 Feb 2013 rev-recd: 27 May 2013 accepted: 30 May 2013 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Michael A. Stearns, Bradley R. Hacker, Lothar Ratschbacher, Jeffrey Lee, John M. Cottle, Andrew Kylander-Clark; Synchronous Oligocene–Miocene metamorphism of the Pamir and the north Himalaya driven by plate-scale dynamics. Geology 2013;; 41 (10): 1071–1074. doi: https://doi.org/10.1130/G34451.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Gneiss domes in the Pamir (Central Asia) and the Himalaya provide key data on mid- to deep-crustal processes operating during the India-Asia collision. Laser ablation split-stream inductively coupled plasma–mass spectrometry (LASS-ICP-MS) data from monazite in these domes yield a time record from U/Th-Pb dates and a petrologic record from rare earth element (REE) abundances. Seven samples from the Pamir and six samples from the north Himalayan gneiss domes yield almost identical monazite dates of ca. 28–15 Ma. Most monazite has invariant heavy REE (HREE) abundances; two samples, however, have older monazite that records progressive HREE depletion and two samples have younger monazite that records progressive HREE enrichment. These variations in HREE are compatible with increasing garnet abundance—prograde metamorphism—until ca. 20 Ma, and decreasing garnet abundance thereafter. The change from HREE depletion to enrichment may record a transition from crustal thickening and heating to dome exhumation and cooling. This documentation of synchronous Barrovian metamorphism within domes of Indian crust along the margin of the orogen (Himalaya) and within domes of Asian crust within the core of the orogen (Pamir) is best explained by a plate-scale driving force rather than by local events. We propose that widespread, synchronous thickening was initiated by the resumption of Indian subduction following slab breakoff and then terminated by a second slab-tearing event—both plate-scale events inferred from tomography. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract Despite the importance of Tethys Himalayan or North Himalayan gneiss domes for discussing extrusive flow of the underlying Greater Himalayan Sequence, these metamorphic domes in general remain poorly documented. The main exception is the Kangmar dome. The Malashan metamorphic complex, a newly documented North Himalayan gneiss dome, is shown to have strong similarities with the Kangmar dome, suggesting that the North Himalayan gneiss domes have the following features in common: (i) Barrovian-type metamorphism with grade increasing towards a centrally located two-mica granite; (ii) the presence of two dominant ductile deformation stages, D 1 and D 2 , with D 2 showing an increasing strength towards the granite contacts; and (iii) the development of a strong D 2 foliation (gneissosity) in the outermost part of the granite cores. In addition, field and bulk-chemical studies show: (i) D 2 is associated with a dominant top-to-the-north sense of shear (in disagreement with the most recent kinematic studies in Kangmar dome); (ii) the deposition age of associated metasediments is upper Jurassic suggesting that the Malashan dome is located not at the base, but within the middle section of the Tethys Himalaya; and (iii) in contrast to the Kangmar granitic gneiss that is interpreted as Indian basement, three granitic bodies in Malashan all formed as young intrusive bodies during the Himalayan orogeny. These results suggest that the formation mechanism of the North Himalayan gneiss domes needs to be re-evaluated to test the rigidity of the hanging wall assumed in channel flow models.
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