Thrust geometries in unconsolidated Quaternary sediments and

Basin Research, 2003

In order to better understand the development of thrust fault‐related folds, a 3D forward numerical model has been developed to investigate the effects that lateral slip distribution and propagation rate have on the fold geometry of pre‐ and syn‐tectonic strata. We consider a fault‐propagation fold in which the fault propagates upwards from a basal decollement and along‐strike normal to transport direction. Over a 1 Ma runtime, the fault reaches a maximum length of 10 km and accumulates a maximum displacement of 1 km. Deformation ahead of the propagating fault tip is modelled using trishear kinematics while backlimb deformation is modelled using kink‐band migration.The applicability of two different lateral slip distributions, namely linear‐taper and block‐taper, are firstly tested using a constant lateral propagation rate. A block‐taper slip distribution replicates the geometry of natural fold‐thrusts better and is then used to test the sensitivity of thrust‐fold morphology to vari...

In fold-and-thrust belts that experienced both pre-orogenic and post-orogenic extension, it may be dicult to establish whether observed normal faults pre-dated, postdated , or were synchronous with thrusting. Geometrical structural patterns may be insucient to constrain the relative chronology of extensional and contractional deformations. The systematic use of kinematic criteria makes it possible to unequivocally de®ne the timing relationships of reverse and normal fault development, and hence to correctly unravel complex structural evolutions. Kinematic analysis in the southernmost Umbria±Marche Apennines of Italy, where both normal and thrust faults are present, revealed a history of repeated tectonic inversion, characterised by two distinct stages of extension separated by an episode of folding and thrusting. Structural overprinting relationships observed at thrust±normal fault intersections were useful for: (i) removing sequentially younger deformations; and hence (ii) separating and quantifying the e€ects of orogenic contraction from those of both pre-orogenic and post-orogenic extension. #

Bulletin of the Seismological Society of America, 2003

Journal of Structural Geology, 1995

Relations between normal faults and pre-existing thrust faults are classically described in terms of three basic situations: normal faults can cross-cut thrust faults; they can branch out from thrust faults at depth on a ddcollement level, or they can entirely reactivate thrust planes. The mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the 'brittle' rocks and silicone putty an interlayered decollement. The models underwent compression, producing thrust faults with variable dips, followed by extension. Three possible ways of interaction are described here: (a) no interaction occurs in the case of low-dip thrust faults (<32' + 1") and normal faults are developed independently, displaying a listric geometry; (b) branching at depth on the decollement level occurs when dip of the thrust faults reaches 32" 5 1". In this case, the dip of the normal faults, whose geometry becomes planar, decreases with increasing thrust dip. We suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre-existing fault zone, where a drop in the friction coefficient is likely to occur; (c) reactivation occurs in brittle material when dip of the pre-existing fault exceeds 41" + 1".

Interpretation, 2015

The 3D kinematic evolution of thrust systems, in which vergence changes along strike, is poorly understood. This study uses 3D seismic data from Big Piney-LaBarge field, Wyoming, to examine the geometry and kinematics of two faults at the leading edge of the Hogsback thrust sheet, the frontal thrust of the Late Cretaceous Sevier fold-thrust belt. These thrusts lie along strike of each another and share an east-vergent detachment within the Cretaceous Baxter Shale. The two thrusts verge in opposite directions: The southern thrust verges eastward forming a frontal ramp consistent with major thrusts of the Sevier belt, whereas the northern thrust verges westward to form a type 1 triangle zone with the Hogsback thrust. The thrusts have strike lengths of 5 km (3.1 mi) and 8 km (5.0 mi), respectively, and they are separated by a transfer zone of less than 0.5 km (0.3 mi) wide. Strata in the transfer zone appear to be relatively undeformed, but reflections are less coherent here, which sug...

2001

We mapped and analyzed two vertical exposures-exposed on the walls of a 3-to 5-m-deep, 70-m-long excavation and a smaller 3-m-deep, 10-m-long excavation -across the 1999 rupture of the Chelungpu fault. The primary exposure revealed a broad anticlinal fold with a 2.5-m-high west-facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture requires initial failure on the basal, east-dipping thrust plane, followed by wedge thrusting and pop-up of an overlying symmetrical anticline between two opposing secondary thrust faults. Net vertical offset is about 2.2 m across the principal fault zone. From line-length changes, we estimate about 3.3 m of horizontal shortening normal to fault strike. The ratio of these values yields a total slip of 4.0 m and an estimate of about 34° for the dip of the fault plane below the excavation. This value is nearly the same as the 35° average dip of the fault plane from the surface to the hypocenter. Restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event. A buried 30-m-wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site. The prior event is also recorded in the smaller excavation, which is located 40 m south and is oriented parallel to the larger excavation. Radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event. Available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 14 C yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret the previous event as the penultimate event relative to the 1999 Chi-Chi earthquake. We estimated the long-term slip rate of the Chelungpu fault to be 10-15 mm/yr during the last 1 Ma, based on previously published retrodeformable cross sections. This rate is, however, significantly higher than geodetic rates of shortening across the Chelungpu thrust where two pairs of permanent Global Positioning System stations suggest 7-10 mm/yr of shortening across the fault. Given the 4 meters of average slip, the long-term slip rate yields an interseismic interval of between 267 and 400 yr for the Chelungpu fault.

