Scarp Degradation Research Articles

Modelling Fault Scarp Degradation to Determine Earthquake History on the Muztagh Ata and Tahman Faults in the Chinese Pamir

The frequency of earthquakes on active faults is usually revealed by palaeoseismic trenching and geological dating. Nonetheless, field trenching and sampling are sometimes limited by difficulties in logistics, therefore impeding palaeoseismological studies. As surface rupture earthquakes create fault scarps that evolve with time, they may also provide important information about the timing of earthquakes. In this study, we provide a new approach to determine earthquake history based on the morphology of fault scarps. We select the Muztagh Ata and Tahman normal faults in the Chinese Pamir as a test area, on which fault scarps are well preserved but the earthquake history has not been quantified yet. We first simulate fault scarp degradation using a nonlinear transport model to obtain the empirical relationship between scarp width and its evolution time. We then measure 68 fault scarps from high-resolution topographic data derived from WorldView-2 stereo imagery. The measured fault scarps are clustered in four groups, each group possibly representing one earthquake event. Combining the time-width relationship and scarp width measurements, we infer that four earthquakes occurred at 0.1±0.4 kyrs before the present (BP), 2.1±0.5 kyrs BP, 2.9±0.4 kyrs BP and 4.9±0.5 kyrs BP, respectively. The first event is likely to be the most recent earthquake occurred on the faults, i.e., the 1895 Tashkurgan earthquake ( ∼0.13 kyrs BP). Based on the estimated earthquake recurrence intervals and vertical offsets, we obtain a vertical slip rate of 2.2±0.3 mm/yr on the Muztagh Ata and Tahman faults, conforming to previous geological rate estimates.

Quantification of Gravitational Mass Wasting and Controls on Submarine Scarp Morphology Along the Roseau Fault, Lesser Antilles

AbstractDue to challenges involved in mapping the seafloor at high‐resolution (e.g., <2 m), data are lacking to understand processes that control the evolution of submarine normal fault scarps, which cover large parts of the global seafloor. Here, we use data from autonomous deep‐sea vehicles to quantify local erosion and deposition associated with a pronounced tectonic surface scarp formed by slip on the submarine Roseau normal fault (Lesser Antilles). We use high‐resolution video imagery, photomosaics, and high‐resolution bathymetric data (0.1–10 m/pixel) to identify active erosional features on the scarp including channels, steep gullies, small scarps, and debris cones. We compare volumes of erosion and deposition and find that under certain depositional conditions, debris cones effectively record the erosion signal of mass wasting from the footwall with a ratio of hanging wall deposition to footwall erosion of 0.80. We use eroded volumes to estimate earthquake‐induced landslide erosion rates for the Roseau fault of 14–46 m Myr−1. Assuming mass wasting of the Roseau fault scarp is mostly coseismic, the erosion rates for the Roseau fault imply that submarine earthquake induced mass‐wasting can occur at similar rates to various terrestrial lithological and tectonic settings. We present a process‐based model of submarine scarp degradation via retrogressive erosion in basement lithology where scarps have a gravitational stability threshold height of 20–40 m and a long‐term average slope of 30°–40°. More generally, the results presented here may be applicable to develop models of submarine landscape evolution based on degradation of normal fault scarps on the seafloor.

Evaluating young fluvial terrace riser degradation using a nonlinear transport model: Application to the Kongur Normal Fault in the Pamir, northwest China

