Miocene rise of the Shillong Plateau and the beginning of the end for the Eastern Himalaya
Introduction
Understanding the relationship between fold and thrust belt deformation developed at a convergent plate boundary, and the propagation of strain, topography and crustal thickening away from that plate boundary to form an orogenic plateau is a first-order question in continental dynamics. The archetypical example of such is the Himalaya fold and thrust belt and the Tibetan Plateau, which have formed in response to ongoing continental convergence between India and Eurasia. Nearly a third of modern plate convergence is neatly concentrated across a few tens of kilometers in the central Himalaya — a relationship that changes dramatically along strike. In the eastern Himalaya system strain is more widely distributed. Also, the downgoing plate is composed of transitional and oceanic lithosphere, and the complexity of oblique subduction beneath the Burma micro-plate is introduced. The unique eastern Himalaya system east of 88°E latitude represents nearly 25% of the length of the arc and yet has been poorly considered to be a significant geodynamic influence on the orogen's development.
A dramatic and unique feature of the eastern Himalaya system is the deformation of the Indian foreland basement beneath the Shillong Plateau. The Shillong Plateau occupies a region between the nearly orthogonal thrust belts of the south-vergent eastern Himalaya and the west-vergent Indo-Burman ranges, which accommodate convergence between India and Eurasia and oblique convergence between India and the Burma micro-plate, respectively (Fig. 1). It is arguably the largest, active basement fold structure in the world, and is 5 to 10 times larger than its commonly-cited analogs found in the Laramide orogeny of the western US or the Sierras Pampeanas of the Andean orogen (Allmendinger et al., 1983, Cross, 1986). Newly available SRTM 90-meter resolution digital topography data of the Shillong Plateau show a smooth, regular erosion surface that defines a doubly-plunging, south-vergent anticline composed of Proterozoic and Archean basement rocks in the core of the range and dipping Cretaceous to Miocene(?) age sedimentary rocks on the limbs (Fig. 2). The crest of this anticline is flat-topped, giving rise to the moniker ‘Shillong Plateau’. Archean and Neoproterozoic granites and gneisses of the peninsular Indian shield are exposed along most of the central and northern portions of the anticline, while up to 6 km of Cretaceous through Miocene, marine to continental sedimentary rocks are preserved unconformably over basement along the eastern, western and southern limbs (Evans, 1964, Das Gupta et al., 1964, Das Gupta and Biswas, 2000, Ghosh et al., 2005) (Fig. 1). The orientation of these sedimentary rocks generally follows the overall topographic trend of the anticline, except in the south where normal displacements occur locally within the Cenozoic strata (Srinivasan, 2005). Rocks of the Shillong Plateau over thrust shelf to basinal facies sedimentary rocks of the Sylhet Trough (Bengal Basin) to the south (Das Gupta et al., 1964) (Fig. 1).
We interpret the anticlinal folding of sedimentary strata and exposure of the basement core to be the result of a blind or emergent reverse fault system at depth (Fig. 1). The existence of thrust or reverse faults beneath the Shillong Plateau is supported by gravity data and compressional earthquake focal mechanisms (Verma and Mukhopadhyay, 1977, Chen and Molnar, 1990, Mitra et al., 2005), although the sense of motion along the southern bounding fault of the Shillong Plateau (Dauki Fault) has been controversial (Oldham, 1854, Oldham, 1899, Evans, 1964, Hiller and Elahi, 1984, Johnson and Alam, 1991, Bilham and England, 2001, Srinivasan, 2005). Modeling of triangulation data following the 1897 Assam earthquake suggests that the northern edge of the Shillong Plateau is controlled by a steeply dipping fault that penetrates most, if not all of the crust, and mirrors motion on the steeply dipping Dauki Fault to the south (Chen and Molnar, 1990, Bilham and England, 2001). A seismogenic lower crust is supported by deep diffuse seismicity (Kayal et al., 2006) and steep bounding faults are a geometry that is commonly observed in analogous basement compressional structures in many other orogens (Narr and Suppe, 1994). Digital topography shows that the exposed basement rocks are intensely fractured and that rivers are deeply incised only in their lower reaches along the southern boundary, and along a west-northwest trending topographic step within the northern limb of the anticline. While some south-draining river canyons reach 1.5 km deep, incision is limited to short, downstream reaches of the major rivers and the landscape overall is not extensively dissected. Lack of fluvial dissection preserves the basement/cover unconformity that defines the surface of the basement fold.
