T53A-0466 1340h
Detrital zircon study along the Tsangpo River, SE Tibet
The interactions among tectonic uplift, river erosion and alluvial deposition are fundamental processes that shape the landscape of the Himalayan-Tibetan orogen since its creation from early Cenozoic time. To better understand these processes around the eastern Himalayan Syntaxis, we conducted a study by systematic sampling riverbank sediments along the Tsangpo River, SE Tibet. Detrital zircons separated from the sediments were subjected to U-Pb dating by the SHRIMP II at the Beijing SHRIMP Center and then in-situ measurements of Hf isotope ratios using LA-MC-ICPMS at GEMOC. These results, together with U-Pb ages and Hf isotope data that we recently obtained for the Transhimalayan plutonic and surrounding basement rocks, allow a more quantitative examination of the provenance or rotosource?areas for the river sediments. Consequently, the percentage inputs from these source areas can be estimated. Our study indicates that, before the Tsangpo River flows into the Namche Barwa Syntaxis of the eastern Himalayas where the River forms a $180\deg$ Big Bend gorge and crosscuts the Himalayan sequences, the Gangdese batholith that crops out just north of the River appear to be an overwhelming source accounting for $\sim$50 % of the bank sediments. The Tethyan Himalayan sequences south of the River are the second important source, with an input of $\sim$25 %. The proportion of sediment supply changes after the River enters the Big Bend gorge and turns to south: $\sim$25 % of detrital zircons are derived from the Greater Himalayas so that the input from the Tethyan Himalayas decreases ($<$ 10 %) despite those from the Gangdese batholith remains high ($\sim$40 %). Comparing with the sediment budget of the Brahmaputra River in the downstream based on literature Sr, Nd and Os isotope information, which suggests dominant ($\sim$90-60 %) but subordinate ($\sim$10-40 %) contributions by the (Greater and Lesser) Himalayan and Tibetan (including Tethyan Himalayan) rocks, respectively, the change is interpreted to be a result of focused erosion along the Tsangpo-Brahmaputra river system that behaves as one of the most active mountain rivers on Earth.
T53A-0467 1340h
Evidence of Cretaceous Foreland Basin Systems in the Lhasa Terrane and the Implications for the Tectonic Evolution of Southern Tibet
Any model attempting to explain the relationship between the Indo-Asian collision and the uplift of the Tibetan plateau must first consider the tectonic circumstances in southern Asia immediately preceding the collision. Current hypotheses regarding Cretaceous tectonism in the Lhasa terrane of southern Tibet range between end-member models involving regional extension and regional contraction. We examined the Cretaceous sedimentary strata of the central Lhasa terrane in order to reconstruct the depositional history and basin evolution of the region, and from these findings, to determine the tectonic conditions during this period. Lower Cretaceous sediments consist of marginal marine and fluvial deposits with abundant quartzite- and granite-clast conglomerates in the northern part of the Lhasa terrane and finer-grained sediments interbedded with occasional quartzite-clast conglomerates in the south. Paleocurrents indicate south- to southwest-directed transport. Overlying these clastic units are widespread Aptian-Albian shallow marine limestones. The thickest limestones occur in the central portion of the Lhasa terrane while to the north and south the limestones are thinner and interbedded with clastic units. Upper Cretaceous strata are preserved primarily in the south and consist of arkosic fluvial red-beds with rare quartzite-clast conglomerates. Paleocurrents in the upper Cretaceous strata indicate north- to northwest-directed transport. We propose that the lower Cretaceous clastic sediments were deposited in a peripheral foreland basin that formed on the Lhasa terrane as it collided with the Qiangtang terrane during the Late Jurassic - Early Cretaceous. The upper Cretaceous sediments were deposited in a retro-arc foreland basin system that was created as the Gandese magmatic arc developed on the southern margin of the Lhasa terrane. The widespread Aptian-Albian limestones mark a significant transition from a peripheral foreland basin in the north to a retro-arc foreland basin in the south. The results of this study substantiate the idea that compressional deformation was occurring on the southern margin of Asia long before the Cenozoic collision with India. Although further study is needed to refine the extent of deformation, these tectonic events should be incorporated as initial conditions into any model addressing the evolution of the Tibetan plateau.
