Listric extensional fault systems - results of analogue model experiments

Abstract Analogue models are a powerful tool for investigating progressive deformation in extensional fault systems. This paper presents exciting new insights into the progressive evolution of hanging wall structures in listric extensional terranes. Analogue models, scaled to simulate deformation in a sedimentary sequence, were constructed for simple listric and ramp/flat listric extensional detachments. For each detachment geometry homogeneous sand, sand/mica and sand/clay models were used to simulate respectively, deformation of isotropic sediments, of anisotropic sediments and of sedimentary sequences with competency contrasts. Roll-over anticlines with geometrically necessary crestal collapse graben structures are characteristic of the steepening-upwards segments of listric extensional fault systems in all of our models. With progressive deformation, crestal collapse grabens show hanging wall nucleation of new faults. Variations in graben size, amount of fault rotation and throw, are dependent on detachment curvature and amount of extension. Individual faults and associated fault blocks may significantly change shape during extension. Complex and apparently conjugate fault arrays are the result of superposition of successive crestal collapse grabens. Ramp/flat listric extensional fault systems are characterized by a roll-over anticline and a crestal collapse graben system associated with each steepening-upwards segment of the detachment and a ramp zone consisting of a hanging wall syncline and a complex deformation zone with local reverse faults. The roll-over anticlines and crestal collapse graben are similar in geometry to those formed in simple listric extensional systems. The models demonstrate that the geometry of the detachments exerts a fundamental control on the evolution of hanging wall structures. Analysis of particle displacement paths for these experiments provides new insights into the mechanical development of roll-over anticlines. Two general models for deformation above simple listric and ramp/flat listric extensional detachments have been erected.     

Quaternary landscape evolution in the San Jacinto fault zone,Peninsular Ranges of Southern California: Transient response to strike-slip fault initiation

Well-constrained case studies of transient landscape response to external forcing are needed to improve our understanding of erosion processes in tectonically active mountain belts. The Peninsular Ranges portion of the San Jacinto fault zone (SJFZ) is an excellent location for such a study because it displays pronounced geomorphic disequilibrium resulting from initiation of a major strike-slip fault in the past 1.0 to 2.5 million years. We recognize two geomorphic domains in this region: (1) a relict low-relief upland domain consisting of broad flat valleys and low-gradient streams and (2) very steep, rough topography with deeply incised canyons and retreating erosional knickpoints. Pleistocene sediments exposed along and near the SJFZ include fluvial conglomerate, sandstone, and mudstone, with weak paleosols and west- to NW-directed paleocurrents. These sediments accumulated in a low-gradient stream system (represented by domain 1) during an early phase of slip in the SJFZ, prior to the modern phase of erosion and degradation (domain 2). Late Pliocene or early Pleistocene initiation of the SJFZ triggered a wave of headward erosion and stream capture that is still migrating NW along the fault zone. Using the total distance that capture points have migrated along the fault zone and a range of possible ages for fault initiation, the rate of knickpoint retreat is estimated at ∼ 12 to 44 km/my.To explore the signal of transient geomorphic response to fault initiation, we analyzed 23 tributaries along an ∼ 20-km portion of the main fault valley within domain 2. The analysis reveals three zones with distinctive morphologies: (1) strongly convex longitudinal profiles in the NW, (2) a large (ca. 5–6 km²) landslide in the central zone, and (3) concave tributaries in the SE with profile complexity decreasing and catchment area increasing from NW to SE. The distribution of these zones suggests close spatial and temporal association of active fault slip, bedrock incision, deep-seated landslides, and erosional modification. The fundamental driving force behind these processes is profound geomorphic disequilibrium resulting from initiation of the SJFZ. We suggest that landslides may have played a significant role in shaping the morphology of this fault zone, and that the influence of landslides may be underestimated in areas where characteristic landforms and deposits are obscured by later erosion and faulting.     

