The Indus Fan, although only about one third of the volume of its giant neighbor in the Bay of Bengal, is one of the largest sediment bodies in the ocean basins, totalling ~5 x 106 km3. Its detrital sedimentary record is an important repository of information on the uplift and erosion of the western Himalaya. New seismic and provenance data from the Pakistan margin now suggest that the Indus River and Fan system was initiated shortly after India-Asia collision at ~55 Ma. The modern Indus drainage basin is dominated by the high peaks of the Karakoram, Kohistan and other tectonic units of the Indus Suture Zone, rather than the High Himalaya, which form the principle sediment sources to the Bengal Fan. The Indus River, which rises in western Tibet near Mount Kailas, follows the Indus Suture Zone along strike before cutting orthogonally through the mountain range to the Arabian Sea. The other tributaries to the Indus, such as the Chenab and Sutlej, do drain the crystalline High Himalayan range but do so in an area where its topography is much reduced (Fig. 1). In contrast, the Bengal Fan's main feeder rivers, the Ganges and Brahmaputra, follow the High Himalaya along strike for much of the length of the orogen. In practice this means that the Bengal Fan is swamped by the large volume of material derived from the rapidly unroofing High Himalaya, while the Indus Fan is dominated by tectonic units adjacent to the suture zone, including western Tibet. This allows their erosional signal to be more readily isolated in the Indus Fan compared to the Bengal.
Age of the Fan
The age of the Indus Fan has been a matter of debate over the past decade, with some workers suggesting that the modern fan might be basically a Late MioceneRecent construction. In this scenario the fan is built as a response to High Himalayan uplift and unroofing starting at ~20 Ma, coupled with the Early Miocene (~22 Ma) uplift of the Murray and Owen Ridges that prevents sedimentation towards the west into the Gulf of Oman. Although the earliest suggested fan deposits sampled at Deep Sea Drilling Project (DSDP) Site 221 in the central Indian Ocean have been dated as Late Oligocene (2530 Ma), the location of the drill site, far from the sediment sources, always implied that fan sedimentation had initiated earlier further towards the north. Paleogene clastic sediments, presumed to be part of the paleo-Indus Fan system, have been found at several locations around the Arabian Sea. Thick sections of PaleoceneEocene turbidites have been identified in the Makran Accretionary Complex in western Pakistan and eastern Iran. These suggest that the Indus Fan was centered more to the west of its present location during the Paleogene. Since that time this part of the early Indus Fan system has largely been accreted to the Asian margin due to ongoing northward subduction along the Makran coast. Those parts of the Paleogene Indus Fan still present in that area are now buried under Neogene clastic sediment eroded from the Makran and Oman margins. Although the center of fan deposition may have been west of the current location, it is clear that some material was deposited in the area of the modern fan during the Paleogene. New seismic data collected on the upper and middle Indus Fan by BGR Hannover (Fig. 2) shows a tilted Paleogene section, predating uplift of the Owen and Murray Ridges in the Early Miocene. BGR line 122-23 shows a ~1.5 km thick section predating uplift in the vicinity of the Murray Ridge, with significant thickening shoreward under the Pakistan Shelf.
In addition to deep buried parts of the Indus Fan, Eocene and Oligocene siltstones and sandstones have been recovered from the Owen Ridge by drilling at DSDP Site 224 and Ocean Drilling Program (ODP) Site 731. These sedimentary rocks are presumed to be parts of the Paleogene Indus Fan elevated by uplift of the Owen Ridge, and saved from the deep burial that makes sampling of this time interval difficult elsewhere offshore. Although distal, muddy turbidites they provide clear evidence of fan sedimentation in the central and northern Arabian Sea back into the Paleogene.
Crucial to supporting the idea of an early Paleogene fan is evidence that these materials are derived from the Himalaya and not just the Indian Shield. Pb isotope character of detrital K-feldspars has previously been used as a provenance tool using conventional mass spectrometry methods. K-feldspars are good provenance indicators because they are common in continental settings and because they typically contain enough Pb to allow measurement of the isotopic ratio using modern probe technology. In a pilot study we have exploited the distinct isotopic character of the High Himalaya, Transhimalaya, Kohistan Arc, Karakoram and South Tibetan Block with respect to Pb isotopes (Fig. 3). The in situ capabilities of a high-resolution ion microprobe, in this case the Cameca 1270 facility at Woods Hole Oceanographic Institution allow the unaltered cores of small detrital grains to be analyzed. Detrital K-feldspars grains in Middle Eocene sandstones recovered at DSDP Site 224 gave characteristics that indicate a source in the Indus Suture Zone, the Transhimalayan batholith or Kohistan Arc (Fig. 3). This observation requires that India-Asia collision had occurred by Middle Eocene times (>45 Ma) and that a proto Indus River system was in existence to bring the material into the ocean basin.
