Wednesday, August 27, 2014

Deformation Event in Cryogenian NeoProterozoic India

Deformation and granite intrusion in the Sirohi area, SW Rajasthan - constraints on Cryogenian to Pan-African crustal dynamics of NW India


de Wall et al


This study provides further evidence for a Cryogenian (∼ 770 - 760 Ma) deformation event in NW India, a region generally regarded tectonically stable during the Rodinia to Gondwana transition. This deformation is prominent in the Sirohi region (southwestern Rajasthan State), previously referred to as the ‘Sirohi Orogeny’, younger to and independent of the ∼1 Ga to 820 Ma Delhi Orogeny. Synkinematic intrusion of the Balda Granite during the NW-directed thrusting and folding of sediments of the Neoproterozoic Sirohi Group is constrained by combined field, microstructural and magnetic fabric analyses. Thrusting (D1) occurred under medium grade deformation conditions along mylonitic to ultramylonitic shear zones featuring quartz and feldspar recrystallization. Steepening of thrust planes and transition from dip-slip kinematics to oblique and dextral strike-slip in high strain areas indicate transpressive kinematics during the late-stage of D1 deformation. A second deformation even (D2) is documented in strain concentration along a conjugate set (N-S & E-W) of shear zones wherein dilation along the N-S trending structure provided space for voluminous synkinematic quartz mineralization. This linear structure, traceable for ∼10 km and named ‘Balda-Paladi Shear Zone’, has overprinted the NE-SW trending magmatic fabrics in the Balda Granite by N-S oriented shearing. Brittle behavior of feldspar constrains this overprint as a low temperature deformation event that also represents the terminal Cryogenian imprint in the Sirohi sector. The findings of this study indicate that an early Cryogenian imprint, seen in three different tectonomagmatic zones, from E to W referred to as South Delhi high-grade metamorphic terrane, Sirohi anatectic terrane and Sirohi fold and thrust terrane, is coeval with the initial closure of the Mozambique Ocean as constrained for central and northern Madagascar. A continuation of early Cryogenian mobile belts extending from southern India, crossing Madagascar into NW India is proposed. Related structures could have served as pathways for heat convection and triggered local late Pan-African thermal resetting in NW India

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