Degradation of unconsolidated Quaternary landforms in the western North America

One of the major goals of geomorphology is to understand the rate of landscape evolution and the constraints that erosion sets on the longevity of land surfaces. The latter has also turned out to be vital in modern applications of cosmogenic exposure dating and interpretation of lichenometric data from unconsolidated landforms. Because the effects of landform degradation have not been well documented, disagreements exist among researchers regarding the importance of degradation processes in the dating techniques applied to exposures. Here, we show that all existing qualitative data and quantitative markers of landform degradation collectively suggest considerable lowering of the surface of unconsolidated landforms over the typical life span of Quaternary moraines or fault scarps. Degradation is ubiquitous and considerable even on short time scales of hundreds of years on steeply sloping landforms. These conservative analyses are based entirely on field observations of decreasing slope angles of landforms over the typical range of ages in western North America and widely accepted modeling of landscape degradation. We found that the maximum depth of erosion on fault scarps and moraines is on average 34% of the initial height of the scarp and 25% of the final height of the moraine. Although our observations are limited to fault scarps and moraines, the results apply to any sloping unconsolidated landform in the western North America. These results invalidate the prevailing assumption of no or little surface lowering on sloping unconsolidated landforms over the Quaternary Period and affirm that accurate interpretations of lichen ages and cosmogenically dated boulder ages require keen understanding of the ever-present erosion. In our view, the most important results are twofold: 1) to show with a large data set that degradation affects universally all sloping unconsolidated landforms, and 2) to unambiguously show that even conservative estimates of the total lowering of the surface operate at time and depth scales that significantly interfere with cosmogenic exposure and lichen dating.     

Spatial distribution of micrometre‐scale porosity and permeability across the damage zone of a reverse‐reactivated normal fault in a tight sandstone: Insights from the Otway Basin,SE Australia

Knowledge of the permeability structure of fault‐bearing reservoir rocks is fundamental for developing robust hydrocarbon exploration and fluid monitoring strategies. Studies often describe the permeability structure of low porosity host rocks that have experienced simple tectonic histories, while investigations of the influence of faults with multiple‐slip histories on the permeability structure of porous clastic rocks are limited. We present results from an integrated petrophysical, microstructural, and mineralogical investigation of the Eumeralla Formation (a tight volcanogenic sandstone) within the hanging wall of the Castle Cove Fault which strikes 30 km NE–SW in the Otway Basin, southeast Australia. This late Jurassic to Cenozoic‐age basin has experienced multiple phases of extension and compression. Core plugs and thin sections oriented relative to the fault plane were sampled from the hanging wall at distances of up to 225 m from the Castle Cove Fault plane. As the fault plane is approached, connected porosities increase by ca. 10% (17% at 225 m to 24% at 0.5 m) and permeabilities increase by two orders of magnitude (from 0.04 mD at 225 m to 1.26 mD at 0.5 m). Backscattered Scanning Electron Microscope analysis shows that microstructural changes due to faulting have enhanced the micrometre‐scale permeability structure of the Eumeralla Formation. These microstructural changes have been attributed to the formation of microfractures and destruction of original pore‐lining chlorite morphology as a result of fault deformation. Complex deformation, that is, formation of macrofractures, variably oriented microfractures, and a hanging wall anticline, associated with normal faulting and subsequent reverse faulting, has significantly influenced the off‐fault fluid flow properties of the protolith. However, despite enhancement of the host rock permeability structure, the Eumeralla Formation at Castle Cove is still considered a tight sandstone. Our study shows that high‐resolution integrated analyses of the host rock are critical for describing the micrometre‐scale permeability structure of reservoir rocks with high porosities, low permeabilities, and abundant clays that have experienced complex deformation.     

