T52A-01 INVITED 10:20h
Strategies to overcome non-uniqueness of detrital age data from Himalayan sediments.
Studies directed at using the detrital record to reconstruct growth history of the Himalayas are made difficult by the relatively homogeneous geology along strike. High temperature methods such as zircon U-Pb dating can distinguish between Tethyan, Greater or Lesser Himalayan sources but generally lack more site specific information. Lower temperature dating methods are better suited to detect localised differences in ages caused by active tectonics, high-relief and variable climate but can also suffer from non uniqueness in detrital age. A case in point is the similarity between fission-track (FT) bedrock ages in the Namche Barwa syntaxis and Holocene detrital FT ages obtained from delta sediments at Barasat, near Kolkata. A syntaxial source for the Holocene samples is in keeping with the observation by Eduardo Garzanti that sites of focused erosion, such as Namche Barwa contribute a disproportionate amount of the total sediment flux reaching the Bay of Bengal. However similar bedrock ages also occur within the Ganges drainages. For example: modern river sediment from the Trishuli River record identical FT age modes to those from Namche Barwa. More robust evidence is required to distinguish source areas. We discuss how the combination of several thermochronometers and geochemical signatures (Hf, Sr and Pb) measured on the same grain, may provide the best opportunity to obtain more specific provenance information.
T52A-02 10:35h
Provenance of early foreland basin sediments, Nepal: constraints to the timing and diachroneity of early Himalayan orogenesis.
In contrast to Eocene Himalayan foreland basin sediments in India and Pakistan, co-eval sediments of the Bhainskati Formation in Nepal contain no petrographic evidence for orogenic input. Such data have been used as evidence to promote models of diachroneity of India-Asia collision. In this paper we provide evidence of orogenic input into the Eocene foreland basin sediments of Nepal, from U-Pb and fission track analyses of detrital zircons. Our data removes the evidence for significant diachroneity of collision as pertains to provenance data in the sediment record in Nepal, and brings the sediment record into better agreement with ages of early thrusting and metamorphism in the orogen. We also use our detrital fission track data to bolster previous age determinations of the overlying Dumri Formation, confirming the basin-wide occurrence of a major unconformity. Comparison of our fission-track dataset with that from the co-eval Dagshai Formation more than 500 kms along strike in India suggests that there is no evidence of significant diachroneity in early stages of exhumation of the orogen.
T52A-03 INVITED 10:50h
The Detrital Record of Himalayan Orogenesis
Recent studies in foreland basins have yielded increasingly detailed detrital records of orogenesis. Along with more traditional petrographic and geochemical analysis, single-crystal dating and isotopic analyses of detrital minerals allow both more refined reconstructions of source areas, erosion rates, and structural evolution within the hinterland, as well as testing of concepts, such as orogenic steady state. Whereas analysis of the detrital record has thus been enhanced in recent decades, the way in which that record is produced has received much less attention. What are the spatial variations in erosion rates within the hinterland? How are these manifested in the detrital record in the foreland? How does the detrital signal in major transverse rivers evolve as it passes through the hinterland? We have focused on these questions through a study of Ar/Ar dating of detrital muscovite in the Marsyandi River of central Nepal. By dating samples from both tributaries and the main stem, we track the downstream evolution of the detrital muscovite signal across the Himalaya. Subsequently, we try to interpret the data with a numeric model that makes two simplifying assumptions: erosional and topographic steady state prevail within each catchment; and a sample's signal is a high-fidelity indication of the true dispersion of bedrock cooling ages in the catchment. If topographic relief is $<$6 km and the erosion rate is $<$3 mm/yr, then muscovite bedrock-cooling ages are approximately a function of erosion rate and height above the closure isotherm. Consequently, the downstream evolution of the detrital signal should be a direct function of the erosion rate in each tributary catchment, its catchment area and hypsometry, and the proportion of the target mineral (muscovite, in this study) in the contributed sediment. Other factors, such as spatial and temporal stability of the detrital signal, or downstream attrition of the target mineral, also need to be addressed. We reconstruct $>$2-fold variations in erosion rates across the orogenic belt, from the Tethyan realm in the north to the Lesser Himalaya in the south. The bulk of the detrital muscovite signal at the mouth of the Marsyandi appears to derive from $<$30% of the catchment. Our mixing models of tributary with main-stem samples emphasize the major impact that the fraction of muscovite has on the calculated erosion rates. This suggests that the often-used assumption in detrital studies of a uniform distribution of a target mineral (e.g. muscovite, zircon, apatite) in the source area needs to be evaluated. Erosion rates are most rapid on the southern flank of the Himalaya, where enhanced rates may be associated either with active deformation in the region of the MCT and/ or with intense monsoonal precipitation.
