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Late Cenozoic metamorphic evolution and exhumation of Taiwan

Beyssac, Olivier and Simoes, Martine and Avouac, Jean Philippe and Farley, Kenneth A. and Chen, Yue-Gau and Chan, Yu-Chang and Goffé, Bruno (2007) Late Cenozoic metamorphic evolution and exhumation of Taiwan. Tectonics, 26 (6). Art. No. TC6001 . ISSN 0278-7407.

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The Taiwan mountain belt is composed of a Cenozoic slate belt (Hsuehshan Range units, HR, and Backbone Slates, BS) and of accreted polymetamorphic basement rocks (Tananao Complex, TC). Ongoing crustal shortening has resulted from the collision between the Chinese continental margin and the Luzon volcanic arc, which initiated ~6.5 Ma ago. The grade and age of metamorphism and exhumation are a key record of the development of the orogenic wedge. Because the Taiwan mountain belt is mostly composed by accreted sediments lacking metamorphic index minerals, quantitative constraints on metamorphism are sparse. By contrast, these rocks are rich in carbonaceaous material (CM) and are therefore particularly appropriate for RSCM (Raman Spectroscopy of CM) thermometry. We apply this technique in addition to (U-Th)/He thermochronology on detrital zircons to assess peak metamorphic temperatures (T) and the late exhumational history respectively, along different transects in central and southern Taiwan. In the case of the HR units, we find evidence for high metamorphic T of at least 340°–350°C and locally up to 475°C, and for relative rapid exhumation with zircon (U-Th)/He ages in the range of 1.5–2 Ma. Farther east, the BS were only slightly metamorphosed (T < 330 °C), and zircons are not reset for (U-Th)/He. From the eastern BS to the inner TC schists, T gradually increases from ~350°C up to ~500°C following an inverted metamorphic gradient. Available geochronological constraints and the continuous thermal gradient from the BS to the basement rocks of the TC suggest that the high RSCM T of the TC were most probably acquired during the last orogeny, and were not inherited from a previous thermal event. Zircons yield (U-Th)/He ages of ~0.5–1.2 Ma. Peak metamorphic T and the timing of exhumation do not show along-strike variations over the TC in the studied area. In contrast, exhumation is laterally diachronous and decreases southward in the case of the HR units. In particular, our data imply that the HR units have been exhumed by a minimum of 15 km over the last few Ma. In the case of the BS, they show far less cumulated exhumation and much slower cooling rates. We propose that most of the deformation and exhumation of the Taiwan mountain belt is sustained through two underplating windows located beneath the Hsuehshan Range and the TC. Our data show significant departures from the predictions of the prevailing model in Taiwan, which assumes a homogeneous critical wedge with dominant frontal accretion. Our study sheds new light on how the mountain belt has grown as a possible result of underplating mostly.

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Additional Information:© 2007 American Geophysical Union. Received 25 October 2006; revised 5 April 2007; accepted 26 June 2007; published 7 November 2007. We are indebted to Georges Rossman and Liz Miura-Boyd for access to the Raman laboratory at Caltech (California, USA) as well as for many instructive discussions. In the field we appreciated the help of Sylvain Bernard (ENS, France), Frank Yang, Chin-Ying Lan, Yu-ting Kuo, and Chung Huang as well as of many other students from NTU (Taiwan). The numerous thin sections were prepared by Roland Caron (ENS, France). The ZHe data were analyzed by Lindsey Hedges (Caltech). This study was improved thanks to the numerous discussions with Kerry Sieh, Bruce Shyu, and Mark Simons (Caltech), as well as with John Suppe (Princeton) and Jacques Malavieille (CNRS Montpellier). Mark Simons also collected the Chipan gneiss samples. Steve Kidder (Caltech) provided constructive comments on an early draft. This manuscript also benefited from the constructive and thoughtful reviews of Tim Byrne and Sean Willett, and from the comments of the Associate Editor K. Stuewe. This project was funded by the IT program (INSUCNRS, France) and by the Gordon and Betty Moore Foundation. This is Caltech Tectonics Observatory contribution 70.
Group:Caltech Tectonics Observatory, Caltech Tectonics Observatory. Taiwan Tectonics and Seismicity
Funding AgencyGrant Number
Institut National des Sciences de l'Univers/Centre National de la Recherche Scientifique (INSU/CNRS) (France)UNSPECIFIED
Gordon and Betty Moore FoundationUNSPECIFIED
Subject Keywords:Taiwan; exhumation; metamorphism
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Caltech Tectonics Observatory 70
Record Number:CaltechAUTHORS:20101115-095303150
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Official Citation:Beyssac, O., M. Simoes, J. P. Avouac, K. A. Farley, Y.-G. Chen, Y.-C. Chan, and B. Goffé (2007), Late Cenozoic metamorphic evolution and exhumation of Taiwan, Tectonics, 26, TC6001, doi:10.1029/2006TC002064
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:20797
Deposited By: Tony Diaz
Deposited On:16 Nov 2010 00:00
Last Modified:26 Dec 2012 12:38

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