Structure of the footwall of a listric fault system revealed by 3D seismic data from the Niger Delta

We use three‐dimensional (3D) seismic reflection data to analyse the architecture of the footwall of a listric fault, in a gravitationally driven extensional system, in the north‐western Niger Delta. In contrast to conventional listric normal fault models with a single master listric fault plane the level of detachment switches from a deeper to shallower level. The footwall evolves through the generation of new master detachment faults and detachments, which transfers hanging wall rocks into the footwall. New detachments form by branching off pre‐existing detachment levels, cutting‐up through stratigraphy to the next mechanical weakness, separating discrete sections of extended strata. As a consequence a deeper, older array of seaward‐dipping, tilted extensional fault blocks is now located in the footwall beneath the master listric detachment fault. The structural complexity located below the master detachment fault highlights extensional episodes on separate detachment faults that are not captured in conventional listric models. We speculate that changes in the level of the detachment are caused by mechanical weaknesses controlled by lithology, pore pressure and episodes of sediment loading related to deltaic progradation.     

Image processing and roughness analysis of exposed bedrock fault planes as a tool for paleoseismological analysis: results from the Campo Felice fault (central Apennines,Italy)

Morphologic investigations along the Campo Felice (CF) fault (central Apennines, Italy) have been made in order to develop a procedure for the paleoseismological analysis of bedrock fault scarps. The CF fault has been responsible for the formation of an impressive limestone fault scarp. Geomorphologic work on the CF basin and related fault indicated that the scarp originated from tectonic fault displacements. Three morphologic units have been distinguished along the fault scarp and defined as morphosome M1 (lowest part of the scarp), M2 and M3 (the uppermost part). These units display different karstic features, which are the result of their different duration of exposure to weathering. Micromorphologic analyses focused on the morphosome M1, along which the CF fault plane is exposed for a height ranging between 4 and 7 m. These analyses were aimed at defining differently weathered bands located at various heights, and parallel to the fault scarp top and base. The presence of these bands suggests repeated fault movements. The exposed fault surface displays a low-grade biokarstic weathering due to the action of epilithic and endolithic organisms. The biokarst distribution is, however, inhomogeneous and conditioned by the presence of nourishing elements, moisture and by light intensity. An area preferentially affected by the biokarstic processes develops as a band at the bedrock–soil contact at the base of the scarp. Roughness and colour analyses were made to identify uplifted bands which previously formed at the bedrock–soil contact. The roughness analysis was made using a microroughness-meter along 20-cm long horizontal transects repeated each 20 cm of fault height for the entire morphosome M1, at various sites along the scarp. The roughness variance data, plotted vs. the fault height, failed to identify differently weathered bands of paleoseismological interest. This result is probably due to the complex distribution of biokarst along the investigated fault plane. More reliable results have been obtained by areal analysis of the variation of the colour rendering of the rocks exposed along the fault plane at different sites. Photographic images of large portions of fault surfaces have been processed with standard graphic computer programs. The variations of colour indicated the presence of bands at various heights along the fault plane. Two uplifted bands have been recognised at all the investigated sites suggesting two displacement events (E1 and E2). A preliminary chronological framework for these two events, the youngest of which affected the CF fault, can be derived from the paleoseismological data available for the southernmost branch of the regional fault system that includes the CF fault. According to these data, E1 may have occurred between 860 and 1300 AD, while E2 may have occurred at about 1900 BC. Work is in progress to define surface exposure ages of different parts of the fault plane by means of in situ produced cosmogenic ³⁶Cl. This procedure will give further chronological constraints for the age of E1 and E2 and will also permit to test the validity of the micromorphologic analysis of bedrock fault scarps for paleoseismological aims.     

Clinoform nucleation and growth in coarse-grained deltas, Loreto basin, Baja California Sur, Mexico: a response to episodic accelerations in fault displacement

