Rift Faults Research Articles

Seismotectonic implications of the Berne earthquake swarms west-southwest of Albany, New York

AbstractFive earthquake swarms occurred from 2007 to 2011 near Berne, New York. Each swarm consisted of four to twenty-four earthquakes ranging from M 1.0 to M 3.1. The network determinations of the focal depths ranged from 6 km to 24 km, 77% of which were ≥14 km. High-precision, relative location analysis showed that the events in the 2009 and 2011 swarms delineate NNE-SSW orientations, collinear with NNE trends established by the distribution of the spatially distinct swarms; the events in the 2010 swarm aligned WNW-ESE. Focal mechanisms determined from the largest event in the swarms include one nodal plane that strikes NNE, collinear with the distribution of the swarms and relative events within the swarms.Two, possibly related explanations exist for the Berne earthquake swarms. (1) The swarms were caused by reactivations of proposed blind NE- and NW-striking rift structures associated with the NE-trending Scranton gravity high. These rift structures, of uncertain age (Proterozoic or Neoproterozoic/Iapetan opening), have been modeled at depths appropriate for the seismicity. (2) The NNE-trending swarms were caused by reactivations of NNE-striking faults mapped at the surface north-northeast of the earthquake swarms. Both models involve reactivation of rift-related faults, and the development of the NNE-striking surficial faults in the second model probably was guided by the blind rift faults in the first model. The Berne swarms may be evidence that these faults are seismically capable and, if so, could sustain a maximum event on the order of Mw 5.7–6.6, based on fault segment length.

Structural style and evolution of inversion structures of Horus field, Alamein Basin, northern Western Desert of Egypt

Abstract High-quality three-dimensional (3D) seismic reflection and borehole data are used to characterize in detail the structural style and evolution of inversion structures in Horus oil field. This field lies in the central part of Alamein Basin, northern Western Desert of Egypt, in the hanging-wall of a major Jurassic-Early Cretaceous normal fault. This fault delineates the northwestern margin of the basin and extends in an ENE-WSW direction, bounding a half-graben trough in its hanging-wall. This trough had continued subsidence during Early Cretaceous time with depositional thickening of the Lower Cretaceous Alam el Bueib Formation. Fault displacement had ceased near the top of Barremian level. Aptian-Lower Senonian seismically traced levels show a major NE-plunging asymmetrical anticline overlying the Jurassic-Barremian half-graben. This anticline is sub-parallel to the inherited Jurassic rifting fault. At shallower Cretaceous levels, this fault is replaced by a set of WNW-ESE-trending en echelon faults of considerable displacements, which probably indicates a strike-slip component of deformation. These faults display normal separation on all displaced stratigraphic levels and tip upwards in the upper part of the Upper Cretaceous succession. This marks a considerable change in the tectonic mode of the area. NW-SE extensional faults were developed perpendicular to the fold axis, dissecting the anticline into several blocks entrapping hydrocarbons. Detailed seismic structural analysis of growth strata indicates that the fold initiated in Early Senonian time was associated with the inversion of the earlier Jurassic-Barremian half-graben. This basin inversion is attributed to the Syrian-Arc event that dominated North Africa during the Late Cretaceous time and continued through to the Early Miocene. Low fold amplitude coupled with normal displacements on the deep-seated ENE-WSW-trending Jurassic-Barremian fault indicate a mild phase of positive inversion. The Upper Senonian-Eocene stratigraphic sequences increased in thickness in the back-troughs that were associated with basin inversion.

Effects of Preexisting Structures on the Seismicity of the Charlevoix Seismic Zone

AbstractThe Charlevoix Seismic Zone (CSZ) is located along the early Paleozoic St. Lawrence rift zone in southeastern Quebec at the location of a major Devonian impact structure. The impact structure superimposed major, steeply dipping basement faults trending approximately N35°E. Approximately 250 earthquakes are recorded each year and are concentrated within and beneath the impact structure. Most M4+ earthquakes associated with the rift faults occurred outside the impact structure. Apart from the unique distribution of earthquakes, stress inversion of focal mechanisms shows stress rotations within the CSZ, and in the CSZ relative to the stress orientation determined from borehole breakouts. The primary goal of this research is to investigate the combined effects of the preexisting structures and regional stresses on earthquake activity and stress rotations in the CSZ. We approach this using PyLith, a finite‐element code for simulations of crustal deformation. Adopting the results from recent hypocenter relocation and 3‐D tomography studies, we modify the locations and dips of the rift faults and assess the effect of the new fault geometries on stress distributions. We also discuss the effects of resolved velocity anomalies. We find that the observed stress rotation is due to the combined effect of the rift faults and the impact structure. One‐dimensional velocity models of the CSZ with an embedded impact structure and a combination of 65°‐40°‐40° and constant 70° fault dip models with a very low friction coefficient of 0.3 and cohesion of 0 MPa can explain the observed seismicity and more than 50% of the stress rotations.

