B. P. Duarah

Late Cambrian Reworking of Paleo-Mesoproterozoic Granulites in Shillong-Meghalaya Gneissic Complex (Northeast India): Evidence from PT Pseudosection Analysis and Monazite Chronology and Implications for East Gondwana Assembly

Pressure-temperature (PT) pseudosection analysis of garnet-cordierite-sillimanite metapelites indicates that the granulites in the central part of the Shillong-Meghalaya Gneissic Complex (SMGC) in Northeast India evolved along a clockwise PT path with near isobaric heating (prograde) between 635° and 730°C at ∼5.7–5.5 kb, peak PT conditions of approximately 730°C/5.5 kb, and cooling between 650° and 595°C at 3.4–3.2 kb. Chemical dating indicates that an overwhelming majority of monazite in the metapelites and granites intrusive into the metapelites formed at Ma (). The retrograde PT segment of the Late Cambrian metamorphic PT path in the SMGC is similar to those estimated in the Prydz Bay area of East Antarctica. It is suggested that the Late Cambrian metamorphism, felsic magmatism, and deformation in central SMGC may be correlated with the Pan-African collision between India and Australia-Antarctica during the assembly of East Gondwana, consistent with paleogeographic reconstructions based on paleomagnetic data. By contrast, an overwhelming majority of chemical dates from monazite cores in western SMGC (Garo-Goalpara) metapelites are Paleo-Mesoproterozoic ( Ma, ), with younger rims of some matrix monazite grains dated at 1141–946, , and 649–524 Ma. PT pseudosection analysis of the Garo-Goalpara metapelites indicates a counterclockwise path of metamorphism that probably occurred at ∼1.6 Ga, with the end phase of retrogression possibly in the Neoproterozoic/Early Paleozoic. The final PT conditions at Garo-Goalpara (∼5.8 kb/630°C) are similar to the prograde conditions of Late Cambrian metamorphism in central SMGC. The 820-Ma dates from western SMGC correspond to high-grade metamorphism anatexis in the 876–784-Ma N-trending Eastern Indian Tectonic Zone at the eastern margins of Eastern Indian Precambrian gneissic complexes located further south in a reconstructed pre-Cretaceous configuration of the Indian shield.

Reply to comment by B. S. Sukhija et al. on ''Interpreting the style of faulting and paleoseismicity associated with the 1897 Shillong, northeast India, earthquake: Implications for regional tectonism''

