Alluvial Fan Surfaces Research Articles

Active fault topography along Kangra Valley Fault in the epicentral zone of 1905 Mw7.8 earthquake NW Himalaya, India

The 1905 CE Kangra earthquake (Mw7.8) was one of the most damaging earthquakes in Himalaya. Malik et al (2015) suggested that the Kangra Valley Fault (KVF) from the epicentral zone was the source of the 1905 Kangra earthquake. In this article we discuss the active fault topography along KVF marked by distinct right lateral strike-slip movement, extends for about 60 km between Patka Village in NW and Tanda Village in SE. The prominent offset of streams ranging from 10 m to 340 m have been identified along the fault trace; the lateral offset of 10 m may be indicative of displacement during a single event. Lateral as well as the vertical offset of alluvial fan surfaces and fluvial terraces, resulted in formation of the north as well as south facing fault scarps ranging in height from 1.2 to 14 m. KVF between two major thrust systems i.e., Main Boundary Thrust (MBT) and Jawalamukhi Thrust (JMT), suggest that slip-partitioning is taking place between the thrust and strike-slip faults and is a net result of oblique convergence in NW Himalaya. The end points of the fault traces exhibit typical “Y” shape because of stepping and branching out of rupture from the main fault. Considering the pre-stress axis from the GPS velocity vectors from Banerjee and Bürgman, (2002), the branching scenario we obtained for KVF, is not consistent with the simulated models by Kame et al., (2003) and Poliakov et al., (2002) for other faults. This could be attributed to the ongoing oblique convergence in NW Himalaya. Based on the “Y” shape pattern, and the upthrown and downthrown sides mapped along the fault trace, we suggest two possibilities for surface rupture, (1) the fault rupture initiated from Pathiyar-Malan side and propagated unilaterally towards west, (2) taking into account the “Y” shape on both the end sections of the fault trace, it can be inferred that the rupture during most recent event probably started in the center at Miniana and propagates bilaterally in E-W.

The influence of tectonic subsidence on volcaniclastic sedimentation: The Cretaceous upper Daeri Member, Wido Island, Korea

AbstractThe Daeri Member, a Cretaceous volcanic–sedimentary succession, can be divided into lower, middle, and upper parts based on vertical changes in its lithologic characters. The lower Daeri Member is composed of siliciclastic deposits formed in a semi‐arid floodplain environment, which is overlain by the middle Daeri Member consisting mainly of andesite lava flow. After the emplacement of the andesite, activities of intrabasinal normal faults created accommodation on hanging wall blocks together with the development of intrabasinal topographic relief. The upper Daeri Member occurs only in hanging wall blocks and is composed of rhyolitic volcaniclastic sediments formed during an explosive volcanic eruption. Following the eruption, owing to semi‐arid climatic conditions and the destruction of vegetation, the eruptive materials were easily remobilized and deposited by episodic sediment gravity flows, resulting in deposition of the resedimented volcaniclastic deposits with sheet‐like geometry. Away from the intrabasinal normal faults, the resedimented volcaniclastic deposits show a decrease in grain size together with changes in inferred depositional processes from debris flows to hyperconcentrated flows and supercritical sheetfloods. This suggests that the resedimented volcaniclastic deposits were stacked on alluvial fan environments induced by intrabasinal topographic relief associated with normal fault activities. In addition, episodic movement of the faults gave rise to periodic fluctuation of the accommodation and an increase in gradient of the alluvial fan surface, resulting in the development of coarsening‐upward trends in the resedimented volcaniclastic deposits. The development of the alluvial fan and the coarsening‐upward trends indicate that dynamic tectonic subsidence and concomitant changes in the intrabasinal physiographic relief influenced the depositional processes and sizes of the transported volcaniclastic sediments of the upper Daeri Member. Thus, it is necessary to carefully observe tectonic signatures in volcaniclastic successions, particularly the syneruptive lithofacies, in order to reconstruct the tectonic and volcanic histories of receiving basins.

