Physiographic Transition Research Articles

Spatial patterns in dissolved organic matter composition controlled by watershed characteristics in a coastal river network: The Neuse River Basin, USA

The effect of watershed characteristics (land use land cover and morphology) on spatial variability in dissolved organic matter (DOM) composition, and concentrations of dissolved organic carbon [DOC] and nitrogen [DON] was assessed in a coastal river basin draining into Pamlico Sound in eastern North Carolina, USA. Understanding the factors that influence DOM concentration and composition i.e., structurally complex molecules with high molecular weight versus low molecular weight, simple molecules can provide insights on DOM cycling and water composition implications. Such information is imperative for large coastal river networks undergoing rapid and intense land use and land cover (LULC) changes. DOM composition was estimated using optical indices calculated from DOM absorbance and fluorescence measurements. DOM was derived from terrestrial sources, and ordination analysis indicated that LULC, in particular, % wetland area was the most significant control on DOM composition and concentration. Wetland and agricultural coastal streams were abundant in humic and complex DOM, whereas forested and urban streams were least abundant in humic DOM. We speculate that greater availability of mobilizable DOM in wetland and agricultural watersheds contributed to this observation. In comparison, mixed urbanized and forested streams in North Carolina’s Piedmont region were abundant in [DOC], less complex, low molecular weight DOM, as well as greater amounts [DON] due to higher urban runoff and elevated DOM production in these streams. Our results indicated that physiographic transition from Piedmont to coastal plain and varying LULC influenced the spatial variability in DOM composition and concentration. Our findings highlight that increasing anthropogenic alterations might increase the abundance of reactive DOM in coastal rivers and estuaries resulting in severe water quality issues. This information is important for monitoring and developing land use policies.

Earthquakes in the Garhwal Himalaya of the Central Seismic Gap: A Study of Historical and Present Seismicity and Their Implications to the Seismotectonics

The historical and the present seismicity catalogues of the Garhwal Himalaya have been studied for their spatio-temporal variations and their implications on the seismotectonics of the region. The Micro-Seismicity, Fractal dimensions (Dc) and Frequency Magnitude Distribution (b-value) coupled with the available literature on geology, geomorphology and geophysics have been used to derive the seismotectonics and stress level changes in the region. The seismic cross sections for the relocated micro-seismicity, focal mechanisms and the swath profiles (for the presence of Physiographic Transition 2 (PT2) at the foothill of the Higher Himalaya) indicate the constant presence of the Mid-Crustal Ramp (MCR) in the detachment plane and its active seismogenic nature. The comparison of this scenario suggests the constant presence of seismogenically active MCR structure throughout the Central Seismic Gap. The seismic cross sections further reveal that the sensu stricto Main Central Thrust (Munsiari Thrust) is also a site of generation of the micro-seismicity in few segments due to its reactivation by thrusting along the MCR. The high fractal dimension value (Dc = 1.47) suggests the heterogeneous nature of the region, owing to the presence of local faults and transverse tectonics. The high stress accumulation in the Garhwal Himalaya with low b value (b = 0.70) suggests the high probability of occurrence of a larger or greater earthquake in the near-future. Further, the study also reveals that the 2011 Chamoli earthquake of M 5.0, preceded by a quiescence period of nearly a year shows different stress levels before and after its occurrence, which is well constrained with the increased moderate earthquake activity around the Chamoli region. This increased seismic activity and stress conditions in the Chamoli region suggest the high possibility of the occurrence of major earthquakes, hence the study recommend for a detailed seismic hazard evaluation of the region.