The mountain Geologist, 2022

This study integrates the results of numerical modeling analyses based on outcrop studies and structural kinematic restorations to evaluate the mechanics of thrust fault initiation and development in mechanically layered sedimentary rocks. A field-based reconstruction of a mesoscopic thrust fault at Ketobe Knob in central Utah provides evidence of thrust ramp nucleation in competent units, and fault propagation upward and downward into weaker units at both fault tips. We investigate the effects of mechanical stratigraphy on stress heterogeneity, rupture direction, fold formation, and fault geometry motivated by the geometry of the Ketobe Knob thrust fault in central Utah; the finite element modeling examines how mechanical stratigraphy, load conditions, and fault configurations influence temporal and spatial variation in stress and strain. Our modeling focuses on the predicted deformation and stress distributions in four model domains: (1) an intact, mechanically stratified rock sequence, (2) a mechanically stratified section with a range of interlayer frictional strengths, and two faulted models, (3) one with a stress loading condition, and (4) one with a displacement loading condition. The models show that early stress increase in competent rock layers are accompanied by low stresses in the weaker rocks. The frictional models reveal that the heterogeneous stress variations increase contact frictional strength. Faulted models with a 20° dipping fault in the most competent unit result in stress increases above and below fault tips, with extremely high stresses predicted in a 'back thrust' location at the lower fault tip. These findings support the hypothesis that thrust faults and associated folds at the Ketobe Knob developed in accordance with a ramp-first kinematic model and development of structures was significantly influenced by the nature of the mechanical stratigraphy. * σ1 is horizontal † σ3 is vertical (downward) Fault µ = 0.85

Journal of Structural Geology, 2011

Stress inversions are a useful and popular tool for structural geologist and seismologist alike. These methods were first introduced by Wallace (1951) and Bott (1959) and subsequent studies continue to be based on their assumptions: the remote stress tensor is spatially uniform for the rock mass containing the faults and temporally constant over the history of faulting in that region, and the slip on each fault surface has the same direction and sense as the maximum shear stress resolved on that surface from the remote stress tensor. Furthermore, successful implementation requires that slip accumulates on faults of diverse orientation. Many studies employ these methods on isolated faults or on fault systems with limited ranges of orientations, which can lead to erroneous results. We propose a new method that incorporates the effects of mechanical interaction of the entire fault or fault system, and solves the complete mechanical problem rather than employing empirical relationships between slip and stress or strain (or strain rate). The method requires knowledge of the fault geometry and information on at least one slip vector component along portions of the known fault geometry. For example, if throw is known, the strike-slip component can be solved for. We test the method using a single synthetic fault with anisotropic roughness similar to that measured at fault outcrops. While the orientation of remote stress may be determined precisely, the lack of diverse fault orientations introduces a systematic error in the remote stress ratio. We further test the effect of diversity of fault orientations and find that Wallace–Bott type inversions do not perform as well for limited ranges of orientations when compared to the proposed method. Finally, we use published data from the 1999 Chi-Chi, Taiwan, earthquake, and find that the method using surface data only, and surface data with subsurface focal mechanisms, produce similar results. The resulting stress orientations are in good agreement with results from Wallace–Bott inversions. Furthermore, the slip distribution is in general agreement with kinematic slip inversions using coseismic surface deformation. Stress inversion methods using fault slip data can thus be improved upon, significantly in some cases, by solving a mechanical boundary value problem that takes into account the geometry of faults or fault systems. As a bonus, the solution provides the stress, strain, and displacement fields throughout the region and the slip distributions on the faults.► New method for stress inversion incorporates the effects of interaction of faults. ► Errors in tests cases are smaller than those of faultless (Wallace-Bott) inversions. ► Using slip data from Chelungpu fault reproduces slip distribution well. ► New method provides a useful alternative to faultless inversion methods.