Large‐Scale Earthquake‐Hazard Class Mapping by Parcel in Las Vegas Valley, Nevada

Clark County and the city of Henderson, Nevada, completed the United States’ first effort to map earthquake site classifications with systematic, direct measurements through an entire urban area. Urban development, disaster response planning, and especially building code implementation and enforcement motivated map development. The municipalities contracted the Nevada System of Higher Education to classify ∼1500 km2 including urban Las Vegas Valley and exurban areas of future development. The resulting Parcel Map includes over 10,000 surface‐wave array measurements accomplished within three years using Optim’s SeisOpt ReMi refraction microtremor measurement and processing technology adapted for large‐scale data collection. The noisy urban setting necessitated use of microtremor as the seismic source. With a nominal measurement spacing of 300 m or less, the Parcel Map classifies every parcel on the National Earthquake Hazards Reduction Program V S 30 site‐class scale. The most important revelation of the Parcel Map was that 84% of the mapped region was found to be stiffer than the default site class D category previously imposed on the whole of the Las Vegas Valley. Details of the mapping also revealed the high variability of V S 30 over short distances (<200 m) in many areas. We describe a class C+ for sites with class B V S 30 velocities but soft surface soil more than 3 m thick. Correlation between the occurrence of class C+ sites and topographic slope varied spatially from north to south along the western margin of Las Vegas Valley. Of particular significance was the delineation of an alluvial fan surface that was not topographically expressed in the southwest part of the valley. Given the density of velocity measurements required for code enforcement, ReMi proved to cost‐effectively produce the desired Parcel Map within the three‐year contract period. [Electronic Supplement:][1] Table listing the “Priorities for Action” outlined for disaster risk reduction under the Sendai Framework for Disaster Risk Reduction accomplished through the Parcel Mapping Project. [1]: http://www.bssaonline.org/lookup/suppl/doi:10.1785/0120160300/-/DC1

Timing and nature of alluvial fan development along the Chajnantor Plateau, northern Chile

Alluvial systems in the Atacama Desert provide a unique opportunity to elucidate the sedimentary response to climate variability, particularly changes in precipitation, in hyperarid environments. Alluvial fans along the eastern margin of the Salar de Atacama, adjacent to the Chajnantor Plateau in the Atacama Desert of northern Chile, provide an archive of climate-modulated sediment transfer and erosion at an extreme of Earth's climate. Three regional alluvial fan surfaces (Qf1 [oldest] to Qf3 [youngest]) were mapped along the western flank of the Chajnantor Plateau. The alluvial fans were examined with geomorphic and terrestrial cosmogenic 36Cl surface exposure dating methods to define the timing of alluvial fan formation and to determine the role of climatic processes on fan development in a hyperarid environment. Alluvial fans in the study area are comprised of hyperconcentrated flow and boulder-rich debris flow deposits that reflect deposition transitioning between cohesive and noncohesive regimes. Alluvial fan surfaces yield exposure ages that range from 49.6±4.4 to 194±12ka, while debris flow boulders yield exposure ages ranging from 12.4±2.1 to 229±53ka. Cosmogenic 36Cl exposure ages indicate that abandonment of alluvial fan surfaces Qf1, Qf2, and Qf3 date to 175±22.6ka (MIS 6), 134.5±9.18ka (MIS 6), and 20.07±6.26ka (MIS 2), respectively. A 36Cl concentration-depth profile through alluvial fan Qf1 suggests a simple depositional history with minimal nuclide inheritance implying relatively rapid aggradation (6m in ca. 25kyr) followed by surface abandonment ca. 180–200ka. Our data support a strong climatic control on alluvial fan evolution in the region, and we propose that the alluvial fans along the margins of the Salar de Atacama form according to the humid model of fan formation.

Transition from collision to subduction in Western Greece: the Katouna–Stamna active fault system and regional kinematics

Transition from subduction to collision occurs in Western Greece and is accommodated along the downgoing plate by the Kefalonia right-lateral fault that transfers the Hellenic subduction front to the Apulian collision front. Here we present an active tectonic study of Aitolo-Akarnania (Western Greece) that highlights how such a transition is accommodated in the overriding plate. Based on new multi-scale geomorphic and tectonic observations, we performed an accurate active fault trace mapping in the region, and provide evidence for active normal and left-lateral faulting along the Katouna–Stamna Fault (KSF), a 65-km-long NNW-striking fault system connecting the Amvrakikos Gulf to the Patras Gulf. We further show that the Cenozoic Hellenide thrusts located west of the KSF are no longer active, either in field observation or in GPS data, leading us to propose that the KSF forms the northeastern boundary of a rigid Ionian Islands-Akarnania Block (IAB). Cosmic ray exposure measurements of 10Be and 36Cl were performed on a Quaternary alluvial fan offset along the KSF (~50 m left-lateral offset). A maximum abandonment age of ~12–14 ka for the alluvial fan surface can be determined, giving an estimated KSF minimum geological left-lateral slip rate of ~4 mm year−1, in agreement with high GPS slip rates (~10 mm year−1). Despite this high slip rate, the KSF is characterized by subdued morphological evidence of tectonic activity, a gypsum-breccia bedrock and a low level of seismicity, suggesting a dominantly creeping behavior for this fault. Finally, we discuss how the IAB appears to have been progressively individualized during the Pleistocene (younger than ~1.5 Ma).