How do pre‐existing normal faults influence rift geometry? A comparison of adjacent basins with contrasting underlying structure on the Lofoten Margin, Norway

AbstractRecent studies of natural, multiphase rifts suggest that the presence of pre‐existing faults may strongly influence fault growth during later rift phases. These findings compare well with predictions from recent scaled analogue experiments that simulate multiphase, non‐coaxial extension. However, in natural rifts we only get to see the final result of multiphase rifting. We therefore do not get the chance to compare the effects of the same rift phase with and without pre‐existing structural heterogeneity, as we may in the controlled environment of a laboratory experiment. Here, we present a case study from the Lofoten Margin that provides a unique opportunity to compare normal fault growth with and without pre‐existing structural heterogeneity. Using seismic reflection and wellbore data, we demonstrate that the Ribban Basin formed during Late Jurassic to Early Cretaceous rifting. We also show that the rift fault network of the Ribban Basin lacks a pre‐existing (Permian‐Triassic) structural grain that underlies the neighbouring North Træna Basin that also formed during the Late Jurassic to Early Cretaceous. Being able to compare adjacent basins with similar histories but contrasting underlying structure allows us to study how pre‐existing normal faults influence rift geometry. We demonstrate that in Lofoten, the absence of pre‐existing normal faults produced collinear fault zones. Conversely, where pre‐existing faults are present, normal fault zones develop strong “zigzag” plan‐view geometries.

Structure formation peculiarities at early stage of Antarctic–Australia separation based on physical modeling

The paper addresses crustal formation in the Australian–Antarctic basin at the early period of separation of Australia and Antarctica. The study covers long rifting (~160–80 Ma), ultraslow spreading (~80–45 Ma) with the formation of proto-oceanic, mainly ultrabasic crust, spreading (~45-40 Ma), and stationary spreading at medium velocities (after 40 Ma). The different stages of oceanic opening are clearly expressed in the changes of basement morphology (the top of the second oceanic layer) on seismic profiles. Physical modeling is used to reveal the peculiarities in the surface morphology of the oceanic (magmatic) crust which developed in the transitional conditions from ultraslow to slow and medium spreading. Our experiments established that (1) the presence of a stronger block in the pre-breakup model lithosphere in the pathway of the propagating rift faults can significantly affect the geometry of the spreading axis in its vicinity and lead to the development of transversal structures and a highly rugged relief; (2) under the conditions of ultraslow ocean accretion, numerous jumps of the spreading axes occur; (3) the temporary cessation of spreading leads to the development of linear high-amplitude uplifts corresponding to amagmatic ridges in the natural conditions.

The influence of basement faults on local extension directions: Insights from potential field geophysics and field observations