AbstractSelecting an appropriate hillslope transport model and calibrating model parameters are essential for morphological dating of fault and fluvial scarps. In this paper, we refine the method of profile‐based morphologic dating by updating the representation of nonlinearity in sediment flux dependence on the hillslope gradient. We apply this revised method to fluvial scarps bounding fluvial terraces offset along the Kongur Normal Fault in the semi‐arid high‐altitude Pamir mountains, northwestern China. One of these terraces, the T3 surface, is dated to 7.0+1.9/−1.6 kyr using 10Be cosmogenic depth profile analysis. Well‐preserved, dated terraces make this an ideal site to test the utility of morphological modelling in constraining ages of the young terrace risers. To do this, 35 topographic swath profiles across the terrace risers are extracted from a 0.2 m‐resolution digital elevation model produced using structure from motion from photos collected by an unmanned aerial vehicle. The best estimates of morphological age are 13.9 ± 1.3 m2 for the riser T3/T4 and 11.9 ± 1.3 m2 for T2/T3 using a linear diffusion approach. These two morphological ages overlap within uncertainty and fail to distinguish between two young terrace risers. Alternatively, we employed a nonlinear diffusion model, calibrated with transport constant k = 1 m2 kyr−1, nonlinearity n = 2, and critical gradient Sc = tan(33°). This nonlinear model produces ages of 7.3 ± 0.5 kyr for T3/T4 and 4.0 ± 0.2 kyr for T2/T3; these ages are consistent with terrace surface ages deduced by using vertical offset divided by independently determining average throw rate. This comparison shows the advantage of a nonlinear model in defining ages of young scarps. Furthermore, we explored the minor effect of heterogeneous degradation along steep sections of the scarp profiles. The nonlinear scarp modelling scheme we develop in this paper is suitable for studying scarp degradation in other regions. © 2020 John Wiley & Sons, Ltd.

Stable Rate of Slip Along the Karakax Section of the Altyn Tagh Fault from Observation of Interglacial and Postglacial Offset Morphology and Surface Dating

AbstractDigital elevation maps obtained using TanDEM‐X and Pleiades data combined with newly obtained surface age estimates using cosmogenic radionuclide (CRN) and optically simulated luminescence (OSL) methods are used to quantify the slip rate along the western section of the Altyn Tagh Fault in southern Xinjiang. The reconstruction of the conical shape of massive alluvial fans inferred to be from the Eemian (115±7 ka) from CRN dating shows consistent left‐lateral offsets of 300±20 m, yielding a slip rate of 2.6±0.3 mm/year. Successive episodes of incision have left cut terraces inset in wide canyons, 10–25 m below the fans' surface. The incision was followed by the deposition of a broad terrace of early Holocene age, which is reincised by modern stream channels. Near the village of Shanxili, a 200 m wide valley is partially dammed by a shutter ridge displaced by the fault. A fill terrace deposited upstream from the ridge has an OSL age of 8.8±0.6 ka. The 23±2 m offset of the riser incising the terrace indicates a minimum postdepositional movement on the fault, yielding a Holocene rate of 2.6±0.5 mm/year, consistent with the 115‐ka average slip rate. Scarp degradation analysis using mass diffusion reveals a nonlinear relationship between fault displacement and degradation coefficient along the progressively exposed fault scarp, a pattern suggesting either seismic clustering or variable diffusion rate since the Eemian. Together with the Gozha Co‐Longmu Co fault to the south, the Karakax section of the Altyn Tagh Fault contributes to the eastward movement of the western corner of Tibet.

Evidence From High‐Resolution Topography for Multiple Earthquakes on High Slip‐to‐Length Fault Scarps: The Bilila‐Mtakataka Fault, Malawi

AbstractGeomorphological features such as fault scarps and stream knickpoints are indicators of recent fault activity. Determining whether these features formed during a single earthquake or over multiple earthquakes cycles has important implications for the interpretation of the size and frequency of past events. Here, we focus on the Bilila‐Mtakataka fault, Malawi, where the 20 m high fault scarps exceed the height expected from a single earthquake rupture. We use a high‐resolution digital elevation model ( 1 m) to identify complexity in the fault scarp and knickpoints in river profiles. Of 39 selected scarp profiles, 20 showed evidence of either multiscarps or composite scarps, and of the seven selected river and stream profiles, five showed evidence for multiple knickpoints. A near‐uniform distribution of vertical offsets on the subscarps suggests they were formed by separate earthquakes. These independent methods agree that at least two earthquakes have occurred with an average vertical offset per event of 10 and 12 m. This contrasts earlier studies which proposed that this scarp formed during a single event and demonstrates the importance of high‐resolution topographic data for understanding tectonic geomorphology. We use a one‐dimensional diffusion model of scarp degradation to demonstrate how fault splays form multiscarps and estimate the diffusion age of the Bilila‐Mtakataka fault scarp to be m , corresponding to years since formation. We calculate that a continuous rupture would equate to a M earthquake, greater than the largest seismic event previously recorded in East Africa.

