Late Pleistocene Alluvial Fan Research Articles

Exploring the impact of deglaciation on fault slip in the Sangre de Cristo Mountains, Colorado, USA

Few natural examples exist where climate’s influence on tectonics is clear. Based on a study of the Sangre de Cristo Mountains in southern Colorado, we argue that climate-driven changes in ice loads affected spatial and temporal slip patterns on the range-front normal fault. Relict glacial features enable the reconstruction of paleoglacier extents and show variable amounts of footwall ice coverage during the Last Glacial Maximum (LGM). Line load models indicate post-LGM ice melting reduced fault clamping stress by ∼20−55 kPa at seismic depths. Flexural isostatic modeling shows several meters of footwall uplift due to ice unloading with spatial patterns and magnitudes consistent with post-LGM fault throw measured from offset Holocene and late Pleistocene alluvial fans. Post-LGM fault throw rates are at least a factor of five higher than middle and early Pleistocene rates. We infer that climate-modulated ice-load changes can pace fault clamping stress and slip patterns on range-bounding normal faults.

Depositional architecture and aggradation rates of sand-rich, supercritical alluvial fans: Control by autogenic processes or high-frequency climatic oscillations?

Alluvial fans are important paleoclimatic archives, that may record high-frequency climatic oscillations. However, climate signals may be overprinted or even be destroyed by autogenic processes caused by channel avulsion and lobe switching. Here we present new data from two different Late Pleistocene (MIS 3–2) alluvial fan systems in northern Germany and compare these systems to experimental alluvial fans and other field examples. The selected fan systems formed under similar climatic and tectonic conditions, but differ in size, type, and drainage area allowing to estimate the role of climate and autogenic controls on flow processes, facies architecture, and fan-stacking patterns. Luminescence dating is used to determine the timing of fan onset and aggradation. Fan onset occurred in response to climate change at the end of MIS 3 when temperatures decreased and periglacial climate conditions were established in northern central Europe. A related increase in sediment supply and strongly variable precipitation patterns probably promoted fan formation. The major period of fan aggradation was approximately between 33 and 18 ka, followed by fan inactivity, abandonment, and incision during the Lateglacial. The highest aggradation rates occurred during the early stage of fan building, when up to 35 m thick sediment accumulated within a few thousand years. Sand-rich, sheetflood-dominated fans are related to larger, low-gradient fan catchments. Steep depositional fan slopes (5°–17°) and short-lived high-energy floods promoted supercritical flow conditions. Well sorted, sediment-laden, rapidly waning flows favored the deposition and preservation of supercritical bedforms and allowed for the aggradation of stable antidunes. Steep, dip-slope catchments enhanced stream gradients and promoted the transport of coarser sediments. These fans have lower gradient slopes (2–6°) and are dominated by channelized flows, alternating with periods of unconfined sheetfloods. Meter-scale coarsening upward successions, characterized by sandy sheetflood deposits at the base, overlain by multilateral or smaller single-story gravelly channel fills may be related to high-frequency climatic fluctuations or seasonal fluctuations in water and sediment supply. These coarsening-upward successions are commonly bounded by a paleo-active layer, from which ice-wedge casts penetrate downwards. The comparison to experimental fans and other field examples implies that the recurrent pattern of multistory, multilateral and single-story channel bodies with a lateral offset to vertical stacking pattern most probably was controlled by autogenic switch in an avulsion-dominated system. The change in deposition from alluvial-dominated processes to aeolian sedimentation with minor alluvial influences during the Lateglacial records alternation of dry and ephemeral wetter phases that are related to rapid climatic variations. The main phase of aeolian sand-sheet deposition probably correlates with Heinrich event H1 between approximately 18–16 ka and reflects sedimentation in response to aridification and high mean wind speeds.

