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Hanging-wall deformation at the active Sierra Palomera extensional fault (Jiloca basin, Spain) from structural, morphotectonic, geophysical and trench study
Abstract
The NNW-SSE trending Sierra Palomera fault is characterized as an active, nearly pure extensional fault with mean transport direction towards N230°E, consistent with the ENE-WSW extension trajectories of the recent to present-day regional stress field. Its macrostructure is described from surface geology and magnetometric and electromagnetic surveys, which have allowed identifying two subsidiary, nearly parallel normal faults (antithetic and synthetic, respectively). The structural contour map of an extensive planation surface, dated to 3.8 Ma, provides a maximum fault throw s.s. of 330 m for the main fault (480 m including bending), and a net slip rate of 0.09 mm/a (0.13 mm/a including bending). Trench study focussed on the subsidiary antithetic fault shows evidence of its activity during Middle-Late Pleistocene times, offsetting ca. 2.5 m the slope of a well-preserved alluvial fan. Detailed analysis and retrodeformation of the antithetic fault and other minor ruptures in the trench has allowed defining seven deformation events. The lack of a consistent age model for the involved sedimentary sequence makes them almost meaningless in terms of paleoseismic history. However, geometry and sequential development of meso-scale faults (intermediate between seismic-scale and analogue models) allows unravelling the extensional deformation history within the hanging-wall block of the Sierra Palomera fault. Progressive rupture patterns reveal shifting from dominantly synthetic to dominantly antithetic faulting, suggesting both kinematical control linked to rollover growth, and dynamical control by the regional stress field.
... Although none of these faults show outcropping rupture surfaces with slickenlines, we will assume for them a nearly pure dip-slip normal movement based on (i) the regional persistence of this type of fault kinematics (e.g., Simón, 1983Simón, , 1989Simón et al., 2012Simón et al., , 2022Lafuente et al., 2014;Liesa et al., 2019;Peiro et al., 2022), and (ii) the focal mechanisms available in the surrounding area (see Fig. 3c), as we will explain in the Section 6. ...
... Epicentral symbols are colour coded with depth, and sized according to their magnitude. The active faults mapped are those included in the QAFI database (García-Mayordomo et al., 2012), together with others described in detail in more recent publications (Simón et al., 2016Ezquerro et al., 2020;Peiro et al., 2022) and those newly mapped in the present work. (b) Synthetic projection of all hypocentres onto a vertical E-W plane. ...
... The nodal planes strike NNW-SSE to N-S, the fault plane being probably represented by that with steeper dip (N-S striking in the case of the 2011 seism, north of Fig. 3c; NNW-SSE in the other cases). All focal solutions correspond to nearly pure dip-slip normal faults, which is consistent with the kinematics inferred for surface faults throughout the region (e.g., Simón, 1983Simón, , 1989Simón et al., 2012Simón et al., , 2022Lafuente et al., 2014;Liesa et al., 2019;Peiro et al., 2022). The ensemble of focal mechanisms is fully compatible with the regional extensional stress field (σ 1 vertical, σ 3 trending WSW-ENE), evidenced for both recent geological times (Simón, 1983(Simón, , 1989Arlegui et al., 2005;Liesa et al., 2019;Simón et al., 2022) and the present day (Herraiz et al., 2000). ...
... Such is the case of the Iberian Chain, in the eastern Iberian Peninsula (Fig. 1a, b), a region that exhibits low to moderate historical and instrumental seismicity, but contains numerous slow active normal faults. Among the latter, those located at the junction of the Teruel and Jiloca grabens (Sierra Palomera, Concud, Teruel and Valdecebro faults) have been the focus of a number of detailed palaeoseismological studies during the last decade, which have demonstrated their Pleistocene activity and seismogenic potential (Lafuente et al. 2011(Lafuente et al. , 2014Simón et al. 2012Simón et al. , 2016Simón et al. , 2017Ezquerro et al. 2015Ezquerro et al. , 2016Peiro et al. 2020Peiro et al. , 2022. ...
... The Sierra Palomera fault is a 15.5-km-long, NNW-SSE striking normal fault with a nearly pure normal sense of slip. Its activity since Late Pliocene times is mainly revealed by the tilting and offset of regional planation surfaces, which allows the estimation of a throw in the range of 350-400 m García-Lacosta et al. 2014;Peiro et al. 2022). A subsidiary antithetic fault, induced by rollover bending associated with the Sierra Palomera fault, shows evidence of recurrent activity during Late Pleistocene times (Peiro et al. 2022). ...
... Its activity since Late Pliocene times is mainly revealed by the tilting and offset of regional planation surfaces, which allows the estimation of a throw in the range of 350-400 m García-Lacosta et al. 2014;Peiro et al. 2022). A subsidiary antithetic fault, induced by rollover bending associated with the Sierra Palomera fault, shows evidence of recurrent activity during Late Pleistocene times (Peiro et al. 2022). ...
The Calamocha fault is an 18-km-long, NNW–SSE striking pure normal fault that moves down the northern sector of the Jiloca graben with respect to the Neogene infill of the Calatayud basin (central Iberian Chain). Its structure and kinematics are characterized by means of detailed geological mapping, morphotectonic analysis and data recording at the outcrop scale. The Calamocha fault represents the inversion of a previous contractional fault zone under the recent tensional stress field (WSW–ENE trending σ 3 trajectories). The extensional activity started during the Late Pliocene (ca. 3.8 Ma), accumulating a maximum net slip of 190–230 m (long-term slip rate of 0.05–0.06 mm/a). The palaeoseismological study of three artificial exposures near Calamocha town evidenced recurrent slip during the Late Pleistocene, which proves its active character. Analysis of faulted clastic alluvial units, dated by means of optically stimulated luminescence (OSL), reveals at least eight slip events since 145.9 ± 9.1 ka, the last one being younger than 13.8 ± 0.9 ka. Only a few events represent visible accumulated displacement on the main synthetic rupture surfaces; this allows a rough estimate of the short-term slip rate (during the Late Pleistocene) of about 0.1 mm/a, faster than the long-term rate. The Calamocha fault could potentially produce a characteristic earthquake (in the sense of Schwartz and Coppersmith, J Geophys Res 89:5681–5698, 1984) with moment magnitude M w ≈ 6.7 ± 0.3 ( M w ≈ 6.9 ± 0.3 in a scenario of activation of the whole Calamocha–Daroca fault zone), average coseismic displacement of 0.5–1.3 m and average recurrence period under 15 ka.