We investigate the controls on the architecture of coarse‐grained delta progradational units (PUs) in the Pliocene Loreto basin (Baja California Sur, Mexico), a half‐graben located on the western margin of the Gulf of California. Dorsey et al. (1997b) argued that delta progradation and transgression cycles in the basin were driven by episodic fault‐controlled subsidence along the basin‐bounding Loreto fault. Here we test this hypothesis by a detailed analysis of the sedimentary architecture of 11 exceptionally well‐exposed, vertically arranged fluvio‐deltaic PUs, each of which shows lateral facies transition from proximal alluvial facies palaeo‐seaward into distal pro‐delta facies. Of these 11 PUs, seven exhibit a lateral transition from a shoal water to Gilbert‐delta facies associations as they are traced palaeo‐seaward. This transition is characterised by down‐transport development of foresets, which grow in height up to 35 m. Foreset units thicken in a basinward direction, with initially an oblique topset–foreset geometry that becomes increasingly sigmoidal. Each delta is capped by a shell bed that records drowning of the delta top. This systematic transition in delta architecture records increasing water depth through time during individual episodes of progradation. A mechanism that explains this transition is an accelerating rate of fault‐controlled subsidence during each PU. During episodes of low slip rate, shoal‐water deltas prograde across the submerged topography of the underlying delta unit. As displacement rate accelerates, increasing bathymetry at the delta front leads to steepening of foresets and initiation of Gilbert deltas. Subsequent delta drowning results from sediment starvation at the shoreline at high slip rates because of sediment trapping upstream. The observed delta architecture suggests that the long‐term (>100 kyr) history of slip on the Loreto fault was characterised by repetitive episodes of accelerating displacement accumulation. Such episodic fault behaviour is most likely to be because of variations in temporal and spatial strain partitioning between the Loreto fault and other faults in the Gulf of California. A physical explanation for the acceleration phenomenon involves evolving frictional properties on the episodically active Loreto fault.     

Investigating the surface process response to fault interaction and linkage using a numerical modelling approach

In order to better understand the evolution of rift‐related topography and sedimentation, we present the results of a numerical modelling study in which elevation changes generated by extensional fault propagation, interaction and linkage are used to drive a landscape evolution model. Drainage network development, landsliding and sediment accumulation in response to faulting are calculated using CASCADE, a numerical model developed by Braun and Sambridge, and the results are compared with field examples. We first show theoretically how the ‘fluvial length scale’, L_f, in the fluvial incision algorithm can be related to the erodibility of the substrate and can be varied to mimic a range of river behaviour between detachment‐limited (DL) and transport‐limited (TL) end‐member models for river incision. We also present new hydraulic geometry data from an extensional setting which show that channel width does not scale with drainage area where a channel incises through an area of active footwall uplift. We include this information in the coupled model, initially for a single value of L_f, and use it to demonstrate how fault interaction controls the location of the main drainage divide and thus the size of the footwall catchments that develop along an evolving basin‐bounding normal fault. We show how erosion by landsliding and fluvial incision varies as the footwall area grows and quantify the volume, source area, and timing of sediment input to the hanging‐wall basin through time. We also demonstrate how fault growth imposes a geometrical control on the scaling of river discharge with downstream distance within the footwall catchments, thus influencing the incision rate of rivers that drain into the hanging‐wall basin. Whether these rivers continue to flow into the basin after the basin‐bounding fault becomes fully linked strongly depends on the value of L_f. We show that such rivers are more likely to maintain their course if they are close to the TL end member (small L_f); as a river becomes progressively more under supplied, i.e. the DL end member (large L_f), it is more likely to be deflected or dammed by the growing fault. These model results are compared quantitatively with real drainage networks from mainland Greece, the Italian Apennines and eastern California. Finally, we infer the calibre of sediments entering the hanging‐wall basin by integrating measurements of erosion rate across the growing footwall with the variation in surface processes in space and time. Combining this information with the observed structural control of sediment entry points into individual hanging‐wall depocentres we develop a greater understanding of facies changes associated with the rift‐initiation to rift‐climax transition previously recognised in syn‐rift stratigraphy.     

