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1.
Sedimentary deposits in the foreland basin of the northeastern Qilian Mountains are crucial documents recording tectonic activity and climate changes on the Tibetan Plateau. In this study, luminescence dating was used to date alluvial conglomerates and fluvial terrace sediments collected from the Beida River in the Jiuquan Basin, a foreland basin in the Hexi Corridor, northeastern Qilian Mountains. Detailed sedimentology and luminescence ages reveal that alluvial conglomerates accumulated from before 620 ka to 12 ka and that sediment accumulation rates increased at ∼330 ka and ∼35 ka, coinciding with the dates of two tectonic events (∼350 and ∼50 ka) and followed by climate cooling (from marine isotope stage (MIS) 9 to MIS 8 and from MIS 3 to MIS 2). This reveals that variations in the sediment accumulation rates are controlled by the coupling of tectonic uplift and climate cooling. The highest terrace (T7) that developed on the alluvial conglomerate base formed at ∼ 12 ka. The incision rate in the early Holocene was ∼2.1 mm/yr and increased to ∼14.6 mm/yr during the middle and late Holocene. The variations in the river incision rate provide geomorphic evidence for Holocene climate patterns in arid and semiarid areas. Luminescence dating offers a credible temporal framework for the deposits and reveals climate and tectonic effects on the evolution of the foreland basin, northeastern Qilian Mountains.  相似文献   

2.
The actively deformed foreland of eastern Qilian Shan (mountains) contains well‐preserved geomorphic features such as erosion surfaces, river terraces and tectonically uplifted alluvial fans, providing suitable archives for research on regional tectonic activities and palaeoclimatic changes. These geomorphic surfaces are well dated by using a combination of magnetostratigraphy, electron spin resonance, thermoluminescence, infra‐red stimulated luminescence, radiocarbon dating, and correlation with the well‐established loess–palaeosol sequences of China. Our results show that the erosion surface formed about 1·4 Ma ago, and the age of river terraces is 1·24 Ma, 820–860 ka, 780 ka, 420–440 ka, 230–250 ka, 140 ka, 60 ka and 10 ka, respectively. Valley incision rates of c. 0·09–0·25 m ka?1 have been identified. The repetitive stratigraphic and geomorphic pattern of these terraces indicates the fluvial sedimentation–incision cycles are tightly associated with the 100‐ka glacial–interglacial climatic cycles. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

3.
Upstream knickpoint propagation is an essential mechanism for channel erosion, carrying changes in base level, tectonics and climate across the landscape. Generally, the terraces on cross-sections at steady-state conditions have been widely reported. However, many landscapes in the field appear to be in a transient state. Here, we explore the mechanism of knickpoint initiation and fluvial evolution in a transient setting in the northeastern Tibetan Plateau. Analysis of channel profiles and terrace correlation indicates that the Yellow River is adjusted to match the increase in differentiated fault activity and climate change in a regional setting of continuous uplift. Consequently, a series of terraces were formed, and the number of terrace steps increased downstream, in the headwaters of the Yellow River. All terraces were dated using the optically stimulated luminescence method. The top terrace, distributed continuously in the whole basin with a gradient, was deposited during a cold period and abandoned at the climatic transition from cold to warm state, at approximately 14.6–9.5 ka. After that, one terrace formed at around 4.2 ka in the upper reach. In correlation with the continuous topographic gradient surface of this terrace, three terrace steps were formed in the down reach during the period from 9.5 ka to 4.2 ka. This phenomenon might indicate multiple phases of continuous headward migration of fluvial knickpoint waves and terrace formation during the downcutting. It was caused by fault activity and tectonic uplift of the gorge at the outlet of the basin, under influence of the gradual integration of the Yellow River from downstream. This phenomenon shows that the fluvial incision in a transient state along the high relief margin of the orogenic plateau can be caused by fault activity, in addition to widespread surface uplift, climatically driven lake spillover and the establishment of external drainage.  相似文献   

