T13H-01
Th-Pb Monazite-in-Garnet Ages From the Greater Himalayan Sequence of Central Nepal
431 new Th-Pb ages have been determined from rocks of the Greater Himalayan Sequence (GHS) of central Nepal. Some of these ages are reported by Martin et al. (2007, Chem. Geol.), most have not been reported previously. 42 samples were collected from transects along Kali Gandaki, Modi Khola, Seti Nadi, Madi Nadi, Nayu Ridge, and Marsyangdi Nadi in the Annapurna region, with three transects extending across the GHS and three transects concentrated near the base of the sequence. Garnet crystals were extracted from the samples, and monazite inclusions were identified by BSE imaging and then analyzed by LA-MC-ICPMS with a 10 micron laser beam. Where possible, inclusions were sampled from both cores and rims of the garnet crystals. The resulting ages belong to four groups: (1) 3 ages (all cores) between 801 Ma and 1407 Ma that are inherited from GHS protoliths, (2) 42 ages (all cores) from 550 Ma to 400 Ma (peak age of 487 Ma) that record early Paleozoic prograde metamorphism, (3) 102 ages (nearly all cores) scattered between 400 Ma and ~50 Ma that are interpreted as early Paleozoic grains which have experienced either Pb loss or overgrowth of Tertiary monazite, and (4) 284 ages (2/3 cores, 1/3 rims) between ca. 50 Ma and ca. 10 Ma, with cores only slightly (avg of 1.5 m.y.) older than rims. The Tertiary ages consistently young northward/upsection from ca. 35 to ca. 18 Ma, and in Marsyangdi Nadi define two separate panels that are interpreted to be imbricated along a north-dipping thrust fault. This fault is near the base of sillimanite-bearing rocks, similar to the Langtang thrust (Kohn et al. 2005, JMG), and may be partly responsible for the inverted metamorphic gradient discussed by many previous workers. Our preferred structural scenario is that the Tertiary monazite ages record progressive burial of the GHS by shortening in the Tethyan thrust belt between ca. 35 and ca. 18 Ma, termination of this metamorphism due to onset of motion along the MCT, and ca. 10 Ma imbrication of the GHS along the fault described above. This fault operated either as an out-of-sequence south-vergent thrust that merges with the MCT to the north, or a south- dipping backthrust that formed the roof of a south-vergent tectonic wedge of GHS. Argon-muscovite cooling ages in GHS and upper Lesser Himalayan rocks are consistent with either scenario, but geological evidence favors the second explanation.
T13H-02
Structural and Thermochronological Constraints on the Coupling Between Exhumation, Denudation and Tectonics in the Himalaya: Insights From the Ama Drime Massif, Tibet- Nepal
Focused denudation and mid-crustal flow are coupled in many active tectonic settings, including the Himalaya where exhumation of mid-crustal rocks is accommodated by thrust faults and low-angle detachment systems during crustal shortening. New structural data demonstrates that the most recently active tectonic feature in the Mt. Everest region is the Ama Drime Massif (ADM), a trans-Himalayan antiformal structure that protrudes ~ 70 km north from the crest of the Himalaya and displaces the South Tibetan Detachment system. Previous investigations interpreted the N-S striking shear zones and fault systems that bound either side of the ADM as the Main Central thrust. Our data show that these are 100-300 m thick normal-sense shear zones that are kinematically linked to young brittle faults that offset Quaternary deposits and record active E-W extension. Geochemical tracers in hot springs along the western shear zone indicate devolatilization of crustal rocks suggesting active metamorphism and/or melting beneath the ADM. Integration of high-to low-T thermochronometric methods, including U(-Th-)Pb, Apatite Fission Track and (U-Th)/He, yield important information regarding the timing and rate of exhumation of the ADM. These data reveal that rapid exhumation of material from the mid-crust during E-W extension began in the mid-Miocene and that a high rate of uplift / denudation persisted into the Pliocene. The southern end of the ADM is centered on the Arun River gorge, which previous geomorphological instigations defined as a prime candidate for climate-tectonic coupling. We combine our results with these exiting data to propose that active exhumation of deep crustal rocks of the ADM during E-W extension is coupled with denudation in the Arun River gorge. This model provides important quantitative constraints on the dynamic feedbacks between climate and tectonics in collisional orogens as well as the evolution of trans-Himalayan rivers and antiformal structures.
