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Evidences of inclined transpression at the contact between Vinjamuru group and Udayagiri group of Nellore Schist Belt, Andhra Pradesh, India Sankha Das1,∗, Devasheesh Shukla1 and SKMitra2 1Specialised Thematic Mapping, SU: T and AP, Geological Survey of India, Bandalaguda, 500 068, India. 2Geological Survey Of India, 284/1A, NSC Bose Road, Kolkata 700 047, India. ∗Corresponding author. e-mail: [email protected] The Nellore Schist Belt (NSB) is a curvilinear Archaean schist belt, approximately 350 km long and 8–50 km wide. The Nellore Schist Belt is considered to be Neoarchean in age and stratigraphically NSB is classified as the western Udayagiri group (dominated by metasediments) and underlying eastern Vinjamuru group (dominated by metabasalts). There is a long controversy regarding the contact rela- tionship between Udayagiri and Vinjamuru groups. Earlier researchers regarded the contact between two groups as tectonic on the basis of metamorphism. A shear zone and a possible thrust contact between the two groups have also been reported. On the basis of present study, an NNW–SSE trending, westerly dipping inclined transpressional zone is found at the contact between Udayagiri and Vinjamuru groups in the central western part of the NSB. Kinematic analysis of both the hanging wall and foot wall of the westerly dipping thrust zone shows presence of strong S1 schistosity, shear bands and S-C fabric in both strike and dip section along with east-verging overturned fold, westerly dipping inverted beds, suggest- ing partitioning of non-coaxial deformation in strike-slip and dip-slip component along with a pure shear component. Strike-slip is more prominent in the northern part of the contact than the southern part. The presence of steep to moderate northerly plunging non-orthogonal stretching/mineral elongation lineation all along the contact and clockwise shift of plot of the same in stereo net from its orthogonal position and presence of other kinematic indicators in plan suggests a right lateral strike-slip component. As a whole, it is suggested that Udayagiri group is thrusted over Vinjamuru group along a westerly dipping thrust plane with a right lateral strike-slip motion and simultaneous E–W contraction. 1. Introduction Marchini (1984) defined transpression as simulta- neous simple and pure shear along horizontal X and Transpressional zones are a common feature of Y reference coordinate axes respectively and exten- areas of oblique tectonic convergence characterised sion along vertical Z-coordinate axis to accom- mostly by vertical shear zone boundaries and modate the shortening, assuming no extrusion stretching lineations. Simple transpression has along X-coordinate axis (figure 1 of Sanderson and been defined by Harland (1971) as two rigid blocks Marchini 1984). Fossen and Tikoff (1993) defined approaching each other obliquely. Sanderson and transpression, as a combination of simple shear Keywords. Nellore Schist belt; Udayagiri group; Vinjamuru group; inclined transpression. J. Earth Syst. Sci., DOI 10.1007/s12040-016-0704-z, 125, No. 5, July 2016, pp. 1007–1020 c Indian Academy of Sciences 1007 1008 Sankha Das et al. and an orthogonal pure shear, which results in a Udayagiri and Vinjamuru group of NSB, which is non-plane strain deformation. Fossen and Tikoff the main focus of this paper. (1998) accommodated extensions along both the horizontal-X and vertical-Z reference coordinate axes (figure 2 of Fossen and Tikoff 1998) and 2. Geological setting defined five categories of transpressional deforma- tion depending on the angle of convergence, and The Nellore Schist Belt (NSB) is a curvilinear discussed in detail the state of finite strain and ori- schist belt with neo-Archean and younger rocks, entation of stretching lineation for different cate- extending approximately over a strike length of gories. The bulk strain can be of the flattening or 350 km long along its N–S strike and is about constrictional type and the pitch of stretching lin- 8–50 km wide. The NSB is thrust over the Nalla- eation can vary from horizontal to vertical, depend- malai Fold Belt (NFB) on its west, and the East- ing on the angle of oblique convergence (Fossen and ern Ghat Mobile Belt (EGMB) is thrust over NSB Tikoff 1998). Simple shear parallel to any of the on its eastern margin. Saha et al. (2010) have reference coordinate axes, will create monoclinic described a thrusted contact and inclined trans- strain symmetry, but if the simple shear is oblique, pression between NSB and NFB. The author sug- i.e., not parallel to any of the reference coordi- gests top towards west shear on stratum parallel nate axes, then it will create triclinic strain symme- sub-horizontal detachment on an easterly-dipping try. Transpressional deformation in a ductile shear thrust with a left lateral strike slip component sub- zone involves simple shear, parallel to shear zone parallel to the overall N–S trend of Nallamalai fold walls and a simultaneous