The NNW–SSE-trending extensional Río Grío–Pancrudo Fault Zone is a large-scale structure that obliquely cuts the Neogene NW–SE Calatayud Basin. Its negative inversion during the Neogene–Quaternary extension gave rise to structural and geomorphological rearrangement of the basin margin. Geological mapping has allowed two right-relayed fault segments to be distinguished, whose recent extensional activity has been mainly characterized using a deformed planation surface (Fundamental Erosion Surface (FES) 3; 3.5 Ma) as a geomorphic marker. Normal slip along the Río Grío–Lanzuela Fault Segment has induced hanging-wall tilting, subsequent drainage reversal at the Güeimil valley after the Pliocene–Pleistocene transition, as well as morphological scarps and surficial ruptures in Pleistocene materials. In this sector, an offset of FES3 indicates a total throw of c. 240 m, resulting in a slip rate of 0.07 mm a –1 , while retrodeformation of hanging-wall tilting affecting a younger piedmont surface allows the calculation of a minimum throw in the range of 140–220 m after the Pliocene–Pleistocene transition, with a minimum slip rate of 0.07–0.11 mm a –1 . For the late Pleistocene period, vertical displacement of c. 20 m of a sedimentary level dated to 66.6 ± 6.5 ka yields a slip rate approaching 0.30–0.36 mm a –1 . At the Cucalón–Pancrudo Fault Segment, the offset of FES3 allows the calculation of a maximum vertical slip of 300 m for the last 3.5 Ma, and hence a net slip rate close to 0.09 mm a –1 . Totalling c. 88 km in length, the Río Grío–Pancrudo Fault Zone could be the largest recent macrostructure in the Iberian Chain, probably active, with the corresponding undeniable seismogenic potential.
Normal faults have irregular geometries on a range of scales arising from different processes including refraction and segmentation. A fault with constant dip and displacement on a large-scale will have irregular geometries on smaller scales, the presence of which will generate fault-related folds and down-fault variations in throw. A quantitative model is presented which illustrates the deformation arising from movement on irregular fault surfaces, with fault-bend folding generating geometries reminiscent of normal and reverse drag. Calculations based on the model highlight how fault throws are partitioned between continuous (i.e. folding) and discontinuous (i.e. discrete offset) strain along fault bends for the full range of possible fault dip changes. These calculations illustrate the potential significance of strain partitioning on measured fault throw and the potential errors that will arise if account is not taken of the continuous strains accommodated by folding and bed rotations. We show that fault throw can be subject to errors of up to ca. 50 % for realistic down-dip fault bend geometries (up to ca. 40∘), on otherwise sub-planar faults with constant displacement. This effect will provide irregular variations in throw and bed geometries that must be accounted for in associated kinematic interpretations.
Abstract The northern part of the eastern margin of the extensional Neogene Teruel Basin (central-eastern Spain) consists of a non-linear, zigzag fault zone made of alternating ca. 2 km long, NNW-SSE trending segments and shorter NNE-SSW ones. Good outcrop conditions made possible a comprehensive integrated stratigraphic and structural study, especially focused on coarse clastic sediments deposited along the basin margin. Well-exposed stratal relationships with boundary faults, allowed the analysis of tectonic influence on sedimentation. Synsedimentary deformation includes growth faulting, rollover anticlines, and monoclines and associated onlap stratal terminations, angular unconformities, and other complex growth strata geometries. One of them is the onlap-over-rollover bed arrangement described here for the first time, which reveals the competition between tectonic subsidence and sedimentary supply. Both, the structural inheritance (dense Mesozoic fracture grid) and the dominant, nearly ‘multidirectional’ (σ1 vertical, σ2 ≈ σ3), Pliocene extensional regime with σ3 close to E-W, are considered to have controlled the margin structure and evolution. Tectono-stratigraphic evolution includes: (i) reactivation of inherited NNW-SSE faults and development of W-SW-directed small alluvial fans (SAF) while NNE-SSW segments acted as gentle relay ramp zones; (ii) progressive activation of NNE-SSW faults and development of NW-directed very small alluvial fans (VSAF); during stages i and ii sediments were trapped close to the margin, avoiding widespread progradation; (iii) linking of NNW-SSE and NNE-SSW structural segments, overall basin sinking and widespread alluvial progradation; (iv) fault activity attenuation and alluvial retrogradation. The particular structure and kinematic evolution of this margin controlled alluvial system patterns. Size of alluvial fans, directly set up at the border faults, was conditioned by the narrowness of the margin, small catchment areas, and proximity between faults, which prevented the development of large alluvial fans. The size of the relay zones, only a few hundred meters wide, acted in the same way, avoiding them to act as large sediment transfer areas and large alluvial fans to be established. These features make the Teruel Basin margin different to widely described extensional margins models.
El margen oriental de la fosa del Jiloca (Cordillera Ibérica) es el resultado de la disposición escalonada diestra de tres fallas normales de dirección NW-SE: las fallas de Calamocha, Sierra Palomera y Concud. Junto con la falla de Teruel, delimitan tres zonas de relevo con evidencias de fracturación reciente. El análisis de la deformación a escala cartográfica y de afloramiento, y su comparación con la fracturación desarrollada en varios modelos analógicos, ha permitido inferir su relación geométrica, cinemática y dinámica con las macroestructuras y con el campo de esfuerzos de extensión radial o “multidireccional” activo en la región de estudio. En los modelos analógicos se han estudiado las variaciones en el patrón de fracturación resultante, en función de las velocidades de extensión en dos direcciones ortogonales y de la orientación de las fallas principales respecto a dichas direcciones de extensión. Tanto en el área de estudio como en los modelos, la fracturación es longitudinal a las macroestructuras que delimitan los relevos. Está controlada por la dirección de las fallas y, en mayor medida, por la dirección de extensión ENE-WSW dominante en el campo de esfuerzos regional. Todo ello permite definir un nuevo tipo de interacción en zonas de relevo de falla, denominada en este trabajo relevo con fracturación longitudinal distribuida.
The area of Alfambra (NE Spain) is very representative of the relief evolution of the Eastern Iberian ranges. The geomorphological cartography (Main Map) includes the northern sector of the Alfambra-Teruel Neogene depression and its mountainous surroundings. The cartographic process started by using aerial photographs (1:30000 scale) and fieldwork. The structural reliefs, erosion surfaces, karst landforms, Quaternary pediments and terraces, and fluvial network are represented over a lithological background. The main cartography is complemented with two maps of lithological and geomorphological units. The obtained information enabled the main geological and geomorphological stages of the regional relief to be established.
