T31D-0667
Petrology of the Leo Pargil gneiss dome, northwest Himalaya
The Leo Pargil gneiss dome is comprised of intermediate-grade paragneiss, migmatite, calc-silicate rocks, and granitoid dikes and small plutons, including two-mica granite, tourmaline granite, and leucogranite. Petrologic and thermometry data from the Leo Pargil dome constrain the metamorphic thermal history and mineral compositions for tourmaline-rich paragneisses. The gneiss underwent multiple deformation events, evidenced by the alignment of biotite within the foliation plane and shear bands that obliquely cross-cut the biotite. Deformation includes microboudinage, mylonitic layers, and mica fish indicate a top to the northwest sense of shear. The host gneiss contains the assemblage qtz + bt + ms + pl (Ab95-78 An4-21) + tur + weakly-zoned grt (Alm62-70 Pyr10-14 Grs2-11 Sps1-18) ± ky/sil ± st + rt + ap + zrn. Phase equilibria, using the KFMASH system, indicate intermediate pressure conditions of approximately 6-12 kbar and temperatures of approximately 400- 700°C in the garnet amphibolite-facies field. Verification of this phase equilibria estimate was possible with electron microprobe mineral chemistry data and biotite Fe-Mg exchange thermometry, yielding temperatures of 450-660°°C.
T31D-0668
Early Miocene granitoids from the Leo Pargil gneiss dome, northwest Himalaya
The Leo Pargil gneiss dome is comprised of upper amphibolite-facies metasedimentary rocks of the lower Tethyan Himalayan sequence (known as the Haimantas Group), that are intruded by numerous small granitoid bodies and leucogranite dikes. The dome is located in northern India/southwestern Tibet at the junction of the Sutlej and Spiti rivers, west of the Zada basin. U-Pb SHRIMP dating of zircon yield concordant ages ranging from Late Archean to Late Proterozoic for paragneisses (2.6 Ga to 970 Ma) corresponding to the Haimantas group, and Early Oligocene to Middle Miocene ages for granitoid intrusions (33 Ma to 15 Ma). Concordant analyses and lower intercept ages from Tera-Wasserburg concordia plots range from 28.1-17.2 Ma. Late Oligocene to Early Miocene ages are from zircon domains with consistently high U contents (1500 to over 25000 ppm). Linear regression of age vs. U content scatter plots show a trend toward younger ages (22-20 Ma) corresponding to ages from zircon with lower U contents (1000-3000 ppm) and that is consistent with lower intercept ages. These Early Miocene ages for Leo Pargil granitoids correspond to the ages for the widespread leucogranite bodies exposed throughout the Himalaya and granites from the North Himalayan gneiss domes further east.
T31D-0669
Vorticity Studies in Mabja Dome, southern Tibet: Late Eocene-Middle Miocene Ductile Flow in the Middle Crust
Channel flow in the middle crust of southern Tibet is predicted to be top-north simple shear at the top of the channel grading into pure shear toward the center and then into top-south simple shear at the bottom. Channel flow is driven by a low-viscosity middle crust, a pressure gradient between Tibet and India, and surface denudation along the southern flank of the high Himalaya. Mabja Dome, southern Tibet, exposes high-grade (chloritoid-zone to incipient migmatites) middle crustal rocks proposed to have originated from this mid-crustal channel. To test this hypothesis we completed kinematic, vorticity and quartz LPO measurements across a 28 km-long transect from chloritoid-grade rocks (metamorphic depths of 1.5-4.5 kb) to incipient migmatites (metamorphic depths of > 8 kb). Outcrop and thin section kinematic indicators show a downward progression from top-north and top-south shear in chloritoid-zone rocks, dominantly top-south shear in garnet-zone rocks, and solely top-south shear in kyanite-zone and deeper rocks. Rigid porphyroblasts such as chloritoid, garnet, and tourmaline, and quartz grain-shape foliation were used to calculate mean kinematic vorticity numbers (Wm). Wm in schists and orthogneisses varies from 0.71-0.82 (c. 50-38% pure shear) in chloritoid and garnet-zone rocks to 0.52-0.69 (c. 64-51% pure shear) in kyanite-zone and deeper rocks. Quartzites yield Wm that increase downward from 0.90 to 0.99 (c. 29-1% pure shear). A combination of mineral assemblages, quartz microstructures, and preliminary EBSD-generated quartz crystal preferred orientations indicate that deformation temperatures during flow were the same as recorded metamorphic temperatures: ~475°C in chloritoid-zone rocks to ~625° C in kyanite-zone rocks to ~700°C in sillimanite-zone rocks. These data indicate that the vorticity record in these rocks formed during peak metamorphism, implying that these fabrics formed in the middle crust during the late Eocene to middle Miocene. In contrast to channel-flow models, the Mabja Dome shows a more complex flow regime involving (a) an increase in the pure shear component with depth in schists and orthogneisses that may be the consequence of an increasing lithostatic load; (b) a higher simple shear component in quartzites, the mechanically weakest layers; (c) mixed top-north and top-south shear at the highest structural levels that may reflect temporal/spatial variations in flow direction as a result of changes in viscosity.
