Rift Faults Research Articles

Fault growth and interactions in a multiphase rift fault network: Horda Platform, Norwegian North Sea

Physical models predict that multiphase rifts that experience a change in extension direction between stretching phases will typically develop non-colinear normal fault sets. Furthermore, multiphase rifts will display a greater frequency and range of styles of fault interactions than single-phase rifts. Although these physical models have yielded useful information on the evolution of fault networks in map view, the true 3D geometry of the faults and associated interactions are poorly understood. Here, we use an integrated 3D seismic reflection and borehole dataset to examine a range of fault interactions that occur in a natural multiphase fault network in the northern Horda Platform, northern North Sea. In particular we aim to: i) determine the range of styles of fault interaction that occur between non-colinear faults; ii) examine the typical geometries and throw patterns associated with each of these different styles; and iii) highlight the differences between single-phase and multiphase rift fault networks. Our study focuses on a ca. 350 km2 region around the >60 km long, N–S-striking Tusse Fault, a normal fault system that was active in the Permian–Triassic and again in the Late Jurassic-to-Early Cretaceous. The Tusse Fault is one of a series of large (>1500 m throw) N–S-striking faults forming part of the northern Horda Platform fault network, which includes numerous smaller (2–10 km long), lower throw (<100 m), predominantly NW–SE-striking faults that were only active during the Late Jurassic to Early Cretaceous. We examine how the 2nd-stage NW–SE-striking faults grew, interacted and linked with the N–S-striking Tusse Fault, documenting a range of interaction styles including mechanical and kinematic isolation, abutment, retardation and reactivated relays. Our results demonstrate that: i) isolated, and abutting interactions are the most common fault interaction styles in the northern Horda Platform; ii) pre-existing faults can act as sites of nucleation for 2nd-stage faults or may form mechanical barriers to propagation; iii) the throw distribution on reactivated 1st-stage faults will be modified in a predictable manner if they are intersected or influenced by 2nd-stage faults; iv) sites of fault linkage and relay-breaching associated with the first phase of extension can act as preferential nucleation sites for 2nd-stage faults; and v) the development of fault intersections is a dynamic process, involving the gradual transition from one style to another.

Structural architecture of the southern Mount Isa terrane in Queensland inferred from magnetic and gravity data

The basement rocks of the southern Mount Isa terrane are concealed under younger sedimentary units and its crustal architecture is understood using constrained regional potential field analysis. Prominent N-S- to NNW-trending geophysical anomalies extend for ∼250km south of the exposed Mount Isa Inlier and are abruptly terminated by the NE-trending Cork Fault. Palaeoproterozoic basinal successions and major Palaeo- to Mesoproterozoic structures recorded in the Mount Isa Inlier are interpreted to continue southward under the Palaeozoic cover. The intensely positive geophysical signature of the region is mostly attributed to shallowing of the Barramundi-aged basement and distribution of metamorphosed sedimentary and volcanic rocks deposited during the formation of the ca. 1790–1730Ma Leichhardt Superbasin. Regional low magnetic and low gravity responses may reflect basinal sequences deposited during the formation of the ca. 1725–1690Ma Calvert Superbasin and the ca. 1675–1595Ma Isa Superbasin. Short wavelength magnetic anomalies and co-located low intensity Bouguer gravity anomalies are interpreted to represent shallow and variously magnetized granitic intrusions.The geophysical signature of the southern Mount Isa terrane reflects the bipartite fault architecture of the region. In the Western Fold Belt, Palaeoproterozoic multiple basin formation was controlled by west-dipping faults in half graben setting, within the NNW-trending Leichhardt River Fault Trough. To the east of the Pilgrim Fault, thick Palaeoproterozoic sedimentary and volcanic successions were controlled by major NNW-trending, east-dipping normal faults.The Mesoproterozoic Isan Orogeny reactivated the existing extensional fault network. Deformation during the ca. 1595–1580Ma Early Isan Orogeny is not apparent in the southern Mount Isa terrane and may have been confined to the northern part of the terrane. The ca. 1570–1550Ma Middle Isan Orogeny produced inversion of major NNW-trending rift faults and resulted in regional scale culminations. The ca. 1550–1540Ma wrench tectonics is represented by the activation of NW- and NE-oriented strike-slip faults overprinting the prominent N-S- to NNW-oriented structures.The emplacement of elongated batholitic bodies may be coeval to the emplacement of the ca. 1540–1520Ma Williams Supersuite recorded in the Mount Isa Inlier and thus may represent its southern extension. The southern Mount Isa terrane also appears to have been affected by post-Isan tectonics that resulted in deposition and magmatic intrusions focused along the northern side of the NE-trending Cork Fault.

