Modeling of Geological Evolution of the Gulf of Mannar Area, South India, by the Event Bush Method

Introduction

The basin of Gulf of Mannar (Mannar Basin) lies between the south-eastern India and western Sri Lanka (6-9°N and 78-80°E), forming a part of extension of Laccadive Sea of the Indian Ocean to the north (Figure 1).

Figure 1.

Location of the Gulf of Mannar (blue shade) and the red box indicates the study area

Early workers treated the Mannar Basin as southern extension of a larger Cauvery Basin followed from Muthupattinam at the south to Portonovo at the north (Baillie et al. 2002). Later, Rana et al. (2008) considered Mannar as a separate basin, which only shows some structural similarity to Cauvery Basin, mostly by its horst-graben tectonics. Based on the stratigraphic data, these scientists state that in general the Mannar basin remained a separate graben all through its history between the Sri Lankan and Indian cratons (Figure 2).

Figure 2.

Tectonic structure of the northern part of the Mannar basin and adjacent territory

The Indian coast of Gulf of Mannar, from Mandapam at the north to Kanyakumari at the south, has received great attention due to its physiographical diversity observed all along its length of 440 km. The coastal landforms are quite diverse, including large strand plains, deltaic marshes, huge sand dune fields, and ridges built of sedimentary rocks and isolated patches of granulitic rocks. Contemporary tectonic movements have been felt in the area in 1948, including the 6m subsidence of a village near Dhanushkodi (Vaz et al., 2006). Anomalously high heat flow (100 to 180 mw per sq m, quite comparable to the Himalayan hotsprings) was identified in the Ramanad area. The Mannar Basin is known for its highest heat flow value in the peninsular Precambrian region (Longhinos et al., 2010). Towards the south, closer to Kanyakumari, dikes and bodies of olivine basalts of supposedly Late Mesozoic age are reported (Ramasamy et al., 1994; Ramasamy, 1995; Longhinos et al., 2010). The same time, huge (sometimes over 60 m thick) cover of migrating sand, at places semi-consolidated, indicates an ongoing phase of aeolian deposition and redeposition since Pleistocene.

This, along with other processes of vigorous weathering and denudation, represents the main problem in interpretation of the tectonic history and current geodynamic regime of the territory. From one side, the area must represent a perfect case of marginal environment of rift activation, with prominent peripheral half-grabens and other typical associated features, including the nowadays processes. From the other, the sand cover spreading along the coast limits exposure of geological units and may have obscured events that the coast underwent even recently. Only few appropriate exposures can be found there, and the geologist is supposed rather to infer than to observe the neotectonic and contemporary movements in the area. Although, locally there is preserved a direct geological evidence of ongoing uplift seen along the coast. It is expressed in the advance and retreat of sea and vertical displacements of the beach. The same time, huge dunes may imply a retreat of the sea, which may be caused either by uplift of land or by downsagging of the basin coupled with strengthening of wind blowing fine-grained coastal sands into the hinterland. Therefore, a complex interpretation is needed with a very accurate treatment of all cause-effect relations between the hypothesized tectonic “driving force” and observed stratigraphic results.

For this, first, all possible consequences of the suggested movements should be tracked given the (i) geological sequence that is believed to have had existed and (ii) principal physical-geological agents that had to exist in the area and/or exist now (i.e., sun+air, sea, probably vegetation). Then, every known outcrop or another geologically peculiar object in the area must be related to this framework to either support or refute the suggested hypothesis.