Himalayan Geology
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Abstract:

 Geological signatures of the Ropar Tear in Ropar- Nalagarh sector of the
Northwestern Himalaya, India

R.J. SINGH, D.D. JOSHI, PRABHAS PANDE Geological Survey of India, Lucknow, India
Email: ramjivan_singh@yahoo.co.in

Evolution of frontal Siwalik ridges along the southernmost limit of the Northwestern Himalaya represents the last orogenic movement in the Himalayan Belt during Quaternary time. The Himalayan Frontal Thrust (HFT) marks the boundary between
the youngest Siwalik foothills and the Indus (or Punjab) Plain in the NW Himalaya (Nakata 1989; Karunakaran & Ranga Rao 1979; Power et al. 1998; Raiverman et al. 1979, 1993; Malik & Nakata 2003; Malik et al. 2003). Due to ongoing convergence of Indian and Eurasian plates, the zone of deformation within the Himalaya has successively shifted southwards with regional faulting and folding along the Main Central Thrust (MCT), the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT) (Gansser 1964; Scheilling & Arita 1991; Lave & Avouac 2000 & Bilham et al. 2001). The most active deformation zone lies within the Himalayan Frontal Belt between the MBT and HFT, particularly along the HFT and associated transverse faults (Valdiya 1976, 2001, 2003; Jain 1987; Nakata 1989; Kumar et al. 2001; Banerjee 2002 and Yeats & Lillie 1991). A balanced cross section of Kangra reentrant indicates a minimum of 23 km shortening occurred since 1.9-1.5 Ma, along the Sub- Himalayan decollement having 2.5º northward dip and yields a shortening rate of 14± 2 mm/yr.

 It indicates that approximately 25% of the total India-Eurasia convergence at this longitude is accommodated within the Himalayan Frontal Belt or Sub- Himalaya (Power et al. 1998). The HFT is mostly a buried or blind fault, except with few emergent minor splay faults in Ropar sector of the NW Himalayan Frontal Belt. The NW-SE trending Janauri anticlinal ridge and Chandigarh Anticlinal ridge located towards west and east of Ropar (or Rupnagar) town (Long. 76º31’31.65" : Lat 30º58’30.68") respectively, are comprised mainly of the Upper Siwalik rocks. The southernmost and youngest frontal Siwalik ridges are formed as fault propagation folds/ anticlines (frontal foreland fold) and its lateral propagation along the southwest vergent HFT and northeast vergent back fault, as wrinkle ridges. With consistent N-S compression, the differential shortening of fault propagation folds/ anticlines and coalescing segments of anticlinal frontal Siwalik ridges is mainly accommodated by the transfer faults/ tear faults with differential displacement between the anticlines or segments of the anticlines (Jain 1987; Power et al. 1998; Singh et al. 2005; Singh & Tandon 2007; Delcaillau 2006).

The apparent left- lateral shift of the NW-SE trending continuous frontal Siwalik ridges (i.e. Janauri and Chandigarh ridges) and abrupt southerly deflection of the mighty Satluj River at Ropar signify the Ropar Tear, a prominent transverse feature of the region. The outer Siwalik ridges in the Northwestern Himalaya, particularly in the Ropar region comprise dominantly clay, soft sandstone with minor conglomerate rocks of the Upper Siwalik of the Plio- Pleistocene age, along with younger alluvial terraces and fan deposits. From south to north in the area, the alluvial deposits of Satluj River and its tributaries and fan deposits of piedmont zone, the Upper Siwalik molasses of frontal ridges, alluvial deposits of Satluj and Sirsa Rivers and its tributaries, along with northern side thick fan deposits in Soan Dun valley and the Lower Siwalik molasses are exposed in the northernmost part. These litho- sequences are marked by nearly parallel tectonic planes of the NW Himalaya in Ropar- Nalagarh sector, from south to north named as the HFT, back fault of the frontal ridge and Nalagarh Thrust (NaT), along with few NE-SW to ENE-WSW trending cross faults/ transverse faults of the Ropar Tear zone (Fig. 1). The NW-SE trending, longitudinal Soan-Pinjaur Dun valley is an intramontane, backlimb depression and comprises of isolated hillocks of the Upper Siwalik, fluvial fan and terrace deposits initiated before 96 ka and continue up to present (Kumar et al., 2007 and Suresh et al. 2007). The elongated outer Siwalik ridges in southern part followed by tectonic, intramontane ridge.



