ALBERT

Notes: Abstract Geological relationships and geochronological data suggest that in Miocene time the metamorphic core of the central Himalayan orogen was a wedge-shaped body bounded below by the N-dipping Main Central thrust system and above the N-dipping South Tibetan detachment system. We infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the Indian foreland, thereby moderating the extreme topographic gradient at the southern margin of the Tibetan Plateau. Reaction textures, thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the Nyalam transect in southern Tibet (28°N, 86°E) imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core. Amphibolite facies metamorphism and anatexis at temperatures of 950 K and depths of at least 30 km accompanied the early stages of displacement on the Main Central thrust system. Our findings suggest that the late metamorphic history of these rocks was characterized by high-T decompression associated with roughly 15 km of unroofing by movement on the South Tibetan detachment system. In the middle of the metamorphic core, roughly 7–8 km below the basal detachment of the South Tibetan system, the decompression was essentially isothermal. Near the base of the metamorphic core, roughly 4–6 km above the Main Central thrust, the decompression was accompanied by about 150 K of cooling. We attribute the disparity between the P–T paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued (and probably accelerated) underthrusting of cooler rocks in the footwall of the Main Central thrust at the same time as movement on the South Tibetan detachment system.

Notes: Whereas geologists have known for three-quarters of a century that there was significant crustal thickening in the central East Greenland Caledonides, the crucial role of extensional faulting during Caledonian orogenesis has only been recognized during the past decade. In this paper, new petrographic and thermobarometric observations are presented from migmatitic metasedimentary gneisses of the Forsblad Fjord region (c. 72.5°N). Samples of the Krummedal Sequence, collected from the footwall of the upper of two significant splays of the main extensional fault system in the region—the Fjord Region Detachment (FRD)—enable us to establish a relative sequence of metamorphism. Our pressure (P)–temperature (T) results imply a clockwise loop in P–T space. As recorded by mineral assemblages in the Krummedal gneisses, prograde metamorphism involved a net increase of c. 4 kbar and 250 °C, with peak conditions of c. 10.5 kbar at 785 °C. Early burial and heating was followed by near-isothermal decompression of 4.5 kbar, a process which is attributed to roughly 18 km of tectonostratigraphic throw on the upper splay of the FRD. Combining data reported here with the published data, it is estimated that the approximate tectonostratigraphic throw along the lower splay of the FRD was c. 16 km. In situ U–Th–Pb-monazite electron microprobe dating suggests that the earliest phase of metamorphism recorded in the Krummedal Sequence gneisses of Forsblad Fjord occurred during the Caledonian orogeny. Furthermore, the combination of our new data with existing conventional TIMS U-Pb and 40Ar/39Ar data imply that: (1) movement along the uppermost splay of the FRD (c. 425–423 Ma) occurred at maximum time-averaged slip-rates equivalent to c. 9 mm of vertical displacement per year; and (2) that the final stages of metamorphism occurred prior to c. 411 Ma, although part of this denudation was likely accommodated on overlying extensional structures that may have been active more recently. There is close agreement between our data and results from the Krummedal Sequence north of the field area (72.5°−74°N), and rocks of the Smallefjord Sequence (75°−76°N) that are suggested to correlate with the Krummedal Sequence. This leads us to infer that the events recorded in the Forsblad Fjord region are of orogen-scale significance.

Notes: The metamorphic core of the Himalaya in the Kali Gandaki valley of central Nepal corresponds to a 5-km-thick sequence of upper amphibolite facies metasedimentary rocks. This Greater Himalayan Sequence (GHS) thrusts over the greenschist to lower amphibolite facies Lesser Himalayan Sequence (LHS) along the Lower Miocene Main Central Thrust (MCT), and it is separated from the overlying low-grade Tethyan Zone (TZ) by the Annapurna Detachment. Structural, petrographic, geothermobarometric and thermochronological data demonstrate that two major tectonometamorphic events characterize the evolution of the GHS. The first (Eohimalayan) episode included prograde, kyanite-grade metamorphism, during which the GHS was buried at depths greater than c. 35 km. A nappe structure in the lowermost TZ suggests that the Eohimalayan phase was associated with underthrusting of the GHS below the TZ. A c. 37 Ma 40Ar/39Ar hornblende date indicates a Late Eocene age for this phase. The second (Neohimalayan) event corresponded to a retrograde phase of kyanite-grade recrystallization, related to thrust emplacement of the GHS on the LHS. Prograde mineral assemblages in the MCT zone equilibrated at average T =880 K (610 °C) and P =940 MPa (=35 km), probably close to peak of metamorphic conditions. Slightly higher in the GHS, final equilibration of retrograde assemblages occurred at average T =810 K (540 °C) and P=650 MPa (=24 km), indicating re-equilibration during exhumation controlled by thrusting along the MCT and extension along the Annapurna Detachment. These results suggest an earlier equilibration in the MCT zone compared with higher levels, as a consequence of a higher cooling rate in the basal part of the GHS during its thrusting on the colder LHS. The Annapurna Detachment is considered to be a Neohimalayan, synmetamorphic structure, representing extensional reactivation of the Eohimalayan thrust along which the GHS initially underthrust the TZ. Within the upper GHS, a metamorphic discontinuity across a mylonitic shear zone testifies to significant, late- to post-metamorphic, out-of-sequence thrusting. The entire GHS cooled homogeneously below 600–700 K (330–430 °C) between 15 and 13 Ma (Middle Miocene), suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels.

Notes: Quantitative thermobarometry in pelites and garnet amphibolites from the Bitterroot metamorphic core complex, combined with U–Pb dating of metamorphic monazite and zircon from footwall rocks, provide new constraints on the P–T –t evolution of footwall rocks. The thermobarometric and geochronological results, when correlated with observations from other regions bordering the Bitterroot batholith, define a regional metamorphic history for the northern margin of the Bitterroot batholith consisting of three distinct events beginning with early prograde metamorphism (M1) coincident with arc-related magmatism and crustal shortening at c. 100–80 Ma. Magmatism and crustal thickening led to regional upper-amphibolite facies metamorphism (M2) and anatectic melting between 64 and 56 Ma. Mineral textures related to high-temperature isothermal decompression (M3), coincident with late stages of magmatism in the Bitterroot complex footwall (56–48 Ma), are only preserved in areas adjacent to extensional structures. The close temporal relationship between peak metamorphism and the onset of footwall decompression indicates that thermal weakening was an important factor in the initiation of Early Eocene regional extension and tectonic denudation of the Bitterroot complex and possibly the Boehls Butte metamorphic terrane. The morphology of the decompressional P–T –t path derived for Bitterroot footwall rocks is similar to other trajectories reported for Cordilleran core complexes and may represent a transition in the deformational style of core-bunding detachments responsible for exhumation.