Lateral variation in structural style along an evaporite-influenced rift fault system in the Halten Terrace, Norway: Influence of basement structure and evaporite facies

The Halten Terrace is underlain by a Triassic evaporitic package, resulting in vertically decoupled normal fault systems following subsequent extension. Four structural domains are identified along the eastern margin of the Halten Terrace, characterised by: i) thick-skinned normal faults affecting both sub-salt and supra-salt cover, ii) basement-restricted normal faults associated with fault-propagation folds, iii) thick-skinned, distributed normal faults, and iv) thick-skinned, localised normal faults. A fault domain boundary associated with an NE–SW striking basement fault corresponds to an abrupt change in style in the north of the Halten Terrace. Summed throw and estimated strain measurements show that throw and strain accommodated by the fault system increase southward, corresponding to a transition from distributed to localised faulting. The evaporite package is variable in thickness, but those variations do not correspond spatially to variations in structural style. Wells that penetrate the evaporite package, and volume attribute analysis of 3D seismic data, suggest variable evaporite facies. A change in seismic attributes from high-amplitude, low variance to low-amplitude, high variance corresponds to a change from decoupled to thick-skinned faulting. The sub-evaporite fault template, amount of strain accommodated across the fault system, and facies variations in the evaporite package are key influences on structural style.

The inverted Triassic rift of the Marrakech High Atlas: A reappraisal of basin geometries and faulting histories

The High Atlas of Morocco is an aborted rift developed during the Triassic–Jurassic and moderately inverted during the Cenozoic. The Marrakech High Atlas, with large exposures of basement and Triassic early syn-rift deposits, is ideal to investigate the geometries of the deepest parts of a rift, constituting a good analogue for pre-salt domains. It allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another. A detailed structural study of the main Triassic basins and basin-margin faults of the Marrakech High Atlas shows that only a few rift faults were reactivated during the Cenozoic compressional stage in contrast to previous interpretations, and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems. Preserved extensional features demonstrate a dominant dip-slip opening kinematics with strike-slip playing a minor role, at variance to models proposing a major strike-slip component along the main basin-bounding faults, including faults belonging to the Tizi n'Test fault zone. A new Middle Triassic paleogeographic reconstruction shows that the Marrakech High Atlas was a narrow and segmented orthogonal rift (sub-perpendicular to the main regional extension direction which was ~NW–SE), in contrast to the central and eastern segments of the Atlas rift which developed obliquely. This difference in orientation is attributed to the indented Ouzellarh Precambrian salient, part of the West African Craton, which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution. As for the Cenozoic inversion, total orogenic shortening is moderate (~16%) and appears accommodated by basement-involved large-scale folding, and by newly formed shortcut and by-pass thrusting, with rare left-lateral strike-slip indicators. Triassic faults commonly acted as buttresses.

Brittle reactivation of ductile shear zones in NW Namibia in relation to South Atlantic rifting

Rifting has occurred worldwide along preexisting mobile belts, which are therefore thought to control rift orientation on a large scale. On a smaller scale, shear zones within mobile belts are reactivated as rift faults. In NW Namibia, shear zones of the Neoproterozoic Kaoko Belt run subparallel to the present-day continental passive margin and are inferred to have been reactivated during the opening of the South Atlantic Ocean. However, the extent of this reactivation and the influence of the reactivated shear zones on South Atlantic rifting are largely unknown. A combined remote sensing and field study was conducted to quantify offsets that are a direct function of shear zone reactivation. The shear zones of the Kaoko Belt are partly overlain by the Parana-Etendeka volcanic rocks, which were emplaced shortly before or simultaneously to the Atlantic rifting. Faulting within these volcanic rocks can be linked to synrift or postrift movements. Along the shear zones, downfaulting of the basalts is widespread along listric faults where half-graben form in the hanging wall. At three sites we could determine vertical offsets of ~1180 m, ~470 m, and ~70 m. Although many shear zones were reactivated as faults, these are isolated, and offsets are small, suggesting that reactivation occurred only as a side effect of the rifting and that the Kaoko Belt shear zones have not exerted a significant influence on the rift orientation.