Luminescence dating of sediments from central Atacama Desert, northern Chile

Abstract This study examines the feasibility of applying luminescence dating methods to quartz and potassium feldspar (K-feldspar) grains from Quaternary continental deposits of the Mejillones Peninsula and Coastal Cordillera in Central Atacama, northern Chile. Sediment burial ages were calculated using single-aliquot regenerative-dose (SAR) protocols applied to the optically stimulated luminescence (OSL) signal of quartz as well as infrared stimulated luminescence (IRSL) and post-infrared infrared stimulated luminescence (post-IR IRSL) signals of K-feldspar. Considering that the target sedimentary deposits comprise the Late Pleistocene age range, K-feldspar grains were dated using fading-corrected IRSL signals measured at 50 °C (IR50) and post-IR IRSL measured at 225 °C (pIRIR225) to minimize the potential effect of residual doses on calculated ages. The results of the analytical procedures indicate that quartz grains extracted from the studied sediments present a very weak or even no fast OSL component. The combination of a low OSL sensitivity, signal instability and equivalent dose distributions with high overdispersion (>40% for most samples) hinder reliable age estimation using quartz aliquots. Bleaching test results show that the IR50 signal from K-feldspar aliquots is well reset after 20–24 h of light exposure, while the pIRIR225 signal may present residual doses corresponding to between 10 and 15% of the natural signal. IR50 fading rates are ∼6–7%/decade for most studied samples, with exception of one sample which yielded a g-value of 18.77 ± 2.06%/decade. Fading rates for the pIRIR225 signal yielded variable results among sampling sites, with g-values ranging between 0.70 ± 0.24 and 6.77 ± 1.05%/decade. The dating results point out that quartz OSL ages are largely underestimated in relation to K-feldspar fading-corrected IR50 and pIRIR225 ages. Fading-corrected pIRIR225 K-feldspar ages indicate that the alluvial sedimentation in the downthrown block of an active normal fault in Mejillones Peninsula occurred between 163.4 ± 18.4 ka and 87.4 ± 6.6 ka. K-feldspar ages also allow to constrain the age of fault scarp degradation and fault reactivation in two main branches of the Atacama Fault System. In the Naguayan Fault, pIRIR225 ages indicate that faulting occurred after 21.4 ± 3.2 ka while in the Salar del Carmen Fault, faulting is recorded after 14.7 ± 1.0 ka.

Regional‐Scale Detection of Fault Scarps and Other Tectonic Landforms: Examples From Northern California

AbstractFault scarps and fault‐related landforms provide important information about fault zone activity over timescales that are not captured by instrumental measurements or historic records. Semiautomated methods for delineating these landforms using topographic data from light detection and ranging (lidar) and spaceborne imaging systems offer the opportunity to characterize fault zones on a global scale. We present a computationally efficient method for extracting scarp‐like landforms from high‐resolution (≤2 m), regional‐scale (≥ 100‐km‐long) digital topographic data sets. We identify fault‐related landforms using a curvature template based on the diffusion model for scarp degradation and extract scarp heights and morphologic ages at each pixel. The method was applied to the GeoEarthScope Northern California data set, an airborne lidar acquisition imaging nearly 2,500 km2 of the northern San Andreas Fault system, by adapting the algorithm to use cloud computing resources. Template results and fault trace mapping show spatial agreement in active fault zones with clear topographic expression, including detection of fault scarps, shutter ridges, and elongated drainages. Comparison of the method against field‐based morphologic dating of scarps along the southern San Andreas reveals a trade‐off between template window size and morphologic age contrasts resolved between strike‐slip fault scarps of different relative ages. Detection performance suggests that window size and orientation constraints may play a key role in improving the accuracy of methods for semiautomated fault zone mapping. As data availability grows, these methods could constrain key earthquake simulation parameters such as damage zone width or rupture length and improve fault maps worldwide.