The Iberian Chain is an intraplate mountain range, oblique to the northern (Pyrenean) and southern (Betic) active margins of the Iberian Plate. It developed by positive inversion of the extensional Mesozoic Iberian basin, caused by compression both normal (NNE to NE) and parallel (SE to SSE) to its boundaries. The main, NNE to NE compression (Middle Eocene to Late Oligocene in age) was responsible for the principal folds and thrusts. The SE to SSE compression was mainly active during the Early Miocene, as the convergence between Europe and Africa was transferred from the Pyrenean to the Betic margin of Iberia, though it formed few new macrostructures. This work mainly reviews and examines the inversion structures associated with Cretaceous normal faults that determined the development of four paleogeographical domains into the Iberian Basin, each one characterized by particular structural and sedimentary features, and also divided into subbasins. Examples are mainly related to inversion of the two main Cretaceous basins (Cameros and Maestrazgo), showing different Cenozoic compressional evolution. Associated with basin inversion, the development of coeval Cenozoic continental basins (e.g., Ebro, Montalbán, Aliaga) was produced, their tectonosedimentary relationships allowing to know the kinematics of some inversion structures. Other inversion examples related to Permian and Triassic extensional structures are also reported. The great variety of cases studied along the Iberian Chain allows us to propose a classification of inversion structures that can be extrapolated to other intraplate areas. In addition, this variety of examples has permitted to analyse some of the main factors controlling tectonic inversion and the resulting compressional structures, such as the location (depth) of the main (basal) detachment, usually in Paleozoic basement rocks, the presence of intermediate detachement levels within the cover (mainly Middle and Upper Triassic gypsum and lutites), or the direction and geometry at depth of the previous extensional faults. Based on changes in geometry of the main inversion structures along the chain and its surroundings, thick-skinned, basement-involved thin-skinned, and thin-skinned inversion tectonics have been distinguished. They usually vary from central to marginal parts of major extensional basins that contain an intermediate detachment level within the cover.
A new trench excavated at the southern sector of the Concud Fault provided evidence of three palaeoseismic events dated to ca. 21, 18 and 13–3 ka BP, respectively. The two youngest ones had not been detected in previous studies. The results extend the total recorded palaeoseismic succession for the fault: eleven events since ca. 74 ka BP to the present day, with an average recurrence period between 7.1 ± 3.5 and 8.0 ± 3.3 ka; total net accumulated slip of about 20.5 m, with average coseismic slip of 1.9 m. The displacement pattern shows alternating periods of fast slip (up to 0.53 mm/a) and slow slip (0.13 mm/a), resulting in average slip rate of 0.29 mm/a. Using this palaeoseismic information, as well as the potential magnitude previously attributed to the characteristic earthquake at the Concud Fault (M ≈ 6.5–6.6), a simple probabilistic seismic hazard analysis has been performed. The estimated probability of occurrence of the characteristic earthquake within the next 500-year period ranges from 2.3 to 26.1 %, according to distinct hypotheses on the elapsed time derived from the uncertainty about the age of the youngest event.
Among the conspicuous extensional structures that accommodate the onshore deformation of the Valencia Trough at the central-eastern Iberian Chain, a number of large faults show evidence of activity during Pleistocene times. At the eastern boundary of the Jiloca graben, the Concud fault has moved since mid Pliocene times at an average rate of 0.07-0.08 mm/y, while rates from 0.08 to 0.33 mm/y have been calculated using distinct stratigraphic markers of Middle to Late Pleistocene age. A total of nine paleoseisms associated to this fault have been identified between 74.5 and 15 ka BP, with interseismic periods ranging from 4 to 11 ka, estimated coseismic displacements from 0.6 to 2.7 m, and potential magnitudes close to 6.8. The other master faults of the Jiloca graben (Calamocha and Sierra Palomera faults) have also evidence of Pliocene to Late Pleistocene displacement, with average slip rates of 0.06 and 0.11-0.15 mm/y, respectively. At the eastern boundary of the Teruel graben, the Sierra del Pobo fault has been active since Late Miocene times, at slip rates of 0.06-0.11 mm/y. Quaternary activity its better constrained for the Teruel fault, which offsets two fluvial terraces, with an estimated slip rate of 0.12 mm/y since 76 ka BP. A widespread, NNE-SSW trending fault system extends over the easternmost Iberian Chain (Maestrat sector), with abundant proofs of activity during Early to Middle Pleistocene s.l. times. Nevertheless, such proofs are mainly geomorphologic, while dated stratigraphic markers allowing precise assessment of slip rates are absent.
The results of geomorphic analysis of the Concud fault-generated mountain front (central Iberian Chain, Spain) are introduced into classifications of fault activity proposed by previous authors, and compared with slip rates calculated from geologic markers. The Concud fault is an extensional structure active since the mid Pliocene times. It gives rise to a 60 to 120 m high mountain front, where footwall rocks belonging to the Triassic and Jurassic (north-western sector) and Miocene (south-eastern sector) crop out. Conspicuous triangular facets are preserved on Jurassic rocks of the central sector, while short, generally non-incised alluvial fans make the piedmont. The value of the Mountain-front sinuosity index is Smf = 1.24 for the whole mountain front (1.17 and 1.32, respectively, for both segments showing distinct footwall lithology), as obtained by the most conservative procedure. Average valley floor width/height ratios calculated for seventeen gullies crossing the fault are Vf = 0.30 (250 m upstream from the fault trace) and Vf = 0.22 (500 m upstream). These geomorphic indices, together with qualitative features of the escarpment and piedmont landscape, indicate moderate to rapid fault activity. The range of slip rates estimated from such morphotectonic classification (0.03 to 0.5 mm/y) encloses the range calculated from offset Late Pliocene and Pleistocene stratigraphic markers (0.07 to 0.33 mm/y). Nevertheless, the highest potential slip rate (0.5 mm/y) clearly represents an overestimate: the mountain front could give the impression of an anomalously high level of activity owing to episodic rejuvenation caused by base level drop.
The geomorphological analysis of regional Neogene erosion surfaces is of great geological interest as it helps in the reconstruction of the environmental evolution of continental areas. Moreover, the structural analysis of their deformation is a good approach to the identification of the main neotectonic trends at a regional scale. The Iberian Range is an excellent example of the application of this kind of study. It shows four stepped Neogene erosion surfaces, well preserved in general and whose ages range from the middle Miocene to the Villafranchian. Their formation is related to planation processes in semi-arid conditions. The stepping of surfaces is related to distensive Neogene tectonic pulses of sufficient magnitude for the planation processes to be interrupted.