T53A-0468 1340h
Tertiary Shortening at the Northeastern Margin of the Tibetan Plateau
Although the Indo-Asian collision is the quintessential example of continental collision, the timing, extent, and distribution of Tertiary deformation around the current margins of the plateau are not well known. Two contrasting hypotheses for plateau growth exist: 1) shortening and crustal thickening occurred over a wide area during the Paleocene and Eocene, such that the plateau reached its present-day extent by Oligocene time, and 2) deformation progressively migrated north and east, such that the current margins of the plateau were not established until Late Miocene-Pliocene time. One means of testing between these two models is to establish estimates for the timing and magnitude of deformation at the edges of the plateau. At the northeast margin of the plateau, the Linxia basin experienced deposition of terrestrial sediments beginning in Oligocene time and continuing up into the Pliocene (Fang et al., 2003); however, little is known about the degree of deformation at the southern margin of the basin. Here we present new geologic mapping and preliminary stratigraphic observations along the southern margin of the Linxia basin. The basin margin is defined for nearly 90 km along strike by an E-W striking, north-vergent thrust system. Along the western section of the fault (near the Daxia river), Mesozoic and Paleozoic sediments are thrust over Tertiary basin sediments. A lack of cutoffs precludes a precise estimate of shortening. However, along the eastern segment (near the Tao river), Tertiary sediments are exposed both in the basin and in the hanging wall of the fault. Mapped relations indicate that the Tertiary conglomerates are folded over the basin-bounding structure and delineate significant deformation within the hanging-wall block. Stratal geometries exhibit up-section decreases in limb angle, indicative of progressive tilting during synchronous fold growth and sediment accumulation. We interpret geologic relations along this section of the Linxia Basin margin to reflect progressive deformation above a blind fault tip. Restoration of balanced cross-sections allows a preliminary estimate of the magnitude of Tertiary shortening along this margin of the Tibetan Plateau.
T53A-0469 1340h
Continental Collision Model Development: Example from Kohistan, NW Himalayas
In order to develop a comprehensive model for the processes involved in continental collision and accretion, it is essential to integrate data and models from a wide variety of disciplines. Studies of the spatial-temporal compositional relationships of post collisional magmas in this regard are very valuable. We are therefore examining the evolution of the Kohistan lithosphere by studying the timing of tectonic and magmatic events, and mantle enrichment-depletion history. A model for the evolution of Kohistan and its interaction with Karakoram and Indian plates will serve to better understand Himalayan orogeny. It is widely accepted that the drift of India was coeval with the development of the Kohistan arc, which collided with Asia along the Shyok Suture, sometime between 75 and 95 Ma (ago). The southern margin of Asia including the Kohistan arc, then became an Andean-type convergent margin that lasted for 20-40 million years, until India collided with Asia. Thrusting of the Kohistan terrane south-ward over the north Indian margin along the Main Mantle Thrust (MMT) was started by 55 Ma. On the basis of the K/Ar and 40Ar/39Ar ages, some authors concluded that the calc-alkaline volcanism, the Utror and Teru Volcanic Formations, continued until 55 Ma. These widely accepted views are inconsistent with our findings on two samples of relatively young 40Ar/39Ar ages for the Teru Volcanic Formation, which suggests that volcanism continued until at least 33 Ma. This interpretation conflicts with earlier models that classify these rocks as pre-collisional. Results giving evidence for the presence of post-collisional volcanic event(s) in Kohistan might indicate previously unrecognized widespread volcanic activity throughout the Himalayas after the collision of the Indian plate with Asia.