Elucidating the geometry of the active Shanchiao Fault in the Taipei metropolis, northern Taiwan, and the reactivation relationship with preexisting orogen structures

The Shanchiao Fault is an active normal fault with documented paleoearthquakes in the Taipei metropolis, Taiwan. While posing direct seismic threat on the multimillion population, its crustal-scale fault plane configuration has not been constrained. This study presents the first attempt to resolve the fault plane dip changes of the Shanchiao Fault within the upper crust by forward modeling late Quaternary deformation. Tectonic subsidence over the last ~23 ka is estimated from vertical displacements of a rapidly formed alluvial fan horizon deformed into a dramatic rollover monocline. A 2-D profile across the Shanchiao Fault is chosen for elastic half-space dislocation modeling, and the results suggest that the fault is listric in the shallow crust with an abrupt change from subvertical ramp (85°–75°) to near-horizontal flat (10°–15°) at 3–4 km depth, consistent with an origin from the inversion of an orogen-related thrust detachment. Given the presence of rift-related fabrics in the underthrust Chinese Continental Margin basement beneath the Taiwanese orogenic wedge, listric ramp-flat-ramp models with a second deeper bend to 60° dip are also tested. Reasonable fits with the geological observations are produced when the lower ramp is located at greater than 8 km depth, which correlates with the hypocentral location of a moderate earthquake in 2004. Joint reactivation of preexisting thrust and rift faults by the Shanchiao Fault is therefore plausible with implications for seismic hazard in the Taipei area.

Structure and kinematics of the Taupo Rift, New Zealand

The structure and kinematics of the continental intra-arc Taupo Rift have been constrained by fault-trace mapping, a large catalogue of focal mechanisms (N = 202) and fault slip striations. The mean extension direction of ~137° is approximately orthogonal to the regional trend of the rift and arc front (α = 84° and 79°, respectively) and to the strike of the underlying subducting Pacific Plate. Bending and rollback of the subduction hinge strongly influence the location, orientation, and extension direction of intra-arc rifting in the North Island. In detail, orthogonal rifting (α = 85–90°) transitions northward to oblique rifting (α = 69–71°) across a paleovertical-axis rotation boundary where rift faults, extension directions, and basement fabric rotate by ~20–25°. Toward the south, extension is orthogonal to normal faults which are parallel to, and reactivate, steeply dipping basement fabric. Basement reactivation facilitates strain partitioning with a portion of margin-parallel motion in the overriding plate mainly accommodated east of the rift by strike-slip faults in the North Island Fault System (NIFS). Toward the north where the rift and NIFS intersect, ~4 mm/yr strike slip is transferred into the rift with net oblique extension accommodating a component of margin-parallel motion. The trend and kinematics of the Taupo Rift are comparable to late Miocene-Pliocene intra-arc rifting in the Taranaki Basin, indicating that the northeast strike of the subducting plate and the southeast extension direction have been uniform since at least 4 Ma.

Present‐day kinematics of the East African Rift

The East African Rift (EAR) is a type locale for investigating the processes that drive continental rifting and breakup. The current kinematics of this ~5000 km long divergent plate boundary between the Nubia and Somalia plates is starting to be unraveled thanks to a recent augmentation of space geodetic data in Africa. Here we use a new data set combining episodic GPS measurements with continuous measurements on the Nubian, Somalian, and Antarctic plates, together with earthquake slip vector directions and geologic indicators along the Southwest Indian Ridge to update the present‐day kinematics of the EAR. We use geological and seismological data to determine the main rift faults and solve for rigid block rotations while accounting for elastic strain accumulation on locked active faults. We find that the data are best fit with a model that includes three microplates embedded within the EAR, between Nubia and Somalia (Victoria, Rovuma, and Lwandle), consistent with previous findings but with slower extension rates. We find that earthquake slip vectors provide information that is consistent with the GPS velocities and helps to significantly reduce uncertainties of plate angular velocity estimates. We also find that 3.16 Myr MORVEL average spreading rates along the Southwest Indian Ridge are systematically faster than prediction from GPS data alone. This likely indicates that outward displacement along the SWIR is larger than the default value used in the MORVEL plate motion model.

