Spatial and temporal trends in exhumation of the Eastern Himalaya and syntaxis as determined from a multitechnique detrital thermochronological study of the Bengal Fan

Abstract

The Bengal Fan provides a Neogene record of Eastern and Central Himalaya exhumation. We provide the first detrital thermochronological study (apatite and rutile U-Pb, mica Ar-Ar, zircon fission track) of sediment samples collected during International Ocean Discovery Program (IODP) Expedition 354 to the mid–Bengal Fan. Our data from rutile and zircon fission-track thermochronometry show a shift in lag times over the interval 5.59–3.47 Ma. The oldest sample with a lag time of <1 m.y. has a depositional age between 3.76 and 3.47 Ma, and these short lag times continue to be recorded upward in the core to the youngest sediments analyzed, deposited at <1 Ma. We interpret the earliest record of short lag times to represent the onset of extremely rapid exhumation of the Eastern Himalayan syntaxial massif, defined as the syntaxial region north of the Nam La Thrust. Below the interval characterized by short lag times, the youngest sample analyzed with long lag times (>6 m.y.) has a depositional age of 5.59–4.50 Ma, and the zircon and rutile populations then show a static peak until >12 Ma. This interval, from 5.59–4.50 Ma to >12 Ma, is most easily interpreted as recording passive erosion of the Greater Himalaya. However, single grains with lag times of <4 m.y., but with high analytical uncertainty, are recorded over this interval. For sediments older than 10 Ma, these grains were derived from the Greater Himalaya, which was exhuming rapidly until ca. 14 Ma. In sediments younger than 10 Ma, these grains could represent slower, yet still rapid, exhumation of the syntaxial antiform to the south of the massif. Lag times <1 m.y. are again recorded from 14.5 Ma to the base of the studied section at 17 Ma, reflecting a period of Greater Himalayan rapid exhumation. Mica 40Ar/39Ar and apatite U-Pb data are not sensitive to syntaxial exhumation: We ascribe this to the paucity of white mica in syntaxial lithologies, and to high levels of common Pb, resulting in U-Pb ages associated with unacceptably high uncertainties, respectively.

Coupled Zircon-Rutile U-Pb Chronology: LA ICP-MS Dating, Geological Significance and Applications to Sediment Provenance in the Eastern Himalayan-Indo-Burman Region

“…Similar to the zircon data, the rutile U-Pb data in Bengal Fan sediments younger than 10 Ma [111] track the erosion of Himalayan sources, such as the ~500-600 Ma source (identified in modern detritus eroded from the Lhasa terrane, Figure 6b, but also present in the modern Dhansiri River draining the IBR, Figure 6m) or the 10-15 Ma GH source (Figures 6d-l and 7). In 2015, the IODP expedition 354 drilled seven sites along a proximal ( • 8N) east-west transect in the Bay of Bengal (Figure 5a) covering the Late Paleogene to present sedimentation of the Bengal Fan [146].…”

“…However, elevated common Pb (which is typically divalent [148]) is observed in apatite compared to rutile, likely due to preferential substitution into the apatite crystal lattice of common Pb 2+ for Ca 2+ , with radiogenic Pb (typically tetravalent) presumably hosted interstitially or in defects [149], hence more subject to diffusion out of the lattice. As pointed out by Najman et al [111], the high common to radiogenic Pb ratio in apatite results in high age uncertainty especially in young grains (e.g., absolute age uncertainty up to 50% for 10-100 Ma apatites). This is interpreted to be the most likely cause for apatite grains with short (<1 Myr) lag times not being observed in the Bengal Fan samples or in syntaxially-derived modern river sediment, in contrast to rutile.…”

“…The chemical and mechanical robustness of rutile and zircon and their occurrence as accessory phases in a wide range of crustal rocks makes both minerals ideal provenance proxies. The first applications of rutile U-Pb chronology coupled to zircon U-Pb chronology (and other thermo-and geochronometers) to provenance studies in the Eastern Himalayan-Indo-Burman region [42,84,[109][110][111] have proven the effectiveness of the coupled provenance proxy in tracking tectono-thermal events in the source areas across a wide range of temperatures, from medium-to high-T igneous and metamorphic crystallization to cooling through~600 • C. This approach is key to characterize detritus sourced from areas with complex poly-phase metamorphic histories such as collisional orogens.…”

“…It is only in recent years that U-Pb dating of detrital rutile has been employed in multi-technique provenance studies of Eastern Himalayan-and Indo-Burman-derived sediments [42,84,[109][110][111]. In this section, modern sediment and bedrock detrital rutile U-Pb data from these studies are summarized and compared to zircon data determined on the same samples to show the ability of the coupled U-Pb zircon-rutile provenance proxy of tracking multiple thermal events in the sediment source areas.…”

“…the modern Siang signature in Figure 6o). Najman et al [111] interpreted the earliest record of short lag times to represent the onset of extremely rapid exhumation of the Eastern Himalayan syntaxial massif starting sometime between 5.59 and 3.47 Ma. Figure 5a.…”