4.
Effective river management strategies require an understanding of how fluvial processes vary both spatially and temporally. Here, we examine the natural range of variability in the Conejos River Valley, southern Colorado, through documentation of terrace morphostratigraphic and sedimentological characteristics as well as through investigation of sediment contributions from headwaters, hillslopes and tributary streams. Additionally, soil development and radiocarbon ages, together with local and regional paleoclimate reconstructions, were used to infer the range of processes acting in this system. Since de‐glaciation, the Conejos River has fluctuated between episodes of bedrock strath formation, aggradation and vertical incision. Morphostratigraphic relationships, soil development and radiocarbon ages enable us to propose a chronology for periods of alluvial deposition (around 8·9–7·6 ka, 5·5 ka and from 3·5 to 1·1 ka), separated by intervals of fluvial incision. We infer potential forcing mechanisms by utilizing multiple working hypotheses. Specifically, we discuss the potential for increases in sediment supply during periods of (1) para‐glacial adjustment, (2) climatic cooling, (3) increased frequency of climate change and (4) increased fire frequency or severity. We also consider the effects of changes in stream discharge and extreme storm occurrence. We conclude that combinations of these processes, operating at different times, have contributed to sediment mobilization since de‐glaciation. Stream and landform morphology also varies longitudinally due to the influence of remnant glacial topography. In particular, valley bottom overdeepening at tributary junctions has resulted in incision and strath formation into unlithified glacial deposits (i.e. fill‐cut terraces) rather than bedrock in some reaches. Overall, the Conejos fluvial system has varied significantly both temporally and spatially since de‐glaciation and appears to be sensitive to changes in sediment supply related to Holocene scale climate fluctuations. This natural range of variability must therefore be a key consideration in any future stream management policies. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

5.
The watersheds along the north coast of California span a wide range of geologic settings, tectonic uplift rates, and historic timber harvest activity. Known trends in how each of these factors influence erosion rates provides an opportunity to examine their relative importance. We analyzed 71 watersheds within nine larger river basins, investigated the factors influencing suspended sediment rating curves (SRCs), investigated how SRCs varied among our study watersheds, and used Random Forest modeling (RFM) to determine which environmental characteristics and land management metrics influence SRC shapes, vertical offsets, and slopes. While SRCs typically take the form of a power function, they also can exhibit threshold or peak relationships. First, we found both power and threshold relationships for the SRCs within our study watersheds. Second, the SRC offsets and slopes systematically varied with regional tectonic uplift. Third, SRC offsets increased in several watersheds following intensive timber harvest events and SRC slopes decreased due to a greater relative increase in suspended sediment concentration at lower flows than higher flows. Our RFM correctly classified 96% of the SRC shapes using two near-channel metrics; near-channel precipitation-sensitive deep-seated landslide susceptibility and near-channel soil erodibility. Our RFM models also showed that timber harvest activity and near-channel local relief can explain 40% of the variability in SRC offsets, whereas tectonic uplift rates, millennial-scale erosion rates, and precipitation patterns explain 40% of the variability in SRC slopes.  相似文献   

6.
Late Quaternary stratigraphy of a 50 km2 catchment on the south-eastern highlands of Australia reveals processes and history of denudation, and helps resolve a long-standing debate about factors controlling episodic valley aggradation and degradation during Holocene times. Valley sedimentation occurred when swampy vegetation fully colonized valley floors and obliterated all channels, promoting aggradation for periods of several thousand years, with most incoming sediment being trapped in swampy meadows. Much of the sediment was reworked from late Pleistocene alluvial fan and valley fill deposits, and primary hillslope erosion was minor during the Holocene. Differing sedimentation patterns between the Late Pleistocene, Holocene and Post-European settlement periods reflect regional changes in sediment supply and transport capacity as a result of major environmental change. Within the Holocene, however, valley fill stratigraphy is controlled by massive, episodic gully erosion terminating aggradation. Gully initiation appears to be controlled more by thresholds of incision into vegetated valley floors than by changes to sediment supply. Whether the thresholds are exceeded because of climatic change, autonomous change or extreme events cannot yet be determined. Overall, the Holocene history represents continuing complex response to events of the Late Pleistocene, and does not support the K-cycle concept, which has strongly influenced late Quaternary geomorphology in Australia.  相似文献   