Scaled analogue models of listric fault systems display a characteristic rollover anticline and crestal collapse graben features. Faults nucleate in sequence into the hangingwalls of the crestal collapse graben and towards the footwall of the master fault. Superposition of crestal collapse grabens produces complex fault arrays. When the basal detachment surface is tilted a characteristic panel of unfaulted synfault stratigraphy forms adjacent to the detachment. These experiments produce structures with close similarities to listric growth fault systems on prograding delta slopes. Synthetic seismic sections of the experimental models have been produced using Kirchhoff integral techniques. These illustrate the complexities in trying to image complex structures in listric fault systems but also show good agreement with natural seismic examples. -Authors
The region studied is located in the Iberian small-plate, between the Pyrenees and the Betic chain, south of the Ebro basin. The aim of this paper is to de scribe its Alpine compressive structures and to discuss its dynamics and evolution.
This region has a stockwerk structure with a lower level made up by the Hercynian base ment and the Permian and the lower Triassic rocks
and an upper level comprising the Jurassic, Cretaceous and Tertiary cover. Both levels are separated by a décollement within middle and upper Triassic beds; the décollement is, in fact, the sole thrust of the co ver at the roof of the basement.
The basement structure is determined by convergent wrench-faults (sinistral NE-SW faults in the Catalan coastal range and dextral NW-SE
faults in the Iberian range) and by E-W north-verging thrusts (Sierra de la Damanda). Cover structures are parallel to basement structures; there are imbricate-fan thrusts and folds striking NW-SE, NE-SW, E-W and N-S; there are also NE-SW sinistral and NW-SE dextral strike-slip fauIts. The sense of displacement of the cover is centrifuge, towards the Ebro basin in the north and towards the Tajo-La Mancha basin in the south. The amount of shortening in the basement and in the cover must be the same, because, in the central zone of the lberian range, the cover is not affected by extensive structures synchronous to compression. Both mapping relations between structures of different directions and relations of Tertiary deposits with the se structures, allow us to deduce a synchronism between structures of the different directions. Compressive deformation took place during Paleogene and lower Miocene.
From the kinematics and the synchronism of the different structures, a N-S direction of shortening is deduced in this region during Alpine
compression.
Normal fault zones play a major role in the development of basins and in the migration and trapping of hydrocarbons. The mapping of normal fault systems using seismic data requires careful correlation of faults on adjacent sections, a procedure that often leads to the interpretation of faults as having long, continuous, sinuous traces. Recent work involving detailed mapping of fault traces, first by using land exposures but more recently using three-dimensional seismics, has demonstrated that faults are usually made up of many overstepping segments, linked by areas of complex deformation, termed transfer zones or relay ramps. Relay ramps occur between normal fault segments that overstep in map view. The geometry and evolution of exposure-scale relay ramps are described from the Somerset coast, England, and are compared with larger scale ramps from elsewhere. Relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments; these groups are interpreted as being evolutionary stages. In stage 1, the segments do not interact. Stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp. In stage 3, connecting fractures start to break the relay ramp. Stage 4 is when the relay ramp is destroyed to produce a single fault that has an along-strike bend. These evolutionary stages can develop through time, but they can also be seen spatially. A branch line between normal faults or an along-strike bend may represent a stage 4 relay, with progressively earlier stages occurring updip or downdip. Characteristic variability in displacement-distance profiles for fault segments and linked faults accompanies the interaction and linkage processes. Displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps. Relay ramps often contribute to a minimum in total fault displacement at a linkage point. 47 refs., 16 figs.
Epiglyptic thrusts can be misinterpreted as paleolandslides since they underwent displacement on the Earth surface. The Daroca Thrust has been object of controversial interpretations owing to (i) the particular style of its frontal structure (a thin slab of Cambrian dolostones overthrusting syntectonic Miocene deposits), and (ii) its relatively young age with respect to the overall compressional building of the Iberian chain. While several authors have interpreted its frontal flat as a synsedimentary landslide, a number of key features evince that it represents an epiglyptic thrust: (i) recumbent folds in competent Cambrian dolostones; (ii) thick damage zone with foliated fault rocks; (iii) physical continuity between the frontal flat and the main thrust ramp; (iv) transport directions towards NNE and E consistently recorded in both of them. Geometric reconstruction in cross section indicates that the syntectonic deposits of the Daroca area lie below the well-dated succession of the Aragonian stratotype (Middle Miocene). Nevertheless, by extrapolating the age model resulting from the complete succession of mammal sites, it can be inferred that the slip on the Daroca Thrust still took place during Middle Aragonian time.
The NNW-SSE trending Teruel Basin rift is the largest Late Miocene-Quaternary extensional intracontinental structure located within the central-eastern Iberian Chain (Spain). The structural and morphotectonic study carried out in the central-northern part of this half graben basin (north of Teruel city) has allowed us to analyse rift segmentation, deformation partitioning and rift evolution. Results are based on vertical displacement calculations (fault throw and bending) of the main border and intrabasin fault zones. We use two geomorfological-stratigraphical markers, the Intramiocene Erosion Surface (IES; 11.2 Ma) and the Fundamental Erosion Surface (FES; 3.5 Ma). While the first marker reveals rift initiation under an E-W extension, the late marker records vertical displacements associated to a second, Late Pliocene–Quaternary rifting stage characterized by a nearly multidirectional extension regime with prevailing ENE-WSW trending ó3. Despite the along-axis rift segmentation into three structural domains (northern, central and southern) and the distribution of deformation among border and intrabasin faults in the central and southern domains, a consistent average slip rate (post-IES) of 0.09 mm/a has been calculated on distinct transects across the basin, suggesting a homogeneous crustal-scale extension process in the region. The results also reveal that slip rates during the Late Pliocene-Quaternary (0.12–0.16 mm/a) are higher than the Late Miocene-Early Pliocene (0.05–0.07 mm/a). Slip rate increase is caused by (i) a westward propagation of deformation from the Valencia Through, and (ii) a change in the regional stress field, both enhanced by crustal doming affecting central-eastern Iberia, as well as progressive fault linkage. Throw vs. distance graphs suggest that the main faults are in a transient stage towards coalescence, less advanced within the southern domain. Regional Late Pliocene-Quaternary uplift, concomitant with increasing slip rates in the Teruel Basin rift, has caused the basin to rise, so that synrift sedimentation only took place in rapidly subsiding residual basins until the region became exorheic and the basin was incised by the present-day fluvial network.
