Syn-rift Fill Research Articles

Does the syn- versus post-rift thickness ratio have an impact on the inversion-related structural style?

Abstract. Many extensional basins worldwide are modified by subsequent compressional episodes, which lead to inverted structures. The structures associated with the reactivation of pre-existing faults are critically important in the formation of suitable subsurface traps for hydrocarbons. Major concerns regarding inverted structures are the trap integrity and fault seal. In general, the preferred structures have simple four-way closures due to the minor forced folding of the post-rift cover, as opposed to (reverse) fault-related structures, which have a higher risk of breaching. Such reverse-fault-bounded structures have been generally observed in basins with a thick syn-rift fill and a relatively thin post-rift sequence at the time of inversion (Mode I). In contrast, gentle/open forced folds have been described in basins with thicker post-rift sequences than the syn-rift basin fill (Mode II). Five physical sandbox models, coupled with particle image velocimetry (PIV) analysis, have been performed to address the influence of the syn- versus post-rift thickness ratio on the inversion-related structural style of the post-rift cover. The main control on the development of Mode I or Mode II inversion structures within the post-rift sequence appears to be different due to the amount of compressional displacement accommodated by the inherited listric fault and the thickness of the post-rift cover. These observations do have a direct impact on the understanding of the geo-energy systems associated with inverted structures and are broadly comparable with natural data examples from Aotearoa/New Zealand, Israel, Romania, and the Republic of Türkiye.

Intrarift fault fabric, segmentation, and basin evolution of the Lake Malawi (Nyasa) Rift, East Africa

AbstractThe Lake Malawi (Nyasa) Rift, in the East African Rift System (EARS), is an ideal modern analogue for the study of extensional tectonic systems in low strain rate settings. The seismically active rift contains the 700-m-deep Lake Malawi, one of the world’s oldest and largest freshwater lakes with one of the most diverse endemic faunal assemblages on Earth. Modern and reprocessed legacy multichannel seismic-reflection data are constrained by velocity information from a wide-angle seismic experiment to evaluate variability in extension, segmentation, and timing of fault development along the 550-km-long rift zone. Fault geometries and patterns of synrift sediment fills show that the Lake Malawi Rift is composed of three asymmetric rift segments, with intervening accommodation zone morphologies controlled by the degree of overlap between segment border faults. Most extension occurs on the basin border faults, and broadly distributed extension is only observed at one accommodation zone, where no border fault overlap is observed. Structural restorations indicate a weakly extended rift system (∼7 km), with diminishing values of extension and thinner rift fill from north to south, suggesting a progressively younger rift to the south. There is no evidence of diking, sill injection, or extrusives within the synrift fill of the Lake Malawi Rift, although the volcanic load of the Rungwe magmatic system north of the lake and related subsidence may explain the presence of anomalously thick synrift fill in the northernmost part of the lake. The thickest synrift depocenters (∼5.5 km) are confined to narrow 10- to 20-km-wide zones adjacent to each rift segment border fault, indicating concentration of strain on border faults rather than intrarift faults. Intrarift structures control axial sediment delivery in the North and Central rift segments, focusing sediment into confined areas resulting in localized overpressure and shale diapirs. The asymmetric, basement-controlled relief was established early in rift development. When overprinted with frequent high-amplitude hydroclimate fluctuations, which are well documented for this basin, the resulting highly variable landscape and lake morphometry through time likely impacted the diverse endemic faunas that evolved within the basin. New seismic-reflection data, augmented by wide-angle seismic data and age constraints from drill core, offer the most highly resolved 3D view to date of latest Cenozoic extensional deformation in East Africa and provide a foundation for hazards analysis, resource assessments, and constraining deformation in a low strain rate, magma-poor active rift.

