Active Orogenic Belts Research Articles

Climate-driven formation of fluvial terraces across the Tibetan Plateau since 200 ka: A review

The causes of terrace formation in active orogenic belts have long been studied. Here, we present a synthesis of 147 available papers on the ages of Quaternary terraces within the Tibetan Plateau and along the surrounding orogenic belts. More than 1500 ages were collected to establish a database for exploring the causes of fluvial terrace formation. Because the age data were collected from different settings, ages younger than 200 ka were grouped into three regional climate systems: the westerlies, Indian summer monsoon, and East Asian summer monsoon. We further examined the relationship between terrace formation and climate change, as well as the contribution of tectonic uplift. Our results indicate that the intervals of fluvial terrace formation are synchronous with transitions in global climate from cooler to warmer conditions (stadials to interstadials). We propose that the formation of fluvial terraces across the Tibetan Plateau has been controlled mainly by cyclical climate changes, although there is not a one-to-one correspondence.

Spatial variations of river incision rate in the northern Chinese Tian Shan range derived from late Quaternary fluvial terraces

Abstract The rate of erosion is critical for characterizing landscape evolution and understanding the interactions between tectonic activity, climate, and surface processes in active orogenic belts. This work focuses on the spatial pattern of river incision rate in the northern Tian Shan range, an active orogenic belt in NW China, based on fluvial geomorphic investigations. Both remote and field-based observations have been used to define the terrace sequences of four selected rivers preserved at the mountain front. For each analyzed river, one of the defined terraces (i.e. terrace T4 of the Kuitun River, terrace T5 of the Jingou River, terrace T5 of the Manas River, and terrace T4 of the Urumqi River) is used as a reference surface from which to reconstruct the paleogeomorphic surface (i.e. the previous riverbed) which was abandoned as a terrace as a result of fluvial incision. By comparing the present-day topography with the reconstructed (pre-incision) landscape, and after a porosity correction, the volume of material eroded by river incision and the depth of incision have been estimated. When combined with the formation age of each reference terrace, the average rates of river incision in the mountain range have been estimated at ~4.1 ± 1.5 mm/yr (Kuitun River), ~3.5 ± 1.4 mm/yr (Jingou River), ~3.1 ± 1.5 mm/yr (Manas River), and ~1.9 ± 1.3 mm/yr (Urumqi River). The above rates display a notable decreasing trend from the Kuitun (west) to the Urumqi (east). Our analyses indicate that lithology and climate might not be the main factors controlling the spatial variation of incision rates in the northern Chinese Tian Shan range. Instead, the observed pattern is in close agreement with changes of slope, relief, and N-S crustal shortening across the range. We thus propose that tectonic activity is the primary factor controlling the erosion and landscape evolution of the northern Tian Shan range, NW China.

Longitudinal profiles and geomorphic indices analysis on tectonic evidence of fluvial form, process and landform deformation of Eastern Himalayan Rivers, India

ABSTRACTThe drainage pattern and the morphology of the piedmont zone of the Himalayas are clear indicators of the active orogenic belt of most recent origin formed by the collision of Indian and Eurasian plate. The foothills of the Himalayas in West Bengal are zones of active tectonics drained by the rivers belonging to the Brahmaputra system. The present study is conducted for the left bank tributaries and sub-tributaries of Tista which bear the imprint of active tectonics of the region as they lie in the zone of Himalayan Frontal Fault, the most active thrust belt of Himalayas. Data and subsequent field experiences which showed that the region is constantly acted upon by recent diastrophism. Various tectonic indices were calculated to evaluate the evidence of tectonism. This include hypsometric integral (HI), fractal dimension (FD), basin asymmetry factor (AF), basin shape index (Bs), stream-length gradient (SL), mountain front sinuosity (Smf) and valley-floor width to valley height ratio (Vf). The results of these indices are used to prepare an index of active tectonics with three classes which is represented in a map. Then the field evidences of deformed landscape are matched with the areas showing high tectonic index values.

