Lincoln S. Hollister

Ductile extrusion of the Higher Himalayan Crystalline in Bhutan: evidence from quartz microfabrics

Quartz textures measured from deformed quartz tectonites within the Lesser Himalaya and Higher Himalaya Crystalline of Bhutan show similar patterns. Orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense: the asymmetry of c-axis and a-axis patterns consistently indicates top-to-the-south shearing. The obliquity of the texture and the inferred finite strain (plane strain to moderately constrictional), suggest the strain regime had a combination of rotational and irrotational strain path. In most of the samples from the Bhutan Himalaya, the inferred deformation mechanisms suggest moderate- to high-temperature conditions of deformation that produced the observed crystallographic preferred orientation. Much higher temperature of deformation is indicated in the quartz veins from a leucogranite. The observed ductile deformation is pervasively developed in the rocks throughout the investigated area. The intensity of deformation increases only slightly in the vicinity of the Main Central Thrust. Simultaneous southward shearing within a large part of the Higher Himalaya Crystalline near and above the Main Central Thrust and normal faulting across the South Tibetan Detachment, is explained by the tectonically induced extrusion of a ductily deforming wedge. The process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes. Channel flow accounts for some observed phenomena in the Himalayan orogen such as inverted metamorphic sequences near the Main Central thrust, not related to an inversion of isotherms, and the syntectonic emplacement of leucogranites into the extruding wedge, locally leading to an inversion of isotherms due to heat advection.

Himalayan metamorphic sequence as an orogenic channel: Insight from Bhutan

The Bhutan Himalayas differ from the rest of the Himalayas in two major ways: (i) low-grade sedimentary rocks lie above the Greater Himalayan Sequence as klippen (i.e. erosional remains of the South Tibetan Detachment); and (ii) an out-of-sequence thrust, the Kakhtang thrust, lies structurally above the klippen, and it doubles the exposed thickness of the Greater Himalayan Sequence. Our field observations and geochronological data constrain the main kinematic events in the Bhutan Himalayas. Crystallisation ages of leucogranite dykes deformed by the Main Central Thrust and the South Tibetan Detachment indicate that these two structures operated together between 16 and 22 Ma. The out-of-sequence Kakhtang thrust was active at 10–14 Ma and was concurrent with reactivation of the South Tibetan Detachment. Restoration of the Bhutan Himalayas prior to the out-of-sequence thrusting shows that the Greater Himalayan Sequence was the core of a long, low-viscosity crustal channel extending under the Tibetan plateau. We propose that the gravity-driven southward extrusion of the channel material from underneath the Tibetan plateau caused the inverted metamorphic sequence in the Lesser Himalayan Sequence and in the Greater Himalayan Sequence. This process also led to occurrences of present-day surface rocks that were derived from variable distances down dip, but from similar crustal depths. Such an exhumation pattern can explain the similar peak pressures for the Greater Himalayan Sequence along the length of the Himalayas.

Melt-enhanced deformation: A major tectonic process

Convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks. Anatectic and/or mantle-derived melts combine to form a melt-weakened zone in the thickened lower crust. The accumulation of melt eventually leads to crustal failure along melt-lubricated shear zones. Rapid (>1 mm/yr vertical component) movements of large crustal blocks result. We refer to these crustal displacements as tectonic surges. The melt-lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons. Rocks that formed at different crustal levels are juxtaposed across these shear zones. One result of surges with large lateral component of movement is metamorphic inversion with high-pressure and high-temperature metamorphic rocks structurally over lower pressure and temperature assemblages. Large and rapid vertical surges may displace crust containing abundant melt and may result in high T/P (including granulite facies) metamorphism and preservation of metamorphic textures caused by rapid decompression.

Phase equilibria in fluid inclusions from the Khtada Lake metamorphic complex

Abstract The Khtada Lake. British Columbia, metamorphic complex consists of high grade amphibolite and metasedimentary units with development of gneiss, migmatite and homogeneous autochthonous plutons. Maximum metamorphic conditions are estimated to have exceeded 5 kbar and 700°C. Fluid inclusions in matrix quartz are highly variable in density and composition, ranging from apparently pure CO 2 (gas or liquid or both at room temperature) through CO 2 + H 2 O ± CH 4 mixtures to inclusions which are entirely aqueous. They occur along cracks, in groups without planar features and as isolated inclusions. The latter and some which occur in groups, are interpreted to most nearly approximate, in density and composition, the fluids present during the peak of metamorphism. The density and fluid composition data are derived from direct observations of phase changes between − 180 and + 380°C and from the application of published experimental data in the system CH 4 -CO 2 -H 2 O-NaCl. The most dense, pure CO 2 inclusions indicate a pressure of entrapment at 5 kbar, if a temperature of 700°C is assumed. This is in close agreement with the minimum P - T estimates from the mineral assemblages. Methane was positively identified in inclusions in graphite-bearing specimens. Salt content is concluded to be about 5–6 wt% NaCl equivalent in the aqueous phase in both aqueous and CO 2 + H 2 O inclusions. There is evidence of immiscible separation of CO 2 -rich and H 2 O-rich fluids at temperatures at least as high as 375°C.

