Rift-related Normal Faults Research Articles

Imaging Pleistocene volcanic edifices along the Egyptian Red Sea margin: Insights from reflection seismics and 3D constrained inversion of gravity and magnetic data

Several volcanic buildups have been documented using gravity and magnetic data at specific locations in the Northern Red Sea (NRS). Most of these volcanoes were never sampled, and only a few were imaged by seismic or bathymetry data. Furthermore, the confidentiality of commercial datasets does not allow adequate knowledge of these structures and their morphology and physical properties. In this study, 2D and 3D seismic reflection data and 3D constrained inversion of magnetic and gravity data have been used to infer densities and magnetic susceptibilities of volcanic features in the central part of the NRS and to evaluate the distribution and geometry. This study indicates that the results of the 3D inverted models show a good fit with the volcanic structures inferred from seismic data and, thus, that the inversion technique adopted can be a reliable tool for detecting other edifices where seismic data are missing. The 3D inverted density and magnetic susceptibility models suggest that at least eight isolated structures with similar gravity and magnetic properties exist in the study area. They are associated with high densities and magnetic susceptibility anomalies with average values of 2700 kg/m3 and 0.025 SI, respectively. These magmatic structures are deeply rooted in the sedimentary cover and emerge at elevations of 100 to >650 m relative to the seabed. They developed in a sector interpreted as containing thinned continental crust, accompanied by the ascension of magma through prominent rift-related normal faults. Magmatic activity likely started in the Pleistocene and continues to the present day.

Relations between inherited basement fabric and fault nucleation in a continental setting: The Rio do Peixe Basin, NE Brazil

The control of normal fault growth by preexisting basement structures is still a worldwide matter of debate. The objectives of the present study are (i) to investigate where rift faults nucleate in continental-scale ductile shear zones and how these zones influence fault architecture; (ii) how subvertical basement fabric controls listric fault geometry and evolution; (iii) how long reactivation of faults lasts; and (iv) which are the implications of fault reactivation for the oil industry. We contribute to these issues by defining the 3-D relationship between preexisting ductile strike-slip shear zones, the geometry and evolution of subsequent rift-related normal faults using airborne magnetic, 2-D and 3-D seismic reflection, and structural data from the Rio do Peixe Basin, a Cretaceous intracontinental basin in northeastern Brazil. Our results indicate that Precambrian mylonitic foliation and faults share a common strike at the surface, but not a common dip, mostly because the Precambrian shear zones are planar and the reactivated faults are listric (curved). The combined magnetic anomaly and seismic reflection help understand the 3-D geometrical relation between inherited structures and Cretaceous brittle reactivation, which is not obvious from the field data. As inferred from subsurface information, the mylonitic foliation (sub-vertical dip) and shear zone boundary (ca. 60° dip, parallel to brittle faulting) are not parallel. The basin-bounding faults reactivated along ductile shear zones that separate major terranes with mechanical contrast. We conclude that the map view spatial coincidence of Precambrian ductile shear zones and main brittle faults can be deceiving. Mylonitic foliation and faults are parallel along strike but not along dip, indicating that the geometry of rift faults is not controlled by the mylonitic foliation, but by the mechanical contrast between tectonometamorphic units separated by ductile shear zones.

Influence of fault geometry, kinematics and growth rate on syn-tectonic stratigraphic pattern: Insights from the 2D move-on-fault technique in MOVE software

Rift-related normal faults generally have various geometries, and cause the syn-rift stratigraphy in their hanging wall to be characterized by different infilling patterns. Our understanding of the tectonic control on the syn-rift stratigraphy mainly stems from the analysis of fault kinematics, stratigraphic styles and depositional facies. However, questions remain about how fault geometry, kinematics and slip rate influence the style of a rift-related half-graben and the syn-rift stratigraphy, and what enlightenment that relationship has for the hydrocarbon exploration in rifts. Using the 2D MOVE Software, this study attempts to simulate the development of a fault-bounded half-graben, with the aim of investigating the aforementioned questions. The modeling results show that, a planar, listric and ramp-flat-ramp border fault controls a tabular, wedged-shaped, and composite wedge-shaped growth package, respectively, but exceptionally, the planar fault bounds a wedge-shaped package if the fault rotates with movement. In addition, rollover structure forms when the border fault has a listric or ramp-flat-ramp shape, indicating that the fault geometry and kinematics dominate the syn-tectonic stratigraphic pattern. Besides, fault slip rate relative to sediment supply affects the scale, dip and infilling pattern of the syn-tectonic stratigraphy. In details, reduction of fault slip rate or increment of sediment supply results in: i) the extent of the syn-tectonic stratigraphy enlarging, ii) the dip angle of the syn-tectonic stratigraphy decreasing, and iii) the water depth in the related half-graben getting shallower. This study provides insights into understanding the origin of the variable styles of rift-related half-graben and syn-rift stratigraphy. Also, our modeling results have an important implication for evaluating source rock quality in rifts. Finally, we give a preliminary example of using the MOVE Software to investigate the geometry and evolution of fault-bounded basin, which is well worth being further explored.