A new type of fault relay zone in extensional contexts, dominated by distributed along-strike or longitudinal fractures, is defined. It contrasts with the classical models reported in the literature, in which transverse connecting faults controlled by the own relay kinematics prevail. The new model is based on structural features of the Teruel graben system, as well as on analogue modelling. Relay zones between the NW-SE to NNW-SSE striking faults that delimit the eastern boundary of the Jiloca Graben (Calamocha, Sierra Palomera and Concud
faults), together with the Teruel Fault, have been studied. All of these relay faults show recent (Neogene-Quaternary) ruptures at different scales, mostly parallel to the macrostructural trend and to the maximum horizontal stress (SHmax) trajectories (i.e., orthogonal to the ENE-WSW regional extension direction that characterises
the nearly biaxial or radial stress regime active during Upper Pliocene-Quaternary times). Transverse ruptures are almost absent, with the exception of the northern relay zone (Calamocha-Sierra Palomera), where an incipient NE-SW striking connecting fault does exist. Analogue models have been run under a biaxial extension regime similar to the regional one. They allowed analysing the main factors controlling fracture propagation, depending on the ratio of extension velocities and the orientation of the master faults relative to extension directions. Laboratory fracture patterns, as in the natural studied examples, are mostly controlled by the inherited anisotropies and, in a greater extent, by the imposed extension trajectories, which results in a clear prevalence of longitudinal fractures. Such external controls, usually disregarded in numerical and analogue modelling, tend to induce fault coalescence through along-strike (parallel or at very-low-angle) propagation resulting in a final braided fault pattern.
Abstract
The official seismic hazard models in Spain used in the seismic building codes do not incorporate Quaternary faults, largely due to insufficient data for their proper characterization. There is an obvious need to conduct investigations in most of the recognised Quaternary faults to unambiguously demonstrate their Quaternary tectonic activity and assess their seismogenic potential. This work illustrates the integration of cartographic, tectonic, geomorphological, paleoseismological and geophysical methods for the characterization of the slow-moving extensional Daroca Fault, related to the negative inversion of the Alpine Daroca Thrust, Iberian Chain, NE Spain. Cartographic data indicate that the 27 km long Daroca Fault and the 17 km long Calamocha Fault, separated by a stepover 1.9 km wide, can be considered as segments of the same structure that might rupture jointly and generate Mw7 earthquakes. A long-term slip rate of 0.06–0.02 mm/yr has been estimated for the 27 km long Daroca Fault using an ESR-dated (Electro Spin Resonance) offset pediment. The work discusses why this slip rate is significantly lower than those estimated in nearby normal faults using OSL ages (Optically Stimulated Luminiscence), but comparable with those derived from offset early Pliocene limestones. A trench excavated across the Daroca Fault exposed evidence of the MRE (most recent event) on the fault, with bracketing ages of 2354–1544 cal yr BP (404 BC – 386 AD). This event likely caused the destruction and abandonment of Roman cities in the vicinity of the fault. Several explanations are proposed for the anomalously low vertical displacement of this surface faulting event recorded in the central sector of a 27 km long fault segment: multi-strand rupture, full-segment rupture, partial segment rupture, spillover rupture, and secondary sympathetic rupture.
The E–W trending, nearly pure extensional Valdecebro fault zone is a transverse structure at the central sector of the N–S Teruel graben. It was activated by the Late Ruscinian (Early Pliocene, ca. 3.7 Ma), giving rise to structural rearrangement of the graben margin. Until the Late Pleistocene, it has accommodated a net slip ca. 205 m, with slip rate of 0.055 mm/a. Paleoseismicity has been analysed in a 29-m-long, 5-m-deep trench excavated through a fault branch that offsets a Pleistocene pediment surface. The paleoseismic succession includes a minimum of 6–7 events occurred since ca. 142 ka BP, although a model with 12 events could be more realistic. The following paleoseismic parameters have been inferred, assuming a minimum of 6 and a maximum of 12 events: average coseismic slip = 58–117 cm; recurrence period = 8.4–28.4 ka; potential moment magnitude Mw = 5.8–5.9. The recorded displacement since ca. 142 ka BP totalizes 7.0 m, with slip rate of 0.05–0.07 mm/a. Slip on the transverse Valdecebro fault zone has critically contributed to bulk deformation under a prevailing ‘multidirectional’ extensional regime. Drainage patterns have been rearranged, recurrently switching between westward and southward directions as a consequence of diverse slip episodes at the Valdecebro fault zone (E–W) and the neighbouring La Hita (N–S) and Concud (NW–SE) faults. The ultimate westward drainage of the Valdecebro depression incised and dismantled a southward-sloping Pleistocene pediment sourced at the Valdecebro mountain front, representing a capture by the Alfambra river occurred between 124 and 22 ka BP.
The Teruel Basin is a NNE-SSW trending intracontinental extensional basin located in central-eastern Iberia. It is asymmetrically bounded to the east by a major fault zone, but intrabasinal faults with diverse orientation (NNE-SSW to NE-SW, E-W, or NW-SE) also appear. Offsets of the successive sedimentary units and of two planation surfaces reveal that tectonic activity initiated at the border faults, while intrabasinal ones mainly developed in a later stage. Fractures on a map scale show a prevailing N-S strike in Neogene synrift rocks, while a dense network made of four main fracture sets (NE-SW, E-W to ESE-WNW, N-S and NNW-SSE), likely inherited from Mesozoic rifting stages, is observed in pre-rift units. The results of palaeostress analyses indicate an overall predominance of σ3 directions around E-W, although two stress episodes have been distinguished during the Late Miocene-Pleistocene: (i) triaxial extension with σ3 E-W; (ii) almost ‘radial’ extension (σ1 vertical, σ2 ≈ σ3) with a somehow prevailing σ3 ENE-WSW. A scenario in which the evolving extensional stress field was able to gradually activate major basement structures with different orientation, inherited from previous tectonic events, is proposed as responsible for the evolution and overall pattern of both the eastern active margin and central parts of the central-northern sector of the Teruel Basin.