Tectonic, topographic and rock-type influences on large landslides at the northern margin of the Anatolian Plateau

High Anatolian orographic margins have large variations in terms of topographic relief, precipitation, and uplift rate. These variations lead to the dynamics of mass movements and surface runoff, which are the dominant geomorphological processes in ice-free mountain landscapes. There is growing recognition that large landslides are important agents of landscape evolution, resulting in massive slope failures, which can cause extensive and rapid topographic changes in many active orogenic belts. Unlike the cognatic orogenic plateau margins in the world, there are no studies available on the large landslides and their geomorphic impact at the margins of the Anatolian Plateau. This study presents results from a regional-scale inventory of 1290 large landslides (> 1 km2) that allow the characterization of spatial distribution and landslide-dominated landscapes in the northern margins of the Anatolian Plateau. The majority of large landslides are clustered in three main zones that correspond to the Western, Central, and Eastern Pontides, which is an east-west-trending orogenic belt that represents a coalesced tectonic entity in the northern section of Turkey. Nearly 80% of large landslides have occurred in a terrain with a mean hillslope relief of > 1000 m in those three landslide-dominated landscapes. The results of regional comparisons reveal that in addition to hillslope relief and steepness, lithotectonic differences largely control the abundance of landslides along the northern margins of the Anatolian Plateau. In this respect, the spatial distribution and abundance of large landslides imply a landscape in which lithological and tectonic controls on hillslope erosion are more significant than climate. The study further shows that the parallel or perpendicular position of the landslides with respect to the direction of the drainage network is effective as positive or negative feedback in response to fluvial dissection of the plateau margins. On the other hand, there is certainly a need for more comprehensive radiometric dating studies to understand the contribution of large landslides on the erosional decay rate of Anatolian Plateau margins. Furthermore, the presence of these large landslides and the derived deposits in this dynamic terrain provide a unique opportunity for deciphering the past climatic and seismic events.

Topography as a proxy for inter-plate coupling

The topography of active mountain belts is shaped by long-term crustal deformation and strain from tectonic plates. To test this hypothesis, we correlated mean topography, topographic slope and GPS derived strain rates along the active orogenic belts of South America (Ecuador and northern Peru segments), where relatively rapid convergence occurs between the Nazca and South America plates. Our results show that the Ecuador and northern Peru segments follow well-defined power-law relation with distinct exponents that are consistent with the nonlinear deformation associated with dislocation-creep occurring at depth. We propose that the depth of dislocation-creep and distinct power-law exponents have first order control over mean topography, and that the topographic slope can be used as proxy for inter-plate coupling.

Using Raman Spectroscopy of Carbonaceous Materials to track exhumation of an active orogenic belt: An example from Eastern Taiwan

The burial and exhumation history of an orogen can provide insights into the thermal history of rocks in the mountain belt. Asynchronous collision will exhume materials to the surface at different times and expose different metamorphic grades along the orogenic axis. By tracking the thermal signals of eroded materials, it is possible to compare spatial patterns of exhumation along the mountain belt. Previous studies such as bulk mineralogy of sandstones, crystallinity of clay minerals, and fission track thermochronology all suggest that the Pliocene to Pleistocene meta-sedimentary deposits within the preserved basins of the Coastal Range reflect the unroofing history of the Central Range in Eastern Taiwan. In this study, modern sediments from six drainages of the east Central Range (Liwu, Mugua, Laku- Laku, Luye, Jinlun, and Hsuhai Rivers) and three basin sections of meta-sedimentary deposits from the Coastal Range (Shuilien, Chimei, and Madagida sections) were sampled to reconstruct the thermal signals of each drainage along the modern orogenic axis and reconstruct temporal variations of thermal signals in preserved Plio-Pleistocene sedimentary rocks.The temperature index (T.I.) of modern sediments determined by Raman spectroscopy of carbonaceous materials are medium to high (0.4–0.8). In conjunction with bio- and magneto-stratigraphy, our data suggest that the northern Shuilien area has been receiving detritus of medium metamorphic grade from the exhumed orogeny in the north from 3.4 Ma and higher grade since 2 Ma; meanwhile, the southern Madagida section has been mostly taking in juvenile detritus from the south part of the orogen until 1 Ma. The central Chimei section has started collecting mature detritus since 1.24 Ma. The data suggest that the Plio-Pleistocene basins contain a good record of orogen evolution.

