GSA Connects 2021 in Portland, Oregon

Paper No. 132-5
Presentation Time: 9:05 AM


BOSE, Subham, Structure and Tectonics Division, Wadia Institute of Himalayan Geology, 33, G.M.S Road, Dehradun, Uttrakhand, Dehradun, 248001, India and GUPTA, Saibal, Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India

The penetrative granulite-facies planar (S1/S2) and linear (δ2) fabrics of the Eastern Ghats Province (EGP), India were transposed along extensional shears (D3) at 550-600 ℃ during the mid-Neoproterozoic. Across the D3 shear zone variation in preservation potential of stretching lineation is consistently detected. The low-strain zones preserve the earlier fabrics (S1/S2) whereas a penetrative S3 foliation is observed in the moderate and high strain zones. The quartzo-feldspathic lithologies in the high strain zones preserve a prominent down-dip stretching lineation (L3) on the S3 foliation that is not consistently observed on S3 in the moderate strain zones. Vertical XZ sections in high strain zones, preserve monomineralic ribbons of quartz and feldspar along with a sillimanite mineral lineation. However, sillimanites are randomly aligned in the moderate D3 strain zones although quartz has significantly higher aspect ratio in vertical section than in plan indicating a consistent down-dip X-direction on S3. Nevertheless, feldspar grains are equidimensional and therefore, did not accommodate considerable strain unlike that in the high strain zones. As a result, bulk strain was accommodated by quartz that anastomoses around the more competent feldspars, thereby forming high amplitude warps. Owing to the anastomosing nature of the quartz ribbons, the entire ribbons never got completely exposed on the foliation surface. Parts of the quartz ribbons frequently went behind equidimensional feldspathic clasts, which being exposed along the foliation surface did not show any linear anisotropy. The higher amplitude warps defined by quartz around the near-equidimensional feldspars in the moderate strain zones indicates a competency contrast between the two phases that is significantly reduced in the high strain zones where both feldspar and quartz forms ribbons. Thus, in the high D3 strain zones a linear anisotropy is noticeable on almost every part of the S3 foliation surface. The apparent competency contrast between two deforming phases reportedly reduces at higher bulk strain rates and vice versa. The implication is that, preservation of stretching lineation in polymineralic lithologies is positively correlatable with higher bulk strain-rates.