The relationship of independence, interaction or linkage between two neighbouring intraplate active extensional faults, the Teruel and Concud faults, are investigated from structural and paleoseismological data, and the results are discussed to improve seismic hazard assessment for the region. This paper provides the structural and paleoseismological characterization of the almost unknown Teruel Fault from detailed mapping and trench analysis, and discusses its kinematic and dynamic relationships with the Concud Fault. Four individual events occurred between 76.0 ka and 9.2 ka BP have been recorded at two branches of the Teruel Fault. Unfortunately, these only represent a small fraction of its overall activity during such time lapse, and their time constraints do not allow correlating them with those at the Concud Fault. The Teruel and Concud faults are independent structures from the geometric and kinematic point of view, as evinced by their distinct (i) transport directions (N275°E and N220°E, respectively), and (ii) average coseismic displacements (0.5 m and 1.9 m, respectively). These displacements are consistent with their respective lengths (9.0 km and 14.2 km) and significantly smaller than those expected for a hypothetically joint Concud-Teruel, 23 km-long fault. However, their displacement gradients close to the relay zone indicate that both faults undergo dynamic interaction, thus suggesting a transient stage from independence to linkage. We hypothesize that slip on both structures occurred, at the scale of the seismic cycle, in a broadly alternating manner, which induced strain partitioning between them and allowed accommodating bulk biaxial extension in the region. Such deformation pattern would have increased the earthquake frequency with respect to the scenario of a hypothetically linked Concud-Teruel Fault, but diminished the potential seismic magnitude.
Rollover is the folding of the hanging-wall sedimentary record in response to slip on listric normal faults, and is a common feature of sediment-rich, gravity-driven tectonic provinces. Rollovers have been extensively studied by means of geometric reconstruction and numerical and analogue modeling. However, the detailed interaction between the kinematics of bounding listric normal faults and their hanging-wall deformation is not yet fully understood. In this study, we use three-dimensional seismic-reflection data from the Forcados-Yokri area, western Niger Delta, Nigeria, to study the lateral linkage and landward backstepping history of an array of listric normal faults, particularly focusing on their influence on the development and evolution of hanging-wall rollovers. Five individual, partly overlapping rollover structures have been studied with respect to their relative initiation and decay time, their spatial distribution and their relation to the tectonic history of the respective bounding fault. We demonstrate that the studied rollovers are highly dependent on the development of their bounding faults in terms of initiation time, lateral linkage, internal structural development, and decay. Fault-rollover interaction is dynamic and changes through time depending on the temporal evolution of listric faults. Four genetic types of fault-rollover interaction were identified in this study: 1) the rotation of a rollover/crestal-collapse system, controlled by changing lateral bounding-fault orientation during fault growth 2) a stepwise shift of rollover/crestal-collapse systems associated with rollover abandonment, controlled by initiation of a new fault in the footwall of an older structure; 3) a gradual shift of successive rollovers controlled by branching main faults; and 4) a general landward and upward migration of crestal-collapse faults within a rollover above stationary listric main faults.
Listric normal fault systems are one of the most important sites for petroleum exploration in extension basins. The geometry of these systems at depth is commonly uncertain because of poor seismic data. Thus several techniques have been developed to construct the shape and position of the master fault at depth using one or more shallower horizons. Antithetic and synthetic faults commonly disrupt the continuity of bedding in the deformed hanging wall and contribute to the overall extension. Current models either neglect or do not adequately account for the effect of these faults in estimating the geometry of the master fault. We suggest that the extension of individual subsidiary faults should be added to the amount of extension on the master fault during the bed-fault construction. The extension on subsidiary faults is supposed to be transferred into the main fault, along the inclination of shear, although the linking mechanism between the small and main faults is unclear. The inclined shear angle can be estimated by orientation of the antithetic faults. Occurrence of synthetic faults may slightly distort the particle motion trajectories (inclined shear) relative to a hanging-wall reference frame but does not change the overall relative motion pattern. Application of our modification of the inclined shear model to unpublished and published seismic data, as well as an analog experiment model, agrees well with observed fault geometry. Negating the amount of heave on subsidiary faults results in an overestimation of depth to detachment.
The Concud Fault is a ~14-km-long active fault that extends close to Teruel, a city with about 35,000 inhabitants in the Iberian Range (NE Spain). It shows evidence of recurrent activity during Late Pleistocene time, posing a significant seismic hazard in an area of moderate-to-low tectonic rates. A geophysical survey was carried out along the mapped trace of the southern branch of the Concud Fault to evaluate the geophysical signature from the fault and the location of paleoseismic trenches. The survey identified a lineation of inverse magnetic dipoles at residual and vertical magnetic gradient, a local increase in apparent conductivity, and interruptions of the underground sediment structure along GPR profiles. The origin of these anomalies is due to lateral contrast between both fault blocks and the geophysical signature of Quaternary materials located above and directly south of the fault. The spatial distribution of anomalies was successfully used to locate suitable trench sites and to map non-exposed segments of the fault. The geophysical anomalies are related to the sedimentological characteristics and permeability differences of the deposits and to deformation related to fault activity. The results illustrate the usefulness of geophysics to detect and map non-exposed faults in areas of moderate-to-low tectonic activity where faults are often covered by recent pediments that obscure geological evidence of the most recent earthquakes. The results also highlight the importance of applying multiple geophysical techniques in defining the location of buried faults.
This is the first book to deal comprehensively with Spain's tectonic and sedimentary history over the past sixty or so million years. During Tertiary times, Spain had suffered compressional collision between France and Africa, and its Atlantic and Mediterranean coasts had been further modified by extensional rifting. This study will therefore be of interest to earth scientists generally because of the insights it provides into continental crustal deformation. Spain contains some of the best exposed outcrop geology in Europe. Because it includes sectors of two separate foreland basins, and an intervening craton with basins that have been influenced by extensional and strike-slip deformation, it provides excellent material for the development and testing of theories on the study of sedimentary basin formation and filling.
The southern Makran fold-thrust belt, Pakistan, displays unique outcrop examples of well-exposed, kilometre-scale, listric growth faults that displace Miocene-age deltaic growth strata by several hundreds of metres to kilometers. The largest growth faults are counter-regional (landward-dipping), bounding major clastic depocentres exposed over areas > 1000 km2. Stratal offset along these faults can exceed 1.5 km. Fault-zone thicknesses range between ca. 100 and 400 m, and average fault thickness-displacement ratios are around 1:10. High-resolution satellite data show in unprecedented detail the faults and the stratigraphic architecture of associated growth sequences, which comprise kilometre-scale progradational clinoforms, thick mudstone units and basinwards wedging sandstone-shale deposits. The true vertical thickness of the syn-kinematic record is, in places, up to 8 km, making the outcrop examples equivalent to major growth faulted successions known from seismic data of large deltas, and at least an order of magnitude larger than other outcrop examples. A comparison of the Makran outcrops with seismic-reflection examples offshore NW Borneo reveals distinct similarities in the gross depocentre geometries and internal architecture. The key control for growth faulting is interpreted to result from sedimentary loading, with rapid sedimentary progradation causing the development of rollover synclines by differential compaction and fluid expulsion, and counter-regional growth faults preferentially forming on the basinward side of these synclines. The data and interpretations presented can be used to assess the key parameters that contribute to the development of growth faults and growth successions above shale, reinforcing structural and stratigraphic observations from seismic interpretation and modelling studies in demonstrating their occurrence in exposure.