Late Quaternary drainage evolution in response to fold growth in the northern Chinese Tian Shan foreland

Alluvial units are important in understanding the interactions of antecedent drainage evolution with fold growth along the flanks of active orogenic belts. This is demonstrated by the Anjihai River in the northern Chinese Tian Shan foreland, which at present flows northward cutting sequentially through the Nananjihai anticline, the Huoerguos anticline, and the Anjihai anticline. Three episodes of alluviation designated as fans Fa, Fb, and Fc are identified for the Anjihai River. These three alluvial terrain features comprise a series of terraces, where the topographic characteristics, geomorphologic structure, and up-warped longitudinal profiles indicate continuous uplift and lateral propagation of the Halaande anticline and the Anjihai anticline over the past 50ky. Shortly after ~3.6ka when the oldest terrace during the period of the fan Fb sedimentation was formed, significant rock uplift at the overlapping zone of the Anjihai anticline and the Halaande anticline led to the eastward deflection of the antecedent Anjihai River. A series of local terraces with elevation decreasing eastward indicate the gradual eastward migration of the channel of the Anjihai River during the period of the fan Fc sedimentation. Finally the Anjihai River occupied the previous course of the Jingou River when the latter was deflected eastward in response to rock uplift of the Anjihai anticline, presently flowing across the eastern tip of the Anjihai anticline.

Cenozoic exhumation in the Qilian Shan, northeastern Tibetan Plateau: Evidence from detrital fission track thermochronology in the Jiuquan Basin

AbstractThe India‐Asia collision resulted in the Cenozoic framework of faults, ranges, and tectonic basins and the high topography of the northeastern Tibetan Plateau, but how and when these features formed remains poorly understood, leading to conflicting tectonic models. However, information on the tectonic evolution of these active orogenic belts is well preserved in synorogenic basin sediments. In this study, we carefully analyze the detrital apatite fission track ages of Cenozoic synorogenic sediments from the Jiuquan Basin to decipher the entire exhumation process of the adjacent Qilian Shan throughout the Cenozoic. Our data indicate that initially rapid Cenozoic exhumation occurred in the Qilian Shan during the late Paleocene‐early Eocene (~60–50 Ma), almost synchronous with the India‐Asia collision. The Qilian Shan subsequently experienced long‐lived exhumation that continued until at least the middle Miocene (~45–10 Ma). During this period of exhumation in the Qilian Shan, tectonic deformation occurred throughout the northeastern Tibetan Plateau. The early Cenozoic deformation in the northeastern Tibetan Plateau may have been caused by the transfer of tectonic stress from the distant India‐Asia collision boundary through the complex lithospheric environment of the Tibetan Plateau. The present tectonic configuration and topography of the Qilian Shan and the northeastern Tibetan Plateau likely became established since the middle Miocene and after the long‐lived deformation began in the early Cenozoic.

Architecture of the crust and uppermost mantle in the northern Canadian Cordillera from receiver functions

AbstractThe northern Canadian Cordillera (NCC) is an active orogenic belt in northwestern Canada characterized by deformed autochtonous and allochtonous structures that were emplaced in successive episodes of convergence since the Late Cretaceous. Seismicity and crustal deformation are concentrated along corridors located far (>200 to ~800 km) from the convergent plate margin. Proposed geodynamic models require information on crust and mantle structure and strain history, which are poorly constrained. We calculate receiver functions using 66 broadband seismic stations within and around the NCC and process them to estimate Moho depth and P‐to‐S velocity ratio (Vp/Vs) of the Cordilleran crust. We also perform a harmonic decomposition to determine the anisotropy of the subsurface layers. From these results, we construct simple seismic velocity models at selected stations and simulate receiver function data to constrain crust and uppermost mantle structure and anisotropy. Our results indicate a relatively flat and sharp Moho at 32 ± 2 km depth and crustal Vp/Vs of 1.75 ± 0.05. Seismic anisotropy is pervasive in the upper crust and within a thin (~10–15 km thick) sub‐Moho layer. The modeled plunging slow axis of hexagonal symmetry of the upper crustal anisotropic layer may reflect the presence of fractures or mica‐rich mylonites. The subhorizontal fast axis of hexagonal anisotropy within the sub‐Moho layer is generally consistent with the SE‐NW orientation of large‐scale tectonic structures. These results allow us to revise the geodynamic models proposed to explain active deformation within the NCC.