Late Pleistocene Slip Rates on an Active Normal Fault in the Northwestern Ordos Block, China

The slip rates of normal faults on the northern margin of the Ordos Block are critical for understanding the deformation of the Ordos Block and adjacent areas. In this study, we obtain the late Pleistocene slip rate of an active normal fault, the Zhuozishan West Piedmont Fault (ZWPF), in the northwestern corner of the Ordos Block. Satellite imagery interpretation and field investigations suggest that the fault displaces late Pleistocene alluvial fans and forms west-facing fault scarps. To quantify the vertical slip rate of the ZWPF, we measured the displacements along the fault scarps using differential GPS and an unmanned aerial vehicle system and dated the displaced geomorphic surfaces using optically stimulated luminescence (OSL) dating methods. The vertical slip rate of the fault is constrained to 0.1 ± 0.02 mm/a, which suggests a horizontal extension rate of 0.05 mm/a for a dip of 65°. This rate is consistent with those on similar structures in the northwestern Ordos Block. Combining this result with published slip rates along other active faults, we infer that the NW-directed extension rate across the northwestern corner of the Ordos Block is ∼0.6–1.3 mm/a. This rate is comparable with the geodetic strain rate. Given the extension rate, we believe that extensional deformation is mainly concentrated on the western side of the basin.

Early development of strike‐slip faulting: palaeoseismic study along the Petersen Mountain fault, northern Walker Lane, Nevada

ABSTRACTCharacterising youthful strike‐slip fault systems within transtensional regimes is often complicated by the presence of tectonic geomorphic features produced by normal faulting associated with oblique extension. The Petersen Mountain fault in the northern Walker Lane tectonic province exhibits evidence of both normal and strike‐slip faulting. We present the results of geologic and geomorphic mapping, and palaeoseismic trenching that characterise the fault's style and sense of deformation. The fault consists of two major traces. The western trace displaces colluvial, landslide, and middle to late Pleistocene alluvial fans and is associated with aligned range front saddles, linear drainages, and oversteepened range front slopes. The eastern trace is associated with a low linear bedrock ridge, a narrow graben, right deflected stream channels, and scarps in late Pleistocene alluvial fan deposits. A trench on the eastern trace of the fault exposed a clear juxtaposition of disintegrated granodiorite bedrock against sand and boulder alluvial fan deposits across a steeply east‐dipping fault. The stratigraphic evidence supports the occurrence of at least one late Pleistocene earthquake with a component of lateral displacement. As such, the Petersen Mountain fault accommodates part of the ~7 mm/yr of dextral shear distributed across the northern Walker Lane.

Basement Characteristics of Jakarta Groundwater Basin Based on Satellite Gravimetry Data

Jakarta groundwater basin is one of the most developed basins in Indonesia as Jakarta city is located within the basin, with an elevation ranging from 0 to 1000 m above sea level. The study of the basement characteristics of groundwater basins still needs further study in the Jakarta Groundwater Basin. The objective of this study is to examine the basement characteristics of Jakarta Groundwater Basin by satellite gravimetry. Gravity forward models were constructed using Oasis Montaj and 2-D GM-SYS software for two north-south sections, A and B, and west-east section C-D. Regional anomalies in the study area with a value range of 36.7 to 53.2 mGal, while the residual anomaly values in the study area were in the range of -7.0 - 10.0 mGal. There are four rock formations based on forwarding modeling. Holocene Beach Ridge Deposit formation has a density of 2.0 gr/cc. The thickness of this formation is estimated to be around 10-20 meters. Late Pleistocene Alluvial fan has a density of about 2.2 gr/cc. The depth of this layer ranges from 10-50 meters. Early Pleistocene Formation, has a density of 2.35 gr/cc. The thickness of this formation is at a depth of 50-200 meters in the cross-section. The Tertiary Marine Sediment Formation is a basement of the Jakarta Groundwater Basin. This formation has a rock density of 2.45 gr/cc.