Fault-scarp degradation in the central Exmouth Plateau, North West Shelf, Australia

Abstract Latest Triassic–earliest Late Jurassic domino-style extensional faulting in the central Exmouth Plateau, North West Shelf of Australia, exhibits footwall degradation scarps with up to 1.8 km of scarp retreat of the Upper Triassic Mungaroo Formation. Extensional fault-propagation folding, rotation and uplift produced gravitationally driven scarp collapse of the incompetent and mudstone-dominated uppermost Mungaroo Formation. Scarp degradation occurred along the entire extent of the footwalls of three major faults within the research area. Individual segments display listric fault surfaces in cross-section and scoop-shaped scars in three dimensions. The listric collapse faults dip towards the erosional scarp and sole out at different levels within the upper Triassic Mungaroo Formation. Footwall crestal collapse formed coalesced, scoop-shaped degradation scarps with Mungaroo Formation debris deposited as wedges within the adjacent hanging-wall synclines. Maximum scarp degradation occurred at the fault centres and decreased towards the fault tips. This study proposes new three-dimensional evolutionary structural models for the fault-scarp degradation in the central Exmouth Plateau.

Denudation outpaced by crustal thickening in the eastern Tianshan

The modern high topography of the Tianshan resulted from the reactivation of a Paleozoic orogenic belt by the India/Asia collision. Today, the range exhibits tectonically active forelands and intermontane basins. Based on quantitative morphotectonic observations and age constraints derived from cosmogenic 10Be dating, single-grain post-infrared infrared stimulated luminescence (p-IR IRSL) dating and modeling of fault scarp degradation, we quantify the deformation in the Nalati and Bayanbulak intermontane basins in the central Eastern Tianshan. Our results indicate that at least 1.4 mm/yr of horizontal crustal shortening is accommodated within these two basins. This shortening represents over 15% of the 8.5 ± 0.5 mm/yr total shortening rate across the entire range at this longitude. This shortening rate implies that the Eastern Central Tianshan is thickening at a mean rate of ∼1.4 mm/yr, a rate that is significantly higher than the average denudation rate of 0.14 mm/yr derived from our cosmogenic analysis. This discrepancy suggests that the Tianshan range has not yet reached a steady-state topography and remains in a transient state of topographic growth, most likely due to limited denudation rates driven by the arid climate of Central Asia.

Morphometric and geometric characterization of normal faults on Mars

Using three different approaches (fault plane fitting, 3D crater rim palinspatic restorations and fault scarps morphometric analysis) we investigate the geometry and degradation history of Martian normal faults in two distinct areas. The three independent methods produce similar results, indicating that the average dip angle of the normal faults on these two locations is probably below the value that is usually assumed for Mars (∼60°). Our best estimate for this average dip angle is 46.8±9.8°, which is a value comparable with the mean dip angle inferred on Earth for seismically active normal faults. This lower average dip angle implies that all the rift strain estimates performed until now might be underestimated.From the comparative analysis of the two faulted regions (Phlegethon Catena and Claritas Fossae), we show that local and regional dip variabilities may exist on Mars. This reinforces the idea that the amount of extension associated with Martian rifts must be reconsidered.We also demonstrate the advantages of performing a comparative morphometric analysis of fault scarps. This approach enables the reconstruction of the faults scarps degradation history and can be used to evaluate how environmental conditions changed through time. After modeling the degradation of the fault scarps at the two sites we conclude that the observed morphometric variations are mainly due to the different faulting ages in an environment characterized by low scarp degradation rates (4×10−3 m2/kyr) over the last 3 Ga.

High-resolution spatial rupture pattern of a multiphase flower structure, Rex Hills, Nevada: New insights on scarp evolution in complex topography based on 3-D laser scanning