CHARACTERISTICS OF CHINESE CONTINENT CURIE POINT ISOTHERM

AbstractBased on the latest compiled 1/100 million aeromagnetic map data, the Curie point depth was estimated by power spectrum method. The calculation obtained 8004 Curie point depth values in Chinese continent. Eventually, we completely compiled the Chinese continental Curie point isotherm map. This map first fully shows the characteristics of Chinese continental Curie point depth. The study shows that Curie point isotherm is characterized by depression inside the stable blocks with the depth of 28∼45 km, such as the Tarim Basin, Junggar Basin, Qaidam Basin, Hoh Xil‐Bayan Har Depression area, the Yangtze Basin area, North China basin area, Songliao Basin, Erlian Basin, Bayan‐Wuwei‐Chaoshui basin, Pearl River Estuary‐southeast Hainan basin. The Curie depth of North China basin area is shallower than Tarim continent block and Yangtze continent block, which may be related to the fact that North China block suffered a complex post‐transformation which resulted in the asthenosphere upwelling and lithospheric thinning. Hoh Xil‐Bayan Har block is a NWW trending Curie point isotherm depression area in northern Tibetan Plateau, where developed a large area of Triassic sedimentary strata and experienced less magmatism. The stable blocks all have uplifted Moho and depressed Curie point isotherm. Conversely, the Curie point isotherm of active orogenic belt is characterized by uplift at the depth of 18∼26 km, such as the Mountain (Mtn.) area of northeast and northwest China, Qinling‐Dabie area, West Kunlun‐Tibet‐Sanjiang‐Kangdian area, the southeast coastal areas and so on. The uplift reflects the geothermal gradient difference caused by tectonic and magmatic activities. A collection of 816 heat flow data from published literatures are used to study the relationship between Curie point isotherm and heat flow. The results show that there is not a simply linear relationship between the depth and heat flow. When the depth is greater than 30 km, the heat flow values are less than 100 mW·m−2. While the Curie point depth is less than 30 km, the heat flow values vary in a wide range. The high heat flow values are mostly found in the east coast of China, southern Tibet‐Sanjiang area, Qinling‐Dabie area, and the east of Liaoning province, which are all characterized by uplift of Curie point isotherm. These areas are important for future geothermal exploration.

Changes in gravitational parameters inferred from time variable GRACE data—A case study for October 2005 Kashmir earthquake

The earth's gravity changes are attributed to the redistribution of masses within and/or on the surface of the earth, which are due to the frictional sliding, tensile cracking and/or cataclastic flow of rocks along the faults and detectable by earthquake events. Inversely, the gravity changes are useful to describe the earthquake seismicity over the active orogenic belts. The time variable gravimetric data are hardly available to the public domain. However, Gravity Recovery and Climatic Experiment (GRACE) is the only satellite mission dedicated to model the variation of the gravity field and an available source to the science community. Here, we have tried to envisage gravity changes in terms of gravity anomaly (Δg), geoid (N) and the gravity gradients over the Indo-Pak plate with emphasis upon Kashmir earthquake of October 2005. For this purpose, we engaged the spherical harmonic coefficients of monthly gravity solutions from the GRACE satellite mission, which have good coverage over the entire globe with unprecedented accuracy. We have analysed numerically the solutions after removing the hydrological signals, during August to November 2005, in terms of corresponding monthly differentials of gravity anomaly, geoid and the gradients. The regional structures like Main Mantle Thrust (MMT), Main Karakoram Thrust (MKT), Herat and Chaman faults are in closed association with topography and with gravity parameters from the GRACE gravimetry and EGM2008 model. The monthly differentials of these quantities indicate the stress accumulation in the northeast direction in the study area. Our numerical results show that the horizontal gravity gradients seem to be in good agreement with tectonic boundaries and differentials of the gravitational elements are subtle to the redistribution of rock masses and topography caused by 2005 Kashmir earthquake. Moreover, the gradients are rather more helpful for extracting the coseismic gravity signatures caused by seismicity over the area. Higher positive values of gravity components having higher terrain elevations are more vulnerable to the seismicity and lower risk of diastrophism otherwise.