Enrichment of CO2 in fluid inclusions in quartz by removal of H2O during crystal-plastic deformation

Abstract Ductile strain-induced leakage of H 2 O from mixed CO 2 + H 2 O fluid inclusions in quartz is proposed as a mechanism for producing occurrences of pure CO 2 fluid inclusions in metamorphic rocks. The H 2 O needed for hydrolytic weakening of quartz under stress may be provided by the H 2 O in fluid inclusions. With dislocation creep, as dislocations nucleate on the walls of fluid inclusions or intersect them, H 2 O can be transported with the dislocations from the inclusion to the grain boundaries. The process should continue so long as there is stress on the host quartz and the inclusion contains H 2 O. The H 2 O taken to the grain boundaries would, due to its wetting properties, be wicked out along the grain boundaries. If the H 2 O is totally removed from an inclusion by a flux of dislocations through the quartz during the crystal-plastic flow, then a residual inclusion bearing components other than water should remain. A mixed CO 2 + H 2 O inclusion would, as a consequence of the process, become a pure CO 2 fluid inclusion. During dynamic recrystallization, the CO 2 would collect at grain boundary triple junctions and, following grain boundary migration, would become incorporated into the recrystallized quartz as fluid inclusions. This mechanism of generating pure CO 2 inclusions could result in CO 2 fluid inclusions having densities appropriate for the pressure—temperature conditions during deformation. Using estimates of temperature of deformation, the total confining pressure during deformation could be determined from the density of the CO 2 fluid in these inclusions.

The role of melt in the uplift and exhumation of orogenic belts

Abstract In some orogenic belts, rapid pulses of uplift occur when melt is present. Detailed work on the relation of melt to structural and metamorphic features in several localities shows that hot metamorphic rocks, and especially anatectic migmatite, can be physically transported from the lower to middle crust on decollements. The decollements form in the lower crust because, even though the lower crust is structurally weak throughout due to thermally activated crystal plastic deformation mechanisms, the crust is substantially weakened where melt forms or is intruded; melt in the lower crust is present long enough to accomodate strain, whereas in the upper crust it crystallizes too fast to be a factor in strain localization. Following crustal loading due to thrusting, exhumation results in the formation of additional melt in the migmatites due to decompression melting. Support of this sequence of events includes: (1) evidence for near isothermal decompression of high temperature metamorphic rocks, with early high pressure minerals having formed pre or synchronous with penetrative deformation and with lower pressure minerals having formed after the deformation, (2) the occurrence of magma sheets that occur along decollements between high temperature metamorphic rocks and lower temperature metamorphic rocks (in either thrust or normal configurations), (3) nearly concordant KAr cooling dates for biotite and hornblende suggesting rapid cooling rates, and (4) “post-tectonic” intrusions of leucogranite or leucotonalite that may represent the products of decompression melting. The crystalline slabs of the high Himalaya, the compressional surges of the Coast orogen of British Columbia and Alaska, the Cordillera of southern British Columbia, and the southern belt of the Hercynian chain are examples where melt assisted rapid pulses of uplift appear to have occurred. During the terminal phases of uplift, and following uplift, these orogenic belts were exhumed during extension while melt was still present.

Role of melt in the formation of a deep-crustal compressive shear zone: The MaClaren Glacier Metamorphic Belt, south central Alaska