T53A-0470 1340h
Field Evidence of Active Uplift in the Central Bhutan Himalaya
The Bhutan Himalaya is distinguished by an east-west trending anomalously flat swath of topography bounded by abrupt physiographic transitions (PT2a and PT2b) to the north and south, respectively. These physiographic transitions manifest themselves as prominent knickpoints on the longitudinal profiles of major rivers in Bhutan. Analyses of longitudinal profiles of rivers using a bedrock incision model indicate discrete zones of steep channels adjacent to the physiographic transitions. Mapping of bedrock geology shows that these knickpoints do not mark lithologic contacts, and therefore, patterns in channel response may not reflect differential erosion. In addition, climatic factors, such as precipitation, are spatially invariant in Bhutan. Thus, the morphology of rivers in Bhutan at the physiographic transitions may reflect spatial gradients in tectonic uplift. Kinematic analysis of brittle faults and shear zones indicate potentially recent deformation with thrust sense and normal sense movement along PT2a and PT2b, respectively. Surveys of fluvial terraces show deformation of Quaternary deposits and provide direct evidence for local and regional tectonic uplift. The age of the organic material within the terraces obtained by 14C method will place constraints on the age of the terraces and the rate of Quaternary deformation in the region.
T53A-0471 1340h
Extremely Rapid and Localized Erosion in the Himalaya Recorded in Sediments of the Bengal Fan
Vigorous erosion during mountain building is now recognized as a significant factor for integrated climate-tectonics-erosion studies in Earth system science. In this context, the ages of detrital grains in sediments both define the depositional age and provide direct evidence for the tempo of erosion. In the eastern syntaxis of the Himalaya, the youngest peak identified by BINOMFIT in detrital zircons from fluvial sediments of the modern Brahmaputra River is 0.6 Ma, and significantly, it includes 47% of the entire sand-sized zircon population. The youngest grains are ~ 0.1 Ma, and a significant subset has a peak age of 0.4 Ma. The youngest peak in apatite fission-track ages from the same samples is 0.4 Ma and includes 39% of the grains. These ages are astonishingly similar to bedrock cooling dates from their source in the Tsangpo gorge, where the Yarlung-Brahmaputra River slices through the Namche Barwa-Gyala Peri massif in southeast Tibet. The Tsangpo gorge is particularly significant because it is a region with exceptionally young bedrock ages, including zircon fission-track dates (0.2 Ma), biotite 40Ar/39Ar cooling ages (1.0 Ma), zircon [(U-Th)/He] cooling ages (0.3 Ma), and migmatite crystallization ages ($<$3.0 Ma). These data are all compatible with an estimated exhumation rate of about 7 mm/yr in the Tsangpo gorge, and provide an actualistic model for interpreting the distribution of grain ages in older sediments; clustering of grain ages from different geothermometers about the time of deposition is an indicator of exceptional exhumation rates, and, if sustained in the longterm, accompanying rapid uplift. The Bengal Fan is a repository for debris eroded from the Himalaya. Grains of K-feldspar, muscovite, and apatite are abundant in sediments of the Bengal Fan. Fission-track and 40Ar/39Ar dates on apatite, K-feldspar and muscovite recovered from DSDP Sites 717 and 718 constrain deposition on the outer fan to about the past 12 m.y. With 2 exceptions, duplicate determinations of the age of the youngest grain from identical depositional horizons within the fan are essentially concordant. This remarkable synchronicity in 40Ar/39Ar and fission-track ages requires erosion and transport from the outcrop to the fan in an astonishingly short time. It requires deposition of first-cycle material with essentially the identical age at the time of deposition, despite the significantly different cooling histories that each of these systems records. This concurrence requires exceptional erosion in a setting that exposes grains of essentially "zero-age," and provides in-situ evidence that extremely rapid and localized erosion, such as that now occurring in the vicinity of the Namche Barwa-Gyala Peri massif and the Tsangpo gorge, has been a factor in exhumation of the Himalaya for at least the past 12 m.y.