AbstractComplex arrays of faults in extensional basins are potentially influenced by pre‐existing zones of weakness in the underlying basement, such as faults, shear zones, foliation, and terrane boundaries. Separating the influence of such basement heterogeneities from far‐field tectonics proves to be challenging, especially when the timing and character of deformation cannot be interpreted from seismic reflection data. Here we aim to determine the influence of basement heterogeneities on fault patterns in overlying cover rocks using interpretations of potential field geophysical data and outcrop‐scale observations. We mapped >1 km to meter scale fractures in the western onshore Gippsland Basin of southeast Australia and its underlying basement. Overprinting relationships between fractures and mafic intrusions are used to determine the sequence of faulting and reactivation, beginning with initial Early Cretaceous rifting. Our interpretations are constrained by a new Early Cretaceous U‐Pb zircon isotope dilution thermal ionization mass spectrometry age (116.04 ± 0.15 Ma) for an outcropping subvertical, NNW‐SSE striking dolerite dike hosted in Lower Cretaceous Strzelecki Group sandstone. NW‐SE to NNW‐SSE striking dikes may have signaled the onset of Early Cretaceous rifting along the East Gondwana margin at ca. 105–100 Ma. Our results show that rift faults can be oblique to their expected orientation when pre‐existing basement heterogeneities are present, and they are orthogonal to the extension direction where basement structures are less influential or absent. NE‐SW to ENE‐WSW trending Early Cretaceous rift‐related normal faults traced on unmanned aerial vehicle orthophotos and digital aerial images of outcrops are strongly oblique to the inferred Early Cretaceous N‐S to NNE‐SSW regional extension direction. However, previously mapped rift‐related faults in the offshore Gippsland Basin (to the east of the study area) trend E‐W to WNW‐ESE, consistent with the inferred regional extension direction. This discrepancy is attributed to the influence of NNE‐SSW trending basement faults underneath the onshore part of the basin, which caused local re‐orientation of the Early Cretaceous far‐field stress above the basement during rifting. Two possible mechanisms for inheritance are discussed—reactivation of pre‐existing basement faults or local re‐orientation of extension vectors. Multiple stages of extension with rotated extension vectors are not required to achieve non‐parallel fault sets observed at the rift basin scale. Our findings demonstrate the importance of (1) using integrated, multi‐scale datasets to map faults and (2) mapping basement geology when investigating the structural evolution of an overlying sedimentary basin.

Linked thick- to thin-skinned inversion in the central Kirthar Fold Belt of Pakistan

Abstract. The Kirthar Fold Belt is part of the transpressive transfer zone in Pakistan linking the Makran accretionary wedge with the Himalaya orogeny. The region is deforming very obliquely, nearly parallel to the regional S–N plate motion vector, indicating strong strain partitioning. In the central Kirthar Fold Belt, folds trend roughly N–S and their structural control is poorly understood. In this study, we use newly acquired 2-D seismic data with pre-stack depth migration, published focal mechanisms, surface and subsurface geological data, and structural modelling with restoration and balancing to constrain the structural architecture and kinematics of the Kirthar Fold Belt. The central Kirthar Fold Belt is controlled by Pliocene to recent linked thick-skinned to thin-skinned deformation. The thick-skinned faults are most likely partially inverting rift-related normal faults. Focal mechanisms indicate dip-slip faulting on roughly N–S-trending faults with some dip angles exceeding 40∘, which are considered too steep for newly initiated thrust faults. The hinterland of the study area is primarily dominated by strike-slip faulting. The inverting faults do not break straight through the thick sedimentary column of the post-rift and flexural foreland; rather, the inversion movements link with a series of detachment horizons in the sedimentary cover. Large-scale folding and layer-parallel shortening has been observed in the northern study area. In the southern study area progressive imbrication of the former footwall of the normal fault is inferred. Due to the presence of a thick incompetent upper unit (Eocene Ghazij shales) these imbricates develop as passive roof duplexes. In both sectors the youngest footwall shortcut links with a major detachment and the deformation propagates to the deformation front, forming a large fault-propagation fold. Shortening within the studied sections is calculated to be 18 %–20 %. The central Kirthar Fold Belt is a genuine example of a hybrid thick- and thin-skinned system in which the paleogeography controls the deformation. The locations and sizes of the former rift faults control the location and orientation of the major folds. The complex tectonostratigraphy (rift, post-rift, flexural foreland) and strong E–W gradients define the mechanical stratigraphy, which in turn controls the complex thin-skinned deformation.

Effects of initial rift inversion over fold-and-thrust development in a cratonic far-foreland setting

