Micah J Jessup, University of Tennessee–Knoxville
It is a privilege to share some of my ongoing research with the structural geology and tectonics community in celebration of the 125th Anniversary for the Geological Society of America. My research group’s interests in structural geology and tectonics are aimed at continent- and micro-scale processes of deformation, metamorphism, partial melting, and exhumation during the evolution of convergent orogens. In this essay, I will highlight some of the techniques that we employ, while giving a few examples of their application to mid-crustal rocks exposed in the Himalaya.
The Himalaya is a convergent orogen that formed in response to the collision between the Indian and Eurasian Plates (Gansser, 1964), beginning in the Eocene and continuing today (for reviews see Hodges, 2000; Yin and Harrison, 2000; Yin, 2006). Convergence resulted in high elevations near the crest of the Himalaya where peaks exceed 8 km as well as the Tibetan plateau where a broad area maintains an average elevation of 5 km (Fig. 1). The Himalayan foreland is dominated by thrust faults that accommodated crustal shortening, while the Tibetan Plateau contains graben and strike-slip faults that accommodated extension (Fig 1).
In the transitional zone between the Himalaya and the southern portion of the Tibetan plateau erosion and exhumation created windows into the processes that occurred in the mid-crust during convergence (Fig. 1). This history of mid-crustal processes can be interpreted by integrating structural geology, metamorphic petrology, (U-Th)/Pb geochronology, and thermochronometry. These data constrain the kinematic evolution of shear zones, pressure-temperature-time paths of metamorphic rocks, and the timing and duration of partial melting of the mid-crust.
The Indus-Tsangpo suture zone marks the boundary between rocks of the Eurasian Plate to the north and Indian Plate to the south (Fig. 2). The Himalaya contain rocks from the Indian Plate, and can be simplified into three north-dipping lithotectonic units that are bounded by shear zones and fault systems. From north to south these are (Fig. 2): 1) The Tethyan Sedimentary Series, which was deformed by the northern fold and thrust belt; 2) the Greater Himalayan Series, a sequence of metamorphic and igneous rocks separated from the Tethyan Sedimentary Series by the South Tibetan detachment (Fig. 3); and 3) the Lesser Himalayan Series, which is bounded at the top by the Main Central thrust and at the base by the Main Boundary thrust. The fault at the base of the Tethyan Sedimentary Series was reactivated as the South Tibetan detachment (Fig. 3), an orogen-parallel, north-dipping system. The South Tibetan detachment juxtaposed the Greater Himalayan Series in the footwall with low-grade to unmetamorphosed Tethyan Sedimentary Series in the hanging wall during north-directed shearing (Burchfiel et al., 1992). The Lesser Himalaya Series was juxtaposed with the base of the Greater Himalayan Series by the Main Central thrust during south-directed movement. The Main Boundary thrust at the base of the Lesser Himalaya Series is the orogen-bounding fault.
While the majority of major fault systems strike parallel to the orogen and perpendicular to the direction of convergence (Fig. 3), there are several deviations from this system, including extensional shear zones that bound the Leo Pargil dome and Ama Drime massif, which formed at a high angle to the strike of the Himalaya (Fig. 1). Because these domes formed at different times and positions relative to the main fault systems, they provide alternative insights into mid-crustal processes. The atypical orientation of these mountain ranges is attributed to syn-convergent extension of the crust in an orientation that is parallel to orogenic strike. North-striking shear zones and active fault systems that are located approximately 40-km-east of Mount Everest bound the first example, the Ama Drime massif (Figs. 1 and 4; Jessup et al., 2008). The second example, Leo Pargil dome (Thiede et al., 2006), is located in NW India where the strike of the Himalaya is oblique to the convergence direction of the Indian Plate (Fig.1).
Ama Drime Massif
The Ama Drime massif is an elongate mountain range that extends northward from the crest of the Himalaya (Fig 4). This north-trending mountain range is a rare occurrence in a convergent orogen where the vast majority of the faults are oriented parallel to orogenic strike (e.g., South Tibetan detachment and Main Central thrust, Figs. 1 & 4). In the vicinity of the Ama Drime massif, the map-trace of the South Tibetan detachment is displaced (see orange-brown contact with dashed line on Fig. 4), indicating that exhumation of the massif followed the cessation of displacement on the South Tibetan detachment during the Middle Miocene (Jessup and Cottle, 2010). The eastern side of the range defines the western margin of the Dinggye rift, which extends north into the Tibetan plateau (Fig. 5) (Burchfiel et al., 1992).