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Five differential equations that interrelate uplift, erosion, and deposition along stream systems that cross the mountain fronts of the northern Mojave Desert were used to appraise three classes of Quaternary tectonism. Class 1 (active tectionism) terrains are characterized by mountain-front sinuosities of 1.2 - 1.6, unentrenched alluvial fans, elongated drainage basins with narrow valley floors and steep hillslopes even in soft materials. Class 2 (moderate to slightly active tectonism) terrains are generally characterized by mountain-front sinuosities of 1.8 - 3.4, entrenched alluvial fans, large drainage basins that are more ciruclar than class 1 basins in similar rock types, steep hillslopes and valley floors ;that are wider than their floodplains. Class 3 (tectonically inactive) terrains are characterized by mountain- front sinuosities of 2 to 7, pedimented mountain fronts and embayments, steep hillslopes only on resistant rock types, and few large integrated stream systems in the mountains. Marked contrasts of landscapes, that are due to different relative rates of base level fall, are present north and south of the strike-slip Garlock fault. -from Authors
Clay models have been used to study the effects of fault shape and displacement distribution on deformation patterns in the hanging wall of a master normal fault. The experimental results show that fault shape influences the style of secondary faulting and folding. Most antithetic normal faults form above concave-upward fault bends, whereas mostly synthetic normal faults form above low angle fault segments and convex-upward fault bends. Generally, secondary faulting and folding are youngest at fault bends and become progressively older past fault bends. The observed variability with depth of the distribution and intensity of deformation is incompatible with homogeneous, inclined simple shear as the hanging-wall deformation mechanism. -from Authors
Three dimensional seismic-reflection data from the western Niger Delta were used to investigate the segmentation and linkage of a syn-sedimentary normal fault array and to estimate the influence of a pre-existing normal fault on the geometry and growth of younger faults. The nucleation, growth and linkage of a regional (seaward-dipping) deltaic fault system were analyzed on reflectivity time-/horizon slices and vertical seismic sections. In the deep subsurface, a master fault that consists of two segments (northwestern, NW, and southeastern, SE) grew through time into a single fault by lateral tip propagation reaching a final length of about 15 km. After attaining this length, displacement along the fault system developed non-uniformly through time. The analysis of the hanging-wall sediments of the deep-seated master fault shows two different processes of vertical linkage above the NW and SE segment. The SE segment links vertically to several younger faults contemporaneously with displacement accumulation on the master fault; in contrast, fault linkage above the NW segment occurred only after an interval of master-fault inactivity connecting the deep-seated structure upwards to a single syn-sedimentary normal fault. The observed differences in fault development suggest that although multi-segment deltaic faults form single fault systems after segment linkage, individual pre-linkage characteristics can be preserved, supporting a possibly diverse upward growth and connection to younger faults in the overburden. The geological interpretations presented highlight the influence of large deep-rooted structures on the development, location and geometry of shallow deltaic faults, documenting the influence of an older structural grain on delta tectonics.
State-of-the-art analysis of geological structures has become increasingly quantitative but traditionally, graphical methods are used in teaching. This innovative lab book provides a unified methodology for problem-solving in structural geology using linear algebra and computation. Assuming only limited mathematical training, the book begins with classic orientation problems and progresses to more fundamental topics of stress, strain and error propagation. It introduces linear algebra methods as the foundation for understanding vectors and tensors, and demonstrates the application of geometry and kinematics in geoscience without requiring students to take a supplementary mathematics course. All algorithms are illustrated with a suite of online MATLAB functions, allowing users to modify the code to solve their own structural problems. Containing 20 worked examples and over 60 exercises, this is the ideal lab book for advanced undergraduates or beginning graduate students. It will also provide professional structural geologists with a valuable reference and refresher for calculations. © Richard W. Allmendinger, Nestor Cardozo and Donald M. Fisher 2012.
Several faults in the Teruel and Jiloca grabens (Iberian Chain, NE Spain), particularly the targeted Concud fault, show evidences of recent, continuous activity, despite their scarce instrumental and historic seismic record.
Three trenches are studied in two locations (central and southeastern sectors of the Concud fault, respectively). After comparing with previous works, we reconstruct a palaeoseismic succession with nine events distributed along a maximum time lapse bracketed between 81.6 and 14.0 ka. This succession involves and average recurrence interval of 7.4 ± 2.8 ka, with individual interseismic periods between 4 and 11 ka. The calculated coseismic displacements range from 0.6 to 2.7 m, with an average value of 1.9 m that results in a slip rate of 0.26 mm/a. Due to the incomplete sedimentary record for Holocene times, we cannot affirm that the youngest event detected was actually the last one. We conjecture that some other event may have occurred during the period between 15.0 and 3.4 ka.
Temporal and spatial variations have been detected in palaeoseismic activity, specifically in the distribution of coseismic displacements. First, a non-steady slip rate is evidenced during Plio-Pleistocene times: a long-term tendency towards increasing slip rate is modulated in detail by the occurrence of minor cycles, as the sequence of increasing/decreasing activity recorded within the studied time window suggests. Secondly, an asymmetric distribution of coseismic slip along the fault trace is observed, paralleling the distribution of total fault throw, which shows an absolute maximum close to the southeastern tip. A combination of factors is proposed to explain this: branching of the main fault; dominant, remote-stress-driven slip towards N 220° E on the NW-SE fault segment; guided movement on the passive, NNW-SSE segment giving rise to an oblique roll-over monocline; and decoupling of the hanging-wall block owing to the transverse Los Mansuetos-Valdecebro fault zone.