The effects of initial rift inversion over the geometry of a far-foreland intracratonic fold-and-thrust belt are investigated. A total of 25 seismic reflection lines and stratigraphic data from 9 wells are used in association to digital elevation and field geological data, to investigate the geometric relationship between shallow thrust development and initial positive inversion of buried rift faults in a cratonic environment, where inversion tectonics is not common. The study case is conducted in the meeting region of the opposed-verging Brasília and Araçuaí foreland fold-and-thrust Neoproterozoic belts, located in central Brazil, formed in the first stages of West Gondwana amalgamation. Seismic reflection sections are used to map buried paleorift faults and trust ramps. The best 5 sections are also interpreted in detail in terms of fold and thrust geometry and their relationship to the inverted rifts is analyzed. They show high correlation between shallow fold-and-thrust geometry and the shortening of synrift strata, caused by rift inversion. Results indicate that initial positive rift fault reactivation exerts a determinant control over the locus of thrust ramp nucleation, by causing syn-rift strata to overfold and drape fold. The passive folding propagates upwards forming gentle folds in the décollement plane, where thrust ramps tend to nucleate. Back-thrust-forethrust pairs tend to form over elevated horsts, where higher topographic plateaus are preserved. Multiple thrusting and lower relief occur over buried rifts with multiple inverted faults. Strata over non-rifted basement remain practically unaffected by thrusting and folding. Moreover, the development of thrust salients and recesses are demonstrated to be conditioned by the morphology of buried rifts. The study also contributes with a more precise mapping of the buried rift system and thrust system in the area, and brings new insights about the structural evolution of the Brasiliano-Pan Africano orogeny affecting the São Francisco craton basement and covers.

Structure-Forming Peculiarities at the Early Stage of Antarctic–Australia Separation Based on Physical Modeling

The paper addresses crustal formation in the Australian–Antarctic basin at the early period of separation of Australia and Antarctica. The study covers long rifting (~160–80 Ma), ultraslow spreading (~80–45 Ma) with the formation of proto-oceanic, mainly ultrabasic crust, spreading (~45-40 Ma), and stationary spreading at medium velocities (after 40 Ma). The different stages of oceanic opening are clearly expressed in the changes of basement morphology (the top of the second oceanic layer) on seismic profiles. Physical modeling is used to reveal the peculiarities in the surface morphology of the oceanic (magmatic) crust which developed in the transitional conditions from ultraslow to slow and medium spreading. Our experiments established that (1) the presence of a stronger block in the pre-breakup model lithosphere in the pathway of the propagating rift faults can significantly affect the geometry of the spreading axis in its vicinity and lead to the development of transversal structures and a highly rugged relief; (2) under the conditions of ultraslow ocean accretion, numerous jumps of the spreading axes occur; (3) the temporary cessation of spreading leads to the development of linear high-amplitude uplifts corresponding to amagmatic ridges in the natural conditions.

The Early Aptian volcanic episode of Gutiolo (N Spain): Expression of the Bilbao Rift Fault Zone

The Gutiolo volcanic unit belongs to the Aptian Bilbao Formation and crops out 6 km SE of Bilbao (N Spain) within marine marlstones. It is 300 m thick and up to 1 km long. The volcanics consist of lava in the lower part and volcaniclastic sediments above, with metatrachytes and metatrachybasalts, similar to others of the alkali Cretaceous sequence in the Basque–Cantabrian Basin. The palaeontological age is top early Aptian (Dufrenoyia furcata Zone) near the Bedoulian–Gargasian substage transition, and the isotopic age (K–Ar) is 123 ± 3 Ma but most probably, 122 Ma. E–W and N–S trending faults bounding the Gutiolo volcanics controlled magma eruption. Other NW‐ and SE‐oriented major faults in the Bilbao area favoured several Valanginian to Santonian eruptions including Gutiolo. The eruptions moved progressively in space and time from SW to NE. These faults make up a 15‐km‐wide tectonic belt, the magma‐leaky Bilbao Fault Zone. A similar magmatic fault zone NE of Bilbao, Gernika‐Elgoibar, poured magma onto the sea floor during the late Albian–Cenomanian up to the Santonian, continuing the north‐easterly younging trend of volcanic inceptions recognized in Bilbao. All this Cretaceous volcanism is related to the Bay of Biscay opening processes, and the Gutiolo event is correlated with the first appearance of oceanic crust in the Bay of Biscay. From stratigraphic, volcanic, geochemical, and mineralization evidence, we attribute Gutiolo to an early phase of mafic intrusion emplacement in the Bilbao area, underplated by magmatism during the Valanginian to Santonian (Cretaceous). Volcanic hydrothermal fluids associated to the Gutiolo event are considered responsible for metallic ore emplacement in the Basque–Cantabrian Basin during the early Aptian.

