RAPID MIDDLE MIOCENE DENUDATION OF THE EASTERN HIMALAYAS: LASER 40AR/39AR AGE CONSTRAINTS ON MIOCENE SEQUENCES FROM THE BENGAL BASIN

The Bengal basin is a modern collisional foreland basin, which contains both pre- and syn-tectonic stratigraphic sequences that links the eastern Himalayas with the Bengal fan. These sequences provide detrital information on the eastern Himalayas that record a significantly older history than that available from ODP and DSDP cores recovered from the Bengal fan.

Petrographic, mineral-chemistry and subsurface studies reveal that orogenic sedimentation had already begun in the Bengal basin by the early Miocene. Laser ⁴⁰Ar/³⁹Ar age determinations were made for detrital muscovite grains from the lower-to-middle Miocene Bhuban Formation using the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL). The laser fusion ages range from ca. 12 Ma to 516 Ma, and thus suggest derivation from a combination of sources: the Himalayas, Indo-Burman ranges and the Indian shield. Numerous individual ages, however, peak around 16 Ma, 18 Ma, and 26 Ma suggesting continued unroofing of the Higher Himalayas since the early Miocene. Detrital micas of such an early age (16 Ma) for the Bhuban Formation indicate very little time span between the isotopic closure of 40Ar in muscovite and their ultimate deposition in middle Miocene strata.

The prominent detrital ages of ca. 16 and 22 Ma in this study are younger than those reported in the western Himalayan foreland basins. These younger detrital ages are consistent with rapid middle Miocene unroofing and erosion as has been proposed for crystalline rocks of the eastern Himalayas.

This result emphasizes the utility of detrital chronology (and the youngest ages determined for a particular sample) as a tool in evaluating stratigraphic age. Additional work on more samples may be able to provide the constraints needed to develop a basin-wide stratigraphy in the Bengal basin.