7.
Holocene and Pleistocene tectonic deformation of the coast in the Mexico subudction margin is recorded by geomorphic and stratigraphic markers. We document the spatial and temporal variability of active deformation on the coastal Mexican subduction margin. Pleistocene uplift rates are estimated using wave-cut platforms at ca. 0.7?C0.9?m/ka on the Jalisco block coast, Rivera-North America tectonic plate boundary. We examine reported measurements from marine notches and shoreline angle elevations in conjunction with their radiocarbon ages that indicate surface uplift rates increasing during the Holocene up to ca. 3?±?0.5?m/ka. In contrast, steady rates of uplift (ca. 0.5?C1.0?m/ka) in the Pleistocene and Holocene characterize the Michoacan coastal sector, south of El Gordo graben and north of the Orozco Fracture Zone (OFZ), incorporated within the Cocos-North America plate boundary. Significantly higher rates of surface uplift (ca. 7?m/ka) across the OFZ subduction may reflect the roughness of subducting plate. Absence of preserved marine terraces on the coastal sector across El Gordo graben likely reflects slow uplift or coastal subsidence. Stratigraphic markers and their radiocarbon ages show late Holocene (ca. last 6?ka bp) coastal subsidence on the Guerrero gap sector in agreement with a landscape barren of marine terraces and with archeological evidence of coastal subsidence. Temporal and spatial variability in recent deformation rates on the Mexican Pacific coast may be due to differences in tectonic regimes and to localized processes related to subduction, such as crustal faults, subduction erosion and underplating of subducted materials under the southern Mexico continental margin.  相似文献   

8.
This study explores paleoflood deposits of the Siang River, known as the Tsangpo in Tibet. The river that often experiences large floods brings down huge amount of sediment and water that adversely affect the downstream regions with large human populations in the states of northeast Himalaya and its foreland. Along it's ~300 km mountainous stretch we collected samples for sedimentological, petrographic and Sr–Nd isotopic study to explore sediment provenance and dated the paleofloods (via optically stimulated luminescence, OSL). Geomorphic indices including precipitation and a geomorphic swath profile across the Brahmaputra catchment were studied to understand the interplay of mountain relief and rainfall that determine potential zones of high erosion and sediment supply. The OSL technique indicated the Siang River experienced at least eight large floods between 7 and 1 ka, possibly under the influence of warm and wet climatic conditions. The petrographic and isotopic data suggests that the eastern Himalayan syntaxis, which has the highest uplift and exhumation rate in the area, is not always the highest sediment producing zone. In some instances, the Tibetan plateau produces higher fluxes of sediments via glacial and landslide lake outburst floods (GLOFs and LLOFs). © 2020 John Wiley & Sons, Ltd.  相似文献   

9.
A large amount of the total sediment load in the Chinese Yellow River is transported during hyperconcentrated floods. These floods are characterized by very high suspended sediment concentrations and rapid morphological changes with alternating sedimentation and erosion in the main channel, and persistent sedimentation on the floodplain. However, the physical mechanisms driving these hyperconcentrated floods are still poorly understood. Numerical modelling experiments of these floods reveal that sedimentation is largely caused by large vertical concentration gradients, both in the channel during the rising stage of the flood, as well as on the floodplains, during a later stage of the flood. These vertical concentration gradients are large because the turbulent mixing rates are reduced by the increased sediment‐induced density gradients, resulting in a positive feedback mechanism that produces high deposition rates. Erosion prevails when the sediment is largely held in suspension due to hindered settling, and is strengthened by the reduced wetted cross‐section caused by massive sedimentation on the floodplain. Observed patterns of erosion and sedimentation during these floods can be qualitatively reproduced with a numerical model in which sediment‐induced density effects and hindered settling are included. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