Critical porosity of melt segregation during crustal melting: Constraints from zonation of peritectic garnets in a dacite volcano

The presence of leucogranitic dikes in orogenic belts suggests that partial melting may be an important process in the lower crust of active orogenies. Low seismic velocity and low electrical resistivity zones have been observed in the lower crust of active mountain belts and have been argued to reflect the presence of partial melt in the deep crust, but volcanoes are rare or absent above many of these inferred melt zones. Understanding whether these low velocity zones are melt-bearing, and if so, why they do not commonly erupt, is essential for understanding the thermal and rheologic structure of the crust and its dynamic evolution. Central to this problem is an understanding of how much melt can be stored before it can escape from the crust via compaction and eventually erupt. Experimental and theoretical studies predict trapped melt fractions anywhere from <5% to >30%. Here, we examine Mn growth-zoning in peritectic garnets in a Miocene dacite volcano from the ongoing Betic–Rif orogeny in southern Spain to estimate the melt fraction at the time of large-scale melt extraction that subsequently led to eruption. We show that the melt fraction at segregation, corresponding approximately to the critical melt porosity, was ∼30%, implying significant amounts of melt can be stored in the lower crust without draining or erupting. However, seismic velocities in the lower crust beneath active orogenic belts (southern Spain and Tibet) as well as beneath active magmatic zones (e.g., Yellowstone hotspot) correspond to average melt porosities of <10%, suggesting that melt porosities approaching critical values are short-lived or that high melt porosity regions are localized into heterogeneously distributed sills or dikes, which individually cannot be resolved by seismic studies.

Crustal permeability: Introduction to the special issue

The topic of crustal permeability is of broad interest in light of the controlling effect of permeability on diverse geologic processes and also timely in light of the practical challenges associated with emerging technologies such as hydraulic fracturing for oil and gas production (‘fracking’), enhanced geothermal systems, and geologic carbon sequestration. This special issue of Geofluids is also motivated by the historical dichotomy between the hydrogeologic concept of permeability as a static material property that exerts control on fluid flow and the perspective of economic geologists, geophysicists, and crustal petrologists who have long recognized permeability as a dynamic parameter that changes in response to tectonism, fluid production, and geochemical reactions. Issues associated with fracking, enhanced geothermal systems, and geologic carbon sequestration have already begun to promote a constructive dialog between the static and dynamic views of permeability, and here we have made a conscious effort to include both viewpoints. This special issue also focuses on the quantification of permeability, encompassing both direct measurement of permeability in the uppermost crust and inferential permeability estimates, mainly for the deeper crust. The directly measured permeability (k) of common geologic media varies by approximately 16 orders of magnitude, from values as low as 10 m in intact crystalline rock, intact shales, and fault gouge, to values as high as 10 m in well-sorted gravels. The permeability of Earth’s upper crust can be regarded as a process-limiting parameter, in that it largely determines the feasibility of advective solute transport (k ~ >10 20 m), advective heat transport (k ~ ≥10 16 m), and the generation of elevated fluid pressures (k ~ ≤10 17 m) – processes which in turn are essential to ore deposition, hydrocarbon migration, metamorphism, tectonism, and many other fundamental geologic phenomena. The hydrodynamics of fluids in the brittle upper crust, where topography and magmatic heat sources dominate patterns of flow and externally derived (meteoric) fluids are common (e.g. Howald et al. 2015) are distinct from the ductile lower crust, dominated by devolatilization reactions and internally derived fluids (e.g. Connolly & Podladchikov, 2015). The brittle–ductile transition between these regimes occurs at 10–15 km depth in typical continental crust. Permeability below the brittle–ductile transition is non-negligible, at least in active orogenic belts (equivalent to mean bulk k of order 10 19 to 10 18 m) so that the underlying ductile regime can be an important fluid source to the brittle regime (e.g. Ingebritsen & Manning 2002). The overall objective of this special issue is to synthesize current understanding of static and dynamic permeability through representative publications from multiple disciplines. The objective of this introduction to the special issue is to define crucial nomenclature and the ‘static’ and ‘dynamic’ permeability perspectives and to briefly summarize the contents of this special issue, which is divided into the following sections: the physics of permeability, static permeability, and dynamic permeability.