Holocene Paleoearthquakes and Early–Late Pleistocene Slip Rate on the Sulmona Fault (Central Apeninnes, Italy)

Abstract The 23 km long Sulmona normal fault flanks the southwestern slope of the Mount Morrone range in the central Apennines. To date, the recent activity of this structure is uncertain, as the faulting evidence concerns only undated deposits and the fault cannot be associated with any of the strong historical earthquakes ( M w >6.5) of the region. Our observation from ∼1 Ma offset slope breccias, coupled with new tephrochronological data on faulted early Last Glacial lacustrine infilling of the Sulmona basin, implies a vertical slip rate of ∼0.5 mm/yr over both the long term and middle term. Moreover, at the apex of a Late Pleistocene alluvial fan, radiocarbon dating of offset stratigraphy uncovered in four paleoseismological trenches shows that repeated earthquakes resulted in more than 4 m of vertical offset since ∼9 ka, providing again a minimum Holocene vertical slip rate of ∼0.5 mm/yr. In combination with the results of the trench exposure, we define four faulting events in the past ∼9 ky B.P., and at least one before then. The most recent is constrained by robust radiocarbon dates as the middle of the second century A.D. The penultimate event was around the middle of the fifth millennium B.P., whereas the other two events are dated in a time between 8.5 ky to 6.5 ky B.P. and around 9 ky cal B.P. The oldest occurred before 9.5 ky B.P. As well as revealing the unexpected paleoseismic history of the Sulmona silent fault, our data provide a recurrence time for M w ≥6.5 earthquakes of ∼2.4 ky versus an elapsed time of ∼1.85 ky since the last event. The latter matches with an earthquake previously hypothesized through archaeoseismic clues collected from several Roman settlements of the Sulmona Plain, all of which were dated to halfway through the second century A.D. Online Material: Table of major‐element compositions, field photos, and Harker diagrams.

Assessing the reliability of U-series and 10Be dating techniques on alluvial fans in the Anza Borrego Desert, California

We combine 10Be exposure dating with U-series dating of pedogenic carbonate from subsurface clast-coatings to date deposition of mid-Holocene to late Pleistocene alluvial fans at four sites in the Anza Borrego desert of California. Since each dating technique is subject to distinct assumptions, their systematic uncertainties are largely independent. We therefore consider 10Be and U-series dates that are consistent to be highly reliable. If post-depositional erosion has not significantly modified a fan surface, exposure dates should yield maximum ages when no correction is made for inheritance. U-series dating, in contrast, provides minimum dates because pedogenic carbonate forms after fan deposition. Accordingly, agreement between dates from the two techniques (or lack thereof) provides a stringent test of their reliability. Our results show that: (1) concordant 10Be and U-series dates were measured for most fans, thereby providing robust age estimates, (2) U-series dating of pedogenic carbonate can be successfully applied to alluvium as young as mid-Holocene and (3) 10Be inheritance of the studied Holocene fan deposits appears to be significantly lower than implied by 10Be concentrations in active wash deposits.

230Th/U dating of a late Pleistocene alluvial fan along the southern San Andreas fault

U-series dating of pedogenic carbonate-clast coatings provides a reliable, precise minimum age of 45.1 ± 0.6 ka (2σ) for the T2 geomorphic surface of the Biskra Palms alluvial fan, Coachella Valley, California. Concordant ages for multiple subsamples from individual carbonate coatings provide evidence that the 238 U- 234 U- 230 Th system has remained closed since carbonate formation. The U-series minimum age is used to assess previously published 10 Be exposure ages of cobbles and boulders. All but one cobble age and some boulder 10 Be ages are younger than the U-series minimum age, indicating that surface cobbles and some boulders were partially shielded after deposition of the fan and have been subsequently exhumed by erosion of fine-grained matrix to expose them on the present fan surface. A comparison of U-series and 10 Be ages indicates that the interval between final alluvial deposition on the T2 fan surface and accumulation of dateable carbonate is not well resolved at Biskra Palms; however, the “time lag” inherent to dating via U-series on pedogenic carbonate can be no larger than ∼10 k.y., the uncertainty of the 10 Be-derived age of the T2 fan surface. Dating of the T2 fan surface via U-series on pedogenic carbonate (minimum age, 45.1 ± 0.6 ka) and 10 Be on boulder-top samples using forward modeling (preferred age, 50 ± 5 ka) provides broadly consistent constraints on the age of the fan surface and helps to elucidate its postdepositional development.