Architectural framework of the NW border of the onshore Potiguar Basin (NE Brazil): An aeromagnetic and gravity based approach

On the basis of qualitative and quantitative geophysical interpretations, studies on tectonostratigraphic evolution were carried out in NW onshore Potiguar basin. Airborne magnetic and terrestrial gravity data were acquired in order to investigate tectonostratigraphic relationships in a poorly studied area of this oil-bearing Early Cretaceous basin. The present integrated study determined the main geophysical lineaments, basin internal geometry and depth of intra-basement magnetic and gravity sources, characterizing geological domains in terms of lithostructural elements. The depth of geophysical sources was estimated using 2D and 3D Euler deconvolution. 2D forward gravity modeling was also performed along three transects. The results unravel crustal partitioning, characterized by NE-SW lineaments with local E-W to NW-SE inflexions. The geophysical patterns are directly related to basement grain, which is Brasiliano in age. In fact, the Jaguaribe shear zone, that is not clearly marked on the surface, appears much more pronounced in the various geophysical maps and gravity models. The Ponta Grossa and Fazenda Belém shear zones show similar geophysical signatures to the Jaguaribe shear zone and appear to limit a low-related gravity features. Another important magnetic lineament was revealed by 2D Euler deconvolution and was named Retiro shear zone. 2D gravity modeling shows the basin geometry in depth, which presents shallow depocenters that may be associated with hydrocarbon reservoirs to the east, such as the Fazenda Belém oil field. An evolutionary tectonic model of this reservoir is proposed comparing our results and previous geological studies. This study indicates that a few faults, which occur in the NW edge of Potiguar Basin and form depocenter boundaries, oblique to the main transform continental margin in NE Brazil, have the orientation, kinematics and geometry as the main rift faults. Thus, our final model suggests that the grabenlike depocenter could be the westernmost expression of the NE Brazilian Rift System that generated a series of rift basins along the Borborema Province in the Early Cretaceous. The still unknown deposition of rift-related sequences to the west of Fazenda Belém oil field is probably associated with raised area that remained active when the eastern sector of the Potiguar basin presented overall subsidence. It is likely this basement structural configuration is topographic highs and lows, keeping geodynamics relationships with strike-slip fault regimes installed in the Atlantic Equatorial Margin during Aptian.

Seismic Evidence for Plume‐ and Craton‐Influenced Upper Mantle Structure Beneath the Northern Malawi Rift and the Rungwe Volcanic Province, East Africa

AbstractP and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd Tanzania project and data from previous networks in the study area. The main features of the models are a low‐velocity zone (LVZ) with δVp = ~−1.5–2.0% and δVs = ~−2–3% centered beneath the RVP, a lower‐amplitude LVZ (δVp = ~−1.0–1.3% and δVs = ~−0.7–1%) to the southeast of the RVP beneath the center and northeastern side of the northern Malawi rift, a shift of the lower‐amplitude anomaly at ~−10° to −11° to the west beneath the central basin and to the western side of the rift, and a fast anomaly at all depths beneath the Bangweulu Craton. The LVZ widens further at depths >~150–200 km and extends to the north beneath northwestern Malawi, wrapping around the fast anomaly beneath the craton. We attribute the LVZ beneath the RVP and the northern Malawi rift to the flow of warm, superplume mantle from the southwest, upwelling beneath and around the Bangweulu Craton lithosphere, consistent with high 3He/4He values from the RVP. The LVZ under the RVP and northern Malawi rift strongly indicates that the rifted lithosphere has been thermally perturbed. Given that volcanism in the RVP began about 10 million years earlier than the rift faulting, thermal and/or magmatic weakening of the lithosphere may have begun prior to the onset of rifting.

Basement control on fault formation and deformation band damage zone evolution in the Rio do Peixe Basin, Brazil

The objective of this study is to analyze the influence of brittle reactivations of continental-scale Precambrian ductile shear zones on the evolution of deformation band damage zones. Our results indicate that the Rio do Peixe Basin was formed by the Cretaceous reactivation of its basement EW- and NE-SW-striking major fault segments. These fault segments are composed of a footwall with Precambrian crystalline mylonites and a hanging wall with Cretaceous fine to coarse sandstones. In the hanging wall, deformation bands occur in poorly sorted, fine to very coarse sandstones. These deformation bands occur as single bands or clusters mostly within damage zones ~84 m from the basement faults and are widespread in structural highs. Our data plot slightly above the medium fault displacement vs. the damage zone width solution but falls within the point cloud of global patterns. The slip surfaces strike EW and NE-SW and are parallel or slightly oblique to the basement rift faults. The kinematics of slip surfaces are consistent with those of the reactivated basement faults. In addition, a logarithmic decrease in deformation band frequency occurs away from the basement slip surface. The deformation band frequencies peak close to the fault core with values as high as 22–48 bands/m. The lithology grain size also influences deformation as clusters of deformation bands and slip surfaces are only observed in coarse sandstones. This new case of deformation bands in damage zones of basement faults is characterized by a strong attitude and geometric and kinematic inheritance but follows the deformation band frequency of global patterns.