Sediment flux from an uplifting fault block

The stratigraphy of rift basins is a direct result of sediment liberation and transport through catchment–fan systems whose dynamics are controlled by both external and internal factors. We investigate the response of catchment–fan systems established across an active normal fault to variations in both tectonic and climatic boundary conditions. Numerical experiments show that the ratio of fan area to catchment area provides a sensitive indicator of tectonic activity. A step decrease in fault slip rate results in a delayed response by the catchment–fan systems; the response time is ∼50 kyr for a variety of parameter values. Decreased slip rate also gives rise to an abrupt but transient pulse in sediment discharge from the fans due to a drop in the hangingwall subsidence rate. In contrast, variations in climatic activity, using precipitation rate as a proxy, produce extremely rapid responses throughout the catchment–fan system. Thus, high-frequency climatic changes will overprint lower frequency tectonic variations in the stratigraphic record of fan deposits. Finally, we map out possible combinations of fault geometry, fault slip rate and precipitation rate that allow fan progradation and high rates of sediment discharge from the system.     

Implications of stress concentration on a strike-slip fault in an elastic plate subject to basal shear stress

Summary We consider a long strike-slip fault in a lithosphere modelled as an elastic slab. To the base of the slab a shear stress distribution is applied which simulates the viscous drag exerted by the asthenosphere. The resulant stress on the fault plane may directly fracture the lithosphere in its brittle upper portion; alternatively it may give rise at first to a stable aseismic sliding in the lower portion. In the latter case, stress concentration due to the deep aseismic slip is the relevant feature of the pre-seismic stress acting on the upper section of the lithosphere. The two cases are examined by use of dislocation theory and their observable effects compared. Different depths of the aseismic slip zone and the presence or absence of a uniform friction on the seismic fault are allowed for. If the model is applied to the San Andreas fault region, where a steady sliding condition actually seems to be present at shallow depth, it turns out that the slip amplitudes commonly associated with large earthquakes are consistent with average basal stress values which can be substantially lower than a few bars, a value often quoted as the steady state basal stress due to a velocity gradient in the upper asthenosphere.     

Neotectonics and Quaternary geology of the Hunter Mountain fault zone and Saline Valley region,southeastern California

The Hunter Mountain fault zone strikes northwesterly, is right-lateral strike-slip, and kinematically links the northern Panamint Valley fault zone to the southern Saline Valley fault zone. The most recent displacement of the fault is recorded in the offset of Holocene deposits along the entire length of the fault zone. Right-lateral offsets of drainage channels within Grapevine Canyon reach up to 50 to 60 m. Initial incision of the offset channels is interpreted on the basis of geomorphic and climatic considerations to have occurred approximately 15 ka. The 50 to 60 m of offset during 15 ka corresponds to a right-lateral fault slip rate of 3.3–4.0 mm/year within Grapevine Canyon. Further to the north along the Nelson Range front, the fault is composed of two sub-parallel fault strands and the fault begins to show an increased normal component of motion. A channel margin that is incised into a Holocene surface that is between 10 and 128 ka in age is offset 16–20 m, which yields a broad minimum bound on the lateral slip rate of 0.125–2.0 mm/year. The best preserved single-event displacements recorded in Holocene deposits range from 1.5 to 2.5 m. In addition to faulting within Grapevine Canyon and the main rangefront fault along the southwest edge of Saline Valley, there also exist normal fault strands within the Valley that strike northeasterly and towards Eureka Valley. The northeasterly striking normal faults in the Valley appear to be actively transferring dextral slip from the Hunter Mountain fault zone north and east onto the Furnace Creek fault zone. Separations on northerly trending, normal faults within Saline Valley yield estimates of slip rates in the hundredths of millimeters per year.     

Observation and modelling of fault-zone fracture seismic anisotropy - I. P, SV and SH travel times

Summary. Three-component VSP borehole seismograms taken in the vicinity of an active normal fault in California show strong systematic shear-wave splitting that increases with proximity to the fault. Using Červený's method of characteristics for ray tracing in anisotropic heterogeneous media and Hudson's formulation of elastic constants for media-bearing aligned fractures, we have fitted a suite of P, SV and SH hanging-wall and foot-wall travel times with a simple model of aligned fractures flanking the fault zone. The dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault. The increase in fracture density is non-uniform (power law or Gaussian) with respect to distance to the fault. Although the hanging-wall and the foot-wall rock are petrologically the same unit, the fracture halo is more intense and extensive in the hanging wall than in the foot wall. Upon approach to the fault plane, the fracture density or fracture-density gradient becomes too great for the seismic response to be computed by Hudson–Červený procedures (the maximum fracture density that can be modelled is about 0.08). Within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low-velocity waveguide. We observe production of trapped waves within the confines of the intense fracture interval.     