Late Pleistocene intraplate extension of the Central Anatolian Plateau, Turkey: Inferences from cosmogenic exposure dating of alluvial fan, landslide, and moraine surfaces along the Ecemiş Fault Zone

Here we documented the vertical displacement, slip rate, extension rate, and geochronology of normal faults within a narrow strip along the main strand of the Ecemis Fault Zone. The Kartal, Cevizlik, and Lorut Faults are normal faults that have evident surface expression within the strip. Terrestrial cosmogenic nuclide geochronology revealed that the Kartal Fault deformed the 104.2 ± 16.5 ka aged alluvial fan surface and the Cevizlik Fault deformed the 21.9 ± 1.8 ka old moraine and talus fan surfaces. Our topographic surveys indicated 120 ± 10 m and 13.1 ± 1.4 m surface-breaking vertical displacements along the Kartal and Cevizlik Faults, respectively. Accordingly, we suggest a 1.15 ± 0.21 mm a−1 slip rate and 0.66 ± 0.12 mm a−1 extension rate for the last 104.2 ± 16.5 ka on the Kartal Fault, and a 0.60 ± 0.08 mm a−1 slip rate and 0.35 ± 0.05 mm a−1 extension rate for the last 21.9 ± 1.8 ka on the Cevizlik Fault. We believe that these structures are an integral part of intraplate crustal deformation in the Central Anatolia. They imply that intraplate structures such as the Ecemis Fault Zone may change their mode through time; presently, the Ecemis Fault Zone has been deformed predominantly by normal faults. The presence of steep preserved fault scarps along the Kartal, Cevizlik, and Lorut Faults point to surface-breaking normal faulting away from the main strand and particularly signify that these structures need to be taken into account for regional seismic hazard assessments.

Controls on valley spacing in landscapes subject to rapid base‐level fall

AbstractWhat controls the architecture of drainage networks is a fundamental question in geomorphology. Recent work has elucidated the mechanisms of drainage network development in steadily uplifting landscapes, but the controls on drainage‐network morphology in transient landscapes are relatively unknown. In this paper we exploit natural experiments in drainage network development in incised Plio‐Quaternary alluvial fan surfaces in order to understand and quantify drainage network development in highly transient landscapes, i.e. initially unincised low‐relief surfaces that experience a pulse of rapid base‐level drop followed by relative base‐level stasis. Parallel drainage networks formed on incised alluvial‐fan surfaces tend to have a drainage spacing that is approximately proportional to the magnitude of the base‐level drop. Numerical experiments suggest that this observed relationship between the magnitude of base‐level drop and mean drainage spacing is the result of feedbacks among the depth of valley incision, mass wasting and nonlinear increases in the rate of colluvial sediment transport with slope gradient on steep valley side slopes that lead to increasingly wide valleys in cases of larger base‐level drop. We identify a threshold magnitude of base‐level drop above which side slopes lengthen sufficiently to promote increases in contributing area and fluvial incision rates that lead to branching and encourage drainage networks to transition from systems of first‐order valleys to systems of higher‐order, branching valleys. The headward growth of these branching tributaries prevents the development of adjacent, ephemeral drainages and promotes a higher mean valley spacing relative to cases in which tributaries do not form. Model results offer additional insights into the response of initially unincised landscapes to rapid base‐level drop and provide a preliminary basis for understanding how varying amounts of base‐level change influence valley network morphology. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Phases of alluvial fan development in a loess area, Lublin Upland, E Poland