Herpetofauna del sitio arqueológico Iglesia Vieja, Costa de Chiapas, México

Se realizó un listado de las especies de anfibios y reptiles del sitio arqueológico Iglesia Vieja. Este sitio se localiza entre la costa y la Sierra Madre de Chiapas a 711 msnm. Los muestreos se realizaron mensualmente entre junio de 2014 y mayo de 2015 abarcando las temporadas de lluvias y secas en dos tipos de vegetación (herbácea y arbórea). La herpetofauna estuvo representada por 61 especies (12 anfibios y 49 reptiles). Los anfibios se agruparon en ocho familias, 10 géneros y 12 especies; para los reptiles, el grupo de las lagartijas se constituyó por 13 familias, 16 géneros y 20 especies; mientras que las serpientes estuvieron representadas por siete familias, 25 géneros y 29 especies. Los estimadores de riqueza Jackknife 1 y Bootstrap mostraron una adecuada representación de la composición. Las curvas de abundancia reflejaron una mayor dominancia de especies generalistas o asociadas a perturbación (Hemidactylus frenatus, Basiliscus vittatus, Rhinella horribilis). El 35% de las especies registradas se encuentra bajo alguna categoría de riesgo de acuerdo a la normativa federal. Aunque el área se localiza en una zona de transición fisiográfica, la herpetofauna mostró una alta afinidad con la Planicie costera del Pacífico (66.53%). Las áreas destinadas a preservar el patrimonio arqueológico constituyen también espacios que contribuyen a mantener los procesos biológicos. Estos espacios rara vez cuentan con inventarios faunísticos. Este trabajo representa una contribución al conocimiento de la biota asociada a sitios arqueológicos en Chiapas, y el inicio de una línea base para el manejo de una zona arqueológica y su biodiversidad, así como para futuras investigaciones sobre la ecología de un grupo prioritario.

Observations of Landslides Caused by the April 2015 Gorkha, Nepal, Earthquake Based on Land, UAV, and Satellite Reconnaissance

Thousands of landslides occurred during the April 2015 Gorkha earthquake in Nepal. Previous work using satellite imagery mapped nearly 25,000 coseismic landslides. In this study, the satellite-based mapping was analyzed in three areas where field deployment was also conducted—the Budhi Gandaki, Trishuli, and Indrawati river valleys—to better characterize the landslides. Unmanned aerial vehicles (UAVs) were deployed to map the three-dimensional (3-D) geometry of failed slopes using photogrammetry, as well as to characterize rock structure and strength. The majority of landslides were rock slides along the ridges and the steeper portions of the basins primarily involving the weathered rock zone. Additional landslides included rock falls and soil failures. Satellite imagery analysis indicated that landsliding was concentrated north of the physiographic transition, in steep areas, and in close proximity to the major rivers. The Trishuli area experienced the lowest landslide density in terms of number of landslides compared to the Budhi Gandaki and Indrawati areas, although all three areas had similar density in terms of total landslide area and other landslide statistics.

Current strain accumulation in the hinterland of the northwest Himalaya constrained by landscape analyses, basin-wide denudation rates, and low temperature thermochronology

Rupture associated with the 25 April 2015 Mw 7.8 Gorkha (Nepal) earthquake highlighted our incomplete understanding of the structural architecture and seismic cycle processes that lead to Himalayan mountain building in Central Nepal. In this paper we investigate the style and kinematics of active mountain building in the Himalayan hinterland of Northwest India, approximately 400km to the west of the hypocenter of the Nepal earthquake, via a combination of landscape metrics and long- (Ma) and short-term (ka) erosion rate estimates (from low temperature thermochronometry and basin-wide denudation rate estimates from 10Be concentrations). We focus our analysis on the area straddling the PT2, the physiographic transition between the Lesser and High Himalaya that has yielded important insights into the nature of hinterland deformation across much of the Himalaya. Our results from Northwest India reveal a distinctive PT2 that separates a Lesser Himalaya region with moderate relief (∼1000m) and relatively slow erosion (<1mm/yr) from a High Himalaya with extreme relief (∼2500m), steep channels, and erosion rates that approach or exceed 1mm/yr. The close spatial similarity in relative rates of long- and short-term erosion suggests that the gradient in rock uplift rates inferred from the landscape metrics across the PT2 has persisted in the same relative position since at least the past 1.5Ma. We interpret these observations to suggest that strain accumulation in this hinterland region throughout at least the past 1.5Ma has been accomplished both by crustal thickening via duplexing and overthrusting along transient emergent faults. Despite the >400km distance between them, similar spatiotemporal patterns of erosion and deformation observed in Northwest India and Central Nepal suggest both regions experience similar styles of active strain accumulation and both are susceptible to large seismic events.