T53A-0472 1340h
Late Cenozoic Conglomerate Progradation in the Southwestern Chinese Tian Shan: Tectonic, Climate or Erosion Control?
The southwestern Chinese Tian Shan of Central Asia is an actively deforming part of the Indian-Asian collision with its associated foreland basin along the northern margin of the Tarim Platform. The Xiyu Formation, an upper Cenozoic massive conglomerate, is widely distributed along the margins of the Tian Shan. The commonly assigned age of the Xiyu Formation is based on lithostratigraphic correlations from the adjoining region (e.g. conglomerate = Xiyu Formation = Pleistocene) and the relationships of such facies to changes in hinterland tectonism, climate, or erosion rates are often unclear. As a result, geologists have assumed that the conglomerate exposed along the margins of the Tian Shan represents synchronous deposition and indicates a late Plio-Pleistocene initiation of deformation (e.g. Huang et al., 1980) or a change in either climate (e.g. Burchfiel et al., 1999; Molnar et al., 1994) or a climate-affected erosion rate (Zhang et al, 2001). However, our data have shown the large-scale stratigraphy of southwestern Chinese Tian Shan foreland basin is punctuated by major episodes of gravel progradation and any claim for synchronous deposition of the upper Cenozoic conglomerate is erroneous. Four units of upper Cenozoic conglomerates could be identified after detailed field mapping. The northernmost conglomerate lies in the piggyback basin atop the Tuotergongbaizi thrust fault and has experienced only slight tilting and folding. The lowest unit, the basal conglomerate (1-25 m-thick), consists of pearl-like, well-rounded pebbles (dominated by quartz and cherts which are reworked from underlying Cretaceous rocks) and lies unconformably above Cretaceous alluvial strata, giving a definitive marker for initiation of basin deposition. The third conglomerate is a thick (2-3 km?), but highly confined, middle-to-upper Miocene fault-proximal syntectonic facies. These strata lie on the southern side of the present basin-bounding Tuotergongbaizi fault and comprise poorly sorted, debris-flow facies near the fault, which implies the development of significant erosional topography along the Tuotergongbaizi fault prior to 5.2 Ma. About 10 km south of the Tuotergongbaizi fault, the fourth coarse unit consists of a well-sorted, thick (2-3 km ?) fluvial conglomerate and forms a wedge that progrades both ~20-30 km southward and ~100 km eastward where the conglomerate pinches out into the distal silt and sand deposits in the Tarim Basin. Our preliminary magnetostratigraphic studies show that, these coarse strata have great temporal and spatial variability. Although this thick conglomerate is as old as 5.2 Ma in the north, age of the basal conglomerate varies by 1-4 My across distances of 15-30 km. The facies geometry is controlled by the interference between major southward-flowing transverse rivers and a major eastward-flowing axial river: the Kezile Su. This time-transgressive conglomerate wedge was initially formed during active faulting. Continued southward-migrating deformation caused the conglomerate to prograde over the more distal basin deposits. Although initially syntectonic in nature, conglomerate progradation may be responding to both tectonics and climate change since onset of global glaciation.