T52A-04 11:05h
Rapid Exhumation of the Greater Himalaya Persisted Until 10 Ma: New Evidence Concerning Himalayan Evolution During the Neogene From a Detrital Investigation of the Siwalik Group in SW Nepal
We have applied 40Ar-39Ar dating of detrital white micas, petrography, and whole-rock Sr and Nd geochemistry to three contemporaneous sedimentary sections (Tinau Khola, 12-6.3 Ma; Surai Khola, 12-1 Ma; Karnali, 16-5 Ma) through the Siwalik molasse of SW Nepal. Mica ages suggest rapid exhumation of the Greater Himalaya persisted until 10 Ma. Subsequent exhumation was slower or had ceased with no evidence for late-stage reactivation of the Main Central Thrust. Rapid uplift of the Greater Himalaya between 12-10 Ma may be responsible for the well documented increase in sediment flux to the foreland between 11-9 Ma as opposed to initiation of the Main Boundary Thrust. We also conducted apatite and zircon fission-track dating on samples from the immediate hanging wall of the Main Boundary Thrust in central Nepal. Surprisingly old zircon ages could reflect rapid cooling along the structure as early as 20 Ma. An abundance of carbonate, sedimentary, and low-grade metasedimentary material testifies to the exposure of the Lesser Himalaya by at least 16 Ma. There is no evidence for perturbations in the detrital signals or sedimentology during the last 10 Ma despite the dramatic climate change indicated by various studies. Our findings are consistent for all three Siwalik sections, indicating the reliability of the data and rendering any potential detrital `hazards' such as sediment ponding unlikely. This work has major implications for the timing and duration of regional Himalayan structures and their relationship to the erosional history of the orogen and also for the patterns of hinterland reorganisation throughout the Neogene.
T52A-05 11:20h
Denudation Rates from Bedrock and Detrital Cooling-Age Elevation Signals: Effects of Post-Closure Deformation in the Marsiyandi Valley, Central Nepal
A bedrock sample's cooling age is a function of elevation and denudation rate in a tectonic block with a simple thermal structure that undergoes uniform unroofing, As a consequence, age-elevation relationships can be used to estimate long-term unroofing rate. Detrital mineral cooling ages from modern rivers have also been used as a proxy for unroofing rates because they integrate bedrock cooling ages from the contributing basin. Both types of estimates are reliable only if deformation within the block ceased before closure, and only if all minerals used in the analysis grew before unroofing began. If the relationship between a basin's detrital mineral cooling-age signal and its hypsometric curve is to be used to infer denudation rates, the basin must have been at topographic and thermal steady state since the closure interval. Moreover, its detrital signal must represent a high-fidelity sampling of bedrock ages in proportion to area. These assumptions can be tested through statistical comparisons of cooling-age distributions and hypsometric curves. Based on our interpretation of new bedrock and detrital muscovite $^4^0$Ar/$^3^9$Ar dates from the Marsiyandi Valley of central Nepal in the context of recent detailed structural mapping, we suggest that, although the sedimentary signal reasonably represents bedrock cooling ages from the catchment and may be consistent with steady-state assumptions, the fundamental assumption that bedrock cooling ages vary simply as a function of sample elevation is compromised by activity of multiple post-closure thrust sheets. Dates for bedrock muscovites collected over 3 km of topographic relief are not correlated with elevation in a straightforward way. Some summit ages are younger than ages observed in valleys. Some anomalously young ages might be explained by local hydrothermal muscovite growth. However, mapped Quaternary thrust faults which juxtapose units with different cooling histories are likely to be responsible for the complex pattern of bedrock dates. Given this, more sophisticated models that take into account post-closure deformation are needed to interpret both bedrock and detrital thermochronologic data in terms of unroofing rates.
T52A-06 11:35h
The "Cumulative Age Distribution": a potential tool for paleo-elevation measurements and quantitative geomorphology using detrital apatite fission track thermochronology
Apparent apatite fission track (AFT) ages of exhumed fault blocks generally increase with elevation. In such cases, we can predict the AFT age for any given elevation. Likewise, we can calculate the elevation corresponding to any given AFT age. Hence, it is possible to reconstruct the "provenance elevations" of detrital AFT age distributions. Implementing this idea, using a "Cumulative Age Distribution" (CAD) methodology, it might be possible to reconstruct (paleo-) topography from detrital AFT age distributions. This is illustrated using a published AFT dataset from Bullen et al. (2001,2003). The assumptions behind the CAD method are tested on a structurally and geologically simple drainage basin in the White Mountains, California. This test reveals the influence of geomorphology, depositional setting and sampling strategy on detrital AFT age distributions.
T52A-07 11:50h
Evidence For Tilting Of Two Fans Along The MBT In The Pinjaur Dun, Northwestern Himalayan Foothills.