The Vallo di Diano Fault System: New Evidence for an Active Range-Bounding Fault in Southern Italy Using Shallow, High-Resolution Seismic Profiling

Range-bounding normal faults can presentsignificant challenges for seis- mic exploration. This is the case of the fault system bounding the Vallo di Diano, the largest intermountain basin in the southern Apennines seismic belt. Industry reflection profiles define the large-scale structure of the basin but barely image the shallow fault system due to unfavorable topographic and near-surface conditions along the foothills of the eastern range. We present two high-resolution (HR) wide-aperture profiles recorded attheeastern marginof thebasin acrossunreported scarpsthat affect Middle- Late Pleistocene alluvial fans and slope debris. The survey is aimed at identifying possible recent faulting across these challenging terrains and at understanding the re- lationship between shallow structures and the master range-bounding fault at depth. Common depth point processing of wide-aperture reflection data and first-arrival tra- vel-time tomography provide detailed images of the upper 200-300 m and sounding evidence of recent activity along previously unknown splays of the fault system. These splays dissect the Mesozoic limestone bedrock and alluvial-fan sequences, affecting theirdepositionalpattern.VeryhighresolutionVPandreflectivityimagesalsogivehints ofpossiblecoseismicsurfacefaultinginHolocenecolluvia.Theseresultshaverelevant implicationsfortheevaluationoftheseismogenicpotentialoftherange-boundingfault systemandforseismichazardassessmentofthedenselyurbanizedVallodiDianobasin. Online Material: Table of acquisition and data processing parameters, and figures showing schematic stratigraphy of the Vallo di Diano basin, restoration of fault dis- placement along the HR profile, and resolution of the P-velocity tomographic images for the VHR profile.

Climatic controls on late Pleistocene alluvial fans, Cyprus

Alluvial fans are commonly associated with tectonically active mountain ranges and tectonism is frequently held responsible for abrupt coarsening and cyclical sedimentation of alluvial fan sequences. Whilst it is accepted that tectonism provides the opportunity for alluvial fan development through the creation of topography, increasing gradients of fluvial systems supplying sediments, and creating accommodation for the storage of sediment flux, the role of climate in fan development is frequently neglected. The hypothesis that climatically controlled events can produce recognisable sedimentary signatures in alluvial fan deposits is tested in the active supra-subduction zone setting of the late Pleistocene of southern Cyprus. This study demonstrates through architectural analysis and the reconstruction of palaeoflood hydrology a recorded pattern of increasing and decreasing palaeoflow dynamics, with switches from a wetter to drier mode, clearly exhibited by changes in the sedimentology of the fan. At the present day Cyprus has a semi-arid climate and is influenced by a strongly seasonal rainfall pattern, largely restricted to the winter months (plus rare occurrences of summer cyclones). However at precession minima increased activity of western Mediterranean depressions produces wetter summers. Using inference we propose that longer-term increases in rainfall increased river discharge as recorded in the fan palaeoflood hydrology and occurred at minima in the precession. These periods correlate with the deposition of conglomeratic channels and open framework gravels. Drier periods are exhibited by sandier braided fluvial deposits. Shorter term or seasonal change is recorded in the form of 2nd and 3rd low order bounding surfaces. This increased activity of Mediterranean summer depressions increased precipitation to the wider Levantine area and was coincident with increased intensity of the north African and Indian Ocean (SW) monsoons. The resultant increase in river discharges at precession minima from both the Nile (and the wider Levant) resulted in the formation of sapropels in the eastern Mediterranean and is recorded as wet periods in speleothem deposits in the Soreq and Peqiin Caves of Israel. The predominant control of sedimentation on the late Pleistocene alluvial fans of southern Cyprus was climate.