Sedimentary architecture during Carboniferous rift initiation – the arid Billefjorden Trough, Svalbard

Analysis of the sedimentary architecture of the early Billefjorden Trough, an exceptionally well-exposed Carboniferous half-graben located in Svalbard, Arctic Norway, allows detailed reconstruction of fault development and basin geometry. Sedimentary facies distribution reveals facies variability in an early synrift fill, when an array of meso-scale intra-basinal faults segmented the basin into rotated blocks with differential subsidence. Growth of these faults into larger systems impacted the distribution of accommodation space and the drainage pattern expressed in fluvial and deltaic sandstone-dominated units. An axially back-stepping character of the early synrift facies associations (Stages 1–3) is reflected in aggrading homogenous facies types of continental depositional Stage 1, which evolved into back-stepping facies belts of deltaic to nearshore depositional environment, Stages 2 and 3, due to the opening of a connection to the sea. The drainage pattern and accommodation distribution in the early synrift deposits reflect a relatively symmetrical basin that predates a later prominent half-graben configuration of the basin. Supplementary material: Figures presenting lithostratigraphic columns of the well data (Supplementary material 1), location of logs in Ragnardalen (Supplementary material 2) and geological map used in Figure 2b (Supplementary material 3) are available at https://doi.org/10.6084/m9.figshare.c.4293077

Structural inheritance and selective reactivation in the central Andes: Cenozoic deformation guided by pre-Andean structures in southern Peru

Structural, stratigraphic, and geochronologic constraints from the Eastern Cordillera in the central Andean plateau of southern Peru (14–15°S) demonstrate the existence and position of major pre-Andean structures that controlled the accumulation of Triassic synrift fill and guided subsequent Cenozoic deformation. The timing of initial clastic deposition of the Triassic Mitu Group is here constrained to ~242–233Ma on the basis of detrital and volcanic zircon U–Pb geochronology. Regionally distinct provenance variations, as provided by U–Pb age populations from localized synrift accumulations, demonstrate Triassic erosion of multiple diagnostic sources from diverse rift-flank uplifts. Stratigraphic correlations suggest synchronous initiation of extensional basins containing the Mitu Group, in contrast with previous interpretations of southward rift propagation. Triassic motion along the NE-dipping San Anton normal fault accommodated up to 7km of throw and hanging-wall deposition of a synrift Mitu succession >2.5km thick. The contrasting orientation of a non-reactivated Triassic normal fault suggests selective inversion of individual structures in the Eastern Cordillera was dependent on fault dip and strike. Selective preservation of a~4km thick succession of Carboniferous–Permian strata in the down-dropped San Anton hanging wall, beneath the synrift Mitu Group, suggests large-scale erosional removal in the uplifted footwall. Field and map observations identify additional pre-Andean thrust faults and folds attributed to poorly understood Paleozoic orogenic events preserved in the San Anton hanging wall. Selective thrust reactivation of normal and reverse faults during later compression largely guided Cenozoic deformation in the Eastern Cordillera. The resulting structural compartmentalization and across-strike variations in kinematics and deformation style highlight the influence of inherited Paleozoic structures and Triassic normal faults on the long-term history of convergent margin deformation in the Andes.

Alluvial fan deposition along a rift depocentre border from the Neuquén Basin, Argentina

The interaction between hangingwall block rotation and alluvial deposition is examined from Late Triassic–Early Jurassic successions exposed along the Catán Lil half-graben border fault system in the Neuquén Basin, Argentina. Analysis of transport and depositional processes, clast composition and rock body geometry allowed the identification of three distinctive fan-shaped alluvial units. The contrasting lithologic nature of the basement (igneous-metamorphic) and syn-rift fill (volcanic and volcanic-derived) permits detailed studies of clast provenance. The origin of each alluvial system (footwall- vs. hangingwall-derived) can thus be verified. A simple method was implemented to establish the geometry of each alluvial unit by comparing the stereographic projection of its bedding to that of an idealised fan shaped body. Results show that the three alluvial systems occupied the same relative location in the rift depocentre. Unit 1 is interpreted as an alluvial fan orientated transverse to the depocentre border fault system and fed from the footwall. Non-cohesive debris flow deposition was the dominant process in this environment. Unit 2 is interpreted as a mainly hangingwall-fed alluvial fan, parallel to the depocentre border fault system and shows an upward decrease in footwall-derived clasts. Hyperconcentrated flow was the principal transport process. Unit 3 represents a fan delta, parallel to the depocentre border fault system. Its components are completely hangingwall-derived and hyperconcentrated flow deposition was the dominant process. Differences in grain-size, composition, transport directions and fan body geometry are proved to be directly linked to variations in ground tilting induced by the direction of hangingwall block rotation in an endorheic rift depocentre.