Holocene alluvial fans are a record of the evolution of the environment and human impact over the last 10 000 years. The study presents the structure of fans at the mouth of gullies as well as their development in the loess areas of the Lublin Upland. Sediment-soil sequences of seven alluvial fans and the adjoining flood sediments and dry valley bottom sediments were studied in detail. The development of fans occurred in three phases: i) the Eneolithic; ii) the middle and late Bronze Age and the Hallstatt period and iii) the Middle Ages and the modern times. The middle phase consists of two sub-phases while last phase consists of three sub-phases: the early Middle Ages; the Little Ice Age; the 19th and first half of the 20th century. These phases were interspersed with longer or shorter periods of stabilization of the alluvial fan surface. The different course of land use changes in the catchments was the reason for this metachronous development of the alluvial fans. Our studies indicate that heavy and catastrophic rainfall and rapid thaw had a considerable role in the development of the alluvial fans. The establishment of roads along the axes of valleys and across the slopes had a key impact on alluvial fan development. The deforestation of the catchment was not an essential condition for the formation of gullies and alluvial fans.

Alluvial fan surfaces and an age-related stability for cultural resource preservation: Nevada Test and Training Range, Nellis Air Force Base, Nevada, USA

Surface geomorphology studies, measurements of cosmogenic 10Be concentrations, and satellite imagery analysis are employed to construct a surface stability model for the preservation of archeological features in Stonewall Flat, Nevada, USA. Primary fan processes of incised channel floods and sheet floods construct alluvial fans and secondary processes of sheetwash, eolian erosion and deposition, bioturbation, and soil development subsequently modify fan surfaces. Each process leaves distinctive surface features, but collectively darkens and smooth's the surface with age. Concentrations of cosmogenic 10Be in surface and subsurface samples for fans with recently active surfaces have effectively zero ages, whereas concentrations for fans with mature pavements have ages of 35 to 40thousandyears (ka). Surface age data from other Stonewall Flat studies indicate that fans in transition from primary process to secondary process domination are younger than 13ka, and that older fan surfaces are dominated by secondary processes. Image analysis demonstrates a correlation between fan surface darkening and age, and permits age estimations for undated surfaces. Results indicate that surfaces dominated by primary processes are younger than 3ka and unstable for the preservation of older sites; surfaces dominated by secondary processes are older than 10ka and stable for site preservation.

Effects of weathering and lithology on the quality of aggregates in the alluvial fans of Northeast Rivand, Sabzevar, Iran

Alluvial fans as depositional landforms can be considered as potential sources of aggregates. As the age of alluvial fans increases, their constituent sediments are exposed to longer periods of weathering and increased mineral alteration, resulting in a decrease in aggregate quality. In this study, physical properties and point load tests were used to assess the aggregate quality on three alluvial fan surfaces (relict, old and young) in the northeastern part of Rivand village in west of Sabzevar, Northeast Iran. Differentiating young from old and relict fans was carried out based on geomorphic criteria such as weathering features, fan surface morphology and drainage pattern. The young alluvial fan is characterized by sub-rounded and unvarnished clasts, distributary drainage patterns and a relatively flat surface, whereas old and relict fans are characterized by incised and rough surfaces, tributary drainage pattern and highly weathered and varnished clasts due to their long-term exposure to weathering. Due to a range of rock types occurring across each fan surface, lithological studies were performed to eliminate the effect of lithology on aggregate quality. A total of 18 rock types comprising comparable lithologies were sampled from each of the three alluvial fans. Results show that, in almost all 18 rock types, the point load test values increases from relict to young fans whereas porosity and percentage of water absorption decrease, implying that aggregate quality decreases with time as a function of duration of exposure to weathering. Also, the strength of aggregates in all three fans decreases from the fan apex to the fan toe. Data show that micaceous, intrusive igneous rocks, tuffs with high porosity and fine-grained extrusive igneous rocks with some porosity are more sensitive to physical weathering, and therefore have lower strength, particularly on the relict and old fans. Overall, variations in aggregate strength on these fans can be attributed to the relative ages of fans, with relict and old fans containing lower quality aggregates due to the longer-term exposure to weathering.