Apatite fission track constraints on the pattern of faulting in the north Qilian Mountain

The Qilian (祁连) Mountain is an active orogenic belt located at the northeastern margin of the Tibetan Plateau. During the process of continuous convergence between Indian and Eurasian plates, the Qilian Mountain grow correspondingly by means of reaction of old faults and generation of new ones. Here we present apatite fission-track data along a river profile crossing three minor fault (the Minle (民乐)-Damaying (大马营) fault, the Huangcheng (皇城)-Taerzhuang (塔尔庄) fault and the Kangningqiao (康宁桥) fault) which compose the North Qilian fault (NQF) to test the timing and patterns of the fault activities. Apatite fission-track (AFT) results indicate that these minor faults experienced two active phases in the Cretaceous and the Oligocene-Miocene. Further research indicate that the initiation timing of faulting became younger northward in both active periods and the later phase probably more active than the former phase. These tectonic activities might be highly related to the docking of the Lhasa Block to the south in the Cretaceous and uplift and expansion of the Tibetan Plateau in the Cenozoic.

The significance of mid-latitude rivers for weathering rates and chemical fluxes: Evidence from northern Xinjiang rivers

Rivers draining the sedimentary platform of northern Xinjiang (the center of Asian continent) are characterized by low discharge under a temperate and arid climate. The influence of rock mineralogy, climate, relief and human activity on natural water composition and export as a result of weathering is a major scientific concern both at the local and the global scale. While comprehensive work on the controlling mechanism of chemical weathering has been less carried out in the sedimentary platform of northern Xinjiang. Thus, the effects of climate and rock weathering on the inorganic hydrogeochemical processes are not well quantified at this climatic extreme. To remedy this lack a comprehensive survey has been carried out of the geochemistry of the large, pristine rivers in northern Xinjiang, the Erlqis, Yili, Wulungu, Jingou and numerous lesser streams which has not experienced the pervasive effects of glaciation and subsequent anthropogenic impacts. The scale of the terrain sampled, in terms of area, is comparable to that of the Huanghe and includes a diverse range of geologic and climatic environments. In this paper the chemical fluxes from the stable sedimentary basin of the northern Xinjiang platform will be presented and compared to published results from analogous terrains in the monsoon basins of China and world. Overall, the fluvial geochemistry of northern Xinjiang in westerly climate is similar to that of the Chinese rivers (Huanghe and Yangtze) in the East-Asian monsoon Climate, both in property–property relationships and concentration magnitudes. The range in the chemical signatures of the various tributaries is large; this reflects that lithology exerts the dominant influence in determining the weathering yield from the sedimentary terrains rather than the weathering environment. The effect of different rock weathering ranges from rivers dominated by aluminosilicate weathering, mainly of granites, sandstones and shales, to those bearing the signatures of dissolution of carbonates and evaporites and of continental playa deposits. Carbonates are the general predominant lithology undergoing dissolution particularly within the lesser arid areas. The pCO2 in the study rivers is out of equilibrium with respect to atmospheric pCO2, about up to ∼20 times supersaturated relative to the atmosphere but not to such an extent as the Amazon in the floodplain. A roughly positive relationship is observed between solute concentrations and the drought index (DI) for natural waters in the region, indicating a coupled mountain-basin climate has a direct effect. The relative contributions of end-member solute sources to the total dissolved cations from each watershed have been quantitatively estimated using dissolved load balance models, showing the results as evaporite dissolution > carbonate weathering > silicate weathering > atmospheric input for the whole catchment. The areal total dissolved fluxes range from 0.05 to 2.53 × 106 mol/km2/yr, 0.02–2.09 × 106 mol/km2/yr and 0.01–1.04 × 106 mol/km2/yr in the Yili, Zhungarer and Erlqis, respectively, comparable to those of Chinese and Siberia rivers draining sedimentary platforms, even though they are in drastically different climatic regimes. In general, the fluxes from rivers in sedimentary basins are comparable to those from orogenic zones, but are much higher than in the shield regions. The CO2 consumption by aluminosilicate weathering (0.2–284 × 103 mol/km2/yr) is much smaller than in active orogenic belts (19–1750 × 103 mol/km2/yr in similar latitudes and 143–1000 × 103 mol/km2/yr in the tropical basins), but comparable to those of the Chinese (7–106 × 103 mol/km2/yr) and Siberia (16–112 × 103 mol/km2/yr) rivers.