T31D-0670
Miocene Metamorphism and Exhumation of the Granulite-Eclogite Anticlinorium of the Ama Drime Through Normal Faulting Associated to Large Scale E-W Folding
High grade rocks have been exhumed in the core of a foliation dome forming the Ama Drime range west of Dingye graben. This range is bordered on its western and eastern sides by two major presently active normal faults which cut the STDS, the eastern one forming the western border of the Dingye graben. The footwall of this fault consists of sillimanite-garnet bearing gneisses with kyanite relics, locally migmatitic and intruded by deformed and undeformed leucogranites. C/S relations show top to the east ductile normal shearing prior to brittle normal faulting presently active. Petrological study of gneisses via minerals microprobe analysis and pseudosections show minimum pressures of equilibration of 13.5 kbar and temperature of 800°C, followed by decompression to 6kbar and 700°C contemporaneous with the onset of ductile normal shearing and melting. SHRIMP U/Pb dating of monazite from migmatites give middle-miocene ages for the melting event while SIMS U/Pb dating of zircons from orthogneisses from the same unit reveal ages of more than 2 Ga. PT conditions and proterozoic ages indicate that this unit belong to Lower Himalayan Cristalline Series, (LHCS as in Groppo et al., 2007, J Metam. Geol) but also reveal that middle-miocene high grade rocks have been exhumed by the ductile normal fault. The same study was carried out on samples from the hanging-wall of the normal fault which consists of garnet- sillimanite micaschists with staurolite relics, locally intruded by undeformed and deformed tourmaline bearing leucogranites, and are sheared top to North probably in relation with the overlying STDS. U/Pb SIMS dating of zircons on undeformed and deformed leucogranite give middle Miocene ages for the end of the deformation associated with the STDS. Moreoever, Paleozoic inheritage in zircons show that the units located to the east of the active normal fault belong to the High Himalayan Crystallines series. PT paths constrained by garnet isopleths show decompression and reheating from 6 to 4 kbar and 600 to 660°C probably associated with the middle Miocene intrusion of leucogranites. Published and ongoing 40Ar/39Ar dating of ductile fabrics from the normal fault indicates miocene deformation in agreement with petrological evidence inferring that normal faulting was coeval with strong decompression probably through active tectonic unroofing. Regional studies suggest that similar large scale geometries and kinematic associations exist on the western of the Ama Drime range (Groppo et al., 2007) which appears to form a foliation anticlinorium with a core of high pressure-high temperature rocks. The location of these high grade units, far north from the MCT is possibly related with the formation of the N-S Arun anticlinorium. Its formation coeval with the development of ductile normal fault on its flank favoured exhumation and probably uplift of deeply seated metamorphic rocks. In that context ductile normal faulting probably developed on lateral ramps, and played as an accommodation mechanism of compression rather than as a result of extension. The Ama Drime example thus shows that ductile normal faulting may not always be a significant proxy for the onset of presently active extension in southern Tibet.