No surface breaking on the Ecemiş Fault, central Turkey, since Late Pleistocene (~ 64.5 ka); new geomorphic and geochronologic data from cosmogenic dating of offset alluvial fans

The Ecemiş Fault Zone (EF) has been recognized as a major left lateral strike-slip fault in the Central Anatolian Fault Zone (CAFZ) of Turkey. However, its Quaternary slip-rate has been challenging to determine due to the difficulty of dating offset markers. Using high-precision offset measurements and 36Cl cosmogenic nuclide dating, we present the first geochronologically determined Late Quaternary slip-rate for the EF. Our study focuses on the excellent exposures of offset alluvial fan surfaces, originating from the Aladağlar, a Late Quaternary glaciated mountain. Analysis of airborne orthophotogrametry and GNSS (Global Navigation Satellite System) surveys indicates 168±2m left lateral and 31±1m vertical displacements. In-situ terrestrial cosmogenic 36Cl geochronology obtained from eleven surface boulders provides a minimum abandonment/incision age of 104.2±16.5ka for the oldest offset alluvial fan surface. Our geomorphic observations together with Self-potential geophysical surveys revealed the presence of an unfaulted alluvial fan terrace, which allows us to constrain the timing of deformation. The abandonment/incision age of this fan is 64.5±5.6ka based on thirteen 36Cl depth profile samples. Accordingly, we obtained a geologic fault slip-rate of 4.2±1.9mma−1 horizontally and 0.8±0.3mma−1 vertically for the time frame between 104.2±16.5ka and 64.5±5.6ka. Our analysis indicates that the EF has not been producing a major surface breaking earthquake on the main strand at least since 64.5±5.6ka (mid-Late Pleistocene). This could be the result of abandonment of the main strand and accommodation of deformation by other faults within the EF. Nevertheless, a recently occurred (30 September 2011) low magnitude (ML: 4.3) left lateral strike-slip earthquake indicates recent seismic activity of the EF. Comparison of the recent GPS velocity field with the longer slip history along the CAFZ indicates a constant but low strain release without surface breaking and very long large earthquake recurrence intervals.

Rates and style of Cenozoic deformation around the Gonghe Basin, northeastern Tibetan Plateau

The northeastern Tibetan Plateau constitutes a transitional region between the low-relief physiographic plateau to the south and the high-relief ranges of the Qilian Shan to the north. Cenozoic deformation across this margin of the plateau is associated with localized growth of fault-cored mountain ranges and associated basins. Herein, we combine detailed structural analysis of the geometry of range-bounding faults and deformation of foreland basin strata with geomorphic and exhumational records of erosion in hanging-wall ranges in order to investigate the magnitude, timing, and style of deformation along the two primary fault systems, the Qinghai Nan Shan and the Gonghe Nan Shan. Structural mapping shows that both ranges have developed above imbricate fans of listric thrust faults, which sole into decollements in the middle crust. Restoration of shortening along balanced cross sections suggests a minimum of 0.8–2.2 km and 5.1–6.9 km of shortening, respectively. Growth strata in the associated foreland basin record the onset of deformation on the two fault systems at ca. 6–10 Ma and ca. 7–10 Ma, respectively, and thus our analysis suggests late Cenozoic shortening rates of 0.2 +0.2/–0.1 km/m.y. and 0.7 +0.3/–0.2 km/m.y. along the north and south sides of Gonghe Basin. Along the Qinghai Nan Shan, these rates are similar to late Pleistocene slip rates of ∼0.10 ± 0.04 mm/yr, derived from restoration and dating of a deformed alluvial-fan surface. Collectively, our results imply that deformation along both flanks of the doubly vergent Qilian Shan–Nan Shan initiated by ca. 10 Ma and that subsequent shortening has been relatively steady since that time.

Late Quaternary alluvial fans of Emli Valley in the Ecemiş Fault Zone, south central Turkey: Insights from cosmogenic nuclides