Tectonic implications of nonparallel topographic and structural curvature in the higher elevations of an active collision zone, Taiwan

Curves in the topographic grain of active orogenic belts typically parallel faults and lithologic contacts in part because of linkages between uplift rates and the structural development of the overriding plate. However, in the Taiwan collision zone, the topographic grain trends nonparallel to mapped faults and folds in the central portion of the belt along the southwest flank of the Hsuehshan Range. Here, the northern side of the Puli topographic embayment trends ∼345°, forming a topographic break that lies at a high angle to the structural grain but is nearly parallel to an underlying continental margin fracture zone in the downgoing plate. We analyzed fault-slip and other structural data, extracted normalized steepness indices from streams within the Tachia, Peikang, and Mei River basins, and integrated the results with recently published precise leveling data in order to understand the spatial and temporal variation in uplift and the structures that accommodated that uplift. Stress inversion reveals a NW-SE–trending maximum compression direction for an early-stage fault population and an ENE-WSW–trending maximum compression direction for a late-stage fault population. River steepness indices delineate a NW-trending boundary in incision rate that coincides with an increase in rock uplift rates derived from independent geodetic measurements. This NW-trending boundary is consistent with the late-stage maximum compression direction, suggesting that uplift of the Hsuehshan Range relative to the Puli topographic embayment has been accommodated by the late-stage faults. Our results suggest that a continental margin promontory has slowed underplating beneath the Puli topographic embayment and that the topographic grain of active collision zones is closely linked with the architecture of the downgoing plate.

Late Quaternary megaturbidites of the Indus Fan: Origin and stratigraphic significance

The Indus sedimentary basin forms one of the largest “source-to-sink” systems of the Quaternary and extends over 106 km2 offshore. It is characterized by a complex tectonic setting marked by the Himalayan active orogenic belt in the source area, and the active strike-slip India-Arabia plate boundary (Owen Fracture Zone; OFZ) in its distal reaches. This paper focuses on a Late Quaternary channel–levee system from the Indus Fan captured by the recent opening of the 20°N pull-apart basin, located at 850 km off the present-day Indus Delta, along the OFZ. In this area the channel-mouth deposits consist of a set of up to 23 m thick megaturbidites trapped in the basin. These deposits form “ponded” lobe deposits in a tectonically-active confined basin. Age determination from radiocarbon dating and extrapolation of local deformation rates show that the older deposits observed on the seismic profiles are up to 358 ka BP old (MIS 10). The origin of these Late Quaternary deposits are investigated in the context of the Indus “source-to-sink” system and their significance is placed in a sequence stratigraphic framework. Integration of the stratigraphic architecture of the 20°N Basin megaturbidites with previous work in the area suggests that the Indus Fan evolved from a delta-fed turbidite system with several active canyons and channel–levee during the forced regressive conditions of the last falling stage of sea-level (122–25 ka BP), to a point source turbidite system during the sea-level lowstand (Last Glacial Maximum) and early transgressive stages (25–12 ka BP). This work sheds new light on the recent evolution of the Indus sedimentary system and illustrates the importance of the delta/river evolution during the fall of sea-level (e.g., incised valley formation) on the timing of sedimentary transfer and sediment distribution at the basin-scale.

U/Pb dating of a terminal Pliocene coral from the Indonesian Seaway

Pristine detrital Platygyra corals were discovered in an exhumed package of syn-orogenic marine sediments on the island of Timor in the eastern Indonesian region and dated using U–Pb techniques. A single coral from the upper part of the sequence yields a 238U/206Pb–207Pb/206Pb concordia age of 2.66±0.14 (2σ) Ma that is supported by coral 87Sr/86Sr chemostratigraphy and foraminiferal biostratigraphy from bounding strata. Minor U-series disequilibrium is best explained by U mobility within the last ~150ka, as pore water chemistry was altered during exhumation, and is unlikely to have affected 238U/206Pb and the apparent sample age by more than 1–2%. The ability to date corals beyond the limits of 14C and U/Th techniques provides the opportunity to improve the temporal resolution of associated marine chronostratigraphic records. In this instance, we refine the timing of Timor's emergence from beneath the waters of the Indonesian Seaway (IS) and the initiation of turbiditic deposition at the study site to between ca. 3.35 and 2.66Ma. These results have implications for the evolution of topography and IS oceanic pathways in the active orogenic belts along the northern fringes of the Australian Plate.