T31D-0671
Kinematics of the Himalayan Metamorphic Slab: Implications for the Structural Framework of Central Nepal
In the Kali Gandaki and Budhi Gandaki valleys of central Nepal, the Himalayan metamorphic slab comprises amphibolite-to-greenschist facies rocks of the Greater Himalayan series and Lesser Himalayan metamorphic series thrust over unmetamorphosed rocks of the Lesser Himalayan sedimentary series along the Main Central thrust (MCT) fault. Quartz-rich specimens sampled from the lower half of the Himalayan metamorphic slab yield well-defined cross-girdle quartz c-axis fabrics characterized by a dominant top-to-the-southwest sense of shear. These crystallographic preferred orientations reflect pervasive crystal-plastic deformation throughout the Lesser Himalayan metamorphic series and extend more than 8 km structurally below the Greater Himalayan series / Lesser Himalayan metamorphic series contact. The quartz c-axis data, in conjunction with new, detailed structural mapping, constrain the position of the Ramgarh thrust to be coincident with the MCT at the base of the pervasively deformed Lesser Himalayan metamorphic series in central Nepal. Quartz c-axis fabric opening angles from specimens sampled within the Lesser Himalayan metamorphic series suggest deformation temperatures of c. 500 ± 50 ° C. Temperatures increase up to c. 670 ± 50 ° C within the migmatitic Greater Himalayan series. These temperatures generally plot within error of geothermometric estimates from metamorphic assemblages interpreted to be coincident with the extrusion of the Himalayan metamorphic slab. Thus, the quartz c-axis fabrics preserve evidence of the deformation incurred during extrusion of the mid-crustal core of the orogen. Neutral kinematic vorticity numbers (Wn) estimated from within the Lesser Himalayan metamorphic series range between 0.29 and 0.80 (c. 81-41% pure shear) with an average value of 0.65 (c. 55% pure shear). Like the quartz c-axis fabrics, these data are interpreted to reflect strain conditions during the extrusion of the mid-crust between ~ 22 and ~ 18 Ma. The vorticity data indicate that this extrusion was facilitated by a significant component of pure shear strain.
T31D-0672
Orogenic superstructure behaviour and mid-crustal plastic flow in the central Nepal Himalaya
In the central Nepal Himalaya, the Tethyan sedimentary sequence (TSS) forms the superstructure to mid-crustal infrastructure rocks of the Greater Himalayan sequence (GHS); the top-to-the-north South Tibetan detachment system (STDS) defines their contact. North-verging folds, opposite to the main orogenic vergence, structurally dominate the TSS. Although the absolute age of this folding is unknown, structural observations and 40Ar/39Ar thermochronology indicate that it formed between 50-23 Ma, predating the dominant Miocene motion on the STDS. The GHS records a two-stage post-collisional history, marked by ca. 35 Ma burial metamorphism, followed by high-T, low-P, ca. 22 Ma metamorphism. Dominant top-to-the-south shear fabrics developed at peak temperatures at ca. 22 Ma pervasively transpose linear and planar features within the GHS. Vorticity analyses yield kinematic vorticity numbers between 0.29 and 0.80 (81–41% pure shear), with a significant amount of stretch parallel to the flow plane (34-53%). 40Ar/39Ar thermochronological data indicate that southward extrusion of the GHS terminated with cessation of movement on the STDS at 19 Ma. Our data suggest that the orogenic superstructure actively influenced the behaviour of the infrastructure in the early stages of orogenesis through fold-thrust belt formation leading to prograde 35 Ma metamorphism in the GHS. Associated melt weakening in the infrastructure allowed the initiation of southward plastic flow of the GHS, locally modifying the vergence of superstructural folds towards the north. As melt weakening in the middle crust intensified and the rheological contrast between superstructure and infrastructure increased, the upper crust decoupled from the middle crust and deformation in the upper crust temporarily ceased. By 17 Ma the extruded mid-crustal rocks cooled sufficiently to require the upper, brittle component of the STDS to become active. As cooling continued (17-14 Ma), the superstructure and underlying infrastructure (i.e., the upper crust, STDS and exhumed mid-crust) re-coupled and was subjected to localized large-scale buckling. This marked a transient stage of out-of-sequence deformation before the activation of new thrusts structurally below the GHS in late Miocene.
