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doi:10.14379/iodp.sp.355.2014

Background

Link between India-Eurasia collision and southwest monsoon

The southwest monsoon is one of the strongest climatic systems on Earth; it provides a lifeline for more than half the global population. Some have suggested that the southwest monsoon is strongly influenced by the topography of the Tibetan Plateau (Molnar et al., 1993, 2010; An et al., 2001) (Fig. F2). However, the situation is likely more complex, as the importance of the Himalayan barrier rather than the wide Tibetan Plateau in modulating the Asian monsoon has also been invoked as a key control (Boos and Kuang, 2010). Still other workers have highlighted processes such as the retreat of shallow seas in central Asia (Ramstein et al., 1997) and the closure of the Indonesian gateways (Potter and Szatmari, 2009) as crucial constraints. Nonetheless, numerical modeling confirms that introduction of a higher plateau to Asian topography intensifies rainfall over southwest Asia (Huber and Goldner, 2012). Presently, the plateau redistributes the regional mid-latitude circulation and affects the annual precipitation in this region (Molnar et al., 1993; An et al., 2001; Wang et al., 2005; Clift and Plumb, 2008;). The erosion of the Himalaya, largely driven by the southwest monsoon, has resulted in the accumulation of the world’s two largest submarine sedimentary deposits: the Indus Fan in the Arabian Sea and the Bengal Fan in the Bay of Bengal (Curray and Moore, 1982; Kolla and Coumes, 1987; Clift et al., 2001; Curray et al., 2003). The two basins jointly preserve records of past erosion and weathering that are controlled, at least in part, by monsoonal variability that in turn may be linked to the progressive uplift of the Tibetan Plateau (France-Lanord et al., 2000; Wang et al., 2005; Lunt et al., 2010).

The relationship between tectonic uplift and variability in strength of the southwest and East Asian monsoons is highly complex and their phase relationship is not yet understood. A rising Tibetan Plateau could have altered climate in a number of ways, including

1. A rising plateau would have deflected and blocked regional air systems, affecting global atmospheric circulation (Held, 1983).

2. The elevated region would have enhanced pressure-driven atmospheric flows as higher and lower atmospheric pressure systems developed over the plateau during the winter and summer. This would have intensified the southwest monsoon, leading to heavier rainfall along the frontal ranges of the Himalaya (Hahn and Manabe, 1975; Wang et al., 2003, 2005).

3. Higher elevations in eastern Tibet would have increased orographic precipitation in that region, which diabatically heated the atmosphere there. This heating would then force subsidence of air masses to the west, potentially suppressing rainfall in southwest Asia (Molnar and Rajagopalan, 2012).

Interactions between the solid Earth and atmosphere are not limited to the impact of mountains on the monsoon, but also vice versa. It has variously been suggested that the onset of Greater Himalayan exhumation was triggered by monsoon intensification (Clift et al., 2008) or by the relocation of the Intertropical Convergence Zone above the Himalaya as a result of India’s northward motion (Armstrong and Allen, 2011). Further, the enhanced availability of fresh rock surfaces exposed by denudation processes and the increased moisture availability might be predicted to enhance chemical weathering (Raymo and Ruddiman, 1992; Derry and France-Lanord, 1997). During chemical weathering, atmospheric carbon dioxide (CO₂) reacts with rock-forming silicates to produce bicarbonates, which may then be transported to the oceans where they eventually form carbonate rocks (Raymo and Ruddiman, 1992). Because CO₂ is an important greenhouse gas helping to warm the atmosphere, a decrease in the amount of atmospheric CO₂ would lead to global cooling. Raymo and Ruddiman (1992) argued that increased chemical weathering of the Himalaya resulted in decreased global CO₂, which culminated in Northern Hemisphere glaciation after 2.7 Ma.

The erosional records needed to reconstruct past environmental conditions can come from foreland basins as well as from the offshore deltas and fans where most of the sedimentation has occurred. Unfortunately, terrestrial records in this region tend to be poorly dated and incomplete, with a particularly large unconformity that spans the Eocene to early Miocene across the whole of the basin (Burbank et al., 1996; DeCelles et al., 1998). On the other hand, >70% of the total eroded mass from the Himalaya resides in the Indian Ocean (Clift, 2002), making any mass balancing that neglects the fans largely meaningless. As a result, testing competing hypotheses is possible only through analyzing long term quantitative records of erosion obtained from the Indian Ocean submarine fans.