Upper crustal earthquake swarms in São Caetano: Reactivation of the Pernambuco shear zone and trending branches in intraplate Brazil

Seismogenic fault reactivation of continental-scale structures has been observed in a few intraplate areas, but its cause is still a matter of debate. The objective of the present study is to analyze two seismic swarms that occurred along the EW-trending Pernambuco ductile shear zone and in a NE-trending branch, in 2007 and 2010 in São Caetano County, Northeastern Brazil. We studied both epicentral areas using a nine- and a seven-station network during 180 and 54 days, respectively. The results indicate that the 2007 swarm correspond to a right-lateral, strike–slip fault with a normal component of slip (strike 74°, dip 60°, and rake − 145°) and the 2010 swarm corresponds to a normal fault (strike 265°, dip 79°, and rake − 91°). The former reactivated a NE-trending branch, whereas the latter reactivated the main E-W-trending mylonitic belt of the Pernambuco shear zone. These results are consistent with seismogenic reactivation of this major structure, generated by the present-day EW-trending compression and NS-trending extension, as observed by previous studies. This shear zone was reactivated as rift faults in the Cretaceous during the South America–Africa breakup. However, our study confirms that the basement fabric such as continental-scale ductile shear zones, show evidence of crustal weakness outside areas of previous rifting, and it reveals the potential for large earthquakes along dormant rift segments associated with major basement shear belts.

Anomalous passive subsidence of deep‐water sedimentary basins: a prearc basin example, southern New Caledonia Trough and Taranaki Basin, New Zealand

AbstractStratigraphic data from petroleum wells and seismic reflection analysis reveal two distinct episodes of subsidence in the southern New Caledonia Trough and deep‐water Taranaki Basin. Tectonic subsidence of ~2.5 km was related to Cretaceous rift faulting and post‐rift thermal subsidence, and ~1.5 km of anomalous passive tectonic subsidence occurred during Cenozoic time. Pure‐shear stretching by factors of up to 2 is estimated for the first phase of subsidence from the exponential decay of post‐rift subsidence. The second subsidence event occured ~40 Ma after rifting ceased, and was not associated with faulting in the upper crust. Eocene subsidence patterns indicate northward tilting of the basin, followed by rapid regional subsidence during the Oligocene and Early Miocene. The resulting basin is 300–500 km wide and over 2000 km long, includes part of Taranaki Basin, and is not easily explained by any classic model of lithosphere deformation or cooling. The spatial scale of the basin, paucity of Cenozoic crustal faulting, and magnitudes of subsidence suggest a regional process that acted from below, probably originating within the upper mantle. This process was likely associated with inception of nearby Australia‐Pacific plate convergence, which ultimately formed the Tonga‐Kermadec subduction zone. Our study demonstrates that shallow‐water environments persisted for longer and their associated sedimentary sequences are hence thicker than would be predicted by any rift basin model that produces such large values of subsidence and an equivalent water depth. We suggest that convective processes within the upper mantle can influence the sedimentary facies distribution and thermal architecture of deep‐water basins, and that not all deep‐water basins are simply the evolved products of the same processes that produce shallow‐water sedimentary basins. This may be particularly true during the inception of subduction zones, and we suggest the term ‘prearc’ basin to describe this tectonic setting.

Spatial and temporal trends in pre-caldera Jemez Mountains volcanic and fault activity

New 40Ar/39Ar dates from the Jemez Mountain volcanic field (JMVF) reveal formerly unrecognized shifts in the loci of pre-caldera volcanic centers across the northern Jemez Mountains; these shifts are interpreted to coincide with episodes of Rio Grande rift faulting. Early activity in the field includes two eruptive pulses: 10.8–9.2 Ma basaltic to dacitic volcanism on Lobato Mesa in the northeastern JMVF and 12–9 Ma mafic to silicic volcanism in the southwestern JMVF. While 9–7 Ma eruptions persisted in the southern JMVF, a new eruptive center developed on the La Grulla Plateau in the northwestern JMVF (8.7–7.2 Ma), corresponding with a period of rift widening caused by reactivation of Laramide faults in this area. The older 8.7–7.8 Ma mafic lavas emitted from Encino Point and the younger 7.7–7.2 Ma trachyandesite and dacite erupted on the La Grulla Plateau are assigned to a new unit called the La Grulla Formation. The chemical composition of a 640 m stack of lava flows exposed in the northern margin of the Valles caldera changes from dacite to andesite, then back to dacite upsection, becoming slightly more alkalic upward. The shift to more alkalic compositions occurs across a sedimentary break, marking a subtle change in magma source for the older Paliza Canyon Formation and the younger La Grulla Formation lavas. New age constraints from a rhyolite intrusion in the southern JMVF and pumiceous rhyolite deposits in the northern JMVF suggest an episode of localized, 7.6–7.8 Ma rhyolitic volcanism that occurred in the central part of the JMVF between 12–8 Ma Canovas Canyon Rhyolite and 7–6 Ma peak Bearhead Rhyolite volcanism. Younger Bearhead Rhyolite intrusions (7.1–6.5 Ma) are more widespread than previously documented, extending into the northeastern JMVF. Tschicoma Formation dacite erupted at 5 Ma in the Sierra de los Valles and then erupted throughout the northeastern JMVF 5–2 Ma. The more refined geochronology of the JMVF indicates that pre-caldera volcanic centers were characterized by geographically and chemically distinct, relatively short-lived, episodes of activity. Volcanism generally migrated eastward through time in the southern JMVF, but the pattern in the northern JMVF had a more complex east (10–9 Ma) to west (9–7 Ma) to east (5–2 Ma) pattern that reflects the timing of motion on faults. The new ages, coupled with detailed mapping of both volcanic rocks and the Santa Fe Group, document significant pulses of faulting, erosion, and deposition during middle Miocene time and during late Miocene time across the Cañones fault zone in the northern JMVF.