Syn-eruptive/inter-eruptive relationships in Late Neoproterozoic volcano-sedimentary deposits of the Hamid area, North Eastern Desert, Egypt

Nonmarine volcano-sedimentary successions in the Late Neoproterozoic Hamid Basin were studied in order to examine the distinctive characteristics of accumulation during syn-eruptive and inter-eruptive periods in a depocenter associated with active volcanism and both extensional and dextral strike-slip tectonics. In particular, the syn-rift fill in this area comprises a wide range of compositions and transport and depositional processes in which lava flows coexist with pyroclastic and epiclastic deposits in the same accumulation space. Seven different accumulation units were identified in the syn-rift fill: (1) polymictic alluvial fan units, (2) fluvial braid plain units, (3) lacustrine units, (4) coherent volcanic bodies/shallow intrusion units, (5) pyroclastic fall units, (6) phreatomagmatic volcanic units, and (7) pyroclastic density current units. These deposits are organized into several stratal packages with contrasting geometries. Analysis of these units and the relationships between them provided insights into the evolution of the syn-rift sedimentary environments and permitted identification of different stages of effusive activity, explosive activity, and relative quiescence, determining syn-eruptive and inter-eruptive rock units. These units provide important clues to the distribution of, and temporal changes in, accommodation space, and hence the configuration and structural evolution of the Hamid Basin. Two accumulation stages were defined. The underfilled stage occurs when the material supplied to the depocenter during the eruptive events is not enough to level the existing topography, allowing the development of high-gradient alluvial systems during the next inter-eruptive period. The overfilled stage occurs when extensive pyroclastic density current deposits choke the accumulation space during syn-eruptive periods, causing low-gradient sedimentary systems to develop during the subsequent inter-eruptive periods. The Hamid Basin is thus interpreted to have been hybrid in nature, influenced by the dynamic changes of the basin–margin faults, which were either normal or strike-slip.

(U-Th)/He thermochronologic constraints on the evolution of the northern Rio Grande Rift, Gore Range, Colorado, and implications for rift propagation models

The Rio Grande Rift system tapers northward into the center of the southern Rocky Mountains. How rift initiation and evolution are related to development of the Rocky Mountains is not well understood. The Gore Range and adjacent Blue River Valley of central Colorado are the northernmost significant fault-related manifestations of the rift. New apatite (U-Th)/He data from the range record only middle and late Cenozoic exhumation phases, unlike neighboring Rocky Mountain uplifts that record a single phase of Late Cretaceous–early Tertiary unroofing. Middle Tertiary dates require that Gore Range basement remained covered by sedimentary rocks until at least the Middle Eocene. Normal faulting initiated during the Oligocene, inducing exhumation of basement to the surface and deposition of synrift fill in the Blue River Valley. Major cooling and unroofing in the Miocene were restricted to the southern Gore Range and continued until at least 7 Ma, with ∼2.3 km of total displacement along the Blue River normal fault. This middle to late Tertiary unroofing history is strikingly similar to that inferred for other rift flank uplifts to the south, and suggests broad synchroneity in the middle Tertiary onset and subsequent evolution of >700 km of the Rio Grande Rift. The results highlight that the notion of a northward-propagating rift, as suggested by its northward-tapering geometry, is incorrect, and preclude models invoking rift propagation to explain late Cenozoic elevation gain of the Rocky Mountains.

Syn-eruptive/inter-eruptive relations in the syn-rift deposits of the Precuyano Cycle, Sierra de Chacaico, Neuquén Basin, Argentina

The syn-rift volcanic successions of the Upper Triassic–Lower Jurassic Precuyano Cycle (i.e. the Lapa Formation) from the Sierra de Chacaico in the Neuquén Basin, Argentina, were studied in order to address the distinctive characteristics of accumulation during syn-eruptive and inter-eruptive periods in a depocentre associated with active volcanism and extensional tectonics. In particular, the syn-rift fill in this area comprises a wide range of compositions, as well as of transport and depositional processes. Lava flows coexist with pyroclastic and epiclastic deposits in the same accumulation space. In order to analyse the complexities inherent in a volcanic environment subjected to extension, five different accumulation units were identified in the area: Lava Flow/Shallow Intrusion Units, Pyroclastic Units, Volcaniclastic Alluvial Units, Polymictic Alluvial Units, and Lacustrine Units. The analysis of each of these units and of the relationship between them provided meaningful insights into the evolution of the syn-rift sedimentary environments and the identification of different stages of effusive activity, explosive activity and relative quiescence, determining syn-eruptive and inter-eruptive rock units. The relationship between these units was examined, and two accumulation stages were defined. The underfilled stage originates when the material supplied to the depocentre during the eruptive events is not enough to level the existent topography, allowing the development of high-gradient alluvial systems during the next inter-eruptive period. The overfilled stage occurs when extensive pyroclastic density current deposits choke the accumulation space during syn-eruptive periods, causing low-gradient sedimentary systems to develop during the subsequent inter-eruptive periods.

