Himalayan Metamorphism and Its Tectonic Implications

The Himalayan range exposes a spectacular assemblage of metamorphic rocks from the midand deep crust that have fostered numerous models of how the crust responds to continental collisions. Recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one-time events. The pronounced metamorphic inversion across the Main Central Thrust reflects continuum thrusting between ca. 15 and 20 Ma, whereas exposure of ultrahigh-pressure rocks in northwestern massifs and syntaxis granulites reflects singular early (≥45 Ma) and late (≤10 Ma) exhumation events. Multiple mechanisms including wedge collapse and flow of melt-weakened midcrust are debated to explain pressuretemperature trajectories, patterns of thinning, and thermal overprinting. A geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid context to recover specific temperature-time points and paths. Combined chemical and chronologic analysis of monazite is now well established and titanite is particularly promising, but recent zircon data raise questions about anatectic rocks and their use for investigating tectonism. 381 A nn u. R ev . E ar th P la ne t. Sc i. 20 14 . 4 2: 38 141 9. D ow nl oa de d fr om w w w . a nn ua lr ev ie w s. or g A cc es s pr ov id ed b y R ic e U ni ve rs ity o n 04 /1 2/ 15 . F or p er so na l u se o nl y. EA42CH17-Kohn ARI 9 May 2014 15:24 Lesser Himalayan Sequence (LHS): mainly Proterozoic metasedimentary sequence, typically greenschist to amphibolite facies; occurs below the GHS Greater Himalayan Sequence (GHS): Late Proterozoic to Paleozoic metasedimentary sequence, typically upper amphibolite to lower granulite facies; occurs above the LHS and below the THS Tethyan Himalayan Sequence (THS): mainly Late Proterozoic to Mesozoic metasedimentary sequence, typically unmetamorphosed to lower amphibolite facies; occurs above the GHS Main Boundary Thrust (MBT): thrust contact separating the LHS from foreland sediments (Siwaliks) Lesser Himalayan Duplex (LHD): series of in-sequence thrusts in the LHS forming a duplex that arches overlying GHS and THS rocks Main Central Thrust (MCT): thrust contact separating the GHS from the LHS INTRODUCTION Himalayan geology unquestionably has played a central role in shaping our understanding of orogenesis. After all, Himalayan thrust-belt architecture, rock-type distributions, geomorphology, and orogen-parallel and -perpendicular extension have given rise to numerous models— orogenic wedges, lower crustal flow, synorogenic magmatism, eclogite exhumation, origins of gneiss domes and oroclinal bends, etc.—and stimulated research linking geomorphic and tectonic processes. Metamorphic rocks have inspired and continue to inspire many of these models and thus warrant particular consideration. Compared with other orogens, two practical advantages attend Himalayan metamorphic studies. First, along-strike consistency of rock types and structures allows age-equivalent comparisons of different parts of the orogen and helps screen for local anomalies. Second, extreme youth facilitates chronologic analysis. For example, typical microanalytical uncertainties of 2% provide ≤1 Myr resolution in the Himalaya, whereas the same chronologic resolution in Paleozoic or Proterozoic orogens requires other analytical methods that attain <0.1 to 0.5% uncertainties. This review is basically separated into two parts. The first and longer part documents patterns and summarizes regional geology, quantitative pressure-temperature conditions and paths, unusual occurrences of eclogites and granulites, and accessory mineral geochronology and geochemistry. The second part addresses tectonic models, emphasizing how observations distinguish (or not) among potential tectonic drivers. Given that over 600 articles on Himalayan metamorphism have been published, only a minority of studies can be discussed here. Metamorphic domes in southern Tibet and textural links between metamorphism and deformation are not considered. I also emphasize studies whose observations are reliably and quantitatively linked to petrogenesis and pressure-temperature-time evolution. GEOLOGIC BACKGROUND: METAMORPHIC SANDWICHES AND ANOMALIES Regional geology is described at length by Yin & Harrison (2000) and Yin (2006), so the following section focuses on only the most salient points related to metamorphism and its tectonic drivers. The Himalaya span approximately 2,000 km, from Pakistan in the west through northwestern India, Nepal, and Bhutan, to southeastern Tibet and northeastern India (Figure 1). Collision between India and Asia commenced ca. 50–55 Ma (e.g., see summary in Najman et al. 2010), but motion on major thrusts within the metamorphosed portion of the orogenic wedge generally commenced ca. 25 Ma. Overall, the orogen exhibits a distinct arcuate form, with sharp bends at the western (Nanga Parbat) and eastern (Namche Barwa) syntaxes (Figure 1), where major drainages cross the orogen. A tripartite lithotectonic subdivision—Lesser, Greater, and Tethyan Himalayan Sequences (LHS, GHS, and THS, respectively)—frames Himalayan metamorphic studies (Figure 1). The LHS was emplaced above unmetamorphosed foreland sediments (Siwalik Group) along the Main Boundary Thrust (MBT). Numerous smaller thrusts have been proposed within the LHS, especially to form the Lesser Himalayan Duplex (LHD). This duplex is important structurally because it accommodated a significant portion of Indo-Asian convergence and tilted overlying LHS, GHS, and THS rock units to expose obliquely the Himalayan metamorphic core (DeCelles et al. 2001, Robinson et al. 2003). The GHS was emplaced above the LHS along the Main Central Thrust (MCT). Additional thrusts have been proposed within the GHS, but structural repetitions are less clear within this unit. All major thrusts are interpreted to have soled into a master detachment surface—the Main Himalayan Thrust (MHT). The THS shares broad stratigraphic similarities with the GHS, but in most regions was juxtaposed against it along a 382 Kohn A nn u. R ev . E ar th P la ne t. Sc i. 20 14 . 4 2: 38 141 9. D ow nl oa de d fr om w w w . a nn ua lr ev ie w s. or g A cc es s pr ov id ed b y R ic e U ni ve rs ity o n 04 /1 2/ 15 . F or p er so na l u se o nl y. EA42CH17-Kohn ARI 9 May 2014 15:24