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

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 ⁴⁰ Ar/³⁹ Ar 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.