orthogonal pure shear belt along with a simultaneous E–W shortening. In across the shear zone walls (Sengupta and Ghosh most of the cases, plots of stretching lineation show 2004). In a non-extrusive transpressoinal zone, the an anticlockwise shift from its orthogonal position stretching lineation can be parallel to the external (Saha et al. 2010). The Nellore Schist Belt (NSB) X-coordinate direction or Z-coordinate direction is considered to be equivalent of the Sargur group (figure 1 of Sengupta and Ghosh 2004) depending with a protolith age of 3.3–2.5 Ga (Hari Prasad on, whether the zone is simple shear dominated or et al. 1999). Metabasalt of Vinjamuru group (from pure shear dominated. Sengupta and Ghosh (2004) Chaganam and Chundi) of NSB is dated to be defined two parameters a and b (ratio of strain rate 2654 ± 100 Ma from Sm–Nd systematics, which is of pure shear, along X and Y coordinate axes, with considered the preferred time of the formation of strain rate of simple shear) and defined four cases these volcanics, and an Sm–Nd age of 1911 ± 88 Ma of transpressional deformation (depending on the is obtained from the differentiated gabbros intru- values of a and b) that produce transport paral- sive into NSB (Ravikant 2010). lel or vorticity parallel stretching lineation, both Stratigraphically the NSB is divided into (upper?) in case of bulk flattening and bulk constrictional western Udayagiri group/domain and (lower?) east- deformation, which can be distributed into nine ern Vinjamuru group/domain (Ramam and Murty (I–IX) groups (figure 3 of Sengupta and Ghosh 1997; Dobmeier and Raith 2003). However, subse- 2004). Inclined transpression is described within quently (Saha et al. 2013) NSB has been divided inclined boundary zones (non-vertical shear zones) into four tectonic units, viz., the Vinjamuru group, where the non-coaxial strain is partitioned into the Kandra complex, the Kanigiri complex and the strike-slip and dip-slip components, parallel to Xc Udayagiri group from bottom to top. The Udaya- and Zc reference coordinate axes respectively giri group consists mainly of psammite with minor (figure 1 of Jones et al. 2004), along with simul- conglomerate and pelite locally intercalated with taneous contraction. In the case of inclined trans- felsic volcanic rocks and relatively rare basalts and pression, the orientation of all three axes of a Finite limestones which have undergone greenschist facies Strain Ellipsoid (FSE) will vary throughout the of metamorphism. The Vinjamuru group is dom- deformation and the pitch of stretching lineation inated by metabasalt intercalated with psammo- on a foliation plane will vary from 0◦ to 90◦ along pelitic schist, quartzites, gneisses and migmatites the zone (Jones et al. 2004). In inclined transpres- and locally abundant felsic metavolcanics, mar- sional zones, the strain is generally of flattening bles, calcsilicate gneisses and kyanite-sillimanite type (Jones et al. 2004). Strain patterns can be het- schist of amphibolite facies (Vasudevan and Rao erogeneous along and across an inclined transpres- 1975; Narayana Rao 1983; Ramam and Murty sional shear zone and different parts of the shear 1997). However, the exact stratigraphic disposi- zone may show different structures or dominance tion and nature of contact between the above two of certain type of structures, depending on whether groups are debated (Ramam and Murty 1997) that part is dominated by strike-slip, dip slip or (figure 1). On the basis of metamorphism, the compression. The present study finds evidences contact between the Vinjamuru group and the of inclined transpression at the contact between Udayagiri group has been described as tectonic Inclined transpression at the contact between Vinjamuru and Udayagiri groups of NSB 1009 Figure 1. Generalised geological map of the Cuddapah Basin and Nellore schist belt showing the study area. 1010 Sankha Das et al. Figure 2. Geological map of the study area. Inclined transpression at the contact between Vinjamuru and Udayagiri groups of NSB 1011 (Moeen 1998). Vasudevan et al. (1975) (GSI unpublished report) reported a shear zone between the two groups of rocks and a possible thrust contact between them. An easterly dipping thrust at the contact between Vinjamuru group and Udayagiri group has been ruled out, as the foli- ation here shows a steep westerly dip in rocks of both groups at the contact (Saha 2004). So far, no conclusive literature exists describing the exact nature of the contact between Udayagiri and Vinjamuru group on the basis of both structure and metamorphism. 3. Present work The present study was confined to the central west- ern part of the Nellore Schist Belt (figure 1) in parts of Nellore and Prakasam district, Andhra Pradesh, India. In the present study area, the Vinjamur group mainly comprises metabasalt/amphibolites, lit-par-lit injection migmatite, quartzite and intru- sive granite exposed mainly in eastern part of the study area (figure 2).

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