Fault scarps represent the most obvious expression of tectonic activity at the Earth9s surface. Studies on scarp morphology place constraints on fault kinematics and scarp-degradation processes, and were often based on geomorphic dating techniques. Fault scarps exposed in areas of simple topography facilitate data acquisition and interpretation, whereas little work had been done where fault scarps are superimposed on complex, dissected topography. Fault scarps developed in complex topography are commonly observed along flower structures and at tips of strike-slip faults. Such structures are important elements for evaluating the evolution and linking of strike-slip fault systems, and appear to be scale-independent from several meters to hundreds of kilometers. We examined the detailed meter- to hundred meter–scale structure and surface expression of a flank of one fault scarp–bounded pressure ridge (Rex Hills flower structure) by combining field mapping with high-resolution digital elevation model (DEM) analysis. Based on terrestrial laser scanning we generated a detailed DEM and extracted high-resolution topographic cross sections, which enabled us to identify fault scarps and to determine their relative ages and geometry. Our study site is located on the transpressional left-bend between the Pahrump and Amargosa segments of the dextral Stateline fault system. The topography is characterized by alternating valleys and ridges (each ∼100 m long, relief of ∼4 m). We observed the following: the southern Rex Hills slope exhibits three fault scarps related to three reverse fault branches; the basal scarp (scarp 1) is most continuous, and exhibits five segments, the upper two scarps (scarps 2 and 3) are less continuous. Furthermore, fault scarps exposed on ridge crests are more numerous (up to four to five scarps), and smaller (∼5 m high); valleys often exhibit single large (>10 m high), smoothed scarps. To easily detect differences between the scarps, we evaluated the height and slope angle of the scarps using topographic cross sections. Our analysis indicates that scarp shape is influenced by fault dip, lithology, and degradation processes resulting in large scatter and broad overlap in scarp-height–slope-angle space. The analysis further indicates that scarp degradation is stronger in the valleys, and that the preservation potential of small, individual fault scarps is therefore greater on the ridge crests. We compared our fault-scarp data with published, calibrated data yielding an age of ∼2 ka for the Rex Hills scarps consistent with an earlier finding. This suggests that the scarp shape mainly reflects progressive degradation since the most recent surface rupture. Our approach of analyzing high-resolution topographic data of closely spaced fault scarps is promising especially when combined with subsurface data as well as geochronological and paleoseismic data, and it provides a basic scheme for analyzing scarp populations in a complex topographic region. Despite the absence of subsurface data, our approach allowed the study of complex high-resolution fault-scarp morphologies across a flower structure for the first time.

Morphologic dating of fault scarps using airborne laser swath mapping (ALSM) data

Models of fault scarp morphology have been previously used to infer the relative age of different fault scarps in a fault zone using labor‐intensive ground surveying. We present a method for automatically extracting scarp morphologic ages within high‐resolution digital topography. Scarp degradation is modeled as a diffusive mass transport process in the across‐scarp direction. The second derivative of the modeled degraded fault scarp was normalized to yield the best‐fitting (in a least‐squared sense) scarp height at each point, and the signal‐to‐noise ratio identified those areas containing scarp‐like topography. We applied this method to three areas along the San Andreas Fault and found correspondence between the mapped geometry of the fault and that extracted by our analysis. This suggests that the spatial distribution of scarp ages may be revealed by such an analysis, allowing the recent temporal development of a fault zone to be imaged along its length.

Physical modelling of fault scarp degradation under freeze–thaw cycles

AbstractPhysical modelling has been developed in order to simulate the effects of periglacial erosion processes on the degradation of slopes and scarps. Data from 41 experimental freeze–thaw cycles are presented. They attest to the efficiency of periglacial processes that control both erosion and changes in scarp morphology: (i) cryoexpulsion leads to an increase of scarp surface roughness and modifies significantly the internal structure of the active layer; (ii) combined effects of frost creep and gelifluction lead to slow and gradual downslope displacements of the active layer (0·3 cm/cycle); (iii) debris flows are associated with the most significant changes in scarp morphology and are responsible for the highest rate of scarp erosion; (iv) quantification of the erosion rate gives values close to 1 cm3 cm−2 for 41 freeze–thaw cycles. These experimental results are consistent with field data acquired along the La Hague fault scarp (Normandy, France) where an erosion rate of 4·6 ± 1 m3 m−2 per glacial stage has been computed from the volume of natural slope deposits stored during the Weichselian glacial stage. These results show that moist periglacial erosion processes could lead to an underestimation of Plio‐Quaternary deformation in the mid‐latitudes. Copyright © 2006 John Wiley & Sons, Ltd.