Repeated degradation and progradation of a submarine slope over geological timescales

ABSTRACTSubmarine slopes prograde via accretion of sediment to clinoform foresets and degrade in response to channel or canyon incision or to mass-wasting processes. The timescales over which progradation and degradation occur, and the large-scale stratigraphic record of these processes, remain unclear due to poor age constraints in subsurface-based studies and areally limited exposures of exhumed systems. We here integrate 3D seismic reflection and borehole data to study the geometry and origin of ancient slope canyons developed in late Mesozoic strata of the Måløy Slope, offshore Norway. Slope degradation and canyon incision commenced during the late Kimmeridgian, coincident with the latter stages of rifting. Later periods of canyon formation occurred during the Aptian to Albian and the Albian to Cenomanian, during early post-rift subsidence. The canyons are straight, up to 700 m deep, and 10 km wide on the upper slope and die out downdip onto the lower slope. The canyons trend broadly perpendicular to and crosscut most of the rift-related normal faults, although syn-filling fault growth locally helped to preserve thicker canyon-fill successions. The headwalls of the oldest (late Kimmeridgian) canyons are located at a fault-controlled shelf edge, where younger canyons overstep this fault, which was inactive when they formed, extending across the paleo-shelf. Downslope, Aptian to Albian canyons either erode into the older, late Kimmeridgian to Barremian canyon fills, forming a complicated set of unconformities, or in the case of the Albian to Cenomanian canyons, die out into correlative conformities. Boreholes indicate that the canyon bases are defined by sharp, erosional surfaces, across which we observe an abrupt upward shift from shallow- to deep-marine facies (i.e., late Kimmeridgian canyons), or deep marine to deep marine facies (Aptian to Albian and Albian to Cenomanian canyons). Missing biostratigraphic zones indicate the canyons record relatively protracted periods (c. 2–17 Myr) of structurally enhanced slope degradation and sediment bypass, separated by > 10 Myr periods of deposition and slope accretion. The trigger for slope degradation is unclear, but it likely reflects basinward tilting of this tectonically active margin, enhanced by incision of the slope by erosive sediment gravity flows. The results of our study have implications for the timescales over which large-scale slope progradation and degradation may occur on other tectonically active slopes, and the complex geophysical and geological record of these processes. We also show that canyon formation can cause large volumes of margin-derived sediment to bypass proximal sub-basins within rifted terranes, an important process not currently captured by marine rift-basin tectono-stratigraphic models.

Tectonically controlled carbonate-seated maar-diatreme volcanoes: The case of the Volsci Volcanic Field, central Italy

Quaternary carbonate-seated maar-diatremes in the Volsci Range are one of the most intriguing products of the west-directed subduction of the Adriatic slab that drove the development of the Apennine mountain belt in Central Italy. The Volsci Volcanic Field is characterized by phreatomagmatic surge deposits, rich in accidental carbonate lithics, and subordinate Strombolian scoria fall deposits and lava flows, locally sourced from some tens of monogenetic eruptive centers (at least fifty tuff rings and scoria cones). We investigate the subsurface maar-diatreme processes in terms of relationships between faulting and explosive magma-water interaction, as well as the distribution pattern of the eruptive centers. With this aim, we present the following new data: i) description of the fold-and-thrust belt structure and associated eruptive centers, ii) componentry of volcanic rock-types, iii) determination of grain-size, degrees of whiteness and roundness of carbonate lithic inclusions, iv) micropaleontological analysis of carbonate lithics. We show that the clustering of eruptive centers is controlled by tectonic features. A first order control is tentatively related to crustal laceration and deep magma injection along a ENE-trending Quaternary lateral tear in the slab and to Mesozoic rift-related normal faults. A second-order control is provided by orogenic structures (mainly thrust and extensional faults). In particular, magma-water explosive interaction occurred at multiple levels (< 2.3 km depth), depending on the structural and hydrogeologic setting of the Albian-Cenomanian carbonates, which are intersected by high-angle faults. The progressive comminution, rounding and whitening of entrained carbonate lithics allow us to trace multistage diatreme processes. Finally, our study bears implications on volcanic hazard assessment in the region.