[1] The source of the 1897 earthquake is central to longstanding controversies about the genesis of the Shillong Plateau and the shortening of the Indian plate at the foot of the Himalaya. Debate on the location and geometry of the 1897 rupture began during the lifetime of R. D. Oldham, a leading geologist during the British colonial period. For nearly 100 years, the 1897 earthquake was attributed to a hypothetical, north dipping fault proposed to extend from the Himalayan thrust system. Instead, Bilham and England [2001] invoked a south dipping fault, which they called the Oldham fault. They further proposed that the Oldham fault is one in a pair of reverse faults of opposing vergence that raised the Shillong Plateau as a pop-up structure. Our paper [Rajendran et al., 2004], while supporting the south dipping geometry, pointed out that the hypothetical Oldham fault lacks known expression along its supposed trace in the exposed crystalline rocks. We also explored potential alternatives, including a buried fault beneath the Brahmaputra Valley. In his comment, Bilham [2006] defends the Oldham fault by pointing out that the Brahmaputra alternative appears to conflict with old triangulation data. In response, we remind readers that the geodetic model by Bilham and England [2001] is a nonunique solution, which remains unsupported by geology. [2] The model of Bilham and England [2001] is based on two sets of triangulation data of doubtful accuracy The 1898 trigonometrical survey south of the hypothetical Oldham fault, across the Shillong Plateau, failed to meet the triangle closure standards of the Survey of India [Oldham, 1899]. Problems also plagued the postearthquake Assam Valley Triangulation Series, north of the hypothetical fault. Writing for the survey as its superintendent, Bomford [1939, p. 32] of the Royal Engineers stated that the triangulation data from Assam (1859–1937) is suitable for nongeodetic purposes only, provided that ‘‘pairs of stations can be found whose mark-stones can be trusted to have undergone no relative movement.’’ Oldham [1899] speculatively ascribed these errors to postseismic crustal movement. Bilham and England [2001] praised this idea as ‘‘ahead of its time’’ without addressing Bomford’s concerns. [3] If, despite these geodetic uncertainties, the Oldham fault is real, one would expect to see it in the geology and geomorphology of the Shillong Plateau [Rajendran et al., 2004]. To explain the fault’s apparent lack of expression, Bilham proposes that as in the case of the 2001 Bhuj earthquake, the faulting in the 1897 earthquake was blind. The thick sediment-fill in the Kachchh rift favored folding and flexuring above the upper part of the fault rupture in 2001, which occurred on an imbricate thrust fault within the rift [Rajendran et al., 2001]. By contrast, the Precambrian crystalline rocks of the Shillong Plateau are unlikely to inhibit surface rupture, especially on a steep dipping fault (50 ) as proposed by Bilham and England [2001]. Even the small-scale structures that would be expected of a major fault are absent in this region. A recent study by Srinivasan [2003], suggests that only 6–7% of the fractures on the Shillong Plateau strike E-W or ENE-WNW, the direction of the hypothesized Oldham fault. Although the proposed Oldham fault coincides with a change in relief, this change need not represent any faulting. The Shillong Plateau slices across granitic plutons, some of which are evident by remote sensing. Differential erosion along their contacts with the host rocks is known to produce high relief. Bilham’s comment does not acknowledge such geological complexities. Fieldwork by a team including Bilham and two of us (B. P. Duarah and C. P. Rajendran), subsequent to the publication of the papers being discussed here, uncovered no evidence for theOldham fault. By contrast, theDauki fault, conjugate to the Oldham fault, according to Bilham and England [2001], is geologically conspicuous. [4] Like the geodetic evidence used by Bilham and England [2001], gravity and seismic data in this region do not point to a unique tectonic explanation for the 1897 earthquake [Rajendran et al., 2004]. However, while the gravity data do not suggest anything anomalous where the Oldham fault is projected, they give a weak signal farther north [see Rajendran et al., 2004, Figure 4]. The Oldham fault is not apparent, either in our compilation of microseismic data or in a recent larger and better data set (J. R. Kayal et al., Shillong Plateau earthquakes in northeast India region: Complex tectonic model, unpublished manuscript, 2005). As proposed in our paper, this recent TECTONICS, VOL. 25, TC2002, doi:10.1029/2005TC001902, 2006

Geochemistry of Ukhrul limestone of Assam-Arakan subduction basin, Manipur, Northeast India

Geochemical studies of the carbonate rocks of Ukhrul limestone from Hundung and Payoi area of Ukhrul District of Manipur, India have been carried out through investigation of major oxides and trace elements geochemistry to determine their chemical composition, to establish the distribution pattern and mutual relationship of the elements, for chemical classification of the limestones and also to decipher environmental conditions that existed during the time of deposition of the calcareous sediments. The study indicates that the limestones were deposited in open to protected near shore, marginal neritic environment with argillaceous sediment flux derived from a granitic continental crust. Protected basin existed partly in Paoyi area. Presence of marine gastropods and foraminifera fossils and higher concentration of strontium in the studied carbonate rocks suggest relatively shallow marine environment of deposition during the Eocene sedimentation.

Morphodynamics of the Kulsi River Basin in the northern front of Shillong Plateau: Exhibiting episodic inundation and channel migration

The present study is undertaken in the Kulsi River valley, a tributary of the Brahmaputra River that drains through the tectonically active Shillong Plateau in northeast India. Based on the fluvial geomorphic parameters and Landsat satellite images, it has been observed that the Kulsi River migrated 0.7–2 km westward in its middle course in the past 30 years. Geomorphic parameters such as longitudinal profile analysis, stream length gradient index (SL), ratio of valley floor width to valley height (Vf), steepness index (

Understanding the Tectonic Behaviour of the Shillong Plateau, India using Remote Sensing Data

k_{s})

Morphotectonic and Basin Parameters of the Noa-Dihing River, Eastern Himalayas

ks) indicate that the upstream segment of the Kulsi River is tectonically more active than the downstream segment which is ascribed to the tectonic activities along the Guwahati Fault.

Seismic hazard assessment in the Jia Bhareli river catchment in eastern Himalaya from SRTM-derived basin parameters, India

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