The timing of uplift of the Himalaya-Tibetan Plateau is one of the major uncertainties in the linkage between mountain building and climate change in this region. Different views on the style and extent of surface uplift exist. Some advocate rapid late Pliocene–Pleistocene uplift based on mammalian fauna, paleokarst, and geomorphology (Ding et al., 1999; Qiang et al., 2001), whereas others suggest that uplift occurred gradually since the early Eocene, with substantial elevations reached by late Eocene, at least in southern Tibet (Dewey et al., 1988; Tapponnier et al., 2001). This latter view has been supported by oxygen isotopic compositions of paleosols and lacustrine sediment in central and southern Tibet (Garzione et al., 2000; Rowley and Currie, 2006), although these do not discriminate between the stepwise plateau growth favored by Tapponnier et al. (2001) and the more progressive growth modeled by Royden et al. (2008). Others think that major rapid uplift occurred after ~25 Ma and that the plateau attained its present elevation by ~14–15 Ma (Garzione et al., 2000; Harris, 2006), subsequently growing in area principally by expansion to the northeast and southeast (Clark et al., 2005; Schoenbohm et al., 2006; Royden et al., 2008). An alternative view suggests that the present elevation and extensional deformation of the plateau probably resulted from uplift caused by convective thinning of the underlying lithospheric mantle (England and Houseman, 1989), although this model does not make any firm predictions about when the uplift occurred.

Understanding passive rifted margins and continental breakup

The eastern Arabian Sea evolved after the break-up of Madagascar and India in the mid-Cretaceous and between India and the Seychelles during the Late Cretaceous (Norton and Sclater, 1979; Courtillot et al., 1988; White and McKenzie, 1989). The Deccan Traps, one of the best known examples of rapidly emplaced flood basalt, are considered to be imprints of the Réunion hotspot, which was located below the Indian continental lithosphere at ~66 Ma (Courtillot et al., 1988). Subsequently, hotspot activity emplaced magmatic intrusions within the crust of the western continental margin of India (Pandey et al., 1996; Singh, 2002). The interaction between the hotspot and the moving Indian plate also caused formation of a new seafloor spreading center, the Carlsberg Ridge (Storey et al., 1995), which resulted in conjugate rifted margins forming along the western margin of India and the eastern Seychelles (Chaubey et al., 2002a; Royer et al., 2002). The nature of these conjugate margins is poorly constrained, and knowledge about the entire break-up history and its relationship to the impact of the Réunion mantle plume is still elusive in the absence of direct observations.

The western continental margin of India consists of northeast–southwest trending structural highs, namely the Laccadive and Laxmi Ridges (Fig. F1) (Naini and Talwani, 1982; Biswas, 1987; Kolla and Coumes, 1990; Gopala Rao et al., 1992; Krishna et al., 1994; Subrahmanyam et al., 1995). Laxmi Ridge is largely considered to be a continental sliver that separated from India prior to the Seychelles-India break-up. The ridge differs from other oceanic features, aseismic ridges, and continental plateaus by being associated with a prominent negative free-air gravity anomaly (Naini and Talwani, 1982; Krishna et al., 1992; 1994; 2006). In contrast, Laxmi Basin, which is located between Laxmi Ridge and the Indian continental shelf, occupies an area of ~2.4 × 10⁵ km² and is marked by positive gravity anomalies, implying oceanic or strongly extended continental crust. Seismic investigations suggest that Laxmi Ridge has a crustal thickness of >21 km and has continental velocity characteristics similar to those found beneath the Indian Shield (Naini and Talwani, 1982). However, the nature of the crust under Laxmi Basin, with a thickness of ~11 km, is ambiguous because it neither matches a typical continental crust nor a typical oceanic crust seismic velocity model. If the crust in Laxmi Basin is oceanic and similar in age and emplacement mechanism to the Deccan Traps, this would strongly suggest a first-order tie between plume impact and break-up. However, if continental, Laxmi Basin crust should fit in the reassembled Gondwana and could challenge the hypothesis that the eastern Arabian Sea was generated as a volcanic rifted margin linked to the Deccan Traps. Consequently, drilling into Laxmi Basin to sample and date the basement rocks is a secondary objective of this proposal, but one that has the potential to resolve both the precise timing of the break-up as well as the nature of crust across the continent/ocean boundary.

Recently, the Ministry of Earth Sciences (MoES) in India has initiated deep continental drilling (8 km deep hole) onshore in the Deccan Traps region (central western India). This project is conducted in collaboration with the International Continental Scientific Drilling Program (ICDP), and the sites lie geographically opposite to the offshore sites proposed here. The onshore sites in central India are planned to core through the Deccan Traps and the Mesozoic sedimentary rocks underneath to reach metamorphic basement. This development provides an opportunity for the scientists involved in both Expedition 355 and the ICDP drilling project to build collaborations and to create an onshore-offshore lithologic/structural correlation for improved geodynamic understanding of this region.

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