The Maclaren Glacier metamorphic belt is an exhumed portion of a deep-crustal shear zone where hot, upper amphibolite facies rocks were emplaced over cooler, lower grade rocks. It is located in south central Alaska within the collision zone between terranes previously accreted to North America and the Wrangellia superterrane. Crosscutting relationships and orientations of thin granitoid sills within the hanging wall show that melt was repeatedly intruded into the shear zone during overall compression. In addition, a 1 km thick tonalire sill (the Valdez Creek tonalire) was emplaced into the shear zone while it was active; this conclusion is based on the presence of a well-developed foliation within the sill which is concordant to the fabric of the shear zone, the alignment and tiling of plagioclase laths, the presence of highly strained mafic enclaves within a matrix which shows no evidence of any subsolidus deformation, and the preservation of a delicate magmatic texture which indicates that a magmatic fluid was still present after deformation in the sill had ceased. A one-dimensional thermal model for sill-shaped plutons of varying thicknesses, which are emplaced into country rocks at different initial temperatures, indicates that melt can be present for long periods of time in the deep crust (> 1 m.y.). An important factor controlling the length of time that a sill remains molten is the temperature of the surrounding rocks which, in the lower crust, can be as high as the solidus temperature of granitoid melts. If the amount of time for melt to crystallize is sufficiently long, potentially large amounts of strain will be accommodated by zones containing melt due to the low strength of melt compared to rock. In the Maclaren Glacier metamorphic belt, the amount of time needed for the Valdez Creek tona!ite to fully crystallize is calculated to be about 90,000 years. This is a minimum value since the original thickness of the sill is not known. Nevertheless, using the 90,000 year value and assuming that the bulk of convergence between North America and Wrangellia was concentrated within the Valdez Creek tonalire, a displacement of at least !0 km could have been accommodated across the sill while melt was present.

An empirical solvus for CO2-H2O-2.6 wt% salt

The solvus in the system CO2-H2O-2.6 wt% NaCl-equivalent was determined by measuring temperature of homogenization in fluid inclusions which contained variable CO2H2O but the same amount of salt dissolved in the aqueous phase at room temperature. The critical point of the solvus is at 340 ± 5°C, at pressures between 1 and 2 kbar; this is about 65°C higher than for the pure CO2-H2O system. The solvus is assymetrical, with a steeper H2O-rich limb and with the critical point at mole fraction of water between 0.65 and 0.8.

Large‐scale transpressive shear zone patterns and displacements within magmatic arcs: The Coast Plutonic Complex, British Columbia

The Coast Plutonic Complex is the largest magmatic arc of the North American Cordillera, extending from northwestern Washington State to eastern Alaska. It forms the transition between two tectonic domains that are suspected to have undergone several phases of large (several thousands of kilometers) orogen-parallel displacement during the Mesozoic and early Cenozoic. A compilation of fabric data, published isotopic ages, and new structural observations shows that the western Coast Plutonic Complex was affected by subvertical, orogen-parallel, crustal-scale shear zones. These shear zones mainly reflect sinistral transpression and were sequentially active from ∼110 to 87 Ma during the intrusion of voluminous batholiths. Sinistral shearing was roughly coeval with the development of the thrust belts flanking the Coast Plutonic Complex (between ∼101 and ∼85 Ma), suggesting plate-scale transpression was a first-order process in the construction of the Coast Mountains orogen. These shear zones separate panels with distinct plutonic and cooling histories, suggesting the sinistral displacements between crustal blocks were large (greater than tens to hundreds of kilometers). This transpressive shear system likely reflects the Jurassic to early Late Cretaceous migration of outboard Cordilleran terranes to the south suggested by paleomagnetic evidence and plate reconstruction models. This example from the Coast orogen shows how transpression is partitioned between a thermally weakened magmatic arc and outwardly vergent fold-and-thrust belts. Our analysis further shows that the ∼2000-km-long Late Cretaceous to early Tertiary Coast shear zone has a minimum extent toward the south to at least 51°30′N.

Geologic Map of Bhutan

Abstract Please click here to download the map associated with this article. We present a new, 1:500,000-scale geologic map of the kingdom of Bhutan, and surrounding areas of India and southern Tibet. The map is a compilation of the most complete and most recent mapping datasets available, and presents an unprecedented amount of structural data when compared to previous published geologic maps of Bhutan. The map is a combination of: 1) new data presented in this study; 2) compilation of small-scale, published geologic maps of specific areas of Bhutan, Tibet, and parts of India; and 3) compilation of specific areas of published, country-scale geologic maps of Bhutan. Mapping detail is focused primarily on Subhimalayan, Lesser Himalayan, and Greater Himalayan rocks, with a lower level of detail on Tethyan Himalayan rocks. We present new 3-part stratigraphic divisions for the Siwalik Group and the structurally-lower Greater Himalayan section, and compile detailed stratigraphic divisions and structural geometries for the Lesser Himalayan section and Paro Formation. We also compile detailed mapping of the Yadong cross-structure and other structurally-complex areas of southern Tibet, and present new locations for the South Tibetan detachment. Our map compilation also highlights specific areas in Bhutan that would benefit from future geologic mapping. It is our hope that this map will be a valuable tool to be utilized by Bhutanese geologists, future researchers visiting Bhutan, and travelers and trekkers as well. We also hope that this map will serve as a new starting point for future mapping in Bhutan and throughout the Himalayan orogen.