Thomas K. Kelty

Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology, geochronology, and tectonic evolution of the eastern Himalaya

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 interaction 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 field mapping, U-Pb zircon age dating, and 40Ar/39Ar thermochronology 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 first three events also occurred in the northeastern Indian craton, while the last is unique to the Himalaya. 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 flow. 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 40Ar/39Ar 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 flat-lying strata prior to the India-Asia collision, leads to estimated shortening of 775 km (∼76% strain) and 515 km (∼70% strain), respectively. We favor the presence of significant basement topography below the eastern Himalaya based on projections of early Paleozoic 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.

Geologic correlation of the Himalayan orogen and Indian craton: Part 1. Structural geology, U-Pb zircon geochronology, and tectonic evolution of the Shillong Plateau and its neighboring regions in NE India

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.

Development of normal faults during emplacement of a thrust sheet: an example from the Lewis allochthon, Glacier National Park, Montana (U.S.A.)

Abstract Geologic mapping in southern Glacier National Park, Montana, reveals the presence of widespread, E-dipping normal faults within the basal portion of the Lewis allochthon. The displacement along the normal faults increases downward from less than 1 m at the highest exposure, 200–300 m above the Lewis Thrust, to a maximum of 200 m near or at the Lewis Thrust. The normal faults are located below discrete, bedding-parallel shear zones associated with mesoscopic structures characterized by NE- or SW-trending striations on bedding surfaces and asymmetric E-verging folds. These shear zones lie directly below the E-directed Brave Dog Fault, a major bedding-subparallel fault within the Lewis allochthon. The shear zones are interpreted to have formed during the development of the Brave Dog Fault. Striations on the Brave Dog Fault, normal faults and shear surfaces in the shear zones are consistent with the transport direction of the Lewis Thrust. The kinematic compatibility of the normal faults with the Lewis Thrust, the concentration of the normal faults along the basal part of the Lewis plate, and the increase in displacement along them toward the Lewis Thrust, all suggest that their development was synkinematic with eastward emplacement of the Lewis allochthon. The normal faults may have formed as Riedel shears (R) that accommodated a bulk, simple-shear strain within the thrust plate between the simultaneously moving subhorizontal Brave Dog and Lewis faults.

Structural evolution of the Lewis plate in Glacier National Park, Montana: Implications for regional tectonic development

Detailed geologic mapping in southern Glacier National Park, Montana, reveals four episodes of deformation in the hanging wall of the Lewis thrust. (1) Pre-Lewis thrust structures include west- and east-dipping imbricate thrusts, conjugate contraction faults, and west- and east-directed bedding-parallel faults. Although these structures are truncated from below by the Lewis thrust, their development was kinematically compatible with the emplacement of the Lewis plate. Thus, they may have formed during early stages of the emplacement of the Lewis plate. (2) Syn-Lewis thrust structures include the Late Cretaceous-early Tertiary Lewis thrust, west-dipping duplexes, east-dipping normal faults, and the Akamina syncline, a broad fold that lies directly west of the Lewis thrust and extends northwestward for about 120 km from southern Glacier Park, western Montana, to southeastern British Columbia and southwestern Alberta, Canada. The development of the duplexes and the normal faults may have been related to east-verging simple-shear deformation during emplacement of the Lewis plate. The formation of the segment of the Akamina syncline in the study area was the consequence of development of the duplexes in the Lewis plate, because strata above the duplexes are concordant with the syncline. The syncline is, however, disconcordant with the Lewis thrust. This observation contrasts strongly with the well-established concordant relationship between the Lewis thrust and the Akamina syncline in its hanging wall in Canada, about 100 km north of the study area. We propose that the formation of the Akamina syncline on a regional scale was related to the development of duplexes and imbricate thrusts at two structural levels, one above and one below the Lewis thrust. During the development of these duplexes, the Lewis thrust transferred horizontal shortening laterally along the strike of regional compressional structures from its footwall in the Paleozoic-Mesozoic strata to its hanging wall in the Proterozoic strata. We speculate that development of the broad-fold belt, a major structure in the fold-and-thrust belt in the southern Canadian Rocky Mountains and western Montana, was related to duplex formation at deep structural levels below the folds. (3) Post-Lewis thrust contractional structures include a high-angle reverse fault that cuts the Lewis thrust and strikes N70°W, which is about 30°-40° more to the west than the average strike of the syn- Lewis thrust structures. The development of this fault represents a change in compressional direction after emplacement of the Lewis plate. (4) Post-Lewis thrust extensional structures include southwest-dipping normal faults. These faults truncate the post-Lewis thrust reverse fault and are part of the Eocene-Oligocene Rocky Mountain trench normal fault system.

An elastic wedge model for the development of coeval normal and thrust faulting in the Mauna Loa‐Kilauea rift system in Hawaii

A long-standing enigma of the Mauna Loa-Kilauea rift system in Hawaii is the coeval development of normal and thrust faults that are vertically partitioned. To address this question, we developed a simple elastic wedge model that explores plausible boundary conditions in terms of tractions for generating such a fault pattern, Analytical solutions that best simulate the observed faulting style and geodetically determined strain at the surface require that (1) the pore fluid pressure ratio within the wedge (λ) and along the basal decollement (λ b ) must be exceedingly high (i.e., λ = λ b = 0.90-0.95) and (2) a tensile stress of the order of 10-30 MPa must have existed in the very top part of the rift zone at the back side of the wedge-shaped rift flank. The high pore fluid pressure within the rift flank may be induced by pumping of fluids during emplacement of magma, whereas the high pore fluid pressure along the basal decollement may be caused by compaction of water-saturated sediments between the volcanic pile above and the oceanic floor below. Although the predicted tensile stress in the rift zone could be related to the presence of a relatively steep topographic slope, our results show that this is not a prerequisite. Therefore we attribute occurrence of tensile stress to either upward bending of the Hawaiian volcanic pile due to emplacement of magma, or inflation of a shallow magma chamber several kilometers beneath the surface. In any case, the results of our model indicate that magma emplacement in the shallow part of the rift zone may be a passive process, while the deep rift zone experiences forceful emplacement (i.e., active rifting via magma push).