T31D-0673
Geochronoloical Evidence for Correlation between the Eastern Himalayan and northern Indian basement units: Implications for the mechanism of Himalayan Construction
Central to determining the mechanism of Himalayan formation is the tectonic origin of the Greater Himalayan Crystalline Complex (GHC) and its relationships to the Lesser Himalayan Sequence (LHS), Tethyan Himalayan Sequence (THS), and Tibetan and Indian basement rocks. The GHC is hypothesized to be derived from Indian basement, Tibetan lower crust, or an exotic terrane, each making distinctive predictions of their protoliths and implying different kinematic histories and thus dynamic controls for the Himalayan development. To differentiate the above models, we conducted a preliminary U-Pb zircon geochronologic study across the Eastern Himalaya at longitude 91-93E from the Indus-Tsangpo suture in the north to the Shillong plateau in the south. Our results indicate that the orthogneiss units in the GHC and LHS, which are abundant in the Eastern Himalaya, have similar mineral composition and structural fabrics with ages clustered at 1.75 Ga, 837 Ma, and 510 Ma, respectively. The orthogneiss ages coincide with those obtained from the Indian basement rocks in the Shillong plateau and Mikir Hills of NE India, where our own dating and the existing age data indicate that the crystalline basement consists of meta-granites with ages clustered at 1.74 Ma, 1.1 Ga, and 800-860 Ma. Detrital zircon ages of the GHC and LHS from the Eastern Himalaya overlap significantly with one another, having peaks at 1.1 Ga and 1.7 Ga that can be attributed to a source from the Indian basement. The detrital zircon ages of the GHC and LHS are also similar to those obtained from the Proterozoic Shillong Group overlying the Indian basement and intruded by 520-500 Ma granites. The above age correlation suggests that the Precambrian orthogneiss units in the Eastern Himalaya were parts of the Indian crystalline basement and the meta-sedimentary rocks of the GHC and LHS were probably parts of the Proterozoic cover sequence above the basement. Our interpreted lithostratigraphy prior to the Indo-Asian collision across the Eastern Himalaya indicates that the orogen formed by thick-skinned thrusting involving Indian crystalline basement rocks.
T31D-0674
Does Tibetan lower crust flow? Preliminary constraints from a reconnaissance investigation of lacustrine shorelines around Siling Co, Tibet
Although rheology of crust and lithosphere is central to all models of lithospheric deformation during orogenesis, recent propositions that lateral flow of weak mid-to-lower crust is an active participant in the growth and evolution of the Tibetan Plateau demand close scrutiny of whether the appropriate conditions exist for such behavior. Unfortunately, determining crustal rheology from geologic and geophysical investigations remains challenging, and many lines of evidence are equivocal. One of the most definitive approaches to this problem exploits the flexural/isostatic response to changes in surface loads, often lacustrine in origin [Gilbert, 1890]. Here we present the results of a reconnaissance expedition to Siling Co, in east-central Tibet. With a present-day area of > 1800 km2, Siling Co is the second-largest lake in Tibet, and the presence of high shorelines assumed to have formed during glacial lake highstands afford the opportunity to investigate deformation associated with isostatic adjustment to changes in lake levels. Shoreline features around Siling Co are remarkably well-preserved and consist of both erosional (wave-cut cliffs, notches) and constructional (beach ridges, spits, tombolos). The highest of these stands approximately 50m above the lake. Surveys of this shoreline, correlated by landscape position and geomorphic character, appear to be consistent with slight (several meter), but systematic, increases in elevation toward the center of the former lake. These results are preliminary, as they depend on correlations among undated shorelines. Early results from U-Th chronology of lacustrine tufas indicate the presence of a Stage 6 lake, but may not directly constrain the age of the shoreline. Future work with cosmogenic isotopes will allow us to both test shoreline correlations and also place age constraints on the timing of lake level fall . Our preliminary analysis indicates that length scales and the inferred magnitudes of shoreline deflection are appropriate to distinguish between models of middle/lower crustal flow or upper mantle (asthenospheric) flow in central Tibet.