Alluvial fans within the paraglacial Ecemiş River drainages on the Aladağlar Mountains in south central Turkey were studied using geomorphological, sedimentological, and chlorine-36 terrestrial cosmogenic nuclide (TCN) surface exposure dating methods to examine the timing of alluvial fan abandonment/incision, and to understand the role of climatic and tectonic processes in the region. These alluvial fan complexes are among the best-preserved succession of alluvial fans in Turkey and they were offset by the major strike-slip Ecemiş Fault of the Central Anatolian Fault Zone. The alluvial fans are mostly composed of well-lithified limestone cobbles (5 to 25cm in size), and comprise crudely stratified thick beds with a total thickness reaching up to about 80m. TCN surface exposure dating indicates that the oldest alluvial fan surface (Yalak Fan) was likely formed and subsequently abandoned latest by 136.0±23.4ka ago, largely on the transition of the Penultimate Glaciation (Marine Isotope Stage 6, MIS 6) to the Last Interglacial (MIS 5) (i.e. Termination II). The second set of alluvial fan (Emli Fan) was possibly developed during the Last Interglacial (MIS 5), and incised twice by between roughly 97.0±13.8 and 81.2±13.2ka ago. A younger alluvial fan deposit placed on relatively older erosional terraces of the Emli Fan suggests that it may have been produced during the Last Glacial Cycle (MIS 2). These events are similar to findings from other fluvial and lacustrine deposits throughout central Anatolia. The incision times of the Ecemiş alluvial fan surfaces largely coincide with major climatic shifts from the cooler glacial periods to warmer interglacial/interstadial conditions. This indicates that alluvial fans were produced by outwash sediments of paleoglaciers during cooler conditions, and, later, when glaciers started to retreat due to a major warming event, the excess water released from the glaciers incised the pre-existing fan surfaces. An alluvial fan in the study area was also cut by the Ecemiş Fault, highlighting the influence of tectonics on fan development. It was offset vertically 35±3m since at least 97.0±13.8ka, which suggests a 0.36±0.06mma−1 vertical slip-rate of the fault.

宇宙成因&amp;lt;sup&amp;gt;21&amp;lt;/sup&amp;gt;Ne暴露测年方法的初步建立及检验

The stable cosmogenic isotope ²¹Ne provides an extremely powerful tool for surface exposure dating, because of its extremely wide exposure dating range, requirement for comparatively small samples (about 0.2-1.0 g), and relatively simple sample preparation process. Furthermore, in addition to quartz, ²¹Ne analysis is applicable to many other types of silicate, such as pyroxene, olivine, and biotite. Cosmogenic ²¹Ne has become a significant geochronological technique in Quaternary geology; however, because of the high resolution required to separate the ⁴⁰Ar²⁺ peak from that of ²⁰Ne⁺ in mass spectrometry, the development of the cosmogenic ²¹Ne exposure dating technique in China is still in its early stages. A measurement procedure has been established in our laboratory for cosmogenic ²¹Ne dating based on improvement of the purification system of the mass spectrometer by adding a cryogenic pump. A highly enriched ²¹Ne standard material (CREU-1) and an alluvial fan surface sample from Longshou Shan, Inner Mongolia, China, were analyzed, and the data from these two samples have demonstrated the feasibility of our ²¹Ne analytical procedure for quartz. The establishment of this neon analytical procedure provides a promising tool for the development of a cosmogenic ²¹Ne dating method for use in many fields, such as Quaternary geology and active tectonics.

Active faulting in apparently stable peninsular India: Rift inversion and a Holocene‐age great earthquake on the Tapti Fault

AbstractWe present observations of active faulting within peninsular India, far from the surrounding plate boundaries. Offset alluvial fan surfaces indicate one or more magnitude 7.6–8.4 thrust‐faulting earthquakes on the Tapti Fault (Maharashtra, western India) during the Holocene. The high ratio of fault displacement to length on the alluvial fan offsets implies high stress‐drop faulting, as has been observed elsewhere in the peninsula. The along‐strike extent of the fan offsets is similar to the thickness of the seismogenic layer, suggesting a roughly equidimensional fault rupture. The subsiding footwall of the fault is likely to have been responsible for altering the continental‐scale drainage pattern in central India and creating the large west flowing catchment of the Tapti river. A preexisting sedimentary basin in the uplifting hanging wall implies that the Tapti Fault was active as a normal fault during the Mesozoic and has been reactivated as a thrust, highlighting the role of preexisting structures in determining the rheology and deformation of the lithosphere. The slip sense of faults and earthquakes in India suggests that deformation south of the Ganges foreland basin is driven by the compressive force transmitted between India and the Tibetan Plateau. The along‐strike continuation of faulting to the east of the Holocene ruptures we have studied represents a significant seismic hazard in central India.

