Scientific objectives

Expedition 355 is designed to drill deep into the Indus submarine fan at three locations and to sample the underlying volcanic basement at two of these sites. The primary objective is to better understand the erosional and weathering response of the western Himalaya, Karakoram, and Hindu Kush to the changing intensity of the southwest Asian monsoon since the onset of the India-Eurasia collision in the early Paleogene (e.g., Najman et al., 2010). In doing so we aim to understand what feedbacks exist between climatic evolution, mountain building, and surface processes in the global type area for such studies. In addition, coring the basement of Laxmi Basin will allow us to date and characterize the tectonics of continental break-up and the role that mantle thermal anomalies, most notably the proposed Réunion Plume, have played in the break-up and subsequent formation of the Indian Ocean and the emplacement of the Deccan Traps. This particular event has implications for biotic mass extinction events, as well as for continental margin tectonics. Specifically, our objectives are

1. Reconstruct long-term changes in erosion and weathering rates at submillennial to millennial timescales in order to compare with existing records of high frequency climatic variability.

Neogene sedimentary sections from the Indus Fan record the erosional and weathering response of the Indus drainage basin to changing climate, which have already been reconstructed using speleothem (Fleitmann et al., 2003), aeolian dust (deMenocal et al., 1991; Clemens and Prell, 2003), and upwelling/productivity records (Kroon et al., 1991; Prell et al., 1992), largely from the Oman margin on the opposite side of the Arabian Sea. Attempts to understand the response of landscape to climate change in southwest Asia have largely been limited to the last glacial cycle (Bookhagen et al., 2005; Giosan et al., 2012; Alizai et al., 2012); however, our proposed deep coring will permit us to examine the changes spanning many such cycles. Weathering intensity will be reconstructed using bulk sediment geochemical analysis, selected isotope systems (such as Sr), and clay mineralogy. These measurements must be performed in concert with provenance work to establish whether any of the chemical changes could be driven by changes in source composition or drainage capture (cf., Clift and Blusztajn, 2005). Bulk sediment and single grain provenance methods, including heavy mineral studies, U-Pb dating of detrital zircons, Ar-Ar dating of detrital mica grains, and apatite fission track, represent some of the methods known to be effective in this drainage system (Clift et al., 2004, 2012; Garzanti et al., 2005) that will allow changing patterns of erosion caused by waxing and waning of the monsoon to be tracked.

Age control will be central to the success of this objective, especially if we are to estimate the lag times between climate change and the sediment record of the deep basin. This will be achieved using a combination of biostratigraphy and magnetostratigraphy. The current water depths of our proposed sites suggest that calcareous microfossils should be present through much of the section, but we also plan siliceous microfossil studies in case carbonate microfossils are missing or poorly preserved over any part of the section. Palynology, carbon isotopes, and leaf wax organic geochemical studies will provide further information on the evolving onshore landscape, as they are known to be effective in this region (Budziak et al., 2000; Ponton et al., 2012). The data from our expedition can then be correlated with existing climate records to determine links between erosion and climate on shorter timescales.

2. Reconstruct changes in erosion and weathering intensity over tectonic timescales and assess whether any changes occurred at ~23, 15, and 10–8 Ma to test earlier hypotheses that invoke changes in monsoon intensity at those times.