T13H-03 INVITED
Cenozoic tectonic evolution of Qaidam Basin: Structural geology, sedimentation, and implications for regional tectonic reconstruction of the Tibetan plateau
Qaidam basin is the largest topographic depression inside the Tibetan plateau. Regional seismic-reflection profiles reveal its first-order structure as a broad Cenozoic synclinorium, with amplitude decreasing from greater than 16 km in the west to less than 4 km in the east. The synclinorium has expanded progressively eastward across the Qaidam region: starting from the west at 65-50 Ma to the east at about 24 Ma. Its formation was induced by an older thrust system initiated at 65-50 Ma in the north and a younger thrust system initiated at 29-24 Ma in the south. Cenozoic upper-crustal shortening decreases eastward across basin, from greater than 48 percent in the west to less than 1 percent in the east. This observation has two implications: (1) the southern Qaidam margin has rotated clockwise relative to the northern Qaidam margin for about 12 degrees, and (2) the crustal-thickening mechanism shifts progressively from dominantly upper-crustal shortening in the west to dominantly lower-crustal shortening in the east because the elevation and crustal thickness of the basin are rather constant. The diachronous initiation of thrusting in the northern and southern margins of Qaidam basin and the existing inference that the uplift of the Eastern Kunlun Range began at or after 30-20 Ma imply that the Paleogene (65-24 Ma) Qaidam basin and the coeval Hoh Xil basin were once parts of a single topographic depression bounded by the Fenghuo Shan thrust belt in the south and the Qilian Shan thrust belt in the north. The development of this large basin, similar in size to the modern Tarim basin north of Tibet, and its subsequent destruction may have been controlled by pre-existing weakness in the Tibetan lithosphere, creating a highly irregular sequence of deformation across Tibet during Indo-Asian collision.
T13H-04
Structural Development of the Eastern Kunlun Transpressional System, Central Tibet, China
The Himalayan-Tibetan orogenic system is superposed on a vast area assembled throughout the Paleozoic and Mesozoic by collision of several discrete tectonic blocks. The Eastern Kunlun range in central Tibet coincides with the boundary between two of these tectonic terranes: the Songpan-Ganzi terrane to the south and the Eastern Kunlun-Qaidam block to the north. The former is characterized by thick, deformed Triassic flysch; the southern margin of the latter consists of Proterozic gneiss, Paleozoic-Mesozoic marine strata and volcanics, and early Paleozoic and Permian-Triassic granitic intrusions. Convergence between the two terranes was accommodated by northward subduction of the Songpan-Ganzi terrane, with suturing completed by the Early Jurassic. Tectonically, the Eastern Kunlun region comprises a large transpressional system consisting of the E-trending left-slip Kunlun fault and two thrust belts: the Qimen Tagh to the west and the Bayanhar to the east. The Kunlun fault is generally considered to have reactivated the Kunlun suture along much of its length. Each thrust belt is distinctly triangular-shaped, widening away from the central segment of the Kunlun fault. In the Qimen Tagh belt, north-dipping thrusts are expressed at the surface along the southern margins of individual ranges. In contrast, there is no major south-dipping structure along the northern flank of the Eastern Kunlun range. The thrusts are oriented obliquely to the Kunlun fault, trending WNW and merging with the Kunlun fault at an angle of approximately 20-30 degrees. To the west, the Kunlun fault terminates into WNW-trending thrusts and N-trending rifts. Taken together, these observations suggest that the transpressional system may have developed by propagating laterally: as the Kunlun fault extended by reactivating the Kunlun suture, obliquely-oriented thrust faults initiated to accommodate crustal shortening. The thrusts of the Qimen Tagh belt likely root into a regional detachment beneath Qaidam basin, with shortening in the Eastern Kunlun occurring above a tectonic wedge. The development of north-dipping thrusts may reflect the influence of pre-existing fabrics associated with the Kunlun suture which may have been similarly north-dipping.