Pre-existing intra-basement shear zones influence growth and geometry of non-colinear normal faults, western Utsira High–Heimdal Terrace, North Sea

Pre-existing intra-basement shear zones can induce mechanical and rheological heterogeneities that may influence rifting and the overall geometry of rift-related normal faults. However, the extent to which physical and kinematic interaction between pre-existing shear zones and younger rift faults control the growth of normal faults is less-well understood. Using 3D reflection seismic data from the northern North Sea and quantitative fault analysis, we constrain the 3D relationship between pre-existing basement shear zones, and the geometry, evolution, and synrift depositional architecture of subsequent rift-related normal faults. We identify NE-SW- and N-S-striking rift faults that define a coeval Middle Jurassic – Early Cretaceous, non-colinear fault network. NE-SW-striking faults are parallel to underlying intra-basement shear zone. The faults either tip-out above or physically merge with the underlying shear zone. For faults that merges with the basement shear zone, a change from tabular to wedge-shaped geometry of the hangingwall synrift strata records a transition from non-rotational to rotational extension faulting, which we attribute to the time of rift fault's linkage with the shear zone, following downward propagation of its lower tip. N-S-striking faults are oblique to, and offset (rather than link with) intra-basement shear zones. These observations highlight the selective influence pre-existing intra-basement shear zones may (or may not) have on evolving rift-related normal faults.

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.

Extension-related buttress-like folds, the western side of the Gulf of Suez rift, Egypt

The wedge-shaped St. Paul block, western side of the Gulf of Suez rift, exposes Late Cretaceous beds that are folded into transverse folds. The block is bounded from the east and west by east- and southeast-dipping rift-related normal faults, respectively. This study reveals that the transverse folds are extension-related, and formed due to the buttress-like effect that was created during the movement along the rift-related normal faults. The more competent Eocene hanging-wall block of the N–S-striking fault buttressed against the moving less competent Cretaceous hanging-wall block of the NE–SW-striking fault. A localized zone of shortening developed between the two fault trends causing the intervening wedge-shaped block to be crumpled and folded into a series of kilometer-scale, gentle SSE-plunging folds. The buttressing, within block internal localized strain caused by the zigzag geometry of the NE–SW-striking fault, and the effect of the minor cross fault explain the SSE-to-ESE plunges of the mesoscopic folds. This study turns the attention to the buttress-like effect as one of the mechanisms of extension-related folds during rifting.

The ‘Helmut’ geophysical anomaly: A regional left-lateral transtensional shear zone system connecting Santos and Campos basins, southeastern Brazil

In the ultradeep water of Santos and Campos basins, eastwards from the area of present day pre-salt exploration in southeastern Brazil, there is a nearly 1000 km long, curved magnetic anomaly here named ‘Helmut’. On a magnetic map, the anomaly orientation switches from NE–SW in Santos basin to nearly N–S in Campos, mimicking the curved Neoproterozoic shear zone pattern of the basement exposed onshore. From the comparison between the position of such anomaly and gravity-derived modeling and residual maps, it is suggested that the magnetic anomaly also corresponds to a major gravimetric boundary, separating different crustal domains. This boundary is slightly oblique to the rift-related normal fault trend in Santos basin. Based on interpreted 3D seismic data, the NNE trending normal faults are dragged to the NE direction close to the magnetic anomaly in southern Santos basin, where an en echelon array of oblique magnetic anomalies occurs, which resulted mainly from the left lateral component of an overall transtensional displacement field.In regional 2D deep reflection seismic profiles, the ‘Helmut’ anomaly is correlated to major eastward dipping, down-to-the basin stepped faults sometimes arranged in a fan-like fashion in Santos basin, and as a landward dipping normal fault zone in Campos basin, north of the E–W Araruama transfer zone. In some areas, the fault zone displaces the base salt reflector and cross-cuts the entire stretched continental crust. The analysis of potential field and seismic data suggests that the ‘Helmut’ anomaly may be interpreted as a crustal-scale shear zone system, active after salt deposition during the Late Aptian. Regionally, the ‘Helmut’ anomaly connects two approximately N–S trending branches of the South Atlantic Rift System: the northern branch that runs from Espírito Santo basin northwards up to the Camamu (south of the Recôncavo-Tucano-Jatobá aborted trough), and the southern branch running southwards from Santos basin through Pelotas basin in Brazil as far as the basins offshore Uruguay and Argentina basins southwards. Extensional faulting of the salt reflector along early NNE rift-related normal faults and dextral transtension on the NW trending fault zones are all consistent with a sinistral transtensive motion on the NE-trending ‘Helmut’ anomaly during the Late Aptian.