Dynamics of microseismicity and its relationship with the active structures in the western Corinth Rift (Greece)

We analyse the complete earthquake archive of the western Corinth Rift using both cross-correlations between pairs of event waveforms and accurate differential traveltimes observed at common stations, in order to identify small-scale fault structures at depth. The waveform database was generated by the dense Corinth Rift Laboratory network and includes about 205 000 events between 2000 and 2015. Half of them are accurately relocated using double-difference techniques. The novelty of this relocated catalogue is the integration of the recent westernmost earthquakes due to the extension of the network in 2010 to the western extremity of the Corinth Rift and the consideration of the whole database over more than 15 yr. The total relocated seismicity exhibits well-defined clusters at the root of the main normal faults mainly between 5 and 10 km depth in the middle of the gulf and illuminates thin active structure planes dipping north about 20° under the northern coast. Some seismicity is observed in the footwall of the main active faults, along the West and East Helike faults. We also built a multiplet database based on waveform similarity taking into account cross-correlation coefficients weighted by signal-to-noise ratios. Short-term multiplets are concentrated in the middle of the gulf along the Kamarai fault system, in a 1–2 km thick layer at 6–8 km depth, interpreted as a highly fractured geological layer. They are often associated to slow seismic migration velocities occurring in this zone during strong swarm episodes and are thus likely to be triggered by pore pressure variations. On the other hand, most long-term and regular multiplets are located deeper (7–10 km), under the northern coast, within a layer less than 0.3 km thick. They occur at the border of nearly planar structures with low seismicity rate, which we identify as fault planes, and they may be explained by aseismic slip on the fault surface around them. This supports the existence of an immature structure growing downdip towards the north at the base of the active geological layer, which possibly connects to the ductile middle crust around 15 km depth, as suggested by the occurrence of deeper events in the continuity of the 1995–fault plane. The different migration velocities (from 0.05 km d−1 to several km d−1) highlighted during the western 2014–swarms indicate that both pore pressure and creep diffusion are operating in the fault zone. The fast migrations observed in the Psathopyrgos fault zone, where a slow slip event was detected by dilatometers in 2002, compare with that for creeping faults. To the west, from spatial distribution of events, we show that the Rion–Patras fault connecting the western extremity of the Corinth Rift fault system to the Patras Rift, is dipping around 60° north–west with a rake angle of −115°. Finally, we identified two new areas within the central active zone which may correspond to large scale, locked asperities on active fault surfaces, similar in size to the main asperity broken during the 1995, MW 6.3, Aigion earthquake.

Distinguishing rift-related from inversion-related anticlines: Observations from the Abu Gharadig and Gindi Basins, Western Desert, Egypt

Distinguishing the tectonic origin of anticlinal structures is problematic in regions with a complex history of rifting and inversion. We present the results of seismic mapping, in the form of time-depth (isochron) and time-thickness maps to characterize how sedimentary thickness differentials evolved in response to normal faulting and to inversion events on faults within the Abu Gharadig and Gindi Basins in the Western Desert of Egypt. Late Cretaceous rift-related faults in the Abu Gharadig Basin strike NW-SE, W-E and SW-NE. In the eastern part of the basin, a prominent SW-NE trending interbasinal saddle formed in response to preferential subsidence forming half-grabens to its north-west and southeast, during the Mid-Turonian to Santonian interval. Santonian to Palaeogene inversion in the Abu Gharadig Basin developed on its northern basin margin, the absence of SW-NE striking faults in the eastern central basin resulting in any inversion effects being minor. In the central Gindi Basin, Upper Cenomanian to Lower Turonian SW-NE striking rift faults underwent inversion as early as the Mid-Turonian. The orientation of existing rift faults and modification of the local stress fields control the extent to which inversion was taken up in each basin trough time. The Abu Gharadig and Gindi Basins are two of the rift basins developed in West and Central Africa that underwent rifting, inversion and dextral shearing during the Late Cretaceous. We emphasize the value of high-resolution stratigraphic mapping to characterize short-lived and subtle pop-up events that may have gone unnoticed.