T13H-05
Structural Observations From the Tangra Yum Co, Lunggar Shan, and Lopu-Kangri Rift Systems, Southern Tibet
We suggest that boundary conditions exert more control over pre-existing structures in dictating the geometry for north-trending rifts in southern Tibet. We present geologic and neotectonic mapping, and observations made from satellite imagery along several north-trending rift systems in the Lhasa terrane, southern Tibet. From east to west, and to the south they include, 1) Tangra Yum Co, 2) Lunggar Shan, and 3) the Lopu-Kangri rift systems. Our observations bear new light on the geometry and kinematics of active extension in the hinterland of the Indo- Asian collision. Observations from the rift systems in southern Tibet indicate that they share a common trait; there is no obvious surface correlation with pre-existing structures. The architecture of the Lhasa terrane and its prior tectonic history illustrates this point as outlined by the following structural relationships. The Lhasa terrane is comprised of Pre-Ordovician gneiss overlain by Ordovician to Cretaceous strata. Pre-Mesozoic strata were intruded by Late Jurassic - Tertiary plutons and underwent significant shortening by the Cretaceous - early Eocene Gandese retroarc and northern Lhasa thrust belts. The plutons and the approximately E- trending contractional fabric of the Lhasa terrane are cut and offset by the active N-trending rift systems. Even though the southern Tibetan rifts share this first-order characteristic, they also vary markedly in structural style as a function of extension magnitude; most Tibet rifts are internally drained systems bounded by high-angle normal faults that have 10km or less extension, as reflected by the geometry of the Tangra Yum Co rift. However, rifts that have experienced large magnitude extension (>20 km) are bounded by low-angle normal faults (detachments) with mylonitic rocks and Miocene granites in the footwall, and intrabasinal topographic highs are actively developing near areas of inferred maximum extension. The Lopu-Kangri rift system in far southwestern Tibet is more enigmatic in that it trends NNW, yet it exhibits normal and oblique dextral shear indicators in the footwall rocks, and streams along the range front are dextrally offset. The fault system right-laterally separates the Gangdese batholith, Kailas conglomerate, Great Counter thrust, and the Tethyan fold-thrust belt. One explanation as to why boundary conditions appear to exert more control on determining the active rift geometry is that pre- existing structures are oriented at a high angle to the present orientation of the maximum principle stress direction, making them unfavorable to reactivation.
T13H-06 INVITED
Do GPS Vectors "Know" About Crustal Heterogeneities?
Unambiguous elastic rebound recorded by GPS data collected before and after major earthquakes has led to the conclusion that the majority of the tectonic signal captured by GPS is elastic and transitory. Elastic models used to understand plate kinematics commonly assume that the only heterogeneities in the earth are the discontinuities (i.e., faults) between blocks of uniform material properties. While a convenient first approximation, this simplification does not accord with geological observations that different crustal blocks have significantly different properties, inherited from hundreds of millions, if not billions, of years of tectonic evolution. A simpler approach, with no a priori mechanical assumptions, is to calculate strain and rotation rate directly from GPS velocity fields. The results of such analysis show a clear spatial correlation between stable blocks (commonly referred to by geologists as "rigid"), embedded within and at the margins of late Cenozoic orogens around the world, and regions of anomalously low strain. The best examples of these are the Tarim and Sichuan blocks in the Tibet- Himalayan orogen. In the southern Central Andes, significant strain rate gradients and EW-trending belts of constrictional strain are best explained in terms of rheological variation. In contrast, the Altiplano, geologically quiescent for the last 9 Ma, but generally thought to be weak, has a relatively high strain rate due to the geometry and locking depth of the subduction thrust. In the western United States, the overall strain rate is 4-5 times greater to the west of the Paleozoic margin of the continent than to the east, even though the magnitudes of crustal thinning measured by 2D dilation rate, are similar in the eastern and western Basin and Range. In the Andean and western US cases, other factors besides the presence of "rigid" or mobile crust probably help to determine the strain rate patterns. Additionally, in none of the cases studied are the GPS data sufficiently dense to distinguish between changes in block size and changes in block rheology. Thus a region of higher distributed strain could be modeled by either by an aggregate of smaller blocks, or a single larger but weaker block. The difference between these two alternatives may be more semantic than real.
T13H-07
Focusing of 50-80 Percent of Total Arabia-Eurasia Convergence Since 5 Ma Along the Southern Margin of the Greater Caucasus: Effect of Strain Localization Along the Margins of a Rigid Inclusion Within a Young Orogen?
