Strike-slip Stress Regime Research Articles

`Crack–seal', slip: a new fault valve mechanism?

This paper presents a new model of fault development in carbonate rocks involving a crack–seal–slip sequence. The structures of sheared calcite veins from the Les Matelles outcrop, Languedoc (S. France), and the observations used to construct this new model which integrates aspects of `crack–seal' evolution of calcite-filled veins with concepts of fault valve behaviour are described. In our model, hydraulic mode I reopening of an oblique pre-existing vein in an overall strike-slip stress regime is accompanied by precipitation of calcite, but significant fault slip cannot occur initially despite this obliquity because the ends of the pre-existing structure limit further reopening propagation beyond the tips. The rate of aligned calcite precipitation keeps pace with the rate of dilation of the structure, so that calcite cement essentially seals the system. Stress concentrations at the tips are allowed to rise with reopening until failure of the tip zone results in branch crack formation, triggering both slip along the vein and hydraulic pressure drop. This is followed by sealing within the branch cracks. Such a crack–seal–slip cycle may be repeated several times, as evidenced by fault-perpendicular calcite vein growth interlayered with calc-mylonite lamellae within these structures. Later cycles will become less pronounced because strength recovery of the sealed branch cracks does not regain the initial strength of the intact rock. This model could apply at various scales, and could be a mechanism for triggering earthquakes.

Focal mechanisms of small earthquakes in the southeastern Brazilian shield: a test of stress models of the South American plate

Focal mechanisms of small earthquakes with magnitudes of about 3 in the SE Brazilian shield are calculated using S/P amplitude ratios. Low attenuation (Qp from 400 to 800) in the shield upper-crustal layers allowed sharp S arrivals to be recorded up to distances of 100km. Besides P-wave polarities, SH-wave first motions were also used to constrain the nodal-plane orientations. Normal and reverse faulting mechanisms with strike-slip components were found. The inversion of four mechanisms to estimate the stress tensor indicated a strike-slip stress regime with roughly E–W-orientated σ1 and N–S σ3. Both the orientations and the shape factor (φ=0.7) of the inverted stress are in excellent agreement with theoretical predictions for that part of Brazil from the driving-force model of Coblentz & Richardson (1996). Good agreement with the nature of the stress, as well as its orientation, was also found for the model of Meijer (1995). Both of these theoretical models include spreading stresses along the continent/ocean lithospheric transition. Because the earthquakes are more than 300km from the continental shelf they should not be affected by the local flexural forces caused by sediment load in the marginal basins. The agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses.

Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, III: faulting and fluid circulation. Constraints from integration of FMS images, geophysical logs and core data

Abstract Downhole geophysical logs and high-resolution electrical images (FMS) from DSDP/ODP Hole 504B are analysed in combination with core data to obtain an integrated description of oceanic faults met in the hole. About 34 500 fractures were mapped from FMS images over 1672 m of basement. The fracture distribution from FMS confirms the presence of a main fault zone between 800 and 1100 mbsf (metres below sea floor), elsewhere detected from seismic data as well as magnetic, acoustic, and electrical resistivity measurements. The fracture density profile reveals the presence of two other highly fractured zones, (1) between 400 and 575 mbsf and (2) close to the bottom of Hole 504B (1700 to 2100 mbsf). Consequently, we infer that Site 504 was submitted first to an extensional stress regime near the ridge axis, with circulation of high-temperature fluids and pervasive alteration of the basalts. This initial phase is associated with the main fault met in Hole 504B. Similar but less developed deformation was generated off-axis, with lower-temperature parageneses, such as that cored between 400 and 575 mbsf. The present compressional to strike-slip stress regime is expressed in subhorizontal fracturing detected in discrete zones, such as within the main fault zone and the lower fracture zone (1700 to 2100 mbsf) in Hole 504B.