Analyscs of the San Andreas fault,Neo Valley fault system and many active faults in Japan show that infl・ cqucnt, 1」 gh magnitudc carthquakcs arc generated in a bcnd or gap of a fault systcrll of a rcversc step-like pattern characterizcd by comprcssional bends and ttcas of upHftrnent.On the other hand, more frequent,lower magnitudc earthq uakcs are commonly gcneratcd in the cxtcnsional zonc of st五 kc-slip faults of a normal step-likc pattern charactcrized by cn cchclon tcnsion fracturcs,areas of subsidence,cxtensional bcnds or basins,which occur in regions of activc rifting, high volcanicity, and geotherlnal activity. Exact distribution of strcss duc to五 ght or left lateral strike-slip move- ment of a linear or pcnny-shapcd ねult is obtained by an analyticJ solution of threc dilnensional theory of elasticity.The rcsults indicate the development of comprcssion and extension on either side ofthe fault depending on light or left laterJ movement producing upliftment in the compressional zone and subsidcnce in the cxtensional areas.En cchelon arrangements of shcar and tension fractures or fault segments in the maor fault zonc inducing reverse steplike and normal step-like fault :胤鮒 岬 h∬c:響糧 総 i絲 :棚島 盟 肥 胤 reverse stcp… like pattern is rnostlikely to develop in a fault system under moderate effective pressure in rocks in transition between b五 ttlc and ductile failurc.The norrrlal stcp-like pattern is,howevcr,favored under 10w crectivc(or high intcrstitial nuid)prcssure in rocks showing bttttle failure.