Hyperpycnal Rivers and Prodeltaic Shelves in the Cretaceous Seaway of North America

Abstract Despite the historical assumption that the bulk of marine “shelf” mud is deposited by gradual fallout from suspension in quiet water, recent studies of modern muddy shelves and their associated rivers show that they are dominated by hyperpycnal fluid mud. This has not been widely applied to the interpretation of ancient sedimentary fluvio-deltaic systems, such as dominate the mud-rich Cretaceous Western Interior Seaway of North America. We analyze two such systems, the Turonian Ferron Sandstone Member of the Mancos Shale Formation, in Utah, and the Cenomanian Dunvegan Formation in Alberta. Paleodischarge estimates of trunk rivers show that they fall within the predicted limits of rivers that are capable of generating hyperpycnal plumes. The associated prodeltaic mudstones match modern hyperpycnite facies models, and suggest a correspondingly hyperpycnal character. Physical sedimentary structures include diffusely stratified beds that show both normal and inverse grading, indicating sustained flows that waxed and waned. They also display low intensities of bioturbation, which reflect the high physical and chemical stresses of hyperpycnal environments. Distinct “mantle and swirl” biogenic structures indicate soupground conditions, typical of the fluid muds that represent the earliest stages of deposition in a hyperpycnal plume. Hyperpycnal conditions are ameliorated by the fact that these rivers were relatively small, dirty systems that drained an active orogenic belt during humid temperate (Dunvegan Formation) to subtropical (Ferron Sandstone Member) “greenhouse” conditions. During sustained periods of flooding, such as during monsoons, the initial river flood may lower salinities within the inshore area, effectively “prepping” the area and allowing subsequent floods to become hyperpycnal much more easily. Although shelf slopes were too low to allow long-run-out hyperpycnal flows, the storm-dominated nature of the seaway likely allowed fluid mud to be transported for significant distances across and along the paleo-shelf. Rapidly deposited prodeltaic hyperpycnites are thus considered to form a significant component of the muddy shelf successions that comprise the thick shale formations of the Cretaceous Western Interior Seaway.

Dissolved constituents and Sr isotopes in river waters from a mountainous island – The Danshuei drainage system in northern Taiwan

Taiwan is a typical active orogenic belt situated at the collision boundary between the Eurasian Continental Plate and the Philippine Sea Plate. Dissolved major and trace constituents, as well as Sr and Sr isotopes in river waters collected from the Danshuei River basin in northern Taiwan have been studied to evaluate chemical weathering processes. The results of principal component analysis show that the ion sources in these river waters can be categorized into 3 major components: chemical weathering, seasalt contribution and local anthropogenic input. The chemical weathering is the most dominant factor that contributes about 85% of total variances. Significantly increased Na/Cl and Ca/Cl, as well 87Sr/ 86Sr, were observed in most upper stream samples. The Na/Cl and Ca/Cl ratios in the Dahan Stream, however, are much higher than the Shindien Stream. Even though average rainfall is stronger in the Shindien drainage basin, chemical evidence from river waters supports less intense weathering in the region. Selective dissolution of secondary calcites explains the observed high Ca/Cl, Sr/Cl and Ca/Na in the Dahan Stream. These results highlight the potential importance of tectonic factors, such as uplift and physical erosion in studying chemical weathering in an active orogenic belt. The variations of 87Sr/ 86Sr in the Danshuei River are quite large, reflecting some strata that released Sr. Most of the upstream waters exhibit more radiogenic 87Sr/ 86Sr, 0.713243–0.714338, due to weathering of ambient low-grade metamorphic rocks, 0.71678–0.72216. The distributions of Cl, Sr and 87Sr/ 86Sr in the main stream were affected by somewhat conservative mixing with sources varing between ambient rocks and seasalt. In the upper Dahan Stream, heavy 87Sr/ 86Sr ratios were coincident with large deviations of Na/Cl from the average seawater value, as high as 40. The degree of chemical weathering in ambient rocks plays a dominant role in affecting the distribution of Sr and 87Sr/ 86Sr in the Danshuei River. This isotopic characteristic makes 87Sr/ 86Sr an invaluable tracer for studying source mixing, migration pathways and chemical weathering in an active orogenic belt.