Brittle tectonic reconstruction of palaeo-extension inherited from Mesozoic rifting in West Zagros (Kermanshah, Iran)

Abstract: The present-day Zagros mountain belt of SW Iran corresponds to the former Arabian passive continental margin of the southern Neo-Tethyan basin, which existed since a Permian–Triassic rifting episode and underwent later collisional deformation in mid- to late Cenozoic time. In West Zagros, the Kermanshah Radiolarian Trough, separated from the Neo-Tethyan basin by the narrow continental platform of the Bisotun Limestone, resulted from the Triassic–Jurassic rifting of the passive margin. Brittle tectonic analyses of syndepositional normal fault slip data have been undertaken to study the extensional history of the passive margin in terms of palaeostress reconstruction. Dominant east–west trends of the inherited rift-related normal faults in West Zagros indicate an approximately north–south trend of extension that prevailed during the entire Mesozoic syn- to post-rift evolution of the radiolarian trough. This extension characterizes a stretched continental margin similar to the present-day passive margin of the British Isles. Considering the structural pattern of the inherited basement faults, as revealed by the present-day earthquake focal mechanisms, an oblique crustal stretching model is proposed for the rifting process.

Early Tertiary sinistral transpression and fault reactivation in the western Vøring Basin, Norwegian Sea: Implications for hydrocarbon exploration and pre-breakup deformation in ocean margin basins

The Nyk High is an approximately northeast-trending, Late Cretaceous to early Tertiary structural high situated in the western Voring Basin, offshore Norway. It is defined by a thick sequence of Upper Cretaceous to lower Tertiary sediments that dip toward the southeast and are cut by normal faults with throws up to 1500 m (4921 ft). Across-fault sediment thickness variations and stratigraphic onlap relationships show that these faults were active during Campanian to early Paleocene extension, prior to the separation of Norway from Greenland ca. 55 Ma. Sediments preserved in the hanging walls of these rift-related normal faults are deformed by two populations of folds: one set is oriented clockwise of the fault strike; the other is oriented parallel to the fault strike. Stratigraphic relationships show that both sets of folds formed subsequent to rifting during the latest Paleocene to the earliest Eocene. We interpret these folds as having developed during minor sinistral transpression, which was partitioned between the northeast-southwest sinistral strike-slip reactivation of the normal faults and the northwest-southeast coaxial shortening (folding) within the fault hanging walls. The Nyk High is oriented counterclockwise to the inferred line of the continental breakup. We speculate that transpression was driven by north-northwest forces arising from the differential topography along the incipient plate boundary and the presence of hot, buoyant material beneath this elevated axial region. Such deformation will not necessarily impact significantly on source rock maturity or the distribution of reservoir sands but can give rise to unusual structural trap geometries. It is also likely to increase the connectivity of trap-bounding faults.

Structural properties of rift-related normal faults: the case study of the Gulf of Corinth, Greece

In the frame of the Corinth Rift Laboratory project, which aims to understand the relation between stress, strain and fluid flow in an extensional context, three normal faults have been studied and sampled in the south-western sector of the Gulf of Corinth. Two of these faults (the Pirgaki and Helike faults) juxtapose carbonate pre-rift sediments and syn-rift conglomerates at outcrop, and the third (the Aigion fault) has been drilled and cored at 760 m depth. The distance between the faults is around 4 km, and the offsets are approximately 1000, 700 and 170 m, respectively. The fault damage zones and cores of the analysed faults exhibit different geometry, size and textural characteristics. In general, it has been assessed that fault cores behave as near-impermeable barriers to transverse fluid flow, whereas highly fractured damage zones act mostly as conduits for nearly along-strike flow. Different types of fault rocks are exposed along the main faults; these include cataclasite, ultra-cataclasite, fault breccia and gouge, with random or foliated fabrics. Along the Pirgaki fault zone only, fault rock micro-fabrics have been related to multiple tectonic episodes. The results of our work also emphasise that the current conditions of deformation, in this sector of the Gulf of Corinth, are responsible for the development of a system of extension fractures that is only partially sealed. These fractures mainly control the permeability structure of the analysed fault zones and furnish information on the stress field acting in the area. Furthermore, their orientation and distribution indicate that they are consistent with a deformation pattern resulting from active extension.

