41 research outputs found
Miocene gneiss domes in the Pamir: linking graviational collapse of thickened crust to shortening in the Tajik foreland
Abstract HKT-ISTP 2013
A
Anomalously old biotite <sup>40</sup>Ar/<sup>39</sup>Ar ages in the NW Himalaya
Biotite 40Ar/39Ar ages older than corresponding muscovite 40Ar/39Ar ages, contrary to the diffusion properties of these minerals, are common in the Himalaya and other metamorphic regions. In these cases, biotite 40Ar/39Ar ages are commonly dismissed as âtoo oldâ on account of âexcess Ar.â We present 32 step-heating 40Ar/39Ar ages from 17 samples from central Himachal Pradesh Himalaya, India. In almost all cases, the biotite ages are older than predicted from cooling histories. We document host-rock lithology and chemical composition, mica microstructures, biotite chemical composition, and chlorite and muscovite components of biotite separates to demonstrate that these factors do not offer an explanation for the anomalously old biotite 40Ar/39Ar ages. We discuss possible mechanisms that may account for extraneous Ar (inherited or excess Ar) in these samples. The most likely cause for âtoo-oldâ biotite is excess Ar, i.e., 40Ar that is separated from its parent K. We suggest that this contamination resulted from one or several of the following mechanisms: (1) 40Ar was released during Cenozoic prograde metamorphism; (2) 40Ar transport was restricted due to a temporarily dry intergranular medium; (3) 40Ar was released from melt into a hydrous fluid phase during melt crystallization. Samples from the Main Central Thrust shear zone may be affected by a different mechanism of excess-Ar accumulation, possibly linked to later-stage fluid circulation within the shear zone and chloritization. Different Ar diffusivities and/or solubilities in biotite and muscovite may explain why biotite is more commonly affected by excess Ar than muscovite
Low-temperature thermochronology of central and northwestern Pamir gneiss domes
Abstract HKT-ISTP 2013
A
Tectonic and glacial contributions to focused exhumation in the Olympic Mountains, Washington, USA
Alichur Dome, South Pamir, Western India-Asia Collisional Zone: Detailing the Neogene Shakhdara-Alichur Syn-collisional Gneiss-Dome Complex and Connection to Lithospheric Processes
Neogene, synâcollisional extensional exhumation of Asian lowerâmiddle crust produced the ShakhdaraâAlichur gneissâdome complex in the South Pamir. The <1 kmâthick, myloniticâbrittle, topâNNE, normalâsense Alichur shear zone (ASZ) bounds the 125 Ă 25 km Alichur dome to the north. The Shakhdara dome is bounded by the <4 kmâthick, myloniticâbrittle, topâSSE South Pamir normalâsense 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 22â17 Ma Alichurâdomeâinjectionâcomplex leucogranites transition from foliationâparallel, centimeterâ to meterâthick sheets within the ASZ into discordant intrusions that may comprise half the volume of the dome core. Secondary fluid inclusions in mylonites and mylonitizationâtemperature constraints suggest Alichurâdome exhumation from 10â15 km depth. Thermochronologic dates bracket footwall cooling between ~410â130 °C from ~16â4 Ma; tectonic cooling/exhumation rates (~42 °C/Myr, ~1.1 km/Myr) contrast with erosionâdominated rates in the hanging wall (~2 °C/Myr, <0.1 km/Myr). Domeâscale boudinage, oblique divergence of the ASZ and SPSZ hanging walls, and dextral wrenching reflect minor approximately EâW material flow out of the orogen. We attribute broadly southward younging extensional exhumation across the central South Pamir between ~20â4 Ma to: (i) Mostly northward, forelandâdirected 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 nonâunderthrust Central Pamir
Low-temperature thermochronology: methodological studies and application in collisional orogens
Die Spaltspuren-Datierung als wesentliche Methode aus dem Bereich der Niedrigtemperatur-Thermochronologie basiert auf der ZĂ€hlung und Messung geĂ€tzter Spuren unter dem Mikroskop. FĂŒr eine akkurate Altersbestimmung ist daher das VerstĂ€ndnis der Ătzung von gröĂter Bedeutung. Ein atomistisches Modell und eine Monte-Carlo Computersimulation erklĂ€ren Ătzgruben-Formen und deren GröĂenwachstum. Thermochronologie wird in zwei Fallstudien angewendet: eine umfassende Studie ĂŒber die tektonische Entwicklung Zentralamerikas seit dem PalĂ€ozoikum zeigt, wie Geo- und Thermochronologie, Strukturgeologie und Petrologie zusammenarbeiten können, um &gt;400 Ma einer komplexen tektonischen Geschichte zu entrĂ€tseln. Eine thermochronologische Studie im Pamir, Tadschikistan betont vor allem die Möglichkeiten, die sich aus der Anwendung der Thermochronologie auf dem Gebiet der Geomorphologie und Neotektonik eröffnen
Pliocene episodic exhumation and the significance of the Munsiari thrust in the northwestern Himalaya
Miocene gneiss domes in the Pamir : linking graviational collapse of thickened crust to shortening in the Tajik foreland
methodological studies and application in collisional orogens$nElektronische Ressource
Die Spaltspuren-Datierung als wesentliche Methode aus dem Bereich der Niedrigtemperatur-Thermochronologie basiert auf der ZĂ€hlung und Messung geĂ€tzter Spuren unter dem Mikroskop. FĂŒr eine akkurate Altersbestimmung ist daher das VerstĂ€ndnis der Ătzung von gröĂter Bedeutung. Ein atomistisches Modell und eine Monte-Carlo Computersimulation erklĂ€ren Ătzgruben-Formen und deren GröĂenwachstum. Thermochronologie wird in zwei Fallstudien angewendet: eine umfassende Studie ĂŒber die tektonische Entwicklung Zentralamerikas seit dem PalĂ€ozoikum zeigt, wie Geo- und Thermochronologie, Strukturgeologie und Petrologie zusammenarbeiten können, um >400 Ma einer komplexen tektonischen Geschichte zu entrĂ€tseln. Eine thermochronologische Studie im Pamir, Tadschikistan betont vor allem die Möglichkeiten, die sich aus der Anwendung der Thermochronologie auf dem Gebiet der Geomorphologie und Neotektonik eröffnen.thesi