Slip rate on the Dead Sea transform fault in northern Araba valley (Jordan)

The Araba valley lies between the southern tip of the Dead Sea and the Gulf of Aqaba. This depression, blanketed with alluvial and lacustrine deposits, is cut along its entire length by the Dead Sea fault. In many places the fault is well defined by scarps, and evidence for left-lateral strike-slip faulting is abundant. The slip rate on the fault can be constrained from dated geomorphic features displaced by the fault. A large fan at the mouth of Wadi Dahal has been displaced by about 500 m since the bulk of the fanglomerates were deposited 77–140 kyr ago, as dated from cosmogenic isotope analysis (¹⁰Be in chert) of pebbles collected on the fan surface and from the age of transgressive lacustrine sediments capping the fan. Holocene alluvial surfaces are also clearly offset. By correlation with similar surfaces along the Dead Sea lake margin, we propose a chronology for their emplacement. Taken together, our observations suggest an average slip rate over the Late Pleistocene of between 2 and 6 mm yr⁻¹, with a preferred value of 4 mm yr⁻¹. This slip rate is shown to be consistent with other constraints on the kinematics of the Arabian plate, assuming a rotation rate of about 0.396° Myr⁻¹ around a pole at 31.1°N, 26.7°E relative to Africa.     

Carbonate sedimentation in a starved pull-apart basin, Middle to Late Devonian, southern Guilin, South China

ABSTRACT Geological mapping and sedimentological investigations in the Guilin region, South China, have revealed a spindle‐ to rhomb‐shaped basin filled with Devonian shallow‐ to deep‐water carbonates. This Yangshuo Basin is interpreted as a pull‐apart basin created through secondary, synthetic strike‐slip faulting induced by major NNE–SSW‐trending, sinistral strike‐slip fault zones. These fault zones were initially reactivated along intracontinental basement faults in the course of northward migration of the South China continent. The nearly N–S‐trending margins of the Yangshuo Basin, approximately coinciding with the strike of regional fault zones, were related to the master strike‐slip faults; the NW–SE‐trending margins were related to parallel, oblique‐slip extensional faults. Nine depositional sequences recognized in Givetian through Frasnian strata can be grouped into three sequence sets (Sequences 1–2, 3–5 and 6–9), reflecting three major phases of basin evolution. During basin nucleation, most basin margins were dominated by stromatoporoid biostromes and bioherms, upon a low‐gradient shelf. Only at the steep, fault‐controlled, eastern margin were thick stromatoporoid reefs developed. The subsequent progressive offset and pull‐apart of the master strike‐slip faults during the late Givetian intensified the differential subsidence and produced a spindle‐shaped basin. The accelerated subsidence of the basin centre led to sediment starvation, reduced current circulation and increased environmental stress, leading to the extensive development of microbial buildups on platform margins and laminites in the basin centre. Stromatoporoid reefs only survived along the windward, eastern margin for a short time. The architectures of the basin margins varied from aggradation (or slightly backstepping) in windward positions (eastern and northern margins) to moderate progradation in leeward positions. A relay ramp was present in the north‐west corner between the northern oblique fault zone and the proximal part of the western master fault. In the latest Givetian (corresponding to the top of Sequence 5), a sudden subsidence of the basin induced by further offset of the strike‐slip faults was accompanied by the rapid uplift of surrounding carbonate platforms, causing considerable platform‐margin collapse, slope erosion, basin deepening and the demise of the microbialites. Afterwards, stromatoporoid reefs were only locally restored on topographic highs along the windward margin. However, a subsequent, more intense basin subsidence in the early Frasnian (top of Sequence 6), which was accompanied by a further sharp uplift of platforms, caused more profound slope erosion and platform backstepping. Poor circulation and oxygen‐depleted waters in the now much deeper basin centre led to the deposition of chert, with silica supplied by hydrothermal fluids through deep‐seated faults. Two ‘subdeeps’ were diagonally arranged in the distal parts of the master faults, and the relay ramp was destroyed. At this time, all basin margins except the western one evolved into erosional types with gullies through which granular platform sediments were transported by gravity flows to the basin. This situation persisted into the latest Frasnian. This case history shows that the carbonate platform architecture and evolution in a pull‐apart basin were not only strongly controlled by the tectonic activity, but also influenced by the oceanographic setting (i.e. windward vs. leeward) and environmental factors.     