Recent temporal change in the stress state and modern stress field along the North Anatolian Fault Zone (Turkey)

SUMMARY The North Anatolian Fault (NAF) is a dextral strike-slip fault which runs about 1400 km from east to west, from the Karliova triple junction to the Aegean domain. It was active during the Plio-Quaternary as a consequence of the collision between the Eurasian and Arabian plates. The NAF intracontinental deformation zone contributes with the sinistral East Anatolian Fault to the westward extrusion of Anatolia as a consequence of the northward drift of Arabia. The Central NAF zone forms a northward convex bend where the eastern NAF N110°E-trending segment extending from Karliova is connected to the western NAF N75°E-trending segment running to Aegea. Analysing the slip occurring in major earthquakes around the NAF, we show that the present-day stress pattern along the NAF agrees with stress magnitude and strike variations from east to west. The stress regime along the NAF is NNW-trending σHmax (σ1) transpression in eastern NAF; this progressively changes westwards to NW-trending σ1, transtension in central Anatolia. This lateral variation means a relative decrease of the σHmax magnitude, which evolves progressively to an extensional regime (σ1 vertical) around the westernmost NAF, characterized by a NNE-trending σhmin (σ3). The drastic change in the tendency of the regional stress state in the North Anatolian block, from compressional to extensional, is effective within the central NAF bend. The stress regime determined by inversion of slip vectors measured on fault planes of various scales confirms the present-day transtensional regime in the central part of the NAF bend. However, the fault kinematic analysis of Quaternary stress states within the Central NAF indicates that this stress state was not continuous throughout the Quaternary. Indeed, chronologies of fault slip vectors provide evidence for two distinct Quaternary regional strike-slip stress states within this zone. Both states have consistent NE- and NW-trending σ3 and σ1 axes, respectively, but have significantly different R values. The change in the strike-slip stress regime probably occurred in the middle Pleistocene. The older mean stress state is characterized by a N142 ± 8°E-trending σ1, a N52 ± 13°E-trending σ3 and a mean arithmetic Rm value of 0.75, which indicates that the regional stress regime is transpressional. The younger strike-slip regime is characterized by a N142°± 14°E σ1 axis, a N52°± 10°E σ3 axis and a mean Rm of 0.24, which indicates the transtensional character for this regime. The low R values of the stress deviators related to the recent stress state reflect normal-component slips. These temporal and spatial stress changes along the NAF result from the coeval influence of forces due to the Aegean subduction in the west and to the northward drift of Arabia in the east. Over these boundary forces, shear is superimposed along the NAF, which accommodates the Anatolia extrusion. However, the timing of the temporal stress change permits the suggestion that the Quaternary stress regime variation in North Anatolia is mainly due to the influence of the Aegean domain.

Stress transmission across an active plate boundary: an example from southern Mexico

Paleostress tensors calculated from inversion of fault-slip data at 60 stations between Zihuatanejo and Tehuantepec in southern Mexico were separated into four distinct groups and assigned to time intervals between 150 Ma and Recent. The change in orientation of the maximum horizontal stress closely traces the change in direction of convergence between the North American and the Farallon/Cocos plates for the same time interval. Depending on the angle between the convergence direction and the plate margin, both thrust fault and strike-slip stress regimes were active. Based on this comparison, we infer that the stress distribution in the continental crust of southern Mexico has been controlled by interplate stress transmission. Latest Cretaceous-Early Tertiary mylonites at the northern edge of the Xolapa complex portray left-lateral oblique crustal extension and oceanward migration of the upper plate. The orientation of extension at mid-crustal level grossly correspond to the orientation of the minimum compressive stress at upper crustal level. During the last 20 Ma a tensional stress regime as a response to surface uplift has been dominant in southern Mexico.