Late to post-Appalachian strain partitioning and extension in the Blue Ridge of Alabama and Georgia

Structural observations and U-Pb and 40 Ar/ 39 Ar isotopic age dates are reported for shear zones and metamorphic rocks in the southernmost Appalachian Blue Ridge. Two major mylonite zones, the Goodwater-Enitachopco and Alexander City fault zones, have retrograded peak amphibolite facies fabrics and assemblages in rocks of the ancient Laurentian margin. Both faults are within a zone of transition between Carboniferous (Alleghanian) west-directed thrusts in the foreland and synchronous strike-parallel dextral shear zones in the hinterland. The 40 Ar/ 39 Ar hornblende and muscovite dates record late Mississippian cooling and exhumation from the Late Devonian (Neoacadian orogeny, 380–340 Ma) peak. Retrograde mylonites of the Goodwater-Enitachopco fault are of two types. Earlier formed, type 1, upper greenschist to lower amphibolite facies shears are roughly coplanar with the dominant schistosity of the country rock, and show northwest-southeast stretching. Later formed, type 2 shears are discrete, steeply dipping, middle to upper greenschist facies shear zones that cut across the type 1 shears, displacing them in an oblique dextral and normal slip sense. A 366.5 ± 3.5 Ma U-Pb SHRIMP-RG (sensitive high-resolution ion microprobe–reverse geometry) date on zircon from a prekinematic trondhjemite dike that is cut by a type 2 shear zone places a maximum age on the time of movement along the Goodwater-Enitachopco fault. The 40 Ar/ 39 Ar cooling dates place a minimum on the timing of extensional movement along the type 1 shears of between ca. 334 and 327 Ma. The Goodwater-Enitachopco fault coincides at depth with a basement step up that has been interpreted as a Cambrian rift fault formed along the ancient Laurentian margin, possibly reflecting its reactivation during Mesozoic rifting of Pangea. The Alexander City fault zone is a middle greenschist facies, dextral strike-slip fault rather than a west-vergent thrust fault, as was previously thought. This fault zone is obliquely cut and extended by more east-trending, subvertical, cataclastic faults (Mesozoic?) characterized by intense quartz veining. These brittle faults resemble those in other parts of the Blue Ridge, Inner Piedmont, and Pine Mountain terrane and, together with the Goodwater-Enitachopco and Towaliga faults, they appear to form a broad graben-like structure across the entire piedmont. Shoulder rocks flanking the Alexander City fault zone contain earlier formed (peak to late peak metamorphic) dextral shear zones. A 369.4 ± 4.8 Ma U-Pb thermal ionization mass spectrometry date on zircon records the time of crystallization of a dike that cuts one of the shears bracketing the peak metamorphic fabric to between ca. 388 and 369 Ma and places a minimum age for right-slip shearing. Similar kinematics, geometries, tectonostratigraphic positions, and timing indicate that these Devonian shears are more southern counterparts to the system of Neoacadian dextral faults exposed in the North Carolina Blue Ridge. Kinematic analysis of the Goodwater-Enitachopco and Alexander City faults document that dextral strains in the Alabama and western Georgia Blue Ridge are partitioned much farther toward the foreland than is reported to the northeast, likely as a consequence of the southern Appalachian master decollement having passed obliquely across a several kilometer step up along the Cartersville transform. The top-to-the-south-southeast normal-slip component of movement along the Goodwater-Enitachopco fault is unusual, considering its position far toward the foreland. Loose timing constraints for this extensional event (late Carboniferous to Early Jurassic) leave room for several tectonic explanations, but we favor the following. (1) Late Pennsylvanian to Early Permian crustal thickening created a wedge of Blue Ridge rocks bound above by the Goodwater-Enitachopco, below by the decollement, and to the northwest (present-day direction) by a topographically steep mountain front. (2) Further convergence and crustal thickening caused this wedge to gravitationally collapse with southward-driven motion. (3) Mesozoic rifting reactivated some of the faults as the Gulf of Mexico began to open.