The massif is bounded by two oppositely dipping shear zones: the Ama Drime detachment to the west and the Nyönno Ri detachment to the east (Fig. 4). The massif also contains rocks that were exhumed from one of the deepest positions in this portion of the Himalaya. Using yaks and local yak-herders, we crossed this remote mountain range to collect a suite of samples that were the basis for (U-Th)/He apatite thermochronometry conducted in collaboration with J. Spotila at Virginia Tech. These constrained a minimum exhumation rate of approximately 1 mm/yr between 1.5 and 3.0 Ma (Jessup et al., 2008).
Partial Melting and Strain Localization
The internal portion of the Ama Drime massif is dominated by rocks weakened by partial melting at 12-11 Ma (Cottle et al., 2009). Oppositely dipping shear zones are areas of relatively narrow (300 m) solid-state fabric development that overprinted an earlier history of melt-present deformation from deeper crustal positions. These gradients in deformation mechanisms mark an important transition in rheology of the mid-crust. Strain accommodated over wide areas of relatively weak, partially melted (subsolidus) rocks with rigid bodies (e.g., retrogressed mafic eclogite boudins) were overprinted by localized high temperature shear zones bounding areas of crystallized gneiss. To explore these concepts in more detail, I will present some salient features of our research on the Ama Drime detachment, bounding the west side of the massif.
The west-dipping Ama Drime detachment is composed of interlayered marble, calc-silicate, and schist that are located near the base of the Greater Himalayan Series (Fig. 4). The detachment juxtaposed upper amphibolite facies migmatites of the Greater Himalayan Series in the hanging wall with granulite facies Ama Drime orthogneiss in the footwall. Meso- and microscale shear sense indicators consistently record west-directed extension (Fig. 6) – nearly perpendicular to the classic high strain zones that bound the anatectic core of the Himalaya (e.g., South Tibetan detachment and Main Central thrust).
Detailed kinematic analysis involved the scanning electron microscope (SEM) equipped with electron backscatter diffraction (EBSD) at the University of California, Santa Barbara as well as the electron microprobe facility at the University of Tennessee, Knoxville. Deformation temperatures estimated using a combination of quartz and feldspar microstructures, quartz LPO, and asymmetric strain induced myrmekite range between approximately 400-650 °C during top-down-to-the-west sense of shear. When combined with strain estimates, these data constrain a minimum of 21-42 km of displacement during orogen-parallel extension (Fig 7).
A simplified cross section, extending from the summit of Mount Everest to the southern portion of the Ama Drime detachment presents the results of our detailed kinematic investigation (Figs. 4 & 7). The footwall of the Ama Drime detachment contains an abrupt transition from solid-state fabric development preserved in the mylonite to migmatitic gneiss with mafic lenses. At positions near the base of the Greater Himalayan Series (750°C, 7-8 kbar), partial melting and boudinage (Fig. 7) mark the initiation of orogen-parallel extension at 12-11 Ma (Cottle et al., 2009).
An aspect of the Ama Drime detachment that I find particularly interesting is the possibility that it localized along a pre-existing contact near or at the base of the Greater Himalayan Series. The footwall rocks reached at least 7-8 kbar before exhumation, indicating that the shear zone exhumed rocks from the deepest structural position exposed in the central Himalaya. Our estimates for deformation temperatures are linked to top-down-to-the-west shear sense indicators. The high temperature shear zone progressed into a localized system of higher strain mylonites during exhumation. The final stages were accommodated by a brittle detachment that is parallel to the shear plane as well as steep normal faults that cut the foliation. This model implies a high temperature, distributed shear zone exhumed positions in the crust with extensive partial melting. As the zone of partial melting crystallized, deformation was partitioned along the margins into narrow high strain zones that continued to exhume rocks through the brittle-ductile transition.