On the Mechanical Stability of lnclined Wellbores

Summary Consideration of the stress field around an arbitrarily oriented borehole shows that in an extensional stress regime (σv σH>σh), wellbores parallel to the direction of minimum horizontal principal stress are the least prone to compressive shear failure (breakout). The most stable deviation angle (from the vertical) depends on the ratio of the horizontal principal stresses to the vertical stresses, and the higher the ratio σH/σV, the higher the deviation angle for minimizing breakout. In a strike-slip stress regime (σv <σH>σh), horizontal wells are the least prone to breakout, and the higher the ratio σH/σ v, the closer the drilling direction should be to the azimuth of σH. A new compressive shear failure criterion, which is a combination of the effective strength concept and the Drucker-Prager criterion, is proposed for quantifying the stresses at which borehole breakout occurs. The lowest mud weight, at and below which breakout will occur, can be predicted by combining this criterion with the stress field around an arbitrarily oriented borehole. The highest mud weight at and above which a tensional or hydraulic fracture is induced can be predicted by combining the tensile strength of the rocks of the wellbore wall with the stress field around an arbitrarily oriented borehole. For the in-situ stress environments considered, the optimally oriented inclined wellbore is less prone to breakout (i.e., allows a lower mud weight) and tensional or hydraulic fracture (i.e., supports a higher mud weight) than a vertical well.

Oblique extension in the Jurassic trough of the central and eastern High Atlas (Morocco)

A field structural analysis has been carried out in the High Atlas Mountains of Morocco. Paleostress solutions provide information on the successive orientations of major stress patterns during Mesozoic and Cenozoic time. An initial rift stage in the Early Jurassic is characterized by normal–oblique faults bounding tilted blocks, which are associated with tensional paleostress patterns whose minimum component trends west-northwest–east-southeast. Faults parallel to this direction are interpreted as paleotransfer faults. We assume that the divergent motion responsible for the opening of the rift system in the Early Jurassic was oriented west-northwest–east-southeast, subparallel to the paleotransfer faults and the trend of the minimum component (σ3) of the paleostress field. The east–west-trending Jurassic central–eastern High Atlas rift opened obliquely, and not in a pure strike-slip stress regime along east–west-striking faults as previously proposed. A later stage of rifting (Middle Jurassic) is characterized by large normal faults and is supposed to accompany local movements, probably due to gravity. The uplift of the High Atlas belt occurred mainly during the Cenozoic period as a consequence of a north–south- to northwest–southeast-directed compression related to collision between Europe and Africa.

A study of the design of inclined wellbores with regard to both mechanical stability and fracture intersection, and its application to the Australian North West Shelf

Knowledge of the in-situ stress field can be applied both in planning most stable drilling trajectories and also in maximizing intersection of teh wellbore with open, natural, and hydraulically-induced fractures in the reservoir. Inclined wells drilled with the optimum drilling direction (azimuth) and deviation (from the vertical), at which the shear stress anisotropy around the wellbore wall is minimized, may be more mechanically stable than vertical wells in various stress regimes. Such mechanically stable trajectories may also maximize open fracture intersection. Combining the objectives of maximum mechanical stability and maximum open fracture intersection • • wells in an extensional stress regime should be highly deviated from vertical (at about 55–70°) and drilled along the azimuth of the least horizontal princial stress; • • wells in a strike-slip stress regime should be horizontal and drilled at 55–70° with respect to the major horizontal principal stress; • • wells in a compressional stress regime should be slightly deviated from vertical (at about 20–35°) and drilled along the azimuth of the major horizontal principal stress. Application of this study to the Wanaea/Cossack field of the Australian North West Shelf, where the in-situ stress field has been constrained, suggests that the most stable drilling direction and that which maximizes potential intersection with any open fractures in the reservoir is horizontal, in the azimuth of the least horizontal principal stress, 005–010°N.