T53A-0473 1340h
Assemblage of Foreland Mineral Populations: Insights From Basin-Scale Mixing of U-Pb Zircon Ages
Fission-track and Ar-Ar dating of detrital minerals such as zircon and muscovite in foreland sediments provide estimates of exhumation rate during different periods of orogen growth. Interpretations of foreland cooling ages often assume that a population of foreland minerals provides a representative sample of the entire orogen. In reality, different mineral concentrations and erosion rates in source lithologies can have a profound effect on the proportion of any given mineral derived from each source region. Where the provenance of mineral cooling ages is often ambiguous, U-Pb ages can provide a lithology-specific fingerprint with which to trace sediment through the fluvial system because U-Pb ages are rarely "reset" in zircon. To better understand the construction of the foreland mineral population, we use mixing of U-Pb zircon ages in modern river sediment to evaluate transport and mixing of zircons in orogen-scale drainages. U-Pb ages were determined from zircons in modern river sands from the Narayani watershed in central Nepal. Tributary samples define the age contribution from each major lithology, whereas samples from main-stem rivers evaluate the relative age contribution from each lithology. The proportion of zircon ages derived from each lithology is determined by least-squares inversion as well as the size and position of component Guassian peaks in the downstream age distributions. The proportion of zircons derived from each lithology is then compared to proportions predicted by combining the exposed area, zircon concentration, and erosion rate of each lithology. Zircon concentrations were measured in bedrock and river sands by grain counting, and modern sediment flux is used as a proxy for erosion rate. Preliminary results suggest that foreland mineral populations are strongly controlled by spatial variations in the mineral concentration and erosion rate of the source region. Even if they are rapidly eroding, regions of low zircon concentration can be underrepresented in the foreland; thus, foreland cooling ages may not representatively record the exhumational history of the entire orogen.
T53A-0474 1340h
New Constraints on the Erosion History of the Andean Plateau Inferred From Detrital Thermochronology Across the Northern Bolivian Thrust Belt
Orogenic plateaus are considered important features that influence global climate and mountain building processes. However, the process of orogenic plateau formation remains poorly understood due to a lack of quantitative information about the kinematic histories of plateau uplift. Current models of plateau growth range from broad regional uplift to outward growth from a narrow orogenic wedge. Here we apply detrital thermochronology to quantify the cooling history of the Bolivian thrust belt marginal to the Andean Plateau in an attempt to differentiate between the two end-member models. Fifteen detrital apatite fission track cooling ages were obtained from Precambrian to Mesozoic rocks along an across-strike transect of the Bolivian thrust belt at 15-16$\deg$S. Deconvolution of sample grain-age distributions show that they range from detrital to fully reset. Many of the samples contain mixed ages corresponding to variable provenance and compositional differences in the apatites. Spatial variations in cooling ages from west to east are as follows. Pooled ages in the Andean Plateau range from 101-2.6 Ma with a youngest statistically significant population of grains in mixed-aged samples ranging from 12-2 Ma. Pooled ages in the Interandean Zone range from 30.8-10.5 Ma. The youngest populations in most of these samples range from 19-10.5 Ma. In the Subandean Zone, pooled ages range from 91-6.6 Ma with a youngest statistically significant population of grains in a few samples ranging from 9-5 Ma. Two additional pooled ages of Tertiary basin samples from the plateau and the Subandean Zone are 37.2 and 116 Ma, respectively. These cooling ages are considered detrital, with the older pooled age having a young population at 30.9 Ma. Assuming the youngest grain-age populations in the samples represent cooling associated with the most recent exhumation event, our results suggest that (1) exhumation magnitude increases from east to west along the plateau margin, and (2) Late Miocene to Pliocene cooling throughout the study area may represent a regional tectonic or erosional event associated with increased shortening rates, climate change, or broad plateau uplift.