The Pinjaur dun is a NW-SE trending intermontane valley in the foothills belt of the Punjab sub-Himalaya. It has a width of 7km in the southeastern part that increases to about 18km towards the northwest. The northeastern boundary of the Pinjaur dun is marked by mountains ranging in elevation between 640-1240m, whereas the southwestern boundary is marked by hills with elevation ranging between 400-620m. The fill of the Pinjaur dun consisting of late Pleistocene and Holocene mudstone, sandstone, and conglomerate, overlies the Lower and Upper Siwalik sequences. Three faults marked by previous workers in the study area are: i) the Barsar Thrust that separates the Pinjaur dun from the mountain ranges in the northeast, ii) the Pinjaur Thrust that occurs within the dun and separates the older Lower Siwalik Subgroup from the Upper Siwalik Subgroup and iii) the Surajpur fault that separates the outermost Siwalik hills from the southwestern margin of the dun. Several streams drain the Pinjaur dun; they mostly originate either in the Lesser Himalayas or in the sub-Himalayas. Of these, the most prominent ones are the Balad nadi, Kiratpur nadi, and the Koshallia rivers. These rivers deposit their sediments in the dun resulting in the formation of either alluvial fans or piedmont aprons. The Balad and the Kiratpur rivers have formed fans in the dun. The Kiratpur fan and the Balad fan have an area of $\sim$28 and $\sim$21 km$^{2}$ respectively; the fans show a typical concave upward longitudinal profile with the surface slopes varying from $1\deg$ to $10\deg$ in their distal and proximal parts respectively. The axial rivers of both the fans are incised along their entire length; and due to this incision the surfaces of the fans do not receive any sediment from their axial streams. Using the method outlined by Keller and Pinter (1995), a tilt towards southwest for the Kiratpur and Balad fans of $0.83\deg$ and $0.57\deg$ respectively has been worked out. Based on these angles of tilt, an uplift of $\sim$41m and $\sim$46m at the fan head has been calculated for the Kiratpur and Balad fans respectively. The incision by the Kiratpur and Balad Rivers at the fan heads is also approximately 40m. The terraces in the proximal part of the Kiratpur fan are unpaired and are well developed on the northern bank supporting the inference of southwestern tilt of the fan. The geomorphic indices (mountain front sinuosity and valley floor width to height ratio) calculated for the mountain front in the dun show low values suggesting that the fault bounding the mountain front (Barsar Thrust) is very active. The stream orientation trends reveal a bimodal distribution for both the fans. The two major trends that the streams follow are $270\deg$ - $280\deg$ and $230\deg$ - $240\deg$ for Kiratpur fan and $260\deg$ - $270\deg$ and $230\deg$ - $240\deg$ for the Balad fan. The Balad and Kiratpur axial rivers flow in southwest direction probably in response to the tilt. Tectonic activity along the MBT and its splays has resulted in fan deposition, followed by SW tilting of the fans. The tilt of the fans has resulted in a vertical uplift of $\sim$40m near the fan head. Fan head incision is of a similar order, and may in part be related to episodic strengthening of monsoons in the Early Holocene.
T52A-08 12:05h
Deformation of Fluvial Terraces over Active Folds, Tian Shan - Tarim Foreland
Near the Chinese city of Kashgar, the southernmost expression of contraction between the Southern Tian Shan and the Tarim Basin is expressed as a series of four folds in Quaternary deposits. Exposed in transverse canyons, the folds reveal a generally coarsening upward suite of sediments, from gypsiferous mudstones in the core to fine grained sandstone dominating the limbs and a capped by the resistant Xiyu conglomerate. Through geologic mapping across seven transects we observed only one major unconformity. The unconformity is temporally variable across the region, occurring just below the Xiyu conglomerate in the west, but expressed in the youngest sediments above the conglomerate 150 km to the east. Magnetostratigraphic dating of the eastern area suggests that folding did not begin there until ~1.2 Ma. Since the growth strata are not sufficiently exposed to allow us to discern the kinematic style of anticlinal growth, we used fluvial terraces across three of the anticlines as kinematic markers for the recent evolution of the folds. We present geometric models of anticlinal growth and the resultant deformation of terraces successively emplaced on the growing folds. Detachment folding by limb rotation and limb lengthening, as well as combinations of these end members, produces very different geometric results. Both the architecture of the deformed terrace flights and the angular relationships between the pregrowth bedding inclination and the terraces are identifying characteristics of the style of fold growth. Application of these models to the Chinese folds and terraces suggests a combination of limb rotation and hinge migration is active. The fixed width of the folds along strike with a 10X range in shortening suggests material passes through fixed hinges at the synclinal axial surfaces. Progressively tilted terraces (up to 8 degrees) towards the core of the tight folds indicate limb rotation, possibly accommodated by hinge migration at the base of the steeply dipping fold limbs. Diapiric movement of gypsiferous mudstone through the core of the folds may facilitate the great vertical growth of the anticlines.