Onshore other parts of this paleo-Indus system have been identified. In the Katawaz Basin of Pakistan and Afghanistan a latest PaleoceneEocene deltaic sequence has been identified, while along the Indus Suture Zone in the Ladakh Himalaya of India fluvial clastic sedimentary rocks (Indus Molasse Group) are exposed. These rocks are poorly dated, due to their continental facies and low grade metamorphic grade, but they are known to predate uplift of the High Himalaya at ~20 Ma, and mostly postdate the last marine incursion along the suture, dated as being uppermost Paleocene >55 Ma). Current flow indicators in these sedimentary rocks show a northward flow prior to the final marine incursion, followed by the establishment of a dominant westward draining system shortly after collision, i.e. during the Early Eocene (Fig. 4). This change is interpreted to reflect the initiation of the Indus River, which is still located in the suture zone, some 55 m.y. after it first flowed through the area.
Northward thrusting of the Indus Molasse at ~20 Ma now results in the Indus River cutting and eroding its own clastic deposits. This relationship demonstrates how difficult it is to stop or reroute a major erosive river, such as the Indus, once it is established. The ability of the river to keep pace with erosion is dramatically demonstrated where it cuts the Nanga Parbat-Haramosh massif in Pakistan, since this is one of the most rapidly uplifting areas in the world. Provenance work on the foreland Siwalik sandstone sequences in western Pakistan supports the concept of the Indus River being basically stationary through time and dominated by erosion of the Indus Suture Zone and surrounding units. Abundant blue-green hornblende has been identified in Middle MioceneRecent (011 Ma) sandstones within Siwalik sequences of the Trans-Indus, Kohat and western Potawar Plateau areas in western Pakistan. This mineral is considered to be uniquely tied to erosion of the Kohistan Arc terrane and thus an Indus River source. Sandstones of this description are not found in the eastern Potawar Plateau and all points eastward in the foreland, a relationship that requires the Indus River to have exited the mountain front in approximately the same position (±100 km) since at least the Middle Miocene (~11 Ma).
The apparent difference in source between the Bengal and Indus Fans has some important implications for tectonic work in South Asia. It demonstrates that different fans provide quite different images of the evolving mountains, an important consideration when dealing with ancient syn-orogenic deposits. In effect the Bengal Fan provide a detailed record of unroofing and erosion in the High Himalaya since ~20 Ma. Presumably the deep levels of that fan record earlier erosion of the Transhimalaya and South Tibet in the east of the orogen, but these deposits are now inaccessible due to burial. Unfortunately no feature equivalent to the Owen or Murray Ridges exists in the Bay of Bengal, which would allow access to deep levels. In contrast, the Indus Fan provides the opportunity to examine the uplift and erosion of the Indus Suture Zone and western South Tibet, together with its dissected equivalents in the Karakoram. The existence of a Paleogene history to Indus Suture Zone and Tibetan uplift, both within the modern Indus Fan and in sections now uplifted on the Murray Ridge, is especially intriguing because of the lack of a well dated Paleogene record available in the Indian foreland, where the entire Oligocene appears to be missing.
Models proposed by Raymo and co-workers that have linked the tectonic uplift of the Tibetan Plateau with global climatic deterioration since Early Eocene times, as well as regional climatic evolution, most notably the South Asian Monsoon, have been difficult to test because of controversy in reconstructing the timing and degree of uplift of the plateau. The sedimentary record on the plateau itself is patchily preserved and poorly dated due to the continental facies of the sedimentary record. Although the marine record cannot provide paleo-altitude information, pulses of uplift of the Tibetan Plateau and Karakoram should be recorded in the Indus Fan due to erosion along the western flank of this feature. New sediment provenance techniques, coupled with single grains thermochronologic methods, should allow changes in the pattern and rates of erosion in these areas to be reconstructed since the initial India-Asia collision. Exploiting the unique sedimentary record of the Indus River and Fan system, onland and offshore, may allow solid earth-atmospheric coupling hypotheses to be tested in this type area.
PDC thanks JOI/USSAC for support to work
on the Arabian Sea and BGR for generous access to their seismic data.
Peter Clift, Nobu Shimizu, and Graham Layne,
Department of Geology and Geophysics, Woods Hole Oceanographic Institution,
Woods Hole, MA 02543
Christoph Gaedicke, Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655, Hannover, Germany
Marin Clark, Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307
Figure 1. Digital topography and GEBCO bathymetry map of the Indus River and Fan system showing the origin of the modern river in western Tibet.
Figure 2. Interpreted seismic lines SO122-23 cutting the Murray Ridge on the middle Indus Fan. Note section of pre-uplift sediment, presumed to be of Paleogene age.
Figure 3. Pb isotopic discrimination diagram showing the Transhimalayan or Kohistani character of detrital Middle Eocene feldspars from DSDP Site 224 and Pleistocene grains from ODP Site 720. The measurements on the small detrital grains were made using in situ ion microprobe methods on the Cameca ism 1270 facility at Woods Hole Oceanographic Institution. Error bars show 2s uncertainties.
Paleo-current indicators from the Indus Molasse, India, showing the switch
to a westerly draining axial river system after the final marine incursion
in the uppermost Paleocene.
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February 18, 2000.
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