The Jiloca basin is a large intramontane, NNWEarly Pliocene age. The geometry of these units is partially controlled by NWQuaternary infill have been interpreted from the geology of the basin margins, borehole data and hydrogeological criteria. The northern and southern sectors of the Jiloca depression are bounded by faults showing measurable hectometric-scale throws (Calamocha and Concud faults). Moreover, in the central sector, the [similar] 350Quaternary extensional evolution of eastern Spain. In contrast, they are hardly compatible with genetic models based on erosional deepening, either topographic lowering by numerous nested Tertiary erosion pediplains, or sub-alluvial Pliocene–Quaternary karstic corrosion.
We use geometric and experimental models to study the development of extensional fault-bend folds. The geometric models show that fault shape, fault displace- ment, and patterns of aggradation/erosion profoundly affect the distribution of growth beds, the magnitude and direction of dip of pregrowth and growth beds, and the location and dip of the outer limit of folding in pregrowth and growth beds. Complex structural and stratigraphic patterns develop if the rate of aggradation/erosion relative to the rate of fault displacement changes through time. The experimental models (with dry sand and wet clay) show that several deformational styles can accommodate extensional fault-bend folding. In sand models, a few, relatively major, secondary antithetic normal faults accommodate most hanging wall deformation. Pregrowth layers, although faulted, remain flat. The effective shear direction parallels the antithetic normal faults, and the shear angle is about 608-658. In clay models, numerous, relatively minor, secondary normal faults (antithetic and synthetic) and cataclastic flow accommodate most hanging wall deformation. The deformed pregrowth and growth layers dip gently toward the main fault. The effective shear angle (358-508) is considerably less than the dip of the antithetic normal faults. In the sand models and geometric models with a large shear angle (608), more displacement occurs on the main normal fault and the hanging wall collapses in a relatively narrow zone. In the clay models and geometric models with a small shear angle (358), less displacement occurs on the main normal fault. Instead, the hanging wall stretches substantially and collapses in a relatively wide zone.
Observations of brittle fractures in the granite of the Massif de la
Borne as well as in other rocks allow us to discuss the different
hypotheses regarding the origin of faults. It seems that all faults
originate by shear along a preexisting plane which can be an important
fracture (joint, former fault) or a microflaw (pore, grain boundary).
The shear on the plane gives rise to an array of en echelon cracks that
makes the rock less resistant and permits the lengthening of the fault.
We also found an arrangement of "horsetail" fractures damping the
movements on the faults. We then propose a pattern for the development
of the faults. They originate on preexisting plane and propagate by
initiating en echelon cracks. The movements are damped by horsetails
which permit, by faults connection in relays, the creation of larger
faults.
Samples of non-striated fracture surfaces within clastic materials of Late Pliocene–Pleistocene age from the Teruel grabens (eastern Spain) have been analysed using a stress inversion method based on observations of slip sense. The results obtained at 21 sites are compared with Late Miocene–Early Pliocene extensional stress tensors previously inferred from striated faults in the same area. The similarity between both sets of stress states suggests that the extensional Miocene–Pliocene stress field essentially continues (with minor changes) into Pliocene–Pleistocene times. The main changes involve (a) the dominant trend of σ3 trajectories, which evolve from ESE to ENE; (b) the waning of the compressional component caused by Europe–Iberia–Africa convergence; and (c) the progressive trend towards a multidirectional extension regime. Stress deflection caused by large-scale extensional faults as well as switching of σ2 and σ3 axes induced by fracture development are common within this stress field. They produce groups of local stress ellipsoids with σ3 axes orthogonal to each other and either orthogonal or parallel to the faults bounding the grabens. The regional consistency of the new results gives support to the new inversion method and demonstrates its utility in research on young sedimentary rocks, where ‘gaps’ in palaeostress records may exist due to absence of striated faults.
Detailed study of mesoscopic faults and joints affecting Neogene and Pleistocene deposits in the Iberian Chain and the Ebro Basin, involving statistical study of fracture directions and palaeostress analysis from striated fault planes, together with analysis of macrostructures, suggest a complex late Cenozoic stress field in which both primary and secondary stress systems are superposed. (1) Regional N-S compression was caused by convergence between the Iberian and African plates. (2) Multidirectional extension was caused by crustal doming in the Iberian Chain. (3) Stress trajectories were deflected near major faults. (4) Small-scale swapping of the horizontal tension axes (σ2 and σ3) was caused by development of fractures at right angles to either the primary or the deflected σ3 axis.
The kinematic and geometric evolution of basement-controlled inversion structures has been analyzed using a plane strain sandbox apparatus. The results of two representative scaled analogue models comprising rigid listric and planar detachment faults that both have cut-off angles of 60° to the horizontal, are described in detail. The hangingwall above these detachment geometries was constructed from alternate layers of sand and mica, in order to simulate an anisotropic sedimentary sequence.In both experiments, initial shortening at the onset of inversion was accommodated by bulk strain. This was followed by reactivation of the basal detachment together with the main basin controlling planar and listric faults which propagate upwards through the post-rift sequence at approximately the same angle as the upper part of the detachment fault (i.e. 60°). Reactivation of relatively steep, intra-basinal extensional faults within the cover was restricted to nucleation of reserve faults and thrusts in the region of the tips during approximately the first 10 to 15% contraction. This resulted in gently dipping thrusts that propagated upwards through the post-rift sequence to produce “harpoon” shaped structures. Later in the inversion phase, after approximately 15% contraction, backthrusts cut through and displaced the original antithetic extensional graben faults, producing downward tapering wedge-shaped structures at the margins of the basin. Although deformation in these experiments was limited to purely dip-slip displacement, the nature of the original extensional faults and their reactivated tip regions resulted in cross-sectional geometries that have a marked resemblance to strike-slip induced flower structures. Uplift was asymmetric and centred above the upper part of the main detachment above the listric fault but was more symmetrical and centred above the soling-out point of the detachment in the case of the planar fault geometry.The results of these experiments show distinct similarities to structures seen on seismic sections form the southern North Sea and other inverted sedimentary basins.
The results of three experimental models of ramp-flat extensional fault systems are presented. The experiments were constructed from sand and mica layers to form an anisotropic sequence that would respond to the imposed deformation by both faulting and folding. The geometry of the underlying detachment controls the deformation in the hangingwall sequence. Ramp-flat detachment geometries produce an upper roll-over with a crestal collapse graben system, a ramp syncline and fold/reverse fault zone, and a lower roll-over with a crestal collapse graben system. Short ramp geometries produce only minor perturbations in a larger roll-over structure. As extension progresses the crestal collapse systems migrate and superimpose earlier systems. The models of ramp-flat extensional structures produce templates for the interpretation of natural ramp-flat listric fault systems.