Quaternary channel-focused rapid incision in the Phung Chu-Arun River in Central Himalaya: Implications for a Quaternary capture event

The Phung Chu-Arun River, one of the few major trans-Himalaya rivers, consists of a large drainage area captured in the northern slope of the Himalaya, and provides a key to understand tectonic, surface processes and their interplay in topographic development of the Himalaya. In this study, we carried out thermochronologic and topographic analysis along the Phung Chu-Arun River in Central Himalaya to reveal its incision development. Apatites from a downstream section along its major tributary yield very young fission track and (U-Th)/He ages of 0.8–1.5Ma. These cooling ages, combined with previously reported data for higher elevations, reveal an abrupt change of the slope in the age-elevation plot suggesting an accelerated channel incision initiated at between ∼1.5 and 2.5Ma. The age-elevation relationship yields a high apparent exhumation rate (AER) of ∼2.6–3.8mm/a during Quaternary, which is spatially focused along the mainstream of the Kharta-Chentang reaches and the associated nearby tributaries. Topographic analysis suggests that this channel-focused incision acceleration was probably facilitated by a Quaternary capture event in upper drainage of the palaeo Arun River. This capture event greatly enhanced the effective discharge draining southward, causing an accelerated incision wave along the mainstream and tributaries of the palaeo Arun River. Furthermore, thrusting at physiographic transition zone is also required to sustain high channel gradient and the long-term rapid incision. Channel morphology presenting common knick points in major tributaries suggests that the drainage basin in Ama Drime Range area likely remain in pre-steady state adjustment of this capture event.

Geomorphology reveals active décollement geometry in the central Himalayan seismic gap

The similar to 700-km-long ``central seismic gap'' is the most prominent segment of the Himalayan front not to have ruptured in a major earthquake during the last 200-500 yr. This prolonged seismic quiescence has led to the proposition that this region, with a population >10 million, is overdue for a great earthquake. Despite the region's recognized seismic risk, the geometry of faults likely to host large earthquakes remains poorly understood. Here, we place new constraints on the spatial distribution of rock uplift within the western similar to 400 km of the central seismic gap using topographic and river profile analyses together with basinwide erosion rate estimates from cosmogenic Be-10. The data sets show a distinctive physiographic transition at the base of the high Himalaya in the state of Uttarakhand, India, characterized by abrupt strike-normal increases in channel steepness and a tenfold increase in erosion rates. When combined with previously published geophysical imaging and seismicity data sets, we interpret the observed spatial distribution of erosion rates and channel steepness to reflect the landscape response to spatially variable rock uplift due to a structurally coherent ramp-flat system of the Main Himalayan Thrust. Although it remains unresolved whether the kinematics of the Main Himalayan Thrust ramp involve an emergent fault or duplex, the landscape and erosion rate patterns suggest that the decollement beneath the state of Uttarakhand provides a sufficiently large and coherent fault segment capable of hosting a great earthquake.