Exhumation of the Inyo Mountains, California: Implications for the timing of extension along the western boundary of the Basin and Range Province and distribution of dextral fault slip rates across the eastern California shear zone

New geologic mapping, tectonic geomorphologic, 10Be terrestrial cosmogenic nuclide, and (U‐Th)/He zircon and apatite thermochronometric data provide the first numerical constraints on late Cretaceous to late Quaternary exhumation of the Inyo Mountains and vertical slip and horizontal extension rates across the eastern Inyo fault zone, California. The east‐dipping eastern Inyo fault zone bounds the eastern flank of the Inyo Mountains, a prominent geomorphic feature within the western Basin and Range Province and eastern California shear zone. (U‐Th)/He zircon and apatite thermochronometry yield age patterns across the range that are interpreted as indicating: (1) two episodes of moderate to rapid exhumation associated with Laramide deformation during the late Cretaceous/early Tertiary; (2) development of a slowly eroding surface during a prolonged period from early Eocene to middle Miocene; (3) rapid cooling, exhumation, and initiation of normal slip along the eastern Inyo fault zone, accommodated by westward tilting of the Inyo Mountains block, at 15.6 Ma; and (4) rapid cooling, exhumation, and renewed normal slip along the eastern Inyo fault zone at 2.8 Ma. Fault slip continues today as indicated by fault scarps that cut late Pleistocene alluvial fan surfaces. The second episode of normal slip at 2.8 Ma also signals onset of dextral slip along the Hunter Mountain fault, yielding a Pliocene dextral slip rate of 3.3 ± 1.0 mm/a, where a is years. Summing this dextral slip rate with estimated dextral slip rates along the Owens Valley, Death Valley, and Stateline faults yields a net geologic dextral slip rate across the eastern California shear zone of 9.3 + 2.2/–1.4 to 9.8 + 1.4/–1.0 mm/a.

Dating alluvial deposits with optically stimulated luminescence, AMS 14C and cosmogenic techniques, western Transverse Ranges, California, USA

In an effort to better understand chronology of alluvial episodes in Cuyama Valley in the western Transverse Ranges of California, USA, we employed optically stimulated luminescence, radiocarbon and cosmogenic radionuclide surface exposure dating methods. Twenty-one optical dates ranging from 0.01 to ∼27 ka were obtained from exposures of late-Holocene axial-fluvial deposits, Pleistocene–Holocene alluvial-fan deposits, and axial-fluvial sands interbedded within a late Pleistocene alluvial fan. These were cross-checked with 37 AMS radiocarbon dates from charcoal and wood from within a and five 10Be surface exposure dates from boulders on alluvial-fan surfaces. The OSL results show generally good stratigraphic consistency, logical comparison with the radiocarbon and cosmogenic data, and appear to be the best method for accurate dating within deposits of this nature because suitable material is fairly easy to find in these environments. The radiocarbon data contained numerous “detrital ages”, but well-bedded lenses of apparently in situ or minimally transported charcoal provide reliable age estimates for the associated alluvium. Radiocarbon dating of detrital charcoal in the older alluvial fan deposits was problematic. Our cosmogenic surface-exposure dating was consistent stratigraphically and with our other data, but we were unable to determine its accuracy due to the limited number of samples and the possibility of inherited radionuclides and post-depositional erosion. In light of our results, we suggest that OSL dating using the latest analytical techniques combined with rigorous methods for estimation of paleodase is reliable and of increasing utility in otherwise difficult-to-date coarse alluvial environments in the southwestern United States and elsewhere.

Temporal variations in slip rate of the White Mountain Fault Zone, Eastern California

The evolution of fault slip through time may yield insight into the geodynamics of deforming lithosphere. Precise determination of temporal variations in fault slip is often hindered, however, by a dearth of markers of varying age from which to reconstruct fault slip. Here we determine slip rates across the White Mountain Fault Zone over the past ca. 0.8 Ma from displaced alluvial deposits preserved along the flank of the White Mountains. Displacement histories inferred from deposits containing the ∼ 760 ka Bishop Tuff contrast strongly with those inferred from Late Pleistocene alluvial fans dated by cosmogenic 36Cl, indicating that the fault has experienced significant temporal variations in slip rate. Oblique-slip rates over the past ∼ 760 ky are determined to have been > 0.9 m/ky parallel to a net slip vector plunging shallowly (< 20°) toward 340°–350°. Displacement of markers in Late Pleistocene alluvial deposits, in contrast, yield slip rates between 0.3 and 0.4 m/ky parallel to a vector plunging somewhat more steeply (20°–40°) toward 320°–340°. These variations in slip rate appear to have operated over timescales > ca. 70 ky, yet occurred on a well-established fault system. Moreover, the timing and magnitude of slip rate variation mimics behavior documented for the northern Fish Lake Valley fault zone. Together, our data provide evidence for coordinated slip rate variations across the Eastern California Shear Zone during the Pleistocene.