Summary of the Ama Drime Massif
- Orogen-parallel extension initiated at 12–11 Ma concomitant with partial melting at 750 °C, 7–8 kbar.
- A shear zone near the base of the Greater Himalayan Series accommodated decompression of granulite facies migmatitic gneiss at <11 Ma.
- During exhumation, a transition occurred from early partial melting to localized solid-state deformation along the bounding shear zones.
- Consistent shear sense is preserved within mylonites on the bounding shear zones, which deformed at a range of temperatures as rocks were exhumed in a kinematic setting that was dominated by orogen-parallel extension.
Leo Pargil Dome
The Leo Pargil dome is a northeast striking, elongate domal structure located in NW India where the strike of the orogen is oblique to the convergence direction (Thiede et al., 2006). Two oppositely dipping high strain zones define its margins: the Leo Pargil shear zone to the northwest (Thiede et al., 2006) and Qusum shear zone to the southeast (Zhang et al., 2000). Steep brittle normal faults cut the mylonites between 16-10 Ma (Thiede et al., 2006) within an area between the South Tibetan detachment to the southwest and the Karakoram fault to the northeast. Our investigation is focused on the southwestern corner of the dome (Fig. 8).
Metamorphic rocks in the Leo Pargil area contain assemblages that are excellent for integration of microstructural analysis, pressure-temperature estimates, and geochronology to construct Pressure-Temperature-time-Deformation paths. We combine compositional maps of phases with chemical data acquired using the Cameca SX-100 electron microprobe at the University of Tennessee. Through collaboration with J. Cottle, these electron microprobe data are combined with in-situ U-Th/Pb monazite geochronology using the Laser Ablation Split Stream ICP-MS facility at the University of California, Santa Barbara.
Barrovian Metamorphism, Partial Melting, and Strain Localization
Subduction of the Indian Plate beneath the Eurasian Plate thickened the crust within the NW Himalaya at 50–40 Ma (Treloar et al., 1989; de Sigoyer et al., 2000). In the upper crust, a fold and thrust belt within the Tethyan Sedimentary Series accommodated shortening (Searle et al., 1997; Steck 2003). At deeper structural positions crustal thickening led to Barrovian metamorphism during the Late Eocene and Early Oligocene. Recumbent folds (Fig. 9A) deformed foliation surfaces with kyanite and staurolite porphyroblasts (Jessup et al., in review). Garnet porphyroblasts preserve chemical zoning patterns that record growth during prograde metamorphism (Fig. 9B). In-situ U-Th/Pb monazite geochronology and thermobarometry indicate that conditions in these pelitic rocks (Haimanta Group) reached 530–630 ºC, 7–8 kbar before Barrovian metamorphism ended at 30 Ma (Chambers et al., 2009; Langille et al., 2012).
At deeper structural positions, the conditions were conducive to regional partial melting of pelitic rocks. The core of Leo Pargil dome contains migmatitic gneiss that fed a network of dikes and sills within an extensive injection complex (Fig. 10). The injection complex contains undeformed leucogranite bodies as well as bodies that were deformed by boudinage, folding, and shearing. Our new U-Th/Pb monazite geochronology, combined with previous data (Leech, 2008) indicate that this system was part of a semi-continuous period of partial melting from 30 to 18 Ma (Lederer et al., in press). Sillimanite and cordierite overgrowths on Barrovian assemblages (e.g., kyanite and staurolite) record near isothermal decompression (Fig. 11A). Tails with high Y-content grew on monazite grains at 23 Ma during west-directed movement on the Leo Pargil shear zone (Fig 11B). We propose, at least locally, that this defines the onset of exhumation within the mid-crust at approximately 23 Ma (Langille et al., 2012). Because the period of partial melting between 30 and 18 Ma spans the onset of decompression at 23 Ma, doming might have involved buoyancy-driven flow (Langille et al., 2012).
The Leo Pargil shear zone (Thiede et al., 2006) is defined as a distributed zone of deformation within leucogranite, schist, marble, and quartzite on the western margin of the dome (Fig. 8). The shear zone juxtaposed mid-crustal rocks in the footwall with Tethyan Sedimentary Series in the hanging wall. Meso-and microscale shear sense indicators consistently record west-directed sense of shear. Quartz LPO data from the central portion of the shear zone yield an early stage of high temperature deformation (500-650 °C) that was overprinted by deformation at lower temperatures (500-280 °C) during continued exhumation (Langille et al., in review). These data were combined to constrain the Pressure-Temperature-time-Deformation (P-T-t-D) history of the Leo Pargil dome and surrounding area (Fig. 12).