Sedimentology of rift climax deep water systems; Lower Rudeis Formation, Hammam Faraun Fault Block, Suez Rift, Egypt

In most marine rift basins, subsidence outpaces sedimentation during rift climax times. Typically this results in sediment-starved hangingwall depocentres dominated by deep-marine mudstones, with subordinate local development of coarser clastics in the immediate hangingwall derived from restricted catchments on the immediate footwall scarp. To highlight the spatial variability of rift climax facies and the controls upon them, we have investigated the detailed three-dimensional geometry and facies relationships of the extremely well exposed Miocene, rift climax Lower Rudeis Formation in the immediate hangingwall to the Thal Fault Zone, Suez Rift, Egypt. Detailed sedimentological analyses allows the Lower Rudeis Formation to be divided into two contemporaneous depositional systems, (1) a laterally continuous slope system comprising, hangingwall restricted (< 250 m wide) slope apron, slope slumps, fault scarp degradation complex and laterally extensive lower slope-to-basinal siltstones, and (2) a localized submarine fan complex up to 1 km wide and extending at least 2 km basinward of the fault zone. Interpretation of individual facies, facies relationships and their spatial variability indicate that deposition in the immediate hangingwall to the Thal Fault occurred via a range of submarine concentrated density flows, surge-like turbidity flows, mass wasting and hemipelagic processes. Major controls on the spatial variability and stratigraphic architecture of the depositional systems identified reflect the influence of the steep footwall physiography, accommodation and drainage evolution associated with the growth of the Thal Fault. The under-filled nature of the hangingwall depocentre combined with the steep footwall gradient result in a steep fault-controlled basin margin characterised by either slope bypass or erosion, with limited coastal plain or shelf area. Sediment supply to the slope apron deposits is controlled in part by the evolution and size of small footwall drainage catchments. In contrast, the localized submarine fan is interpreted to have been fed by a larger, antecedent drainage network. The structural style of the immediate footwall is also believed to exert a control on facies development and stratigraphic evolution. In particular, fault scarp degradation is enhanced by fault propagation folding which creates basinward-dipping bedding planes in the pre-rift footwall strata that large pre-rift blocks slide on.

Dating rupture events on alluvial fault scarps using cosmogenic nuclides and scarp morphology

Scarp morphology evolution has commonly been used to estimate the timing of fault-scarp rupture events. However, rates of scarp degradation depend strongly on the geomorphic diffusivity, a parameter that is difficult to constrain independently. This difficulty may lead to large uncertainties in the estimated ages of rupture events. In this study, we have coupled the accumulation of the cosmogenic nuclide 36Cl to a model for scarp morphology in order to constrain the value of the geomorphic diffusivity and thus determine a more accurate rupture chronology than a rupture history based solely on scarp morphology. We measured depth profiles of 36Cl accumulated in situ within alluvial sediments beneath the surface of the Socorro Canyon fault scarp in central New Mexico. The material analyzed consisted of ∼150 individual gravel clasts from each depth interval, amalgamated into a single sample. The alluvium was sampled in three vertical profiles ∼4 m deep. The first profile was ∼1.5 m downslope of the fault plane on the hanging wall, the second ∼1.5 m upslope of the fault plane on the footwall, and the third ∼27 m upslope of the fault plane on the footwall. The third profile, which served as a control, and which soil geomorphic evidence indicated was beneath a stable surface, showed a simple exponential 36Cl profile from which a depositional age of 122±18 ka was calculated. The second profile (on the footwall) showed a 36Cl deficit relative to the control profile, indicating net erosion. The first profile (on the hanging wall) showed a 36Cl excess relative to the control profile, indicating net deposition. Stratigraphic evidence in the hanging wall indicated two rupture events prior to the Late Holocene. We modeled the accumulation of 36Cl in the vicinity of the scarp, simulating erosional redistribution using a diffusion equation for scarp morphology. The model accounted for redistribution of 36Cl along with the sediment and the effects of the changing fault-scarp morphology on the 36Cl production. By matching both the observed hanging-wall and footwall 36Cl profiles to profiles calculated by the model, as well as the observed topographic profile, we obtained ages of 92+16−13 and 28+18−23 ka for the two ruptures. The results of our study indicate that cosmogenic nuclides can be useful in constraining rupture chronologies of fault scarps in alluvium.