parallel Soan Dun valley and Nalagarh Hill ranges (of the Lower Siwaliks) in the northern part show a typical setup of thrust related anticlines and piggyback tectonic valleys in the frontal active tectonic region (Burbank & Anderson 2000 and Keller & Pinter 2002). Abrupt southward turning of the southeasterly flowing Satluj river with a straight cut to 50 m high (from alluvial plain level) frontal ridges at Ropar water gap area, indicate the presence of a transverse structural weak zone. This gap area within the frontal ridges is highly influenced by the fluvial activities of Satluj River system and covered by terrace, fan and Soan Dun deposits in upstream and alluvial fan, flood plain and terrace deposits in the Punjab plain. Ground signatures of the Ropar Tear are often concealed by these Quaternary deposits. Four prominent transverse tectonic lineaments/ zones have been delineated by study of the remote sensing data like aerial photographs, PAN merged LISS-III digital imagery, digital images of Google Earth site of Internet, digital elevation model (DEM) of SRTM data, geological and seismo- tectonic maps of the area along with geological field works; the prominent lineaments are as i) NE-SW trending Ropar- Nagal- Dhabota- Jhandori, ii) N60ºES60 º W trending Ropar- Manguwal- Nalagarh, iii) ENE- WSW trending Ropar- Nanakpur- Nanowal and iv) NE-SW trending Jhandian- Mandiarpur (Fig. 1). These lineaments are feebly represented near Ropar due to extensive riverine activities of the Satluj River and its tributaries. Along the first lineament, westerly facing, small normal fault with moderate dip has been observed
within the Upper Siwalik rocks near Nagal (Fig. 2).

The crushing of sandstone of the Lower Siwalik Formation along with NE-SW trending criss- cross fractures have been observed within the same tectonic zone/ lineament near Jhandori. Along the second lineament, westerly facing, high angle reverse and normal minor faults have been observed within the Lower Siwalik sandstone and shale rocks near Nalagarh (Fig. 3). Westerly facing, steep normal faults have been observed along the third lineament within the Upper Siwalik rocks near east of Nanowal along Sirsa River section (Fig. 4). Along the fourth lineament, NE-SW trending westerly facing, normal cross faults with moderate dips have been identified near Jhandian at southern side of the Chandigarh  Ridge and Mandiyarpur at northern side of the ridge along Sirsa River section (Fig. 5). Four levels of terraces related to episodic

tectonic upheavals and corresponding incisions by Satluj River have been observed at Ropar. Almost undeformed, the oldest T4 terrace lies at the top of Chandigarh Ridge and Janauri Ridge near Ropar, directly on clay- sandstone rocks of the Upper Siwalik Formation. Beside these exposed transverse faults, many geomorphic evidences like abrupt change in river course, subvertical and linear escarpments, and linear extensive weathering and erosion valleys are observed within these cross fault zones. The beds of the Upper Siwalik rocks of the Janauri Anticline show gentle (5º-20º) northerly dips. In western vicinity of Ropar, the Siwalik rocks of the Janauri Anticline show overturned beds with sub- vertical to steep northerly dips at southern contact of the ridge to the piedmont zone. Within the same marginal zone,

meso-scale fault propagation folds along northerly dipping multiple thrusts planes, crushing, shattering and criss- cross small faults along with slickensides are also observed in the clay and sandstone sequences of the Upper Siwalik Formation. These geological evidences indicate that the Janauri ridge might be formed as fault propagation fold due to southwest verging movement along the blind HFT and its splay faults. On contrary, the clay, sandstone and conglomerate beds of the Upper Siwalik Formation of the Chandigarh Anticline show gentle to moderate (10º-35º) southerly dips east of Ropar. It indicates the blind or buried nature of the HFT at the southern margin of this ridge in Ropar sector. The rocks of the northern edge of the Chandigarh Anticline show low to moderate (10º-40º) southerly dips along with sub- vertical and fairly straight escarpment along the Sirsa River section. An emergent and active back fault/ thrust may be interpreted along the northern margin of this ridge in Ropar- Nalagarh sector. The NW-SE trending axis of the Chandigarh Anticline shows WNW-ESE to E-W axial trace near east of Ropar.


The inflection point of the axial bend lying within the NE-SW trending Jhandian- Mandiarpur transverse lineament cum fault (no.-iv) accommodate the differential compression within the anticlinal ridge. The variation in disposition of the Siwalik rocks and fold axes within the both ridges along with occurrence of cross faults indicate a lateral ramp like situation within the HFT zone to produce the Ropar Tear. Thus, the variation in folding style, disposition of beds within the both ridges along with occurrence of the NE-SW to ENE- WSW trending cross faults with normal and reverse slip components characterize the Ropar Tear zone around Ropar- Nalagarh area.

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