Competing hypotheses exist for the timing of initial monsoon intensification based on a variety of proxies from across Asia, with some invoking the growth of the Tibetan Plateau (Molnar et al., 1993), the rising of the Greater Himalaya (Boos and Kuang, 2010), or the retreat of shallow seas from Central Asia (Ramstein et al., 1997). Unfortunately, most of the existing climate reconstructions do not span tectonic timescales, especially the critical initiation of the Greater Himalaya along the Main Central Thrust at ~23 Ma (Catlos et al., 2001; Godin et al., 2006; Tobgay et al., 2012). Without a long-term reconstruction of monsoon intensity, it is impossible to judge what tectonic processes are responsible for the intensification. Our improved understanding of how the monsoon and erosion/weathering interact on short timescales (Objective 1) will allow us to better use the long-term record to reconstruct environmental conditions through the Cenozoic. Many tectonic models for the Himalaya link intensified erosion, driven by stronger summer monsoon rains, to the start of Greater Himalayan exhumation (Hodges, 2006; Harris, 2007; Clift et al., 2008). However, currently no well-dated erosion record spans this critical interval, so these models remain untested. Correlating changes in Himalayan-Tibetan tectonics with the marine record of erosion and weathering is the key test for these mechanisms. Changes in sediment provenance tracked by a variety of bulk and single grain proxies are needed to show the intensified erosion that is predicted during Greater Himalayan exhumation. The timing of the final unroofing of the Greater Himalaya is also poorly defined (Najman, 2006) and should be indicated by influxes of high-grade metamorphic minerals into the fan. Comparison of detrital mineral cooling ages with depositional ages will allow us to assess changing rates of exhumation in Himalayan source regions in order to see how these are linked to climate change. Combined biostratigraphic and magnetostratigraphic studies will allow the age of these changes to be fixed.

Determining the age of the base of the fan is a key objective, as this is only known in distal (and therefore young) locations at the present time (e.g., Site 221 [Shipboard Scientific Party, 1974]). Provenance methods and mass accumulation rates are expected to show when the first detritus sourced from the northern side of the Indus-Yarlung Suture Zone arrived in the Arabian Sea. This age would provide an important constraint on the much-debated timing of India-Eurasia collision (Aitchison et al., 2007; Najman et al., 2010; Wu et al., 2014). This age is of much more than simple local paleogeographic interest because it constrains how much Indian continental crust has been underthrust into the collision zone. Simple comparison of that volume with the size of the Tibetan Plateau will allow us to assess whether horizontal compression can explain all of the strain accommodation since the onset of collision (England and Houseman, 1986; Dewey et al., 1989), or if major “extrusion” of crust as rigid blocks along major strike-slip faults is needed to accommodate the impact of Greater India (Molnar and Tapponnier, 1975; Replumaz and Tapponnier, 2003).

3. Decipher the nature of basement rocks in Laxmi Basin and constrain the timing of early seafloor spreading and the relationship to the emplacement of Deccan Flood Basalts. Does mantle plume initiation predate or postdate rifting and early spreading?

We plan penetration of 50–100 m of basement at two sites in order to determine the nature of basement rocks and the age of their eruption. Although biostratigraphic and magnetostratigraphic analyses conducted during the expedition will provide an age for the oldest sediment overlying the basement, postcruise radiometric dating, using methods such as 40Ar-39Ar, will be employed to constrain the age of eruption. Because the Deccan Traps have been very precisely dated onshore (Baksi, 1994; Courtillot et al., 2000; Chenet et al., 2008), the relationship between opening of Laxmi Basin and emplacement of the Deccan Traps will be revealed. Considering that the precise timing of the rifting in Laxmi Basin is unknown (Minshull et al., 2008), the indistinct nature of the magnetic anomalies in the basin leave open the possibility that it is floored by hyperextended continental crust (Bhattacharya et al., 1994; Krishna et al., 2006). Shipboard geochemical analyses will allow the composition of the volcanic rocks to be compared with Deccan Flood Basalts or mid-ocean-ridge basalts (MORBs), which would have no linkage to a deep-seated mantle plume or other mantle compositional anomalies. Sediment overlying the basement may potentially show the subsidence of the margin, which can further be used to look at the thermal state of the mantle under Laxmi Basin during its rifting. Many rifted volcanic margins are characterized by subaerial eruption and rapid subsidence (Calvès et al., 2008). If the volcanic sequences of Laxmi Basin are linked to the Deccan Traps, then this would add significantly to their volume and thus to their potential environmental impact and role in the biotic mass extinctions at the Cretaceous/Paleogene boundary (Courtillot et al., 1988; Self et al., 2008). The timing of rifting and bathymetric evolution of Laxmi Basin also have strong implications for precise paleogeographic reconstructions of the Arabian Sea during the Paleogene (Royer et al., 2002).