Geomorphic evidence for an emergent active thrust along the edge of the Po Plain: The Broni‐Stradella fault

We present here geomorphic evidence for an emergent active thrust along the northwestern Apennine front about 50 km south of Milano. Fieldwork combined with SPOT image analysis attests to the presence of a ∼35‐km‐long, en échelon, cumulative fault scarp, that cuts E‐W across Quaternary surface deposits from Casteggio to Sarmato. The scarp offsets vertically alluvial fans and terraces emplaced by tributaries of the Po River that flow northward from the Apennines. Incision by such tributaries occurs only south of the scarp. Valleys of smaller streams stand in hanging position. To the northeast, the buried, east dipping Pavia lateral ramp bounds the San Colombano anticline. We interpret the Montebello‐Sarmato scarp to result from the emergence of an active thrust fault dipping south, the Stradella thrust, which splays eastward from that ramp. Total station profiles leveled perpendicular to the Stradella thrust show variable cumulative surface throws ranging from 2 to 25 m. Scarp degradation analysis indicates fairly recent offset of the terrace surfaces (10–100 kyr) suggesting a minimum uplift rate of 0.3 mm/yr. The underlying fault might thus be one of the most active thrust ramps along this segment of the Apennines. N‐S to N15°W shortening is consistent with both the E‐W strike of this thrust ramp and with coeval oblique sinistral motion on the NNE‐SSW trending Pavia lateral ramp. The Vogherese‐Bobbiese earthquake of 1828 (I ≈ IX) south of Casteggio and Broni may have activated one segment of this lateral ramp rather than the Stradella thrust. Other historical events (I &gt; 7) that caused damage in Pavia, Piacenza, Lodi, and Milano in the last 2000 years (290 A.D., 1249 A.D., 1276 A.D., 1473 A.D.), may have occurred on the Broni‐Stradella thrust or on the blind San Colombano and Lodi thrusts farther north. This makes detailed paleoseismological studies and trenching across its trace mandatory.

Tectono‐climatic evolution of a Neogene intramontane basin (Late Miocene Carboneras subbasin, southeast Spain): revelations from basin mapping and biofacies analysis

ABSTRACTExceptional 3‐D exposures of fault blocks forming a 5 km × 10 km clastic sediment‐starved, marine basin (Carboneras subbasin, southeast Spain) allow a test of the response of carbonate sequence stratigraphic architectures to climatic and tectonic forcing. Temperate and tropical climatic periods recorded in biofacies serve as a chronostratigraphic framework to reconstruct the status of the basin within three time‐slices (late Tortonian–early Messinian, late Messinian, Pliocene). Structural maps and isopach maps trace out the distribution of fault blocks, faults, and over time, their relative motions, propagational patterns and life times, which demonstrate a changing layout of the basin because of a rotation of the regional transtensional stress field. Progradation of early Messinian reefal systems was perpendicular to the master faults of the blocks, which were draped by condensed fore‐slope sediments. The hangingwall basins coincided with the toe‐of‐slope of the reef systems. The main phase of block faulting during the late Tortonian and earliest Messinian influenced the palaeogeography until the late Pliocene (cumulative throw &lt; 150–240 m), whereas displacements along block bounding faults, which moved into the hangingwall, died out over time. An associated shift of the depocentres of calciturbidites, slump masses and fault scarp degradation breccias reflects 500–700 m of fault propagation into the hangingwall. The shallow‐water systems of the footwall areas were repeatedly subject to emergence and deep peripheral erosion, which imply slow net relative uplift of the footwall. In the dip‐slope settings, erosional truncations of tilted proximal deposits prevail, which indicate rotational relative uplift. Block movements were on the order of magnitude of third order sea‐level fluctuations during the late Tortonian and earliest Messinian. We suggest that this might be the reason for the common presence of offlapping geometries in early Messinian reef systems of the Betic Cordilleras. During the late Pliocene, uplift rates fell below third order rates of sea‐level variations. However, at this stage, the basin was uplifted too far to be inundated by the sea again. The evolution of the basin may serve as a model for many other extensional basins around the world.

Morphological dating of cumulative reverse fault scarps: examples from the Gurvan Bogd fault system, Mongolia