10.
Stratigraphic and morphostructural analyses have been carried out in the Mt. Etna volcanic region (eastern Sicily) to investigate in detail the deformation events that have affected the sedimentary successions forming the substratum of the volcano. In the foredeep, Quaternary submarine sedimentation ended 600 ka ago when the whole area emerged in response to homogeneous regional uplift. The irregular distribution of a stratigraphic marker, recognized through the analysis of more than 250 borehole logs, suggests that local dynamics also affected the area. We identify both compressional tectonic dynamics and volcano-related tectonic activity, and discriminate among their associated deformations. In particular, we quantify the vertical deformation component of the compressional structures (thrusts and related folds) and recognize for the first time a vertical component of deformation whose pattern clearly indicates a doming process acting at Mt. Etna. The comparison between long-term and short-term rates suggests that the doming has acted consistently over space and time through the last 600 ka and provides clues to the source of uplift. This component, defined by a specific Quaternary sedimentary horizon, has been compared with vertical deformation obtained by analytical inversion of morphological substratum data, and localizes the source at a depth of ～16 km, at the mantle-crust transition. This uplift may be the consequence of hydration occurring in the altered ocean-like crust.  相似文献   

11.
Fluvial terraces are important geomorphic markers for modern valley development.When coupled with numeric ages,terraces can provide abundant information about tectonic,climatic,paleohydrological and the paleoenvironmental changes.On the basis of the paleomagnetic,electron spin resonance(ESR) and optically stimulated luminescence(OSL) dating,in addition to an investigation of local loess-paleosol sequences,we confirmed that 13 fluvial terraces were formed,and then preserved,along the course of the Upper Weihe River in the Sanyangchuan Basin over the past 1.2 Ma.Analyses of the characteristics and genesis of these terraces indicate that they resulted from the response of this particular river system to climate change over an orbital scale.These changes can further be placed within the context of local and regional tectonic uplift,and represent an alternation between lateral migration and vertical incision,dependent upon the predominance of climatic and tectonic controls during different periods.Most of the terraces are strikingly similar in that they have several meters of paleosols which have developed directly on top of fluvial deposits located on the terrace treads,suggesting that the abandonment of terraces due to river incision occurred during the transitions from glacial to interglacial climates.The temporal and spatial differences in the distribution patterns of terraces located on either side of the river valley indicate that a tectonic inversion occurred in Sanyangchuan Basin at-0.62 Ma,and that this was characterized by a transition from overall uplift to depression induced by fault activity.Synthesized studies of the Basin's terraces indicate that formation of the modern valley of the Upper Weihe River may have begun in the late Early Pleistocene between1.4-1.2 Ma.  相似文献   

12.
Fluvial systems in uplifting terrain respond to tectonic, climatic, eustatic and local base‐level controls modified by specific local factors, such as river capture. The Rio Alias in southeast Spain is an ephemeral, transverse‐to‐structure fluvial system. The river drains two interconnected Neogene sedimentary basins, the Sorbas and Almeria basins, and crosses two major geological structures, the Sierras de Alhamilla/Cabrera and the Carboneras Fault Zone. Regional epeirogenic uplift resulted in sustained fluvial incision during the Quaternary, punctuated by major climatically driven periods of aggradation and dissection, which created a suite of five river terraces. The river terrace sequence was radically modified in the late Pleistocene by a major river capture (itself a response to regional tectonics), localized tectonic activity and eustatic base‐level change. The Rio Alias is defined by four reaches; within each the climatically‐generated, region‐wide, fluvial response was modified by tectonics, base‐level change or river capture to varying degrees. In the upper part of the basin (Lucainena reach), climate was the dominant control on river development, with limited modification of the sequence by uplift of the Sierra Alhamilla and local drainage reorganization by a local river capture. Downstream of the Sierra Alhamilla in the Polopus reach, the climatic signal is dominant, but its expression is radically modified by the response to a major river capture whereby the Alias system lost up to 70% of its pre‐capture drainage area. In the reach adjacent to the Carboneras Fault Zone (Argamason reach), modification of the terrace sequence by local tectonic activity and a resultant local base‐level fall led to a major local incisional event (propagating c. 3–4 km upstream from the area of tectonic disturbance). At the seaward end of the system (El Saltador reach) Quaternary sea‐level changes modified the patterns of erosion and incision and have resulted in steep incisional terrace profiles. The signals generated by regional tectonics and the Quaternary climate change can be identified throughout the basin but those generated by ongoing local tectonics, river capture and sea‐level change are spatially restricted and define the four reaches. The connectivity of the system from the headwaters to the coast decreased through time as incision progressed, resulting in changes in local coupling characteristics. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

