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Conjugate, cataclastic deformation bands in the Lower Devonian Muth Formation (Tethyan Zone, NW India); evidence for pre-Himalayan deformation structures

Erich Draganits, B. Grasemann and C. Hager
Conjugate, cataclastic deformation bands in the Lower Devonian Muth Formation (Tethyan Zone, NW India); evidence for pre-Himalayan deformation structures
Geological Magazine (November 2005) 142 (6): 765-781

Abstract

The purpose of this study is to use the mechanisms of deformation band formation to help with interpreting the timing of phases of deformation in an area with a complex geological history. Deformation bands and zones of deformation bands are described from the quartzites of the Lower Devonian Muth Formation in the Pin Valley, NW Himalayas. Thin-section analyses show that the deformation bands in the Muth Formation formed early in the diagenetic history before porosity was lost. Deformation mechanisms involved cataclasis, translation, rotation of quartz grains and effective porosity reduction. The orientations of the deformation bands cannot be reasonably grouped with the orientations of faults related to Himalayan deformation in the Pin Valley. Additionally, the deformation bands are deformed by Eo-Himalayan (Eocene) folds, which in turn are cut by later faults. The later faults that cross-cut the Eo-Himalayan folds developed in the already-cemented Muth Formation at much higher temperature and pressure conditions by crystal plastic deformation mechanisms, indicated by quartz crystals with undulatory extinction, abundant kink bands, dislocation glide, elongated subgrains, slightly curved deformation lamellae and pronounced shape-preferred orientation. These two completely contrasting deformation mechanisms on the microstructural scale characterize two distinct fault sets that formed at different depths in the crust. Based on these differences, a pre-Himalayan origin of the deformation bands is concluded, thus representing a set of rare pre-Himalayan deformation structures. After unfolding to remove Eo-Himalayan crustal shortening, the orientation of the deformation bands and restored relative offsets of sedimentary bedding are most compatible with approximately E-W-oriented shortening associated with N-S extension. The age of the deformation bands in the Muth Formation is bracketed by an early Devonian sedimentation age of the Muth Formation and a middle Cretaceous age of considerable cementation as deduced from compiled burial histories. Accepting a pre-middle Cretaceous age of the deformation bands, maximum conditions of about 80 degrees C and 60 MPa lithostatic pressure during their formation are estimated from the amount of overburden during the middle Cretaceous. We suggest the deformation bands are a result of either the Neo-Tethys rifting event beginning in the early Carboniferous or the extension related to late Carnian/early Norian rapid subsidence, although a hitherto unknown deformation event cannot be excluded.


ISSN: 0016-7568
EISSN: 1469-5081
Coden: GEMGA4
Serial Title: Geological Magazine
Serial Volume: 142
Serial Issue: 6
Title: Conjugate, cataclastic deformation bands in the Lower Devonian Muth Formation (Tethyan Zone, NW India); evidence for pre-Himalayan deformation structures
Affiliation: Vienna University of Technology, Institute for Engineering Geology, Vienna, Austria
Pages: 765-781
Published: 200511
Text Language: English
Publisher: Cambridge University Press, London, United Kingdom
References: 87
Accession Number: 2006-021487
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, geol. sketch map
N31°55'00" - N32°04'60", E78°00'00" - E78°04'60"
Secondary Affiliation: University of Vienna, AUT, AustriaUniversity of Kansas, USA, United States
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Cambridge University Press. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200611
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