Morphometry and depositional style of Late Pleistocene alluvial fans: Wadi Al-Bih, northern UAE and Oman

Abstract Three types of alluvial-fan settings are recognized in the Wadi Al-Bih area of the Musandam Mountains, northern UAE and Oman; mountain-front fans, tributary-junction fans and steep hillslope debris cones. Three styles of fan geometry, only partly dependent on fan setting, can be recognized: telescopic fans, stacked fans and truncated fans. Each style, together with degree of confinement, reflects the topographic and geological context of the fan and its source area. The mountain-front fans are mostly unconfined fans with telescopic styles. The tributary-junction fans are confined fans, some with stacked or telescopic styles, others that have been truncated by base-level-induced toe trimming. Most of the debris cones are simple hillfoot debris cones. Standard morphometric analyses of fan areas and fan gradients in relation to drainage basin area yield results that compare with other studies, but the relationships differ between the three groups of fans, in part reflecting fan style, especially between mountain-front and tributary-junction fans. The morphometry of the debris cones is only poorly characterized by the morphometric relationships. Cone morphology most strongly reflects source-area lithology. Analysis of the residuals from the regression analyses suggests that the morphometric differences between the three groups reflect fan sedimentary processes, fan setting and fan style, particularly relating to confinement.

Late Pleistocene/Holocene slip rate of the Zhangye thrust (Qilian Shan, China) and implications for the active growth of the northeastern Tibetan Plateau

We derive a slip rate for a thrust at the central Qilian Shan mountain front by combining structural investigations, satellite imagery, topographic profiling, luminescence dating, and 10Be exposure dating. The seismically active Zhangye thrust transects late Pleistocene alluvial fan deposits and forms a prominent north facing scarp. The fault consists of two segments that differ in orientation, scarp height, and age. A series of loess‐covered terraces records the uplift history of the western thrust segment. Loess accumulation on all terraces started at 8.5 ± 1.5 kyr and postdates terrace formation. Gravels from the highest terrace yielded a 10Be exposure age of 90 ± 11 kyr, which dates the onset of faulting. With a displacement of 55–60 m derived from fault scarp profiles, this yields a vertical slip rate of 0.64 ± 0.08 mm yr−1. Along the eastern thrust segment, three 10Be ages from the uplifted alluvial fan constrain that faulting started at ∼31 ± 5 kyr. Together with a displacement of 25–30 m this leads to a vertical faulting rate of 0.88 ± 0.16 mm yr−1. A dip estimate of 40° to 60° for the fault plane combined with lower and upper limits of ∼0.6 and ∼0.9 mm yr−1 for the vertical slip rate gives minimum and maximum horizontal shortening rates of 0.4 and 1.1 mm yr−1 across the Zhangye thrust. Our results are consistent with geologic and GPS constraints, which suggest that NNE directed shortening across the northeastern Tibetan Plateau is distributed on several active faults with a total shortening rate of 4 to 10 mm yr−1.

The role of volcanic activity and climate in alluvial fan growth at volcanic areas: an example from southern Campania (Italy)