The W-NW-trending, ~1000 km long Greater Caucasus Mountains lie between the Black and Caspian Seas to the west and east, respectively, forming the northernmost edge of the Arabia-Eurasia collision zone. This belt lies nearly 550 km north of the Bitlis Suture, and is separated from the Lesser Caucasus and East Anatolian plateau to the south by the Kura-Alazani basin. This topographic depression is the landward continuation of the South Caspian depression to the east. Earthquake and geodetic data together suggest that oblique, NW-SE directed convergence between Arabia and Eurasia is partitioned into range-perpendicular shortening across the Caucasus and right-slip faulting in the East Anatolian plateau to the south. Although recent geodetic investigations indicate that the rate of NE-SW convergence across the southern margin of the Greater Caucasus increases systematically from ~2mm/yr in the west (~42.5°E) to ~14mm/yr in the east (~49°E), the major structures within the Caucasus which accommodate this convergence remain poorly understood. To address this problem, we used the Real-time Interactive Mapping System (RIMS) and ASTER satellite imagery to map active structures within the Kura basin along the southeastern margin of the Greater Caucasus in Georgia and Azerbaijan. We focused on the Kura-Alazani fold-thrust belt, a chain of topographic ridges that defines the northern edge of the Kura basin and parallels the Greater Caucasus to the north, from which it is separated by the piggy-back Alazani basin. Our neotectonic mapping indicates that recent shortening in the eastern Caucasus is concentrated in the Kura-Alazani fold-thrust belt, along a series of SW- vergent folds, inferred to be underlain by gently NE-dipping thrust faults. By combining the fold geometries determined in our neotectonic mapping with previously published geologic maps, borehole data, and isopach maps of the Kura basin, we were able to construct two balanced cross sections across the Kura-Alazani fold- thrust belt to estimate total shortening and initiation age. Our data indicate that the fold-thrust belt shows pronounced west-to east asymmetry: the estimated total shortening, cross-strike topographic width, and density of structures all decrease systematically from west to east. This pattern is opposite that expected from the GPS velocities, which increase from west to east, suggesting that shortening initiated diachronously and propagated eastwards over time. Our shortening estimates indicate that since 5 Ma, the Kura-Alazani fold-thrust belt has accommodated between 12-52 km of total shortening, which accounts for 50-80% of total Arabia-Eurasia convergence during this interval. We hypothesize that the transfer of deformation to the northern edge of the Kura basin is facilitated by the presence of an abnormally rigid/strong section of crust and/or lithosphere underlying 5-7 km of Cenozoic sedimentary fill within the basin. This rigid inclusion may be a remnant of oceanic crust, a buried island arc complex, or a combination of both.
T13H-08
Kinematics of thrust belt development in the external Dinarides, Croatia
Many compressional orogenic systems include foreland fold-and-thrust belts, but the relationship between foreland thrust belt kinematics and hinterland crustal thickening, and their dependence on numerous additional factors such as variable accretionary flux, fault friction, surface processes, crustal flow, and pre-existing structure remains controversial. The Dinaric orogen may provide an important opportunity to isolate the relative importance of some of these factors. The external Dinarides are composed primarily of carbonate strata. Their insensitivity to climatic variability may potentially mitigate this important source of uncertainty relative to many other fold and thrust belts. As a first step toward understanding the kinematics of the external Dinarides, we measured present-day crustal velocity along a N13°E profile across the southern Adria microplate and external Dinarides collision zone. We used these data to develop an elastic dislocation model representing contemporary fold and thrust belt kinematics and decollement geometry. At the latitude of the Dalmatian Islands, the model fault plane reaches the surface some 80~km seaward of mapped SW-verging thrusts of Eocene and perhaps Neogene age along the coastal areas, consistent with SW-migrating deformation in an active fold and thrust belt. South of the Dalmatian Islands, however, the modern deformation front appears to be located within about 1~km of the older mapped faults. The largest known earthquakes in the system (~M7) are associated with this southern stationary-width part of the system. Possible explanations for along-strike variability in thrust belt kinematics include along-strike changes in decollement friction, clastic sediment availability and drainage network connectivity, accretionary flux associated with variable Adria carbonate platform thickness, retroarc exhumation, and subaerial distribution of pro-wedge carbonates.