The Iapetan rifted margin of southern Laurentia

The Iapetan rifted margin of southern Laurentia includes the northeast-striking Blue Ridge, Ouachita, and Marathon rifts, which are offset by the northwest-striking Alabama-Oklahoma and Texas transform faults, framing the Alabama and Texas promontories and the Ouachita and Marathon embayments of the continental margin. Interpretations of the original trace, structural style, and age of the rifted margin rest on identification of synrift rocks and structures, as well as continental-shelf and off-shelf sedimentary deposits on the passive margin. Both late Paleozoic Ouachita-Appalachian allochthons and post-orogenic Atlantic-Gulf passive-margin deposits cover the Iapetan rift margin, necessitating the use of data from deep wells and geophysical surveys along with geologic maps of the exposed Ouachita-Appalachian thrust belts to characterize the synrift and post-rift rocks and structures. The continental margin and passive-margin shelf strata are primarily in the footwall of the Ouachita allochthon; however, some Ouachita thrust faults displaced shelf-margin basement and cover. Appalachian thrust faults imbricate synrift fill of the intracratonic Birmingham graben and the passive-margin shelf. Palinspastic restoration of thrust-belt structures uses balanced cross sections to locate the original trace of the Iapetan margin. Thickness and subsidence history of the passive-margin successions, as well as a general lack of preserved synrift deposits, indicate an upper-plate structure along the Blue Ridge rift on the Alabama promontory and along the Ouachita rift on the Texas promontory. The upper plate on the Texas promontory is conjugate to a lower-plate rift structure on the Argentine Precordillera. Although data are limited, the evolution of the passive margin along the Marathon rift in the Marathon embayment suggests a lower-plate structure. Geophysical modeling supports a steep continental margin along the Alabama-Oklahoma transform, and a similar structure can be inferred for the Texas transform. The Blue Ridge rift north of the Alabama promontory is dated by synrift volcanic rocks as young as 564 Ma, and passive-margin transgression beginning in earliest Cambrian is documented along the Alabama promontory and farther north. The age of the Ouachita rift is documented by the 530–539 Ma synrift volcanics of the transform-parallel intracratonic Southern Oklahoma fault system, by Early Cambrian synrift sediment along the conjugate rift margin in the Argentine Precordillera, and by late synrift graben-fill of Early to early Late Cambrian age in the rift-parallel intracratonic Mississippi Valley and Birmingham graben systems, as well as by subsidence history of the passive margin on the Texas promontory. The diachroniety of rifting reflects an inboard shift from the Blue Ridge rift to the Ouachita rift along the Alabama-Oklahoma transform and rifting of the Argentine Precordillera from the Ouachita embayment.

Sedimentary dynamics and extensional structuring related to early Cretaceous rifting of Neocomian and Barremian deposits of the interior basin of Gabon

Recent field and subsurface data about the early Neocomian N’dombo series and the Neocomian to mid-Barremian Schistes series of the interior basin of Gabon further our understanding of the initial stages of early Cretaceous N40–60°E extensional rifting. The syn-rift series comprise fluvial–lacustrine claystones–sandstones, rare conglomerates, and carbonates. The syn-rift fill begins with braided-stream feldspathic sandstones. These are overlain first by fluvial–lacustrine deposits and then by predominantly lacustrine–palustrine claystones, which are potential petroleum source rocks. The claystones are eroded in part and are capped by the pre-Aptian angular unconformity marking the end of Cretaceous rifting in the interior basin. This change in syn-rift facies and depositional environments reflects a rise in base level in response to accelerated subsidence after the initial stage of rifting. The syn-rift deposits form two fining-upward sequences several 100–1000 m thick.