Rotation of the syn-rift stress field of the northern Gulf of Aden margin, Yemen

Remote sensing and field studies of several extensional basins along the northern margin of the Gulf of Aden in Yemen show that Oligocene–Miocene syn-rift extension trends N20°E on average, in agreement with the E–W to N120°E strike of main rift-related normal faults, but oblique to the main trend of the Gulf (N70°E). These faults show a systematic reactivation under a 160°E extensional stress that we interpret also as syn-rift. The occurrence of these two successive phases of extension over more than 1000 km along the continental margin suggests a common origin linked to the rifting process. After discussing other possible mechanisms such as a change in plate motion, far-field effects of Arabia–Eurasia collision, and stress rotations in transfer zones, we present a working hypothesis that relates the 160°E extension to the westward propagation since about 20 Ma of the N70°E-trending, obliquely spreading, Gulf of Aden oceanic rift. The late 160°E extension, perpendicular to the direction of rift propagation, could result from crack-induced extension associated with the strain localization that characterises the rift-to-drift transition.

Aeromagnetic signatures over western Marie Byrd Land provide insight into magmatic arc basement, mafic magmatism and structure of the Eastern Ross Sea Rift flank

Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10–40 km in wavelength and with amplitudes ranging from 50 to 500 nT could reveal buried Late Devonian(?)–Early Carboniferous Ford Granodiorite plutons. This is suggested by similar magnetic signature over exposed, coeval Admiralty Intrusives of the Transantarctic Mountains (TAM). Geochemical data from mid-Cretaceous Byrd Coast Granite, contact metamorphic effects on Swanson Formation and hornblende-bearing granitoid dredge samples strengthen this magnetic interpretation, making alternative explanations less probable. These magnetic anomalies over formerly adjacent TAM and western Marie Byrd Land (wMBL) terranes resemble signatures typically observed over magnetite-rich magmatic arc plutons. Shorter wavelength (5 km) 150 nT anomalies could speculatively mark mid-Cretaceous mafic dikes of the E7, similar to those exposed over the adjacent Ford Ranges. Anomalies with amplitudes of 100–360 nT over the Sulzberger Bay and at the margin of the Sulzberger Ice Shelf likely reveal mafic Late Cenozoic(?) volcanic rocks emplaced along linear rift fabric trends. Buried volcanic rock at the margin of the interpreted half-graben-like “Sulzberger Ice Shelf Block” is modelled in the Kizer Island area. The volcanic rock is marked by a coincident positive Bouguer gravity anomaly. Late Cenozoic volcanic rocks over the TAM, in the RSR, and beneath the West Antarctic Ice Sheet exhibit comparable magnetic anomaly signature reflecting regional West Antarctic Rift fabric. Interpreted mafic magmatism of the E7 is likely related to mid-Cretaceous and Late Cenozoic regional crustal extension and possible mantle plume activity over wMBL. Magnetic lineaments of the E7 are enhanced in maximum horizontal gradient of pseudo-gravity, vertical derivative and 3D Euler Deconvolution maps. Apparent vertical offsets in magnetic basement at the location of the lineaments and spatially associated mafic dikes and volcanic rocks result from 2.5D magnetic modelling. A rift-related fault origin for the magnetic lineaments, segmenting the E7 region into horst and graben blocks, is proposed by comparison with offshore seismic reflection, marine gravity, on-land gravity, radio-echo sounding, apatite fission track data and structural geology. The NNW magnetic lineament, which we interpret to mark the eastern RSR shoulder, forms the western margin of the “Alexandra Mountains horst”. This fundamental aeromagnetic feature lies on strike with the Colbeck Trough, a prominent NNW half-graben linked to Late Cretaceous(?) and Cenozoic(?) faulting in the eastern RSR. East–west and north–north–east to NE magnetic trends are also imaged. Magnetic trends, if interpreted as reflecting the signature of rift-related normal faults, would imply N–S to NE crustal extension followed by later northwest–southeast directed extension. NW–SE extension would be compatible with Cenozoic(?) oblique RSR rifting. Previous structural data from the Ford Ranges have, however, been interpreted to indicate that both Cretaceous and Cenozoic extensions were N–S to NE–SW directed.

Geometry and scaling relations of a population of very small rift-related normal faults

Exceptionally well exposed normal faults within the Solite Quarry of the Dan River rift basin range in length from a few millimetres to a few metres and are possibly the smallest visible faults studied to date. Displacement is greatest at or near the center of isolated faults and decreases toward the fault tips. Relay structures form between closely overlapping faults. The distribution of fault sizes in the study area follows a power-law (fractal) relation, and the maximum observed displacement scales linearly with fault length. The new fault data extend the global data set to more than eight orders of magnitude of fault length and indicate that there is no significant change in displacement geometry and the linear length-displacement scaling relation between small and large faults.