T31D-0675
S-wave crustal structure in the eastern Tibetan Plateau and its tectonic implications
A teleseismic profile composed of 26 broadband seismic stations was deployed along 30°N latitude in the Eastern Tibetan plateau in 2004-2006. Based on the receiver functions inferred from the teleseismic records, the crustal thickness and Poisson ratio beneath the stations were determined by the H-k slant stacking method. Along the profile, the crustal thickness and Poisson ratio are averaged to be (62.1 km, 0.248) in the Lhasa block, (72.4 km, 0.269) in the Banggong-Nujiang suture, (65.8 km, 0.275) in the Qiangtang block, ( 57.4 km, 0.293) in the Songpan-Garze block, (42.0 km, 0.268) in the Sichuan basin, respectively. The Sichuan basin has a huge thick sedimentary (~10 km in thickness) with low S velocity, which may cause higher crustal Poisson ratio. The inversion of teleseismic P-wave receiver function was used to obtain S-wave velocity structures at stations along the profile, which are characterized by low velocity layer in mid-lower crust (at depth of 30-60 km) in the Qiangtang and the Songpan-Garze blocks, and in upper crust (at depth of 10-20 km) in the Lhasa block. There is a significant lateral variation of crustal thickness along the profile. The maximum crustal thickness (~73 km) along the profile is located at the Bangong-Nujiang suture, which is considered as the continent-continent collision between the Indian and the Eurasian plates. Low S-wave velocity anomaly in lower crust, weak Moho transition, and high crustal Poisson ratio in the Qiangtang and the Songpan-Garze blocks are generally related to hot and weak state in lower crust. The results in this study indicate that the deep crustal material in the eastern Tibetan Plateau is in a state of the lower crustal flow.
T31D-0676
Is Jiali Fault still an active fault in the late Pleistocene?
The Karakoram-Jiali Fault Zone (KJFZ) is the most important active structure system in the Tibetan Plateau. This zone consists of two major faults (i.e., Karakoram fault and Jiali fault) and other minor faults in between. The Karakoram fault strikes NW to SE in the western Tibet, while the Jiali fault roughly EW in the eastern Tibet, According to previous study, the Jiali fault possesses rapid dextral slip rate (15-20mm/yr) and the maximum observed offset is ca. 1.5km. Above mentioned minor faults in the middle of KJFZ can be divided into two groups. One strikes N120°-130°E, such as the Beng Co fault, the Gyaring Co fault, the Lamu Co fault, and the Awong Co fault. They are all previously reported as right-lateral strike-slip faults. The other group striking N70°-80°E seems to be conjugated with the first group. In the east of Beng Co fault and Jiali Fault, there are still several lineations striking similar to Beng Co fault. They are located in 31°-32°N, en echelon in configuration, similar length, and subparallel to each other. In this study they are tentatively regarded as part of the KJFZ. In the spring of 2007, after the feature identification by satellite imagery we conducted a field investigation to western Jiali fault and other minor faults located in its immediate west. Both of our image analysis and field survey found no evidence to indicate the late Pleistocene activity of the main trace of the Jiali fault. One of the minor faults mentioned above, on the contrary, shows lots of active fault evidence, such as ~360m offset of the last glacial moraine, many abandon channels, offset streams, and shutter ridges, etc. Based on a previously published TL date, the slip rate of this minor fault is ca. 15±2 mm/yr. The recently published GPS velocities show a relatively large WNW-ESE extension in the plateau interior (~22±3 mm/yr) and the speeds increasingly toward the east. There is also no symptom across the main trace of the Jiali fault. We therefore would like to conclude that the main Jiali fault is no longer active at least since late Pleistocene, which may be attributed to that the orientation of the Jiali fault is parallel to the extension axis and no differential stress occurs across the fault. On the other hand, the minor conjugated faults, instead, play essential roles to accommodate the stress under certain tectonic system. Based on this findings, the crustal flow model may be preferred, but fault locked model still cannot be entirely ignored.