Scale-dependent influence of pre-existing basement shear zones on rift faulting: a case study from NE Brazil

Rifting of continental crust initiates faults that are commonly influenced by pre-existing structures. We document newly identified faults cutting Precambrian units in the interior of the NE Brazilian margin to assess the effects of structural inheritance on both rift geometry and fault architecture. Stratigraphic and structural data indicate that the faults were active in the main phase of rifting of Gondwana. The influence of pre-existing structures on the Mesozoic rift faulting is scale dependent. Regionally, the faults trend parallel to subvertical, crustal-scale Brasiliano ( c . 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust indicated by shear-wave splitting controlled the overall orientation and kinematics of the rift faults. However, outcrop observations of the faults show that at scales up to hundreds of metres, mylonitic foliations have little influence on fault architectures. Faults cross-cut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. This observation explains the range of focal mechanisms associated with seismicity that coincides with ancient shear zones in intra-continental areas.

Mesozoic fault reactivation along the St. Lawrence rift system, eastern Canada: Thermochronologic evidence from apatite fission-track dating

The St. Lawrence rift system is an active fault zone where reactivation of Late Proterozoic, Iapetus-related faults is believed to occur. It forms both a NE-SW–trending half graben and NW-SE–trending grabens that link and include the Ottawa-Bonnechere and Saguenay River grabens, both interpreted as aulacogens. The rift faults locally preserve kinematic evidence for normal-sense displacement. The lack of isotopic chronological constraints and the absence of strata younger than the Ordovician make it difficult to determine the timing of fault increments. Field relations in the Quebec City, Charlevoix, and Saguenay River regions suggest faulting event(s) younger than the Devonian-age Charlevoix crater. We determined apatite fission-track (AFT) ages for Grenvillian basement rocks from the hanging wall and footwall of typical rift faults at different locations of the St. Lawrence and Saguenay River fault systems along the St. Lawrence rift system. The footwall rocks of major faults yielded Late Triassic–Early Jurassic AFT ages ranging from 235 ± 28 Ma to 184 ± 22 Ma. The hanging-wall samples yielded older ages, between 443 ± 44 Ma and 279 ± 28 Ma along the NW-trending Saguenay River fault system that are consistent with Triassic–Early Jurassic normal faulting at ca. 250–200 Ma based on HeFTy inverse models of fission-track length distributions. Younger Late Jurassic AFT ages, ranging from 152 ± 17 Ma to 149 ± 14 Ma in the hanging wall of the NE-SW–trending St. Lawrence fault system, suggest the occurrence of localized faulting increments during the Late Jurassic. The latter AFT age discontinuities are interpreted as the result of normal faulting at ca. 250–200 Ma, followed by tectonic inversion. Evidence for the reactivation and inversion of preexisting normal faults during or after Jurassic time is also found in the Mesozoic rift-related basins of the Atlantic coast. The NW-SE–trending compressive stress field currently documented along the St. Lawrence River is consistent with tectonic inversion along the St. Lawrence rift system. This study provides geochronological support for Atlantic rift-related extensional deformation followed by compressional deformation in the interior of the Canadian Shield, more than 500 km west of the axis of the Mesozoic Atlantic rift basins.