T53A-0475 1340h
Extensional Basins in a Convergent Margin: Oligocene-Early Miocene Salar de Atacama and Calama basins, Central Andes
The Salar de Atacama Basin (SdAB) is the largest and most persistent sedimentary basin of northern Chile, accumulating nonmarine sediment from Cretaceous to modern times. Its northwestern neighbor, the Calama, was a Cenozoic basin. Although SdAB was in the backarc zone early in the Andean orogeny, both are now forearc basins. Others demonstrated that the basins overlie anomalously cold, strong, and dense crust and lithosphere. We focus on an extensional Oligocene basin stage. Interpretation of the basin-controlling faults is based on seismic reflection studies supported by field relations. The SdAB is limited to the west by the NNE-trending, steeply east-dipping, Paciencia Fault (PF). The PF experienced 5-7 km of down-to-the-east offset during the Oligocene-early Miocene. Syntectonic strata, an arid succession of siliciclastics and evaporites, are asymmetric, with thicknesses of 5000 m and abundant halite adjacent to the PF, and of 1000 m with fine detrital clastic strata 25 km farther east. Relations in conglomeratic growth strata that overlap the PF also demonstrate normal displacement during sediment accumulation. Seismic data reveal that a buried normal fault with 1-1.5 km down-to-the-east displacement limits the western margin of the Oligocene-Miocene Calama siliciclastic basin fill. Regionally, Oligocene-early Miocene margin-parallel strike-slip deformation dominated northwest of the basins, contributing sinistral offset (West Fissure Fault) to the northern segment of the long-lived Domeyko Fault System. The new SdAB and Calama data reveal that a 20,000 km2 domain of extensional basins existed within the dominantly strike-slip region. Even if PF and the fault in the Calama Basin were transtensional, the proportion of extension to strike-slip displacement is much greater in these basins than elsewhere in northern Chile. Further study is required to understand what combination of factors caused this kinematic distinction as well as delayed the onset of CVZ volcanism in the same area.
T53A-0476 1340h
Sedimentary Basin Inversion Without Lithospheric Compression
The inversion of sedimentary basins, because of its implications for petroleum generation and trapping, has received attention in recent years. Because of the common use of lithospheric extension models of basin formation (e.g. Mckenzie, 1978 etc), it is not surprising that models of inversion mechanics have concentrated on the inverse of lithospheric extension, namely compression. Such models of inversion have no direct mechanism of localizing uplift (and thus inversion) at the site of the original basin. It is necessary to postulate mechanisms by which the presence of the basin weakens the lithosphere locally (e.g. by elevated geotherms cuased by the thermal blanketing of the basin). However, such mechanisms are variable in their effectiveness (Sandiford 1999). In this paper, we examine a different context for basin formation and inversion, in which the inversion is automatically localized at the original basin. Specifically, we examine the case where basin formation is driven by intra-crustal anomalous density loads induced subsidence of the elastic upper crust in response to elastic thickness reduction driven by sublithospheric heating. In such a case, if elastic thinning is sufficient, the density load can detach from the elastic upper crust and sink, by ductile flow, rapidly (Glazner, 1994) into the deep crust. The resultant relief permits the elastic upper crust to rebound upwards over the site of the prvious basin. Strikingly, such rebounds can be as much as 7 km (locally erasing a huge part of the detrital record). We demonstrate this by both semi-analytic modelling using an extended version of Kaufman and Royden's (1994) elastic-beam over ductile channel model, and with a full thermal and viscoelastic finite element model. Finite element modelling shows that in this type of inversion, surface horizontal stresses in the basin are compressive during basin formation, and become more tensional during inversion, in direct contrast to the lithospheric extension-compression models.