Age and environmental evolution of the syn-rift fill of the southern coast of the gulf of Corinth (Akrata-Derveni region, Greece)

AbstractThe southern coast of the gulf of Corinth exhibits syn-rift deposits, giving insights into the first stages of continental extension as well as the geodynamic evolution of the surrounding Aegean region. The stratigraphy (relative position, 3D geometry, dating) of these deposits is still subject to controversies. The syn-rift evolution of the central part of the southern coast of the Corinth rift is revisited, based on new sedimentological and paleontological data. While ostracods analysis provides precise information about the paleoenvironments, recent advances in palynology supply a more accurate chronology.For the first time, we document marine evidences and Pleistocene evidences below the well-known giant Gilbert-type fan deltas of the Corinth rift. The syn-rift fill records a three-phase history: (1) the Lower Group corresponds to continental to lacustrine environments passing up progressively to brackish environments with occasionally marine incursions from before 1.8 Ma to some time after 1.5 Ma, (2) the Middle Group corresponds to giant alluvial fans and to Gilbert-type fan deltas prograding in an alternating marine and lacustrine environment from around 1.5 Ma to some time after 0.7 Ma, and (3) the Upper Group corresponds to slope deposits, Gilbert-type fan deltas and marine terraces indicating the emergence of syn-rift sediments along the southern coast from at least 0.4 Ma to the present day, with alternating marine and lacustrine deposition controlled by the position of the Mediterranean sea level relative to the Rion Strait sill.

Using bathymetry to identify basin inversion structures on the English Channel shelf

High-resolution bathymetry is used to derive a new structural interpretation of a submerged inverted graben on the English Channel shelf (northwest Europe). The bathymetry provides a continuous plan-view image of the bedrock geology and resolves fine structural detail, particularly in areas of steeply dipping strata where traditional seismic imaging fails. The imagery, combined with shallow core, deep borehole, and two-dimensional seismic reflection data, shows a marked asymmetry in the bedrock outcrop and structural style, both across and along the axis of the basin. For example, the oldest synrift rocks crop out in a complex south-verging anticline along one margin, whereas more massively bedded younger rocks are folded into a simpler anticline at the other. Comparison of our observations with both recent analog modeling and a neighboring inverted basin highlights the importance of the mechanical properties of the basin stratigraphy in controlling the structural development. We conclude that the inversional asymmetry developed because of the distribution of strong competence contrasts within the synrift fill.

Controls on the structural architecture and sedimentary character of syn-rift sequences, North Falkland Basin, South Atlantic

The Falkland Islands lie in a critical location for our understanding of Gondwana supercontinent break-up. Subsurface data obtained during recent exploration activity provide important new insights into the structure and stratigraphy of the North Falkland Basin's Late Jurassic syn-rift fill history. Results of seismic interpretation indicate that deposition was largely controlled by prominent planar normal faults, which divide the basin into a series of north-south trending half-grabens and minor grabens. Well data (in the form of exploration wireline logs, cores, petrographic thin sections, major and trace element analyses) indicate that their syn-rift fill was dominated by abundant volcaniclastic sediment supplied through debris flows, hyperconcentrated flows and pyroclastic ash-fall. The geochemical affinity of the volcanic sedimentary components shows a direct link to the widespread, Middle Jurassic to Early Cretaceous, silicic Chon Aike Large Igneous Province, in southern South America and Antarctica. Diagenesis has obscured provenance information and severely reduced porosity within the syn-rift sediments, causing them to have poor reservoir potential. The results enable a radically new interpretation of this key period in the North Falkland Basin's evolution, which helps modify and unify previously disparate views. It is now suggested that basin development was the direct result of the Chon Aike back-arc stretching, influenced in part by the underlying Palaeozoic basement trends. This was followed by Cretaceous-Recent post-rift thermal subsidence of the basin, which was only arrested by local structural inversion related to Atlantic opening. Consequently, it is thought that the extension that created the North Falkland Basin was independent of both Early–Middle Jurassic rotation of the Falkland Islands microplate and the evolution of the South Atlantic Ocean in the Cretaceous and Cenozoic.