The Analysis of Sedimentary Evolution Mode of Denglouku-Formation in Gulong Area of Songliao Basin

By making use of the datum of seismic, drilling, logging and cores, this thesis conducted a study on the sequence stratigraphic framework and sedimentary characteristics of Denglouku-Formation in Gulong area, and analysed the sedimentary evolution mode of Denglouku Formation in Gulong area. The results show that Denglouku Formation in Gulong area can be divided into one supersequence Ⅰ, four third-order sequences (Sq1-Sq4). The depositional systems developed in Denglouku Formation in Gulong area mainly include alluvial fan, fan delta, sublacustrine fan, fluvial facies, braided river delta, shallow water delta, and lake sediment and so on. The distribution features and sedimentary evolution pattern of depositional systems of each third-order sequence are significant difference. The formations of Sq2 and lower Sq1 are controlled by rift faulting and their characteristics of sedimentary filling conform to the continental faulted basin sedimentary evolution mode. Sq3 and Sq4 formation and upper formation of Sq2 have characteristics of depression sequence stratigraphic. The upper formation of Sq2 and Sq3 formation belong to the "river - shallow delta" sedimentary model of depression basin. And Sq4 formation conforms to the “flood-overlake” model in depression basin in droughty climate.

Reactive transport modelling applied to the prediction of the dolomitization distribution in carbonate hydrocarbon reservoirs

Reactive transport modelling can be applied to dolomitization to test interpretations of the diagenetic process and to help predict the lateral extent of dolomite reservoirs. Here we report applications to four different cases of dolomitization. The first case simulates the compaction-driven flow model of a drowned palaeohigh and demonstrates that the main limiting constraint is the ratio between the volume of basinal fluids and the rock volume in the palaeohigh. In the second case, hydrothermal dolomitization was simulated in a shallow burial setting above deep-seated rift faults, and shows the importance fault geometry may have in constraining the geometry of the dolomitized body. In the third and fourth cases, two synsedimentary dolomitization processes, thermal convection and mesohaline reflux, were tested in modelling an isolated carbonate platform. Thermal convection produced a dolomitized wedge in the bank margin area, and reflux model simulation shows that some dolomitization can also be produced by mesohaline seawater influx from the top of the platform. In all cases, the simulations suggest that the hydrogeological system is the most important driver for dolomitization, and the final geometry of dolomite bodies is greatly affected by the permeability and the presence of fractures.

Discovery of Yanting-Tongnan trough of late Permian in Sichuan basin and its significance

Reef-bank of Sichuan (四川) basin have been explored along the platform margin belt on both sides of the Kaijiang (开江)-Liangping (梁平) trough for a long time. Correspondingly, a series of major breakthroughs have been obtained and a number of large oil and gas fields have been found. After the breakthroughs of Kaijiang-Liangping trough, how reef-bank exploration will proceed? Is there another “Kaijiang-Liangping trough” in Sichuan basin? Previous research shows that Sichuan basin underwent intracratonic rift and basement fault activities in Late Permian, developing a paleogeography framework of “three uplifts and three depressions” under the background of the regional large ramp, including Exi (鄂西)-Chengkou (城口) trough, Kaijiang-Liangping trough and Yanting (盐 亭)-Tongnan (潼南) trough. Based on sedimentology and tectonics dynamics analysis, data of drillings, outcrops and seismic, Yanting-Tongnan trough has been discovered in central Sichuan basin in our study which distributes from Mianzhu (绵竹), Jiangyou (江油) in western Sichuan basin through Nanchong (南充) to Guang’an (广安), as long as about 210 km, extending in NW-SE nearly paralleling to Kaijiang-Liangping trough, opening to the sea north-westward, and gradually pinchout to the east platform. Similar to Kaijiang-Liangping trough, another platform margin reef belt distributes along a U-shape belt around Yanting-Tongnan trough, which is expected to become another natural gas accumulation zone after the platform margin around Kaijiang-Liangping trough. The discovery of Yanting-Tongnan trough expands favorable reef exploration area from original 2×104 km2 on both sides of Kaijiang-Liangping trough to more than 6×104 km2 in Yanting-Tongnan trough platform margin belt and Kaijiang-Liangping platform margin belt, greatly enlarging the reef exploration field. But compared to Kaijiang-Liangping trough, Yanting-Tongnan trough is relatively shallower and smaller, and the scale and quality of reef-bank reservoirs in its margin belt is a little poor relatively. Therefore, much research should be done timely such as comparative study of two troughs, detailed research of formation and evolution of the trough, regularity of reef migration, meticulous depiction of reef-bank reservoir size and distribution in order to obtain major breakthroughs around the Yanting-Tongnan trough.