T31D-0677
No Evidence for Neotectonic Activity Along the Inferred Northern End of the Karakorum Fault
The active right-slip Karakorum Fault bounds the western margin of the Tibetan Plateau and is a first-order structure within the Cenozoic Indo-Asian collision zone. To date, all maps link this structure at its northern end with active right-slip faults in the eastern Pamir (e.g. the East Pamir fault of Strecker et al., 1996). While much of the trace of the northern Karakorum fault crosses glaciated topography, obscuring evidence of its potential activity, several generations of Quaternary deposits are preserved along the inferred trace of the fault at the southern end of the Tashkorgan valley in the Eastern Pamir. Our analyses of ASTER and CORONA satellite images yield several observations that argue against recent activity along the inferred northern end of the Karakorum fault: 1) Deeply incised northeast-trending valleys show no evidence of lateral offset within the mountains bounding the western side of the valley, where the fault is interpreted to lie; 2) Both glacial deposits at the head of these valleys, interpreted to date from the last glacial maximum, and older glacial deposits preserved in the Tashkorgan valley are undisturbed; 3) An older, topographically high Quaternary (?) surface which predates both generations of glacial deposits is cut by numerous small northwest trending scarps, but streams which flow across these scarps are not laterally displaced. We interpret the scarps to result from slumping of the deposits eastwards into the valley. While these observations are only applicable to a small portion of the slip history of the Karakorum fault they suggest the possibility that active slip along the Karakorum fault to the southeast may not continue into the eastern Pamir to the northwest. Alternatively, this displacement may transfer into strike-slip and thrust deformation within the Karakorum Range (e.g. Zanchi et al., 1996) similar to the transfer of deformation from the left-lateral Altyn Tagh fault to thrust faults of the Nan Shan thrust belt. Another possibility is that the fault may tip out south of the Tashkorgan valley as recently suggested for the eastern end of the Kunlun fault (Kirby et al., 2007).
T31D-0678
Deformation History of the Ayishan, Western Tibet
Geologic mapping and structural analysis of the Ayi shan in western Tibet was conducted in order to better understand the deformation history along the Karakoram fault. Three regional-scale faults were mapped along the length of the Ayi shan for a distance of approximately 200 km. From oldest to youngest they are; the Great Counter thrust, Ayishan detachment, and the Karakoram fault. The Ayishan detachment lies between the Karakoram fault on its east and the Great Counter thrust on its west. The detachment mantles two large doubly plunging gneiss domes that are elongated towards the NW-SE. Our results show that the detachment consists of low-angle top-to-SE extensional faults that overlie a thick (~1500 m) top-to-the SE shear zone which juxtaposes Cretaceous granitoids in its hanging wall against mylonitic rocks in its footwall. Both the faults and underlying shear zone are broadly folded about an axis parallel to the strike of the range. Shear zone fabrics display ductilely deformed feldspar porphyroclasts indicating their formation at mid-crustal depths. The shear zone passes structurally downwards into weakly foliated granitoids. Mineral stretching lineations in the footwall rocks indicate the mean slip direction along the Ayishan detachment is S60&°&E. We correlate the hanging wall granitoids with the Gangdese batholith based on its composition and texture. In the northern Ayi shan the footwall of the detachment consists of mylonitic gneisses, biotite schist and migmatite in the core. Leucogranite sills are pervasive in the footwall above the migmatite. In the central Ayi shan, the footwall of the detachment correlates to that exposed in the northern range with the exception that migmatite is not exposed and leucogranite sills are less common. On the east side of the Ayi shan the detachment merges and shares similar kinematics with the Karakoram fault. Moreover, the detachment forms the master fault of the Neogene Gar valley releasing bend basin. On the west side of the range the detachment dips to the southwest structurally beneath the Great Counter thrust. Field relations suggest that locally the Great Counter thrust and Ayishan detachment merge towards the west, placing rocks we correlate with the Gangdese batholith beneath the Tethyan sedimentary sequence as well as ophiolitic rocks within the Indus-Yalu suture zone. In order to explain these field relations we suggest the Ayishan detachment had an early phase of slip that together with the Great Counter thrust facilitated southwest emplacement of Gangdese batholithic rocks beneath the Tethyan sedimentary sequence. In this scenario the Ayishan detachment operates as a passive roof thrust. This model predicts the presence of a SW- directed thrust, possibly the Gangdese thrust, at structurally deeper levels. Subsequent development of the Karakoram fault in the Middle to Late Miocene reactivated the detachment and facilitated exhumation of the footwall rocks. This two-phase deformation history implies significantly more crustal thickening along the Indus- Yalu suture zone than has been previously estimated and indicates that rocks with an Asian affinity lie, at least locally, southwest of and beneath the surface trace of the suture zone.