13.
Landscapes respond in complex ways to external drivers such as base level change due to damming events. In this study, landscape evolution modelling was used to understand and analyse long‐term catchment response to lava damming events. PalaeoDEM reconstruction of a small Turkish catchment (45 km2) that endured multiple lava damming events in the past 300 ka, was used to derive long‐term net erosion rates. These erosion rates were used for parameter calibration and led to a best fit parameter set. This optimal parameter set was used to compare net erosion landscape time series of four scenarios: (i) no uplift and no damming events; (ii) no uplift and three damming events; (iii) uplift and no damming events; and (iv) uplift and three damming events. Spatial evolution of net erosion and sediment storage of scenario (iii) and (iv) were compared. Simulation results demonstrate net erosion differences after 250 000 years between scenarios with and without dams. Initially, trunk gullies show less net erosion in the scenario with damming events compared with the scenario without damming events. This effect of dampened erosion migrates upstream to smaller gullies and local slopes. Finally, an intrinsic incision pulse in the dam scenario results in a higher net erosion of trunk gullies while decoupled local slopes are still responding to the pre‐incision landscape conditions. Sediment storage differences also occur on a 100 ka scale. These differences behaved in a complex manner owing to different timings of the migration of erosion and sediment waves along the gullies for each scenario. Although the specific spatial and temporal sequence of erosion and deposition events is sensitive to local parameters, this model study shows the manner in which past short‐lived events like lava dams have long‐lasting effects on catchment evolution. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

14.
Six plains cottonwoods along the axis of a meander were excavated to determine if dendrochronology could identify the year and location of germination and date past overbank sedimentation events. Samples from all excavated trees showed clear anatomical changes associated with burial, including increased vessel size, decreased definition of annual ring boundaries, and decreased ring widths. Some of these burial signatures were created by deposition of only a few centimeters of sediment, and most burial events were detected by multiple samples from the same tree. Four of the trees germinated at or near the upper surfaces of bar deposits, while two germinated within thin overbank deposits draped over bar deposits, indicating that germination is closely associated with bars. Dates and inferred thicknesses of overbank sedimentation events are consistent with repeated topographic surveys and data obtained from cesium-137 (137Cs) analyses. However, the record of overbank sedimentation extracted from the trees does not entirely reflect the history of past peak discharges documented by stream gaging, largely because individual trees are progressively less likely to be flooded through time as the river migrates farther away. Germination dates and locations closely track past positions of the river channel. Germination elevations and the elevations of the tops of point bars appear to be decreasing with time as the bend migrates, implying vertical incision by Powder River at a rate of 7.1 ± 4.3 mm/yr. The rate of floodplain growth determined by elevation changes decreases progressively through time, ultimately reaching an apparent plateau after 0.8–1.3 m of vertical accretion. While similar patterns of vertical accretion have previously been interpreted as resulting from decreasing flood probability with increasing floodplain elevation, distance from the channel is also a first-order control on vertical floodplain growth. © 2019 John Wiley & Sons, Ltd.  相似文献   