Transform-normal extension on the Northern Death Valley fault system, California-Nevada

The Northern Death Valley fault zone is a major right-lateral structure that has accommodated 70 km or more of regional transtensional deformation in Tertiary to Recent time. Extension parallel to its north-west transport direction in the Death Valley region of California has produced ‘pull-apart’ structures that are responsible for opening the central Death Valley rhombochasm. In several ranges along the length of the Northern Death Valley fault zone, there is also evidence for extension directed to the south-west, normal to strike-slip movement. Evidence from the Funeral, Grapevine and Cottonwood Mountains suggests that a significant amount of down-dip slip has occurred on the Northern Death Valley fault zone and parallel structures (together referred to as the Northern Death Valley fault system) coeval with the majority of right-lateral slip and transform-parallel extension. As a result of both these components of extension, a separate basin opened in northern Death Valley with an orientation and architecture very different from that of central Death Valley. In addition, the Northern Death Valley fault system may be responsible for the present topography of the Funeral and Grapevine Mountains. Transform-normal extension appears to be the result of a misorientation of the Northern Death Valley fault zone within the regional stress field over the past 6 Myr, as suggested by simple geometric calculations.     

A major seismogenic fault in a 'silent area': the Castrovillari fault (southern Apennines, Italy)

Large historical earthquakes in Italy define a prominent gap in the Pollino region of the southern Apennines. Geomorphic and palaeoseismological investigations in this region show that the Castrovillari fault (CF) is a major seismogenic source that could potentially fill the southern part of this gap. The surface expression of the CF is a complex, 10–13 km long set of prominent scarps. Trenches across one scarp indicate that at least four surface-faulting earthquakes have occurred along the CF since Late Pleistocene time, each producing at least 1 m of vertical displacement. The length of the fault and the slip per event suggest M =6.5-7.0 for the palaeoearthquakes. Preliminary radiocarbon dating coupled with historical considerations imply that the most recent of these earthquakes occurred between 380 BC and 1200 AD, and probably soon after 760 AD; no evidence for this event has been found in the historical record. We estimate a minimum recurrence interval of 1170 years and a vertical slip rate of 0.2-0.5 mm yr^-1 for the CF, which indicates that the seismic behaviour of this fault is comparable to other major seismogenic faults of the central-southern Apennines. The lack of mention or the mislocation of the most recent event in the historical seismic memory of the Pollino region clearly shows that even in Italy, which has one of the longest historical records of seismicity, a seismic hazard assessment based solely on the historical record may not be completely reliable, and shows that geological investigations are critical for filling possible information gaps.     

Characterization of active fault scarps from LiDAR data: a case study from Central Apennines (Italy)

A high-resolution digital elevation model (DEM, 1 ms spacing) derived from an airborne light detection and ranging (LiDAR) campaign was used in an attempt to characterize the structural and erosive elements of the geometry of the Pettino fault, a seismogenic normal fault in Central Apennines (Italy). Four 90- to 280 m-long fault scarp segments were selected and the surface between the base and the top of the scarps was analyzed through the statistical analysis of the following DEM-derived parameters: altitude, height of the fault scarp, and distance along strike, slope, and aspect. The results identify slopes of up to 40° in faults lower reaches interpreted as fresh faces, 34° up the faces. The Pettino fault maximum long-term slip rate (0.6–1.1 mm/yr) was estimated from the scarp heights, which are up to 12–19 m in the selected four segments, and the age (ca. 18 ka) of the last glacial erosional phase in the area. The combined analysis of the DEM-derived parameters allows us to (a) define aspects of three-dimensional scarp geometry, (b) decipher its geomorphological significance, and (c) estimate the long-term slip rate.     