Volcaniclastic-rich alluvial fans developed in the southern Campanian Plain (Italy) during the late Pleistocene and Holocene in an area eastward of the Somma-Vesuvius and Campi Flegrei volcanoes. Meanwhile, bedrock-rich alluvial fans developed in areas unaffected by pyroclastic deposition. Late Pleistocene and Holocene volcaniclastic-rich alluvial fans show some important differences: (i) late Pleistocene alluvial fans were dominated by hyperconcentrated flow deposits, whereas the Holocene ones were dominated by debris flows deposits; and (ii) late Pleistocene fans consist of several superimposed sedimentary bodies, characterized by homogeneous volcaniclastic material, whereas Holecene fans show either volcaniclastic bodies with homogenous lithology or mixed lithology (i.e., juvenile fractions eroded from different tephra layers). These differences are not related to the amount of volcaniclastic supply in time, but seem to be linked to changes in climatic condition between late Pleistocene and Holocene. Rapid remobilization of the pyroclastic material was favored by climatic and vegetation conditions of the study area during the late Pleistocene, when a semiarid setting dominated by steppe-like vegetation prevailed. During Holocene, the general increase in temperature and humidity favored vegetation and soil development and stabilization of the loose volcaniclastic materials. Thus, part of volcaniclastic material was stored in the catchments and was available for erosion a long time after an eruption. Shallow soil slips, active also today, generated volcaniclastic debris flows characterized by mixed lithology of pumice and scoria.

Fault growth, displacement transfer and evolution of the Jiugongkou fault overlap zone on the southern marginal fault zone of the Guangling‐Yuxian Basin, North China

If two normal faults overlap each other in an extensional direction, the overlapping faults and their overlapping area can comprise a fault overlap zone. Fault overlap zones with a separation greater than several kilometres are usually considered to be segment boundaries in the study of earthquake rupture segmentation. The Jiugongkou fault overlap zone located on the southern marginal fault zone of the Guangling‐Yuxian Basin provides an atypical example. The Jiugongkou fault overlap zone has 7 km of overlap length and 2.2–5.7 km of overlap separation. A Late Pleistocene alluvial fan is deposited within the overlap area. Geological and geomorphologic investigation has provided evidence for growth of fault, including abandonment of the Late Pleistocene alluvial fan, vertical displacement of the fan gravels, deposition of fault‐scarp‐derived colluvial deposits, intrusion of dacite magma along the fault, occurrence of antithetic faults, and fracture cleavage developed in loess. Throw analysis indicates that no displacement deficit has occurred within the overlap zone, but that interaction and displacement transfer have occurred between the overlapping faults. These are expressed as asymmetrical throw profiles for both of the main overlapping faults, a symmetrical composite throw distribution for all the fault traces characterised by a single fault, and a significant increase in the throw within the overlap zone. Moreover, dating of the faulted materials shows that the fault growth and displacement transfer occurred in the same time period, starting between 27.4 ± 2.2 ka BP and 26.0 ± 2.1 ka BP, and continuing until after 18.7 ± 1.5 to 13.1 ± 1.0 ka BP. For the overlap zone the displacement transfer can be guaged by two mechanisms, magmatic activity and tectonic tilting, while two evolutionary stages, offset and overlap, are evident. The overlap zone, however, has not yet been breached or linked. Results of research undertaken on the Jiugongkou fault overlap zone indicate that fault overlap zones on normal fault zones are temporary features. These results can be of great importance in assessing seismic hazards, especially in determining the magnitude of potential earthquakes based on calculation of fault lengths, and in carrying out earthquake probability predictions in terms of geological data.

Influence of bedrock geology on sedimentation in Pre-Late Wisconsinan alluvial fans in the Canadian Rocky Mountains

Abstract Late Pleistocene alluvial fan deposits in Jasper National Park, Canadian Rocky Mountains, unconformably overlie Precambrian and Paleozoic clastic and carbonate rocks and stratigraphically underlie Late Wisconsinan glacial deposits. The paleofan sequences were deposited in an inferred, cool temperate, alpine to subalpine paleo-environment. They mainly contain clast-supported diamicton interpreted as noncohesive debris flow deposits. Fluvial gravels are less common and cohesive debris flow deposits (matrix-supported diamictons) are rare. Although noncohesive debris flows dominate the fan sequences, the bedrock geology of the fan source areas exerts a strong control on sedimentation patterns by influencing susceptibility to debris slide initiation, availability of large clasts and matrix fines and drainage basin shape (and hence discharge flashiness). Paleofans in the Main Ranges with mainly gently dipping, clastic source rocks are dominated by thick, bouldery debris flow sediments whereas fans with steeply thrusted, interbedded carbonate and shale source rocks in the Front Ranges show thin, pebbly to cobbly debris flow beds with interbedded flood flow and normal stream flow gravels. These observations indicate that debris flow events were less frequent and of higher energy in the Main Ranges than in the Front Ranges. Front Range basins are long and narrow with complexly interbedded and deformed rocks, resulting in the relatively small grain size of transported materials, a greater susceptibility to small, frequent, debris slides, and locally, more uniform discharges during flood events. The prevalence of high-magnitude, low-frequency, debris flow events in the Main Range paleofans, and lower magnitude, higher frequency events in the Front Range fans may be of relevance to paleoenvironmental interpretations of ancient alluvial fan deposits, as well as to modern hazard studies and mitigation efforts mountainous areas.