Structural Inheritance and Rapid Rift‐Length Establishment in a Multiphase Rift: The East Greenland Rift System and its Caledonian Orogenic Ancestry

AbstractWe investigate (i) margin‐scale structural inheritance in rifts and (ii) the time scales of rift propagation and rift length establishment, using the East Greenland rift system (EGR) as an example. To investigate the controls of the underlying Caledonian structural grain on the development of the EGR, we juxtapose new age constraints on rift faulting with existing geochronological and structural evidence. Results from K‐Ar illite fault dating and syn‐rift growth strata in hangingwall basins suggest initial faulting in Mississippian times and episodes of fault activity in Middle‐Late Pennsylvanian, Middle Permian, and Middle Jurassic to Early Cretaceous times. Several lines of evidence indicate a close relationship between low‐angle late‐to‐post‐Caledonian extensional shear zones (CESZs) and younger rift structure: (i) reorientation of rift fault strike to conform with CESZs, (ii) spatial coincidence of rift‐scale transfer zones with CESZs, and (iii) close temporal coincidence between the latest activity (late Devonian) on the preexisting network of CESZs and the earliest rift faulting (latest Devonian to earliest Carboniferous). Late‐ to post‐Caledonian extensional detachments therefore likely acted as a template for the establishment of the EGR. We also conclude that the EGR established its near‐full length rapidly, i.e., within 4–20% of rift life. The “constant‐length model” for normal fault growth may therefore be applicable at rift scale, but tip propagation, relay breaching, and linkage may dominate border fault systems during rapid lengthening.

Structural framework from gravity and magnetic data in the paleo/mesoproterozoic Araí rift-sag Basin, Central Brazil

The Araí Basin comprises a paleo/mesoproterozoic rift-sag basin inverted in a thick-skinned regime during the Brasiliano Orogeny. Its depth to basement, geometry, and lateral extension remain unknown. We have aimed to determine this information based on geophysical/geologic data to clarify its main structures and gain an evolutional understanding. Gravity and magnetic data allowed us to delineate the rift system — its major faults and adjacent postrift sedimentary structures — over its entire length. The geophysical investigation included radial power spectrum analysis and Euler deconvolution alongside the application of edge structure enhancement filters on magnetic anomaly and matched filter grids. A 2.5D forward gravity model was generated using all results, with geophysical sources at depths of 35–42, 20.9–22.6, 11–15.6, 9.68, 8–2.3, and 0.37–0.54 km, respectively, interpreted as the Mohorovičić Discontinuity, Curie Surface, Conrad Discontinuity, granitic intrusions, basement depths, and superficial faults. The main interpreted tectonic framework of the entire area showed a preferential direction in N40E, comprising structures related to the Transbrasiliano Lineament and its reactivations. The basin magnetic signature trends N50-80E, with structures generated essentially in the Brasiliano Orogeny and those related to extensional rift faults reactivations (belonging to São Jorge-Alto Paraíso-Cormari, Cavalcante-Teresina, and Teresina-Nova Roma systems). These fault systems exhibit a magnetic signature at the average depth of 6 km, converging to a single fault detected up to 21.8 km. The main rift faults are in accordance with the Euler solutions and the matched filter interpretation, confirmed by mapped alluvial fans conglomerates. The current structural framework of the basin goes back to the Brasiliano Orogeny, when its sedimentary filling was limited by crustal blocks with distinct compositional, rheologic, and magnetic characteristics. These intrinsic characteristics of each block along with the resulting kinematics were responsible for delimiting the shape and depth of the basin.