Major Miocene exhumation by fault‐propagation folding within a metamorphosed, early Paleozoic thrust belt: Northwestern Argentina

The central Andean retroarc thrust belt is characterized by a southward transition at ∼22°S in structural style (thin‐skinned in Bolivia, thick‐skinned in Argentina) and apparent magnitude of Cenozoic shortening (&gt;100 km more in the north). With the aim of evaluating the abruptness and cause of this transition, we conducted a geological and geo‐thermochronological study of the Cachi Range (∼24–25°S), which is a prominent topographic feature at this latitude. Our U‐Pb detrital zircon results from the oldest exposed rocks (Puncoviscana Formation) constrain deposition to mainly Cambrian time, followed by major, Cambro‐Ordovician shortening and ∼484 Ma magmatism. Later, Cretaceous rift faults were locally inverted during Cenozoic shortening. Coupled with previous work, our new (U‐Th)/He zircon results require 8–10 km of Miocene exhumation that was likely associated with fault‐propagation folding within the Cachi Range. After Miocene shortening, displacement on sinistral strike‐slip faults demonstrates a change in stress state to a non‐vertically orientedσ3. This change in stress state may result from an increase in gravitational potential energy in response to significant crustal thickening and/or lithospheric root removal. Our finding of localized Cenozoic shortening in the Cachi Range increases the estimate of the local magnitude of shortening, but still suggests that significantly less shortening was accommodated south of the thin‐skinned Bolivian fold‐thrust belt. Our results also underscore the importance of the pre‐existing stratigraphic and structural architecture in orogens in influencing the style of subsequent deformation.

Wedges and buffers: some new structural observations from the Dnieper–Donets Basin, onshore Ukraine

Abstract The onshore Ukraine Dnieper–Donets Basin (DDB) is a large partially inverted late Palaeozoic intracratonic basin. Rifting began in the late Devonian when two evaporite sequences were deposited, and Carboniferous post-rift thermal sag was accompanied in some areas by some extension. Although important quantities of Devonian salt are calibrated by wells and diapirs have been documented, classic salt diapirs are probably not as common as suggested by the literature. Indeed, in several parts of the basin salt movement was stopped and buried by up to 5 km of overburden. Four important tectonic events related to salt movement have been documented over the DDB: Tournaisian–Early Visean extension, Middle Serpukhovian extension, Late Carboniferous–Early Permian movements mostly documented recently as extension and uplift and Alpine compression. Seismic imaging of the basin has not traditionally been of the highest quality, but the reprocessed seismic data made available over several parts of the DDB as part of a Ferrexpo/RDS multi-licence project has enabled three new structural observations of the basin to be made. Firstly, it is proposed that basin margin extension, facilitated by listric faulting with thin-skinned décollement on Devonian salt and exemplified by well-imaged Tournaisian syntectonic wedges, is linked to thick-skinned partial inversion of some basement rift faults. Secondly, it is proposed that these thick-skinned faults acted as obstacles or buffers, where monoclines began to develop. These folds were exaggerated by the later tectonic movements. Thirdly, we propose that these linked tectonic events may have been caused by pulses of sag which punctuated the Lower Carboniferous post-rift thermal sag process. These three observations fit neatly with published interpretations of Tournaisian–Lower Visean and Middle Serpukhovian extension. Finally, however, a literature review and new seismic interpretation also clearly show that Late Carboniferous–Early Permian movements inverted marginal listric faults and their Tournaisian wedges, and also inverted the downdip thick-skinned faults causing the monoclines to further develop prior to the Alpine overprint. This, contrary to many published papers over the last 15 years or so, suggests that this Late Carboniferous–Early Permian event brought shortening rather than extension to the DDB.

Neogene extensional deformation and related stress regimes in central Tunisia

A study of brittle fracturing in onshore Tunisia, including field and seismic data, suggests that the conspicuous NW–SE-trending rift system of the Atlasic domain postdates the main compressional event of Tortonian age (Late Miocene). Therefore, the extensional event would be recent, occurring from the latest Miocene to the Quaternary. From fault-slip data set, we demonstrate that the dominant stress regime coeval with the Neogene rifting was purely extensional, with the extensional stress being oriented NE–SW normal to the average trend of the rift faults, excluding any kind of lateral displacements along the rift boundaries. We discuss the integration of the Tunisian rift-system in the context of the eastward crustal thinning in Tunisia and the development of the offshore Sicilian–Tunisian rift system. The upper-crustal stretching within the Pantelleria–Tunisia domain results probably from the migration of the Ionian block towards the Hellenic subduction zone in the NE.