Late Quaternary alluvial fans at the eastern end of the San Bernardino Mountains, Southern California

Alluvial fans at the eastern end of the San Bernardino Mountains in Southern California provide a record of climate modulated sediment transfer and erosion, and are deformed and displaced in places by active faults. Alluvial fans within two study areas, the Mission Creek and the Whitewater River drainages, were examined using geomorphic, sedimentological, and 10Be terrestrial cosmogenic nuclide (TCN) surface exposure methods to define the timing of alluvial fan formation and erosion, and to examine the role of climatic, tectonic and autocyclic processes. These alluvial fan complexes were studied because they are amongst the best-preserved successions of alluvial fans in southern California and they are located at the mouths of two of the largest drainages, Whitewater River and Mission Creek, in the San Bernardino Mountains and traverse major faults, the Mission Creek and Banning. The alluvial fans comprise bouldery debris deposits that represent deposition dominated by flash flood and debris flow events. TCN surface exposure dating indicates that abandonment/incision of alluvial fan surfaces date to early in the Last Glacial or more likely the penultimate glacial cycle, to marine isotope stage (MIS) 4, and to the Holocene. The lack of alluvial fan ages during the latter part of the Last Glacial (MIS 2 and 3) suggests that there has been little alluvial fan lobe deposition/incision during that time. This is similar to findings for many other alluvial fans throughout the American Southwest, and supports the view that there is a strong climatic control on alluvial fan formation throughout this region. Furthermore, the oldest alluvial fan surfaces in the Mission Creek region are beheaded by the Whitewater River drainage, showing that the oldest alluvial fans in the Mission Creek region underwent significant capture by the Whitewater River drainage. This shows the autocyclic controls are also important on alluvial fan evolution in this region; but the importance of these processes to alluvial fan development in other regions of the American Southwest needs to be more fully assessed. The alluvial fans in the Mission Creek area traverse the Mission Creek fault, but are not deformed by it, which suggests that there may have been little if any movement along this fault since at least MIS 4. In contrast, alluvial fans in the Whitewater River study are displaced by active faults highlighting the influence of tectonism on alluvial fan development in this region. In addition to illustrating the importance of climatic controls on the development of alluvial fans in the American Southwest, a classic region for alluvial fan studies, this study illustrates the complex mixture of autocyclic and allocyclic factors that force alluvial fan development in tectonically active settings.

Mapping Quaternary alluvial fans in the southwestern United States based on multiparameter surface roughness of lidar topographic data

AbstractQuaternary alluvial fans have diverse surface morphologies related to both depositional and weathering processes. Numerous studies have demonstrated that the surface expression and morphometry of alluvial fans can be used as an indicator of their relative age of deposition, but only recently has there been an effort to utilize high‐resolution topographic data to differentiate alluvial fans by surface age with automated and quantifiable routines. We developed a quantitative model for mapping the relative age of alluvial fan surfaces based on multiparameter surface roughness values computed from 1 m resolution lidar topographic data. Roughness is defined as a function of observational scale and integration of slope, curvature, and aspect topographic parameters. Alluvial fan roughness values were computed across multiple observation scales (3 × 3 m to 150 × 150 m) based on the standard deviation (SD) of slope, curvature (tangential), and aspect topographic parameters. Plots of roughness value versus size of observation scale suggest that the SD of each parameter over a 7 × 7 m observation window best identified the signature of surface roughness elements. Roughness maps derived from slope, curvature, and aspect at this scale were integrated using fuzzy logic. The integrated roughness map was then classified into five relative morphostratigraphic surface age categories (active wash to ~400 ka) and statistically compared with a similar fivefold surface age map of alluvial fans developed using traditional field surveys and aerial photographic interpretation. The model correctly predicted the distribution and relative surface age of ~61% of alluvial fan landforms based on traditional mapping techniques.

NEW OBSERVATIONS ON THE AGE AND CONTEXT OF SOANIAN TOOLS FOUND IN POST-URBAN HARAPPAN SITES AND ALONG WTH SE ASIAN TYPE TOOLS ON THE MID-HOLOCENE TERRACES IN THE NW SUB-HIMALAYAS