Architecture and evolution of syn-rift clastic depositional systems towards the tip of a major fault segment, Suez Rift, Egypt

ABSTRACT This paper investigates syn‐rift stratigraphic architecture and facies relationships along a 7 km long strike section towards the tip of a major, basin‐bounding normal fault segment (Thal Fault) in the Suez Rift, Egypt. In this location, the fault is composed of two precursor fault strands, Gushea and Abu Ideimat, linked by a jog or transfer fault. We document a Miocene syn‐rift succession, deposited more than c. 5.5 Myr after rift initiation, that is composed of a range of carbonate‐clastic facies associated with coarse‐grained deltaic, shoreface and offshore depositional systems. Key regionally correlatable stratal surfaces within this succession define time equivalent stratal units that exhibit variability in thickness and architecture, related to the interplay of both regional and local controls, in particular, the evolution of two, small‐scale (<6 km long) precursor fault strands (Gushea and Abu Ideimat). Integration of structural and stratigraphic data indicates that the boundary (relay ramp) between these two fault strands was a relative high during much of the rift event, with hard‐linkage and considerable displacement accumulation not occurring until at least c. 7.5 Myr after rift initiation. This is because: (i) the preserved stratigraphy is thinner in the hanging wall of the strand boundary; (ii) a eustatic sea‐level fall with an amplitude of 100 m generated more than 25 m of incision at the strand boundary, a region that has a final fault displacement of c. 600 m; and (iii) the fault strand boundary persisted as a footwall low and transport pathway for coarse‐grained deltas entering the basin. This study indicates that variability in stratal thickness and stratigraphic architecture towards the tip of the Thal Fault was related to the linkage history of two small‐scale (c. 6 km long) precursor fault segments. We suggest that similar, small‐scale stratal variability may occur repeatedly along the entire length of major basin‐bounding fault segments due to the process of fault growth by the linkage of smaller scale precursor strands.     

Quaternary alluvial-fan development, climate and morphologic dating of fault scarps in Laguna Salada, Baja California, Mexico

Late Quaternary slip across the Cañada David detachment has produced an extensive array of Quaternary scarps cutting alluvial-fans along nearly the entire length (~ 60 km) of the range-bounding detachment. Eight regional alluvial-fan surfaces (Q₁ [youngest] to Q₈ [oldest]) are defined and mapped along the entire Sierra el Mayor range-front. Terrestrial cosmogenic nuclide ¹⁰Be concentrations from individual boulders on alluvial-fan surfaces Q₄ and Q₇ yield surface exposure ages of 15.5 ± 2.2 ka and 204 ± 11 ka, respectively. Formation of the fans is probably tectonic, but their evolution is strongly moderated by climate, with surfaces developing as the hydrological conditions have changed in response to climate change on Milankovitch timescales. Systematic mapping reveals that the fault scarp array along active range-bounding faults in Sierras Cucapa and El Mayor can be divided into individual rupture zones, based on cross-cutting relationships with alluvial-fans. Quantitative morphological ages of the Laguna Salada fault-scarps, derived from linear diffusive degradation modeling, are consistent with the age of the scarps based on cross-cutting relationships. The weighted means of the maximum mass diffusivity constant for all scarps with offsets < 4 m is 0.051 and 0.066 m²/ka for the infinite and finite-slope solutions of the diffusion equation, respectively. This estimate is approximately an order of magnitude smaller than the lowest diffusivity constants documented in other regions and it probably reflects the extreme aridity and other microclimatic conditions that characterize the eastern margin of Laguna Salada.