Summary for the Leo Pargil Dome
- Burial was followed by Barrovian metamorphism that reached 530-630 ºC, 7–8 kbar and ended by 30 Ma.
- The cessation of Barrovian metamorphism marks the onset of a protracted period of semi-continuous partial melting between 30–18 Ma.
- Movement on the Leo Pargil shear zone caused localized exhumation that triggered near isothermal decompression to approximately 4 kbar at 23 Ma during continued partial melting.
- Steep normal faults exhumed the dome through the closure temperature for 40Ar/39Ar muscovite between 16-10 Ma (Zhang et al., 2000; Thiede et al., 2006).
Barrovian assemblages record burial during the Late Eocene and Early Oligocene. Pressure-Temperature-time-Deformation paths derived from rocks around the Leo Pargil dome are valuable to other areas of the orogen where the early history of deformation and metamorphism was overprinted by subsequent stages of decompression and partial melting. Our integrative approach of combining detailed compositional maps of garnet and monazite is adding new timing constraints for the onset and duration of metamorphism. In NW India, Pressure-Temperature-time paths demonstrate that within a large area of Barrovian metamorphism, a localized zone within the Leo Pargil dome was exhumed by 3-4 kbar during near isothermal decompression that began at 23 Ma.
The Ama Drime detachment initiated near the base of the Greater Himalayan Series to accommodate orogen-parallel extension at 15-20 km. Early stages of the shear zone occurred within a high temperature, distributed area that was overprinted by more localized zones of mylonite within a 300-m-thick zone. In contrast, the Leo Pargil shear zone formed as a distributed (> 500-m-thick) zone of high temperature deformation that was exhumed to slightly lower temperatures, but it did not progress to mylonite. These data imply that while both shear zones initiated at high temperatures and, presumably, lower differential stress, the Ama Drime system progressed into a more localized system of mylonites and ultramylonites during continued exhumation.
As the kinematic setting of the Everest/Ama Drime massifs evolved, the Main Central thrust and South Tibetan detachment ceased to extrude the Greater Himalayan Series by the Middle Miocene. The base of the Greater Himalayan Series was reactivated during orogen-parallel extension that exhumed the Ama Drime massif beginning at approximately 11 Ma, recording a progression from south-directed extrusion to orogen-parallel extension. In contrast, the onset of exhumation of Leo Pargil dome in the hinterland was synchronous (23 Ma) with foreland-directed extrusion of the Greater Himalayan Series. Exhumation occurred along similar sets of detachments that were inherited from earlier stages of crustal shortening. The architecture of the orogen that formed in the Eocene and Oligocene contributed to the subsequent juxtaposition of rocks with different pressure-temperature-time paths during the Miocene.
These efforts were funded by grants from the National Geographic Society (NG-CRE-8494-08) and the National Science Foundation (NSF-EAR-0911561) in collaboration with J. Cottle at the University of California, Santa Barbara. Several examples were taken from J. Langille’s Ph.D. dissertation that was completed at the University of Tennessee, Knoxville in 2012. She is currently an Assistant Professor at the University of North Carolina, Asheville. These international projects would be impossible without collaborators Dr. Lingsen Zeng at the Chinese Academy of Geological Sciences and Dr. Talat Ahmad at the University of Kashmir. Discussions with J. Cottle, R. Parrish, G. Lederer, T. Diedesch, D. Newell, A. Willingham, and C. Hughes contributed to this essay.
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Micah J Jessup is an Associate Professor in the Dept. of Earth and Planetary Sciences, University of Tennessee, Knoxville. In addition to his research in the Himalaya, he is involved with projects focused on mid-crustal processes exposed by Proterozoic rocks in Colorado as well as strain localization during syn-convergent extension in the Peruvian Andes. Please see his website (http://web.eps.utk.edu/~faculty/jessup/Welcome.html) for more details about these projects and his research group.