15.
How rock resistance or erodibility affects fluvial landforms and processes is an outstanding question in geomorphology that has recently garnered attention owing to the recognition that the erosion rates of bedrock channels largely set the pace of landscape evolution. In this work, we evaluate valley width, terrace distribution, and bedload provenance in terms of reach scale variation in lithology in the study reach and discuss the implications for landscape evolution in a catchment with relatively flat‐lying stratigraphy and very little uplift. A reach of the Buffalo National River in Arkansas was partitioned into lithologic reaches and the mechanical and chemical resistance of the main lithologies making up the catchment was measured. Valley width and the spatial distribution of terraces were compared among the different lithologic reaches. The surface grain size and provenance of coarse (2–90 mm) sediment of both modern gravel bars and older terrace deposits that make up the former bedload were measured and defined. The results demonstrate a strong impact of lithology upon valley width, terrace distribution, and bedload provenance and therefore, upon landscape evolution processes. Channel down‐cutting through different lithologies creates variable patterns of resistance across catchments and continents. Particularly in post‐tectonic and non‐tectonic landscapes, the variation in resistance that arises from the exhumation of different rocks in channel longitudinal profiles can impact local base levels, initiating responses that can be propagated through channel networks. The rate at which that response is transmitted through channels is potentially amplified and/or mitigated by differences between the resistance of channel beds and bedload sediment loads. In the study reach, variation in lithologic resistance influences the prevalence of lateral and vertical processes, thus producing a spatial pattern of terraces that reflects rock type rather than climate, regional base level change, or hydrologic variability. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

16.
Factors influencing sediment transport and storage within the 156·6 km2 drainage basin of Pancho Rico Creek (PRC), and sediment transport from the PRC drainage basin to its c. 11 000 km2 mainstem drainage (Salinas River) are investigated. Numeric age estimates are determined by optically stimulated luminescence (OSL) dating on quartz grains from three sediment samples collected from a ‘quaternary terrace a (Qta)’ PRC terrace/PRC‐tributary fan sequence, which consists dominantly of debris flow deposits overlying fluvial sediments. OSL dating results, morphometric analyses of topography, and field results indicate that the stormy climate of the Pleistocene‐Holocene transition caused intense debris‐flow erosion of PRC‐tributary valleys. However, during that time, the PRC channel was backfilled by Qta sediment, which indicates that there was insufficient discharge in PRC to transport the sediment load produced by tributary‐valley denudation. Locally, Salinas Valley alluvial stratigraphy lacks any record of hillslope erosion occurring during the Pleistocene‐Holocene transition, in that the alluvial fan formed where PRC enters the Salinas Valley lacks lobes correlative to Qta. This indicates that sediment stripped from PRC tributaries was mostly trapped in Pancho Rico Valley despite the relatively moist climate of the Pleistocene‐Holocene transition. Incision into Qta did not occur until PRC enlarged its drainage basin by c. 50% through capture of the upper part of San Lorenzo Creek, which occurred some time after the Pleistocene‐Holocene transition. During the relatively dry Holocene, PRC incision through Qta and into bedrock, as well as delivery of sediment to the San Ardo Fan, were facilitated by the discharge increase associated with stream‐capture. The influence of multiple mechanisms on sediment storage and transport in the Pancho Rico Valley‐Salinas Valley system exemplifies the complexity that (in some instances) must be recognized in order to correctly interpret terrestrial sedimentary sequences in tectonically active areas. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

17.
Using quantitative geomorphic factors for regional active tectonic evolution is becoming more and more popular. Qilian Mountains-Hexi Corridor which locates in the northern edge of Qinghai-Tibet plateau is the most leading edge of the plateau's northward extension. The uplift rate of all segments and the intensity of influence from tectonic activity are the important evidences to understand the uplift and extension of the plateau. Heihe River Basin is located at the northern piedmont of the western segment of Qilian Mountains, the development of the rivers is influenced by the tectonic activity of the Qilian Mountains, and the unique river morphology is important carriers of the regional tectonic uplift. Geomorphologic indexes such as hypsometric integral, geomorphologic comentropy and river longitudinal profiles were extracted by GIS tools with free access to the Shuttle Radar Topography Mission(SRTM)DEMs, and according to the different expression of the geomorphological indexes in the Heihe River Basin, we divided the drainage basin into two parts and further compared them to each other. Recent studies reveal that obvious differences exist in the landscape factors(hypsometric integral, geomorphology entropy and river profiles)in the east and west part of the Heihe Basin. The structural intensity of the west part is stronger than that of the east, for example, in areas above the main planation surface on the western part, the average HI value is 0.337 8, and on the eastern part the HI value is 0.355. Accordingly, areas under the main planation surface are just on the contrary, indicating different structural strength on both sides. Similar phenomenon exists in the whole drainage basins. Furthermore, by comparing the fitting river profiles with the real river profiles, differential uplift is derived, which indicates a difference between west and east(with 754m on the western part and 219m on the east). Comprehensive comparison and analysis show that the lithologic factors and precipitation conditions are less influencing on the geomorphic factors of the study area, and the tectonic activities, indicated by field investigation and GPS inversion, are the most important element for geomorphic evolution and development. The variation of the geomorphologic indexes indicates different tectonic strength derived from regional structures of the Qilian Shan.  相似文献   