Flexural cantilever models of extensional subsidence in the Munster Basin (SW Ireland) and Old Red Sandstone fluvial dispersal systems

Abstract Flexural cantilever (2D) computer modelling of the palinspastically restored Mid-Late Devonian Munster Basin has been used to appraise quantitatively extensional subsidence and Old Red Sandstone (ORS) stratigraphic geometries. One-dimensional decompaction and (Airy isostatic) backstripping were carried out to constrain syn-rift forward models; these specify the Late Palaeozoic rifting history of the region. Forward modelling showed that listric faults and detachments fail to reproduce restored ORS sediment geometries, but instead indicated that multiple planar, upper-crustal faults are necessary to achieve the correct order of syn-rift subsidence across the basin. Modelled (non-unique) sections transverse to the basin bounding fault (Dingle Bay-Galtree Fault Zone) replicated ORS geometries with cumulative extensions of 27 km in the east (stretching factor β = 1.3) and 59 km in the west (β = 1.48), with effective elastic thicknesses of 7 and 8 km, respectively, starting from a 40 km thick post-Acadian crust. Resultant peak heat-flow anomalies predict well the location of known syn-rift volcano-magmatic centres. Modelling indicated that significant offshore faults are required to achieve subsidence in the west Cork region, implying that the basin continues offshore. Observed ORS (<0.85 km thick) sections on the regional footwall, considered to be the result of thermal (post-rift) subsidence, are not accounted for by modelling, whereas c . 1 km of post-rift ORS is modelled over 5 Ma in the central-southern regions of the basin. These sections buried the principal rift faults during late Famennian time. The ORS of the Munster Basin is dominated by two large-scale transverse fluvial dispersal systems that were largely insensitive to deflection by any extension faults that propagated in the syn-rift fill. A third major system entered in the SW, demarcated by antithetic extension faults south of the depocentre.

Genesis of tectonic inversion structures: seismic evidence for the development of key structures along the Purbeck–Isle of Wight Disturbance

The interpretation of a densely spaced and well-calibrated seismic grid sheds new light on the development and evolution of key regional and local structures in the Wessex Basin. The results help to resolve long-standing controversies concerning the tectonic significance of apparently anomalous outcrop patterns and the role of important, local ancillary structures with respect to the major monoclinal folds with which they are associated. Although the structures are entirely consistent with the effects of contractional reactivation (tectonic inversion) of normal faults, the subsurface data demonstrate the role that original extensional fault segmentation and associated relay ramps had on original depositional patterns, subsequent inversion geometries and resultant outcrop patterns. As well as illustrating regional controls on the formation of structures, the new seismic-based interpretations enable a reassessment of the Lulworth Crumple and the Ballard Down Fault. The Lulworth Crumple is interpreted as a parasitic fold complex generated by internal folding of the inverted, incompetent syn-rift fill in the immediate hanging wall to the Purbeck Fault, a reactivated major normal fault. The Ballard Down Fault’s origin is interpreted to result from the formation of a local, late-stage ‘out of the syncline’ reverse fault which propagated southwards and upwards through a Chalk succession. As the Chalk had already been rotated to form the northward-dipping steep limb of the Purbeck Monocline at Ballard Down, the structure cuts down stratigraphically. The results stress the importance of understanding the nature of original extensional fault geometries and the competence of the sedimentary units incorporated in folds in gaining a full understanding of the genesis and evolution of structural styles in inverted basins.

Sequence Stratigraphy of the Synrift Fill, Danish Central Graben, North Sea

Sequence Stratigraphy of the Synrift Fill, Danish Central Graben, North Sea

Some geometrical characteristics of inversion

Summary The inversion of extensional fault systems results in the reversal of slip on the faults and expulsion of the synrift fill. During inversion the beds in the cover sequence shorten before the net extension at the basement level has been cancelled. Shortening of the sedimentary cover generates folding and backthrusting in the still downthrown hanging wall block. Intracratonic inverted basins in different parts of the Alpine Foreland show similar structural geometries with the major extensional faults which controlled basin development reactivated during subsequent compression. We use examples from the Western Approaches and offshore Holland (Broad Fourteens Basin) to illustrate the structural styles developed during inversion. The fundamental control on compressional structural geometry exerted by pre-existing extensional structures is also visible in more complexly deformed orogenic belts, like the Western Alps and the Pyrenees. In these areas inversion also occurs, but more commonly extensional faults which may not have inverted act as an indirect control on the location of ramps, and/or thrust orientation. Seismic data are normally required to establish these effects with certainty. However, as the body of knowledge builds up, it is possible to recognize certain geometrical characteristics which suggest the control of extensional faults in thrust belts. These include footwall shortcuts, out of sequence structures and arcuate thrust-fold traces.