Shortening strain in the eastern Himalaya is widely distributed over several hundred kilometers and includes basement rocks of the underthrust plate (India), whereas strain in the western and central Himalaya occurs in a more narrowly focused manner across a few tens of kilometers and rarely more than 100 km (Das Gupta et al., 1964, Gansser, 1983, Bilham et al., 1997, Lave and Avouac, 2000, Wobus et al., 2005). Recently measured GPS velocities suggest that possibly as much as 30% of the 15–19 mm/yr of convergence across the eastern Himalayan system occurs across the Shillong Plateau (Bilham and England, 2001, Jade et al., 2004). Furthermore, approximately 30% of the Tibetan Plateau sits north of the eastern Himalaya and adjacent India/Burma plate boundary. Understanding eastward expansion of the Tibetan Plateau may in part depend on understanding how changes at the plate boundary affect strain distribution far within the orogen. Deformation of the Shillong Plateau signals differentiation of the eastern Himalaya from the rest of the Himalaya and a regionally-significant change in how strain is partitioned at the plate boundary. Therefore the timing of Shillong deformation can be used to assess the relationship between deformation at the plate boundary (Himalaya) and the interior of the orogen (Tibetan Plateau).
Section snippets
Timing of deformation and tectonic interpretation from basinal stratigraphy: summary of previous work
Previous estimates for timing of fault motion beneath the Shillong Plateau are based on the sedimentary record of the Sylhet Trough (or Surma Basin), a province of the larger Bengal Basin. These estimates vary considerably from the Oligocene–Miocene boundary to Pliocene time. Basinal strata are also preserved along the up-thrust margins of the Shillong massif itself. However, tectonic interpretation of basinal stratigraphy is hampered by multiple sources of tectonic loading and sediment supply
Timing of deformation from apatite (U-Th-Sm)/He thermochronometry
Samples collected for low-temperature thermochronometry along horizontal and vertical transects can give information about the spatial distribution of erosion and timing of erosional events related to structural activity. We collected 14 samples for apatite (U-Th-Sm)/He dating along a north–south horizontal transect across the central plateau and a vertical transect within a single river gorge that incises the southern margin of the plateau (Fig. 1, Fig. 2). We aim to determine the thickness of
Geomorphology and fluvial analyses
Bedrock river channel gradients are sensitive indicators of variable rock uplift rates in an actively deforming region and can be used qualitatively to identify faulting patterns. This approach can be particularly useful in remote areas, areas of dense vegetative cover, or where the lack of appropriate aged rocks involved in recent deformation inhibit determination of young or active faulting. We utilized the method outlined by Kirby et al. (2003) for determining normalized steepness values for
Constraints on structural geometry
The spatial extent of deformation associated with the Shillong Plateau can be estimated from both the extent of exposed basement and sedimentary rocks and the limit of missing or attenuated foredeep deposits surrounding the plateau (Fig. 1). Thin foredeep deposits exist west of the Shillong Plateau to the Malda-Kishanganj Fault where they abruptly deepen from 500 m to several kilometers across this structure (Geol. Surv. India, 2002). Northeast of the Shillong Plateau, the foredeep eventually
Initiation of faulting, fault slip rate and erosion rate
In a compressional tectonic setting, an increase in apparent exhumation rate on an age/elevation diagram is generally assumed to be an acceleration of erosion rate related to the upward motion of hanging wall rocks (Wagner and Reimer, 1972, Wagner et al., 1977, Fitzgerald et al., 1995, Reiners and Brandon, 2006). Using both thermochronometry data and the stratigraphy of the Sylhet Trough, we consider the initiation of faulting, faulting rates and erosion rates independently. Three aspects of
Comparison of long-term fault slip rates with geodetic rates
Using our proposed fault geometry (Fig. 5) and vertical slip rates determined above, we calculate the horizontal shortening rate across the Shillong Plateau to be 1.0–2.0 mm/yr. Use of a shallower dip for the Dauki Fault (35°) produces higher horizontal shortening rates of about 1.5–2.9 mm/yr; however, lower rates are obtained if the geodetically determined fault dip of 57° ± 8° for the Oldham Fault prevails also in the Dauki system (Bilham and England, 2001) (Fig. 5).
Faulting rates determined
Discussion
Altogether, we consider the area deforming around the Shillong Plateau to encompass a region nearly 600 km east by 150 km north (90,000 km 2), which accounts for more than 25% of the entire length of the Himalayan arc. We argue that deformation encompasses not only the region of elevated, exposed basement of the Shillong Plateau proper, but also includes deformation of a much larger region where Himalayan foredeep rocks are attenuated or missing (Fig. 1). The western extent of the attenuated
Conclusions
New apatite (U-Th-Sm)/He data suggest deformation of the Shillong Plateau initiated in mid- to late-Miocene time, significantly earlier than was previously estimated from the sedimentary record alone. Helium ages collected within 150 m depth from the plateau surface vary systematically with depth between 116 and 14 Ma and are suggestive of slow cooling during this time interval. Samples collected on a vertical transect along the southern limb of the anticline indicate a change to rapid cooling
Acknowledgments
We thank Ken Farley for access to the Caltech Noble Gas Facility in order to produce our helium measurements. We also thank Anjan Battacharyya, Sanjeev Bhattacharyya, B. P. Durah, Nicole Feldl, Vinod Gaur, Malay Mukul, and C. P. Rajendran for their assistance during our field work. Ken Farley, Peter Molnar, Karl Mueller, and Nathan Niemi contributed in the valuable discussions regarding helium age reproducibility, structural geometries applicable to the Shillong Plateau and comparisons between
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