Despite being the largest active collisional orogen on Earth, the growth mechanism of the Himalaya remains uncertain. Current debate has focused on the role of dynamic inter action between tectonics and climate and mass exchanges between the Himalayan and Tibetan crust during Cenozoic India-Asia collision. A major uncertainty in the debate comes from the lack of geologic information on the eastern segment of the Himalayas from 91°E to 97°E, which makes up about one-quarter of the mountain belt. To address this issue, we conducted detailed fi eld mapping, U-Pb zircon age dating, and 40 Ar/ 39 Ar thermo chronology along two geologic traverses at longitudes of 92°E and 94°E across the eastern Himalaya. Our dating indicates the region experienced magmatic events at 1745-1760 Ma, 825-878 Ma, 480-520 Ma, and 28-20 Ma. The fi rst three events also occurred in the northeastern Indian craton, while the last is unique to the Hima laya. Correlation of magmatic events and age-equivalent lithologic units suggests that the eastern segment of the Himalaya was constructed in situ by basement-involved thrusting, which is inconsistent with the hypothesis of high-grade Himalaya rocks derived from Tibet via channel fl ow. The Main Central thrust in the eastern Himalaya forms the roof of a major thrust duplex; its northern part was initiated at ca. 13 Ma, while the southern part was initiated at ca. 10 Ma, as indicated by 40 Ar/ 39 Ar thermochronometry. Crustal thickening of the Main Central thrust hanging wall was expressed by discrete ductile thrusting between 12 Ma and 7 Ma, overlapping in time with motion on the Main Central thrust below. Restoration of two possible geologic cross sections from one of our geologic traverses, where one assumes the existence of pre-Cenozoic deformation below the Himalaya and the other assumes fl at-lying strata prior to the IndiaAsia collision, leads to estimated shortening of 775 km (~76% strain) and 515 km (~70% strain), respectively. We favor the presence of signifi cant basement topog raphy below the eastern Himalaya based on projections of early Paleo zoic structures from the Shillong Plateau (i.e., the Central Shillong thrust) located ~50 km south of our study area. Since northeastern India and possibly the eastern Himalaya both experienced early Paleozoic contraction, the estimated shortening from this study may have resulted from a combined effect of early Paleozoic and Cenozoic deformation.

The Himalayan orogen has experienced intense Cenozoic deformation and widespread metamorphism, making it diffi cult to track its initial architecture and the subsequent deformation path during the Cenozoic India-Asia collision. To address this issue, we conducted structural mapping and U-Pb zircon geochronology across the Shillong Plateau, Mikir Hills, and Brahmaputra River Valley of northeastern India, located 30-100 km south of the eastern Himalaya. Our work reveals three episodes of igneous activity at ca. 1600 Ma, ca. 1100 Ma, and ca. 500 Ma, and three ductile-deformation events at ca. 1100 Ma, 520-500 Ma, and during the Cretaceous. The fi rst two events were contractional, possibly induced by assembly of Rodinia and Eastern Gondwana, while the last event was extensional, possibly related to breakup of Gondwana. Because of its prox imity to the Himalaya, the occurrence of 500 Ma contractional deformation in northeastern India implies that any attempt to determine the magnitude of Cenozoic deformation across the Himalayan orogen using Proterozoic strata as marker beds must fi rst remove the effect of early Paleozoic deformation. The lithostratigraphy of the Shillong Plateau established by this study and its correlation to the Himalayan units imply that the Greater Himalayan Crystalline Complex may be a tectonic mixture of Indian crystalline basement, its Proterozoic-Cambrian cover sequence, and an early Paleozoic arc. Although the Shillong Plateau may be regarded as a rigid block in the Cenozoic, our work demonstrates that distributed active left-slip faulting dominates its interior, consistent with earthquake focal mechanisms and global positioning system velocity fi elds across the region.

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In the Sikkim region of north-east India, the Main Central Thrust (MCT) juxtaposes high-grade gneisses of the Greater Himalayan Crystallines over lower-grade slates, phyllites and schists of the Lesser Himalaya Formation. Inverted metamorphism characterizes rocks that immediately underlie the thrust, and the large-scale South Tibet Detachment System (STDS) bounds the northern side of the Greater Himalayan Crystallines. In situ Th-Pb monazite ages indicate that the MCT shear zone in the Sikkim region was active at c. 22, 14-15 and 12-10 Ma, whereas zircon and monazite ages from a slightly deformed horizon of a High Himalayan leucogranite within the STDS suggest normal slip activity at c. 17 and 14-15 Ma. Although average monazite ages decrease towards structurally lower levels of the MCT shear zone, individual results do not follow a progressive younging pattern. Lesser Himalaya sample KBP1062A records monazite crystallization from 11.5 ± 0.2 to 12.2 ± 0.1 Ma and peak conditions of 610 ± 25°C and 7.5 ± 0.5 kbar, whereas, in the MCT shear zone rock CHG14103, monazite crystallized from 13.8 ± 0.5 to 11.9 ± 0.3 Ma at lower grade conditions of 525 ± 25°C and 6 ± 1 kbar. The P-T-t results indicate that the shear zone experienced a complicated slip history, and have implications for the understanding of mid-crustal extrusion and the role of out-of-sequence thrusts in convergent plate tectonic settings.

Models for the origin and deformation of Himalayan rocks are dependent upon geometric and age relationships between major units. We present fi eld mapping and U-Pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern Himalaya, revealing that: (1) the South Tibet detachment along the Bhutan-China border is a top-to-the-north ductile shear zone; (2) Late Triassic and Early Cretaceous sedimentary samples from the northern Indian margin show a similar age range of detrital zircons from ca. 3500 Ma to ca. 200 Ma, but the Late Triassic rocks are distinguished by a signifi cant age cluster between ca. 280 and ca. 220 Ma and a well-defi ned age peak at ca. 570 Ma, (3) an augen gneiss in the South Tibet detachment shear zone in southeast Tibet has a Cambrian-Ordovician crystallization age, (4) Main Central thrust hanging-wall paragneiss and footwall quartzites from the far western Arunachal Himalaya share similar provenance and Late Proterozoic maximum depositional ages, and (5) Main Central thrust footwall metagraywacke from the central western Arunachal Himalaya has a Paleoproterozoic maximum depositional age, indicated by a single prominent age peak of ca. 1780 Ma. Recent work in the eastern Himalaya demonstrates that in the early-middle Miocene, the Himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south. A proposed subsequent (middle Miocene) brittle low-angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here. The ca. 280-220 Ma detrital zircons of the Late Triassic strata are derived from an arc developed along the northern margin of the Lhasa terrane. Detritus from this arc was deposited on the northern margin of India during India-Lhasa rifting. Along-strike heterogeneity in Main Central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western Arunachal. Nonetheless, the Late Proterozoic rocks in the Main Central thrust hanging wall and footwall in far western Arunachal can be correlated to each other, and to previously analyzed rocks in the South Tibet detachment hanging wall to the west and in the Indian craton to the south. These fi ndings are synthesized in a reconstruction showing Late Triassic India-Lhasa rifting and Cenozoic eastern Himalayan construction via in situ thrusting of basement and cover sequences along the north Indian margin.