Paleorift structure constrained by gravity and stratigraphic data: The Statherian Araí rift case

Gravimetric and stratigraphic data were used to investigate the Paleoproterozoic Araí Paleorift, a failed Statherian continental rift located in the western margin of the São Francisco Craton, where basement and cover were affected by the Neoproterozoic Brasiliano Orogeny. Euler deconvolution, tilt, total horizontal gradient amplitude and upward continuation technics were applied to terrestrial gravimetric data in order to investigate the rift's main faults location, direction and depth, allowing to identify its main horsts, grabens, volcanic and plutonic centers. We found that rift faults occur to a maximum depth of ca. 38 km, but major fault throw occurs from 4 to 8 km deep and attenuates from 8 to 12 km, probably the brittle-ductile transition zone at the time of rifting, practically disappearing at 20 km. Stratigraphic data and basement mapping were used in order to constraint gravimetric results. We classify the Araí Rift as a passive, three-armed failed rift, narrow to divergent type, that produced preferably anorogenic rapakivi-related magmas, most of it still lodged in the crust from surface down to ca. 19 km deep and subsidiary mafic magmatism. The results indicate the deep occurrence of low-density magmas beneath the rift's main axis, detected up to 20 km deep. Correlation to other global Statherian rifts show that the São Francisco Craton was strongly affected by taphrogenesis during the Statherian, together with Siberia, North America and North China cratons. Finally, by comparing our results to recent rifts we found that the Ethiopian rift's morphology is quite similar to the Araí. Surrounding the Tanzanian craton, the Cenozoic East Africa rift system morphology is compared to the Araí-Espinhaço rift system, which surrounds the São Francisco craton. The major contribution of this paper is the recognition of Araí Paleorift surface and subsurface morphology, up to now unknown, over an area of ca. 45.000 km2.

Interplay between inherited rift faults and strike-slip structures: Insights from analogue models and field data from Iceland

Abstract Although structural inheritance is a fundamental factor in the tectonic evolution of the crust, few studies have been aimed at understanding in detail the interaction between pre-existing normal faults in a rift zone and successive dominant strike-slip (transtensional) movements. Firstly, we present the complex pattern of faults and tension fractures of the western, pre-Holocene part of the N-S Theistareykir Fissure Swarm (ThFS - Northern Iceland Rift), crossed by the NW-SE striking, right-lateral, Husavik-Flatey Fault (HFF), and we integrate previous observations with new field data that reveal Holocene motions along the N-S faults near the HFF. Secondly, we propose a set of analogue experiments that reproduce the N-S rifting followed by superimposed transtensional faulting along the HFF. In a first set of experiments, we tested the effect of an initial extension at the shallow crustal level by forming a rift zone. During this rifting phase, we modelled the presence of a sub-orthogonal discontinuity that represents the structural inheritance of the HFF (that is older than the ThFS), but without any imposed “regional” transtensional movement along it. After that, we superimposed a dominant right-lateral movement along the HFF on the modelled rift faults in order to assess the effect of the inheritance of the rift plus the HFF. Our results show that during rifting, a series of elongated and depressed areas develop along the initial HFF discontinuity (that is locally reactivated by the rift opening), in agreement with field data. During the second experimental set of exclusively shear deformation along the HFF, the models show the development of Riedel shears at the HFF, contemporaneous to the local reactivation of normal faults and tension fractures in the previous rift zone. This reactivation occurs by block rotation, incremental dip-slip motions on pre-existing fault scarps, widening of tension fractures and development of new structures. We conclude that transversal basins can develop from the early stages of rift development, and that the structures of a pre-existing rift may reactivate under a local stress state induced by incremental transtensional motions.

Insights into the Early Evolution of the Côte d'Ivoire Margin (West Africa)

Abstract A tectono-stratigraphic analysis of a broadband 3D seismic survey over the outer slope of Côte d'Ivoire margin, west Africa, revealed that Cenomanian and younger strata seal well-developed rift fault blocks up to 15 km across. Growth strata indicate that these were formed during rifting that culminated in seafloor spreading in the late Albian, challenging existing plate reconstructions for the opening of the equatorial Atlantic ocean. A previously unrecognized system of volcanic edifices linked at depth to a network of sill complexes has also been identified. These are aligned along a NE–SW trend, concordant with kilometre-wide ridges, interpreted as folds formed by steep, crustal faults with an oblique-slip component. These trends are similar to those of fracture zones in the region and indicate that the Côte d'Ivoire was a transform margin in the late Albian. These results highlight the potential of offshore Côte d'Ivoire for deep-water rift plays with large traps formed by extensional fault blocks together with prospective Albian reservoirs ponded in their hanging walls. In addition, the volcanoes and ridges generated seabed relief along the newly created transform margin, forming confined basins for potential deposition of Turonian and younger turbidites and the generation of stratigraphic traps.