We relate reverse fault scarp morphology formed by several earthquake dislocations to the average deformation rate, using a morphological dating model based on a diffusion analogue of erosion. Our scarp degradation model includes diffusive erosion during the interseismic period, the gravitational collapse of the coseismic fault scarp just after formation, and the variation of the surface rupture location. Interactions between thrusting and geomorphic processes acting on scarp morphology are analysed along the Gurvan Bogd Range in Mongolia. Four main processes acting on scarp morphology were distinguished: 1) gravitational collapse of the frontal scarp, resetting the diffusive scarp if fault offsets are big and faulting is localized; 2) progressive erosion of the fault scarp during the interseismic period; 3) folding associated with the frontal thrust and backthrusts; 4) competing alluvial deposition on mountain piedmont slopes and abrasion of the fault scarp by wash processes. The growth of cumulative reverse fault scarps is suppressed when they are located in the outwash of major drainage basins. They can grow higher in distance from major catchment discharges. The modelling suggests that the morphology of the scarp and its apparent degradation stage, depend on the parameters controlling the amount of frontal collapse; the magnitude of coseismic offsets, the dip of the fault near the surface and the step distance between faults. Folding associated with thrusting creates a convexity on the upper part of the scarp and increases its height. The comparison of different scarp profiles suggests that folding leads to an overestimate of the morphological age. We estimate a diffusion coefficient at 3.3 ± 1.7 m2 ka−1. Morphological ages calculated with our model confirm that slip rate along reverse faults of the Gurvan Bogd range has not been constant over the last 100 ka.

Morphometric analysis of active normal faulting in slow-deformation areas : examples in the Lower Rhine Embayment

AbstractWe studied the applicability of classical scarp degradation modelling to active normal faults in the Lower Rhine Embayment. Our quantitative analysis was conducted on the frontal Bree fault scarp (Feldbiss fault) in Belgium and the Peel fault scarp near the city of Neer in the Netherlands. Vertical offset and diffusion age of these scarps have been modelled from elevation profiles across the studied faults using the diffusion equation. For that purpose, a computer-program (profil 2000) has been written, providing a sensitivity analysis of the determined parameters in function of the spatial repartition of the elevation measurements along the considered profiles. The results of this morphometric analysis have been validated by a comparison with the geologic record of the tectonic activity observed in the trenches excavated at the sites where the measurements have been conducted.We conclude that the modelling can only be applied to study tectonic activity since the Last Glacial Maximum (±14-19 kyr BP) because the surface expression of older paleoearthquakes in unconsolidated Late Pleistocene sediments has been erased by the strong erosive phase that occurred at the end of this glacial period. Even for Holocene scarps, morphologic dating seems very difficult because man-made perturbations destroyed surface evidence of the very recent fault activity in many sites. Nevertheless, we estimate that an appropriate value for the mass diffusivity constant for~ 1-m-high scarps in the investigated region is 0.002 to 0.010 m2/yr. On the other hand, vertical offsets can be determined with a good precision. These amount to respectively ~1 m and 1,3 m since the Last Glacial Maximum on the Feldbiss fault in Belgium and the Peel fault near Roermond in the Netherlands.

Evolution of terrace risers along the upper Rhine graben inferred from morphologic dating methods: evidence of climatic and tectonic forcing

SUMMARY We show that morphologic dating techniques that have been applied successfully in arid and semi-arid areas are also suitable for slowly evolving scarps that are usually found in temperate climate environments. We have attempted two morphologic approaches, based on diffusion, to relate the present-day shape of an abandoned terrace riser to its age. The first assumes a model of scarp degradation based on a diffusive process (the D method). The second evaluates the state of scarp degradation using the slope distribution (the SD method) along a topographic profile. By using a manmade scarp of known age, we have obtained a mass diffusivity close to 1.4 m2 ka−1 when the area experiences a temperate climate characterized by a continuous vegetation cover. However, this value decreases during glacial episodes, probably on account of the permafrost. Even though the SD method requires an age correction that can be easily computed, only this method reveals that at several profiles a later scarp reactivation event has occurred. Indeed, along several profiles, the slope distribution was best fitted by two offset Gaussian curves, suggesting that some scarps have undergone a complex evolution that cannot be modelled with a single diffusive process. This scarp reactivation may correspond to a new incisive episode and allows one to estimate the vertical incision rate along the terrace riser. Applied to a Wurmian terrace riser of the upper Rhine valley (NE France), this approach reveals that the vertical incision rate ranges from 0.2 to 0.85 mm yr−1 between 35 and 15 ka and that the terrace bevelling occurred during two episodes related to climatic forcing. Moreover, we can identify a component of tectonic forcing evidenced by an increase to the north of vertical incision rate and Rhine stream-power. Another major result is showing that this terrace riser is not isochronous along its strike and that younger portions result from lateral incision of a 30 ka pre-existing scarp.