18.
实地调查表明,山西长治断裂由三段组成,南段和中段为西倾正断层,北段为东倾正断层,总体走向北东25o,长达65km,控制着附近山地与第四纪山间盆地的构造演化和地貌发育;第四纪早期,断裂发生强烈的垂直差异活动,最大垂直活动幅度可达400m左右;第四纪晚期,断裂活动性明显减弱,中段仍有活动,至少发生过两期活动,第—期活动发生于距今10.88万年左右或以后、1.49万年以前,第二期活动发生于距今1.49万年以后,垂直错距0.65m。  相似文献   

19.
The Huoshan piedmont fault is a small watershed region in Shanxi Province. We utilized the high-resolution DEM data and the stream-power incision model which describes the relationship between the tectonic uplift and fluvial incision to analyze the S-A double-log graph, concavity index (θ)and steepness index (logks) of the 64 channels across this fault and discuss their responses to the tectonic movement of the fault. The results show that (1)the S-A double-log graphs all exhibit an obvious convex form, which is the direct expression of the response to the situation that the bedrock uplift rate is higher than the fluvial incision rate. (2)All of the concavity index (θ)values of 64 channels are lower than 0.35 with an average value of 0.223, much lower than the empirical value (0.49)of the rivers in steady state. These low values are the quantitative reflections of the channels' slightly concave profiles. Meanwhile they imply that these channels across the fault are very young. There is no enough time for them to adjust the profiles through the fluvial incision to the steady state because of the fault's frequent and strong tectonic movements. (3)The steepness index values of the channels located in the Laoyeding Mt. are highest, while they are lower in the northern and southern mountains. Moreover, the steepness index values of the channels in the northern mountains, on average, are higher than those of the channels in the southern mountains. To a certain extent, this distribution of the steepness index corresponds to the difference in the uplift rates of the Huoshan piedmont fault. It means that the uplift rate of the middle fault segment in the Laoyeding Mt. is highest, and the uplift rate of the northern segment is higher than that of the southern segment.  相似文献   

20.
— Geodetic data indicate that the northern Metropolitan Los Angeles region is shortening at a rate of 4.5–6.0 mm/yr between downtown Los Angeles and the San Gabriel Mountains. If we assume that all of the uplift of the San Gabriel Mountains is due to the major frontal fault system (the Sierra Madre fault) and use reported values for bedrock uplift, slip per event and recurrence intervals to determine the slip rate on the Sierra Madre fault, we obtain slip rates between 0.6–1.27 mm/yr. Using these slip rates, the horizontal shortening attributable to the Sierra Madre fault accounts for only ~33% of the observed shortening across the basin, leaving ~67% of the shortening to be accounted for elsewhere. Herein we present a suite of models that test possible shortening mechanisms to account for this strain deficit. The models incorporate a range of fault geometries and have a layered structure with variable vertical and horizontal rheologies. The models demonstrate how lower-crust rheology and the presence of a low rigidity, anelastically deforming sedimentary basin affects the dissipation of stress imposed on the viscous layers by elastic failure of the faults. We found that viscoelastic models with a single fault, vertically strong crust and a compliant sedimentary basin yield a horizontal velocity profile that best matches the geodetically observed velocity profile across the Los Angeles Basin. Our models also indicate that we are still not accounting for all of the observed deformation. Therefore, more complex models that include both laterally varying rheologies and frictional properties on faults must be considered.  相似文献   

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