Journal of Geophysical Research, 2001

We investigate fault growth in Afar, where normal fault systems are known to be currently growing fast and most are propagating to the northwest. Using digital elevation models, we have examined the cumulative slip distribution along 255 faults with lengths ranging from 0.3 to 60 km. Faults exhibiting the elliptical or "bell-shaped" slip profiles predicted by simple linear elastic fracture mechanics or elastic-plastic theories are rare. Most slip profiles are roughly linear for more than half of their length, with overall slopes always <0.035. For the dominant population of NW striking faults and fault systems longer than 2 kin, the slip profiles are asymmetric, with slip being maximum near the eastern ends of the profiles where it drops abruptly to zero, whereas slip decreases roughly linearly and tapers in the direction of overall Aden rift propagation. At a more detailed level, most faults appear to be composed of distinct, shorter subfaults or segments, whose slip profiles, while different from one to the next, combine to produce the roughly linear overall slip decrease along the entire fault. On a larger scale, faults cluster into kinematically coupled systems, along which the slip on any scale individual fault or fault system complements that of its neighbors, so that the total slip of the whole system is roughly linearly related to its length, with an average slope again <0.035. We discuss the origin of these quasilinear, asymmetric profiles in terms of "initiation points" where slip starts, and "barriers" where fault propagation is arrested. In the absence of a barrier, slip apparently extends with a roughly linear profile, tapered in the direction of fault propagation. [Davies, 1879, and references therein]. In the last 20 years, interest has expanded, with structural geologists, hydrologists, seismologists, and those who study rock mechanics or make continuum mechanics calculations all participating. Behind this recrudescence has been mainly the need to understand the structure of oil fields in better detail as well as the fracture systems that may control fluid flow in the crust. Understanding fault and fracture systems is also basic to understanding the mechanical behavior of the crust. As a result, a variety of scenarios and models have been proposed to describe fault growth processes [e.g., Walsh and Watterson, 1988; Cowie and $cholz, 1992a, 1992b; Gillespie et al., 1992; $cholz et al., 1993; Biirgmann et al., 1994; Cartwright et al., 1995; Cowie and Shipton, 1998]. Most of them predict that isolated faults grow in such a way that slip is maximum at the fault center and decreases to zero at the fault ends, following either an elliptical (simple linear elastic theory [e.g., Pollard and Segall, 1987]), a "bell-shaped" (elastic-plastic theory [e.g., Cowie and Scholz, 1992a]), or a more complexshaped slip-length profile [e.g., Biirgmann et al., 1994]. The available measurements of lengths and offsets of faults, however, do not totally support any of these models. This may partly be due to two main reasons. First, most measurements were performed on dormant faults, and the slip distribution does not necessarily represent that when these faults were actively 13,667 13,668 MANIGHETTI ET AL.: GROWTH AND PROPAGATION OF FAULTS IN AFAR a Unstretched continental crust Propagating ridges and rifts Ridge jump Coast 11ø40 b [• Water Volcanoes Coast "Asal fault set" 5km "Ghoubbet fault set" 42ø30 and Watterson, 1988; Marrett and Allmendinger, 1990; Gillespie et al., 1992; Cowie and Scholz, 1992b; Dawers et al., 1993; Cartwright et al., 1995; Schlische et al., 1996]. As a result, the question of how cumulative slip distribution along faults and fault systems is related to their time-space growth and propagation history remains open. Our goal is to address such a question by analyzing vertical slip distribution on 255 of the thousands of active, normal faults that dissect the Quaternary, basaltic surface of the Afar depression (Figure la). Afar is well suited to such studies. The region is arid and the rates of deformation are high, so that the effects of erosion and deposition are generally not important and vertical slip can be measured using offset landsurfaces. The large-scale kinematic evolution of Afar is also well established so that, unlike other areas, the deformation boundary conditions are reasonably well known. On the basis of geological, geometrical, geomorphological, and seismological evidence, together with timing derived from radiometric dating and magnetic reversals, it has been demonstrated that for the last 2 Myr, the rift zone containing most of the faults we study has been growing and propagating northwestward [Courtillot, 1980; Tapponnier et al.Audin et al., 2001] as a consequence of the Aden ridge penetrating into the depression (Figure la). The Afar faults, most of them are still active and fast slipping (a few millimeters per year), therefore offer the possibility of deciphering the way their cumulative slip distributions are related to their ongoing growth. 2. Data Sets The 255 faults studied offset the basalt surface of Afar, 215 on land and the others in shallow water (<200 m deep) (Figure 1). All form clear scarps in the topography or the bathymetry, with lengths between 0.3 and 60 km and maximum heights between 6 and 1300 m. On land, only minor erosion and deposition occur, so that the scarps provide well-preserved records of cumulative vertical slip. Owing to syntectonic deposition, underwater, this may not be the case. The scarps were mapped using high-resolution satellite and aerial images on land, and bathymetric and side-scan sonar images for the submarine part. From this body of data, the lengths, geometries, and kinematic relations between faults could be determined. Note that most of this work, in particular fault and fault system kinematic analysis, was done before the present study [e.g., Manighetti, 1993; Manighetti et al., 1997, 1998, 2001; Audin et al., 2001]. We also performed field work in the region to check critical features on the maps and slip profiles. When operating at a range of scales, a term such as "fault" must be treated with caution. For this article, we define an "individual fault" as exhibiting a unique, continuous trace in map view (at a scale > 1/4500), and a "fault system" as a group of individual faults being kinematically coupled to produce one of the peculiar overall geometries recognized at any scale in Afar (such as en •chelon fault systems; horsetail, fish tail or cock's tail-ended fault systems; propagating tip-shaped fault systems, etc. [see Manighetti, 1993; Manighetti et al., 1997, 1998, 2001]). From three high-resolution digital elevation models (DEMs), we extracted elevation profiles following the top and base of each fault scarp. From these profiles we calculated the variation of vertical relative displacement as a function of distance along fault strike. Most scarps are steeper than 70 ø at the surface so that the measured slips are nearly the same as the downdip fault throws (see discussion by De Chabalier and Avouac [1994]). We have analyzed 134 faults in the Asal-Ghoubbet rift (AG, Figures la and lb), 94 on land ("Asal faults") and 40 associated with the continuation of the rift underwater ("Ghoubbet faults"). The Asal-Ghoubbet rift, located at the southwestern termination of the propagating Aden ridge, is one of the most recent and active rift segments of the plate boundary. The site of localized active faulting and volcanism, it accommodates -• 85-90% of the total extension imposed by plate driving forces (total extension rate -• 1.8 + 0.2 cm/yr [e.g., De Chabalier and Avouac, 1994; Manighetti et al., 1998; Chu and Gordon, 1998]). In the last -• 100 kyr, volcanic activity has almost stopped in the rift, while faulting has continued. As a result, most of the faults offset volcanic surfaces (<100 kyr old), with their hanging walls and footwalls being free of any significant subsequent volcanic deposits. The Asal-Ghoubbet rift faults are known to have repeatedly ruptured seismically in the recent times (last major earthquake sequence in 1978), to be young (<100 ka), fast slipping (1-4 mm/yr) [e.g., Ruegg and Kasser, 1987; Stein et al., 1991; De Chaballet and Avouac, 1994] and to participate in the overall northwestward propagation of the rift [Manighetti et al., 1998, Audin et al., 2001] (Figure la). On land, the elevation profiles were extracted from a DEM covering almost the entire land portion of the rift (with a grid of points spaced by -• 15 m, and elevation accuracy of 4-1 m [e.g., De Chabalier and A vouac, 1994]). Taking into account the roughness (<2 m) of the basalt surfaces offset by the faults, we estimate that the throws are measured with a 4-3 m uncertainty. Fault traces were identified from aerial photographs (1:4,500 to 1:20,000 scales), so that fault trace lengths are underestimated by 20 rn at the most. We analyzed the submarine