Dominance of tectonics over climate in Himalayan denudation

Landscape denudation in actively deforming mountain ranges is controlled by a combination of rock uplift and surface runoff induced by precipitation. Whereas the relative contribution of these factors is important to our understanding of the evolution of orogenic topography, no consensus currently exists concerning their respective infl uences. To address this question, denudation rates at centennial to millennial time scales were deduced from 10Be concentrations in detrital sediments derived from 30 small basins (10-600 km2) in an ~200-km-wide region in central Nepal. Along a northward, strike-perpendicular transect, average denudation rates sharply increase from <0.5 mm/yr in the Lesser Himalayas to ~1 mm/yr when crossing the Physiographic Transition, and then accelerate to 2-3 mm/yr on the southern flank of the high peaks in the Greater Himalayas. Despite a more than five-fold increase in denudation rate between the southern and northern parts of this transect, the corresponding areas display similar precipitation rates. The primary parameter that presents a signifi cant co-variation with denudation is the long-term rockuplift rate that is interpreted to result from the ramp-fl at transition along the Main Himalayan Thrust. We propose that, in this rapidly uplifting mountain range, landscapes adjust quickly to changing climatic conditions, such that denudation is mainly limited by the rate at which material is pushed upward by tectonic processes and made available for removal by surface processes. In this particular context, variations in precipitation appear to have only a second-order role in modulating the denudation signal that is primarily set by the background rock-uplift rate. © 2014 Geological Society of America.

Evidence for Pliocene–Quaternary normal faulting in the hinterland of the Bhutan Himalaya

In the central Himalaya, past researchers have identified a distinctive transition from the physiographic Lower Himalayan ranges in the south to the Higher Himalayan ranges in the north. Local relief and hillslope gradient, as well as erosion and surface uplift rates, increase abruptly across this transition to the north. In the eastern Himalaya, the same physiographic transition exists, but it is less dramatic. We describe here a previously undocumented steep, north-dipping, brittle structure that is roughly coincident with this physiographic transition in eastern Bhutan—the Lhuentse fault. Low-temperature (U-Th)/He apatite data suggest that the Lhuentse fault has been active since the Pliocene, and (U-Th)/He dates on offset hydrothermal hematite deposits from within the fault zone demonstrate a component of Quaternary slip. Although we identified no definitive evidence of fault kinematics based on field or petrographic analysis of the fault rocks, the disrupted pattern of (U-Th)/He apatite dates suggests normal-sense displacement, contrary to what was expected given previous studies of an analogous transition in the central Himalaya. We regard the existence and activity of the Lhuentse fault as evidence of (1) recent evolution in the tectonic regime of the eastern Himalaya from one of near-exclusive north-south shortening to one in which both transcurrent and normal faulting are increasingly important in the region north of the Himalayan deformation front, or (2) an active duplex south of the physiographic transition in the middle latitudes of Bhutan.

Evidence for Plio‐Pleistocene north‐south extension at the southern margin of the Tibetan Plateau, Nyalam region

The southern Tibetan Plateau margin between ~ 83°E and 86.5°E is defined by an abrupt change from the low‐relief Tibetan Plateau to the rugged topography and deep gorges of the Himalaya. This physiographic transition lies well to the north of active thrusting, and thus, the mechanism responsible for the distinct topographic break remains the focus of much debate. While numerous studies have utilized thermochronology to examine the exhumation history of the Himalaya, few have done so with respect to variations across the Himalaya‐Tibetan Plateau transition. In this work, we examine the nature of the transition where it is accessible and well‐defined in the Nyalam valley of south‐central Tibet. We employ several new and previously published thermochronologic datasets (with a closure temperature range of ~ 70°C–300°C) in conjunction with river incision patterns inferred by the longitudinal profile of the Bhote Kosi River. The results reveal a sharp change in cooling rate at ~ 3.5 Ma at a location corresponding to a pronounced river knickpoint representing a sharp increase in river gradient and presumably incision rate (a proxy for rock uplift). Margin retreat models for the physiographic transition are inconsistent with the cooling pattern revealed by low‐temperature thermochronologic data. Models invoking passive uplift of the upper crust over a midcrustal ramp and associated duplex to account for the physiographic transition do not explain the observed break in cooling rate there, although they may explain a suggesting in the thermochronologic data of progressively increasing exhumation rates south of the transition. The simplest model consistent with all observations is that passive uplift is augmented by contemporaneous differential uplift across a young (Pliocene‐Quaternary) normal fault at the physiographic transition. Drawing on observations elsewhere, we hypothesize that similar structural relationships may be characteristic of the Tibetan Plateau‐Himalaya transition from ~83°E – 86.5°E.

Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

Two end‐member kinematic models of crustal shortening across the Himalaya are currently debated: one assumes localized thrusting along a single major thrust fault, the Main Himalayan Thrust (MHT) with nonuniform underplating due to duplexing, and the other advocates for out‐of‐sequence (OOS) thrusting in addition to thrusting along the MHT and underplating. We assess these two models based on the modeling of thermochronological, thermometric, and thermobarometric data from the central Nepal Himalaya. We complement a data set compiled from the literature with 114 40Ar/39Ar, 10 apatite fission track, and 5 zircon (U‐Th)/He thermochronological data. The data are predicted using a thermokinematic model (PECUBE), and the model parameters are constrained using an inverse approach based on the Neighborhood Algorithm. The model parameters include geometric characteristics as well as overthrusting rates, radiogenic heat production in the High Himalayan Crystalline (HHC) sequence, the age of initiation of the duplex or of out‐of‐sequence thrusting. Both models can provide a satisfactory fit to the inverted data. However, the model with out‐of‐sequence thrusting implies an unrealistic convergence rate ≥30 mm yr−1. The out‐of‐sequence thrust model can be adjusted to fit the convergence rate and the thermochronological data if the Main Central Thrust zone is assigned a constant geometry and a dip angle of about 30° and a slip rate of &lt;1 mm yr−1. In the duplex model, the 20 mm yr−1 convergence rate is partitioned between an overthrusting rate of 5.8 ± 1.4 mm yr−1 and an underthrusting rate of 14.2 ± 1.8 mm yr−1. Modern rock uplift rates are estimated to increase from about 0.9 ± 0.31 mm yr−1 in the Lesser Himalaya to 3.0 ± 0.9 mm yr−1 at the front of the high range, 86 ± 13 km from the Main Frontal Thrust. The effective friction coefficient is estimated to be 0.07 or smaller, and the radiogenic heat production of HHC units is estimated to be 2.2 ± 0.1 μW m−3. The midcrustal duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr−1. We also run 3‐D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effect of the evolving topography can explain a fraction of the scatter observed in the data but not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumation are likely. It has been argued that the steep physiographic transition at the foot of the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well.

Neotectonics of the central Nepalese Himalaya: Constraints from geomorphology, detrital 40Ar/39Ar thermochronology, and thermal modeling

The southern flanks of the central Nepalese Himalaya correspond to a sharp transition in landscape morphology and bedrock mineral cooling ages that suggests a change in rock uplift rate. This transition can be explained by either (1) accretion of footwall material to the hanging wall across a ramp in the décollement separating India from Eurasia, thereby enhancing rock uplift rates above the zone of accretion or (2) out‐of‐sequence surface thrust faulting at the physiographic transition. Here we use geomorphic data, 649 new detrital 40Ar/39Ar cooling ages, and a simple thermokinematic model to evaluate which of these tectonic configurations is most appropriate for the central Nepalese Himalaya. We first define and delineate the physiographic transition in central Nepal using maps of knickpoints, river steepness indices, local relief, and the distribution of thick alluvial fill deposits. We then report new detrital 40Ar/39Ar data from two trans‐Himalayan transects, each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition. Thermokinematic modeling suggests that either of the two developmental scenarios for the transition is plausible but that an accretion model is viable only under an extremely narrow range of conditions. We contend that the physiographic and thermochronologic data in our study area are most simply explained by recent out‐of‐sequence surface thrusting within the Lesser Himalayan metasedimentary sequence, approximately 15–30 km south of the mapped surface trace of the Main Central Thrust system. An important finding of this work is that there are substantial along‐strike variations in physiography and thermal history that reflect along‐strike changes in the degree and location of out‐of‐sequence surface thrusting.