<a href='http://www.physics.utoronto.ca/~bailey/' >http://www.physics.utoronto.ca/~bailey/
T53A-0477 1340h
Syn-tectonic sequence generation in low-gradient desert margin systems - the Lower Triassic Buntsandstein of the Central European Basin
The Late Permian/Early Triassic succession of the Central European Basin (CEB) was repeatedly affected by tectonic pulses associated with the earliest phases of Tethyan and Arctic-North Atlantic rifting. Effects of differential tectonic subsidence are particularly well recorded by unconformities, which form widespread sequence boundaries. In the lowermost Triassic the Hardegsen Unconformity (H-unconformity) represents the most prominent unconformity that can be traced over a distance of 1200 km from southern Germany as far north as the Irish Sea Basin. This hiatus is best expressed on intra-basinal highs that are associated with "stratigraphic losses" of whole formations down to Permian strata and a calculated erosional downcutting of up to several hundreds of meters. This study investigates the controlling mechanisms of unconformity development in a predominantly continental environment. 3D-seismic data from the eastern margin of the East-Netherlands Palaeo-High reveal that differential subsidence during the Middle and Upper Buntsandstein was induced by basement tectonics. Core- and log-based facies analysis of 18 cores from the Ems Area in northwestern Germany identifies the effects of differential subsidence and growth faulting on the generation of sedimentary sequences of the Middle Buntsandstein. Lineaments of synsedimentary tectonic movements are observed in seismic sections and drillcores. They originate from deep rooting, north-south extending basement faults in Upper Carboniferous strata that were decoupled by intercalated Permian evaporits. The sedimentary sequences of the Middle Buntsandstein Group are characterised by the fining upward sequences of the Volpriehausen-, Detfurth- and Solling Formations. Beginning with basal sandstone units that represent either fluvial or aeolian dune sandstones the depositional environment changes into ephemeral fluvial or playa type deposits. The H-unconformity is expressed by a stratigraphic gap underlying the Solling Formation. It comprises the entire Hardegsen Sequence and the upper Detfurth Sequence. Apart from these obvious "stratigraphic losses" the remaining sequences underlying the H-unconformity are characterised by a reduced thickness on the palaeo-high. The internal stratigraphical organisation of these continental sequences is characterised by the establishment of maximum flooding surfaces that can even represent short-lived marine ingressions. These flooding surfaces are traceable throughout the whole working area and they even occur twice in one unconformity bound main sequence. Such composite sequences give indication of previously unknown sequence boundaries of higher order. Throughout the CEB the H-unconformity is associated with palaeosoil developments, which show characteristic geochemical and textural finger prints, that allow regional correlations. The textural characteristics vary between vertisols (clay-rich types) and calcretes (carbonate-rich types). Analysis and correlation of these sequence boundaries is used to evaluate the relative importance of synsedimentary tectonism in the development of palaeo-topography and the provision of accommodation space.
T53A-0478 1340h
Drainage Evolution in Response to the Migration of the Yellowstone Hotspot
The topography of the Yellowstone region in northwestern USA, preserves a record of the interaction of a hotspot with continental crust. The track of the hotspot has been defined by the time-transgressive migration of caldera forming volcanic centres; a distance of 700 km north-eastward to the present Yellowstone volcanic field since 16 Ma (Pierce and Morgan, 1992). The passage of such a long wavelength deformation field associated with large-scale mantle processes is likely to have exerted a significant effect on the spatial and temporal organisation of river drainage systems, the sedimentological consequences of which form the focus of this investigation. Fritz and Sears (1993) hypothesised that the deformation field associated with passage of the Yellowstone hotspot was responsible for a complete Neogene drainage reversal in southwestern Montana. In order to test this hypothesis, sedimentary deposits preserving a record of potential fluvial disruption in a Mio-Pliocene half-graben in southwestern Montana were analysed in order to investigate changes in river system geometry and dispersal patterns. Integration of a comprehensive sedimentary facies study with detailed palaeocurrent analysis combined with chronostratigraphic constraints from dated ash horizons across the region, enable reconstruction of drainage evolution and palaeogeography. The sedimentary record reveals basin-fill evolution from alluvial fan-dominated deposition during the mid-Miocene, to externally sourced, axial-flow dominated deposition in small isolated streams, during the mid- to late- Miocene. Palaeocurrent data shows clear northeasterly drainage throughout the mid-Miocene to early Pliocene, with no evidence to support the hypothesis of drainage reversal. During the late Miocene to early Pliocene, a distinct change in the axial-fluvial depositional system is recorded by the abrupt transition from pebble-grade, isolated stream channel deposits, to broad, sheet-form, cobble-boulder deposition. Reconstruction of past hotspot positions (based on caldera locations and patterns of faulting) indicate that topographic doming associated with the hotspot would have impacted south-western Montana at $\sim$ 6 Ma, a time coincident with this gravel progradation. We propose that the change in sedimentary style was a consequence of source area uplift and erosion associated with hotspot-related deformation.