Pleistocene paleoenvironmental changes in the high mountain ranges of central China and adjacent regions

Abstract During several joint Chinese–German expeditions, the Quaternary landscape and paleoclimatic evolution of the mountain ranges and grabens surrounding the Loess Plateau of central China was studied. The results are compared with proxy data from the interior Loess Plateau, to help reconstruct atmospheric circulation during Late Pleistocene times. Middle to Late Pleistocene alluvial terraces and fans, capped with loess units and paleosols, show that aggradational episodes correlate with periods of glaciation and increased loess accumulation. In turn, degradation correlates with interglacials or interstadials and increased soil formation. This implies that climate is the primary control on Quaternary paleohydrology and alluvial sedimentation and erosion. The high mountains, which barely reached the last glacial ELA, were only glaciated during the most favourable climatic period of glaciation. The glaciation, the glacial and alluvial forms point to a cool early to maximum glacial period with increased precipitation and to a maximum to final period with cold–dry conditions. During the last glaciation, the maximum advance of the small glaciers took place towards the end of a relatively humid and cold period between ca. 32 and 23 ka. The following dry–cold stage between ca. 23 and 13 ka was rather unfavourable for glacial advance. Proxy data from the Loess Plateau supports the view, that glaciations were more or less dominated by cold–dry winter monsoon climate, whereas during the interglacial/interstadials a warmer and more humid climate prevailed, implying a strengthening of the summer monsoon influence. The results prove variations of the monsoon, that in turn modulates paleohydrology, periglacial dynamics, glacier advances, dust flux and soil formation.

Growth of late Quaternary folds in southwest Santa Clara Valley, San Francisco Bay area, California: Implications of triggered slip for seismic hazard and earthquake recurrence

We combine analysis of geologic and geomorphic data with observations of deformation during the 1989 Loma Prieta earthquake in northern California to evaluate the contribution of aseismic triggered slip and creep processes to cumulative late Cenozoic deformation along the northeastern Santa Cruz Mountains range front. Deformed late Pleistocene alluvial fans and terraces provide evidence for localized late Quaternary uplift above range-bounding reverse faults within the southwestern Santa Clara Valley adjacent to the range front. On the basis of offset of late Quaternary surfaces, the long-term average slip rate on the primary range-bounding structure (Monte Vista fault) is estimated to be ∼0.2 mm/yr. Northeast of the range front are several discontinuous northwest-trending folds, indicated by alignment of late Pleistocene alluvial-fan apices, anomalous stream-channel convexities, and topographic and vegetation lineaments within a 3–5-km-wide, northwest-trending corridor. Subsurface geologic and geophysical data support the interpretation that the surface folds are a result of blind reverse faulting along the Cascade fault beneath the Santa Clara Valley. From stream incision rates we estimate an average uplift rate of 0.2 ± 0.05 mm/yr for the Cascade fault. Measurements of triggered and postseismic slip following the 1989 Loma Prieta earthquake, combined with estimates of the average return period for Loma Prieta–type events, suggest a long-term average deformation rate along the range front of 0.25–0.4 mm/yr associated with these aseismic processes. This range of values is comparable to rates of late Quaternary deformation on the range-front faults derived from geologic and geomorphic data, and it suggests that growth of the overlying folds is at least partially the result of triggered slip and postseismic creep associated with nearby Loma Prieta–type earthquakes. If this inference is correct, then the return period for independent events on the range-front faults probably is greater than that suggested by the long-term average geologic slip rates.