ABSTRACT Most of the alluvial fan surfaces in the northwestern sub-Himalayas were laid down during terminal Pleistocene to late mid-Holocene times and so were the cusp terraces of the streams cutting through Siwaliks and the joining perennial Himalayan rivers. Though the stone tools and other prehistoric artefacts found on a surface have to be younger than the age of that surface but in the absence of any dates, almost all the earlier workers in this region determined the chronology of lithic sites by mere speculations. We have recently made surface collections from almost 30 stream-terrace sites and alluvial fan surfaces in the sub-Himalayas and have excavated two in buried state, all were in dated contexts. We have observed that the Soanian type stone implements existed in the sub-Himalayas up to late mid-Holocene times. This inference is also confirmed by the discovery of such tools from post-Urban Harappan sites and also in association with Harappan potsherds on young terrace surfaces. The discovery of many new tool-types, especially the pitted cobbles and edge-ground lithic specimens known from the mid-Holocene sites elsewhere, especially of Southeast Asia, also hints at the influence of those lithic industries in this region. We briefly present here the lithic assemblages from five sites with new evidences and conclude that the Soanian and many new tool-types were in use in the northwestern sub-Himalayas until the late mid-Holocene times.

Timing and nature of alluvial fan and strath terrace formation in the Eastern Precordillera of Argentina

Sixty-eight 10Be terrestrial cosmogenic nuclide (TCN) surface exposure ages are presented to define the timing of alluvial fan and strath terrace formation in the hyper-arid San Juan region of the Argentine Precordillera. This region is tectonically active, and numerous fault scarps traverse Quaternary landforms. The three study sites, Marquesado strath complex, Loma Negra alluvial fan and Carpintería strath complex reveal a history of alluvial fan and strath terrace development over the past ∼225 ka. The Marquesado complex Q3m surface dates to ∼17 ± 3 ka, whereas the Loma Negra Q1ln, Q2ln, Q3ln, Q4ln, and Q5ln surfaces date to ∼24 ± 3 ka, ∼48 ± 2 ka, ∼65 ± 13 ka, ∼105 ± 21 ka, and ∼181 ± 29 ka, respectively. The Carpintería complex comprises eight surfaces that have been dated and include the Q1c (∼23 ± 3 ka), Q2c (∼5 ± 5 ka), Q3ac (∼25 ± 12 ka), Q3bc (∼29 ± 15 ka), Q4c (∼61 ± 12 ka), Q5c (∼98 ± 18 ka), Q6c (∼93 ± 18 ka), and Q7c (∼212 ± 37 ka). 10Be TCN depth profile data for the Loma Negra alluvial fan complex and Carpintería strath terrace complex, as well as OSL ages on some Carpintería deposits, aid in refining surface ages for comparison with local and global climate proxies, and additionally offer insights into inheritance and erosion rate values for TCNs (∼10 × 10410Be atoms/g of SiO2 and ∼5 m Ma−1, respectively). Comparison with other alluvial fan studies in the region show that less dynamic and older preserved surfaces occur in the Carpintería and Loma Negra areas with only younger alluvial fan surfaces preserved both to the north and south. These data in combination with that of other studies illustrate broad regional agreement between alluvial fan and strath terrace ages, which suggests that climate is the dominant forcing agent in the timing of terrace formation in this region.

Slip-rates along the Chaman fault: Implication for transient strain accumulation and strain partitioning along the western Indian plate margin

The Chaman fault in Western Pakistan marks the western collision boundary between the Indian and Eurasian plates and connects the Makran subduction zone to the Himalayan convergence zone. Geomorphic-scale slip-rates along an active strand of the Chaman fault are added to the sporadic data set of this poorly investigated transform system. Field investigations coupled with high-resolution GeoEye-1 satellite data of an alluvial fan surface (Bostankaul alluvial fan) show ~ 1150 m left-lateral offset by the fault since the formation of the alluvial fan surface. A weighted mean 10Be exposure age of 34.8 ± 3 kyr for the Bostankaul alluvial surface yields a slip-rate of 33.3 ± 3.0 mm/yr. This rate agrees with the geologically defined slip-rates along the Chaman fault, but is approximately twice as large as that inferred from the decade-long global positioning system measurements of 18 ± 1 mm/yr. The contrast in geomorphic and geodetic slip-rates along the Chaman fault, like other major intra-continental strike–slip faults, has two major implications: 1) the geodetic rates might represent a period of reduced displacement as compared to the averaged Late Pleistocene rate because of transient variations in rates of elastic strain accumulation; or 2) strain partitioning within the plate boundary zone. While strain partitioning could be the reason of slip-rate variations within the western Indian plate boundary zone, transient strain accumulation could explain contrasting slip-rates along the Chaman fault at this stage in its poorly understood seismic cycle.