The Contemporary Stress Field of the Barrow-Dampier Sub-Basin and Its Implications for Horizontal Drilling

The interpretation of borehole breakouts suggests that minimum horizontal stress (sh) is regionally oriented approximately N-S in the Barrow-Dampier Sub-Basin. More locally, breakout interpretation suggests that sh orientation is 005°?010° in the Wanaea/Cossack area, and 012°?022° in the Griffin/Chinook/Scindian area. Horizontal stress magnitudes in the Wanaea/Cossack area, derived from modified leak-off tests, together with vertical stress magnitudes derived from density and sonic log data, suggest that the stress (fault) regime in the Wanaea/Cossack area is on the boundary of extension (normal faulting) and strike-slip. The contemporary stress field impacts on planning the drilling direction of deviated and horizontal wells through the issues of mechanical wellbore stability and fracture intersection. In order to maximise intersection with any open, natural fractures, and to optimise any hydraulic fracturing program, wellbores should be horizontal and parallel to ah (minimum horizontal compressive stress) in the extensional (normal fault) and strike-slip stress regimes, and vertical in the compres-sional (reverse fault) regime. In order to minimise the tendency for borehole breakout, the stress anisotropy around the wellbore should be minimised ? i.e. horizontal wells should be drilled in the direction of sh in the extensional regime, and in the direction of sH (maximum horizontal compressive stress) in the compressional regime. In the strike-slip regime, horizontal wells can be oriented such that they are not subject to any stess anisotropy by progressively changing from the sh direction towards the sH direction as the horizontal stresses increase with respect to the vertical. In the Wanaea/Cossack area, wells drilled towards 005°-010° or 185°?190° will maximise the potential for intersection with open, natural fractures (should they be present), and recovery from any induced, hydraulic fracturing will be optimised. Furthermore, wells in this direction will be subject to the minimum stress anisotropy, and are thus least likely to be subject to breakout.

3-D Block Rotation in the West Transverse Ranges: the Key to Structure History and Basin Development: ABSTRACT

Block rotation is widespread in regions of distributed shear. As rigid block-faulted domains rotate, gaps open up along their boundaries and sedimentary basins may develop. The evolution and the geometry of these basins is controlled by the slip history of the faults that bound them. To unravel their history, a three-dimensional (3-D) model was developed that combines the kinematics of block rotation with the mechanics of faulting. As an example, the authors present a 3-D computer simulation for the history of rotation of the west Transverse Ragges domain, southern California. A set of preexisting faults, striking north-northeast, is allowed to slip and rotate in accordance with known friction criteria. Rotation is assumed to occur in the strike-slip stress regime. The principal stress axes are assumed fixed in the present-day orientation throughout the deformation. Therefore, as faults slip and blocks rotate, the sense of motion along the faults changes. This simulation predicts, in agreement with the observations, an initial period of normal motion along the reactivated faults. Upon fault slip and block rotation the same faults go through a phase of strike slip. With further slip and rotation they eventually become the east-west-striking oblique reverse faults that characterize the present-day tectonicsmore » of this domain. The model shows that a single set of faults can experience both dipslip and strike-slip motion throughout its deformation history within a strike-slip stress regime. It is not necessary to appeal to complex and arbitrary changes in the orientation of the stress field. Only by combining a 3-D block rotation model with structural and paleomagnetic data may it be possible to unravel the complex tectonics of distributed deformation and basin evolution.« less

Geology and mineralization of the Jabal Samran-Jabal Abu-Mushut area, Saudi Arabia

This article discusses the structural control of the Precambrian Al Ji'lani basic intrusion, which is one of numerous oval igneous intrusions scattered within different terranes of the Arabian-Nubian Shield. The area in the Ad Dawadimi vicinity exposes 75 km long northwest-trending sinistral en echelon fault systems. During East African Orogen, the master fault segments propagate as sinistral system creating a rhomboidal shape releasing stepover area, defined in this work as Al Ji'lani pull-apart structure. The fault kinematic analysis shows significant variation in stress tensors between data measured outside and inside Al Ji'lani pull-apart structure. The stress tensor, computed from fault population inside the pull-apart, typify a NW-SE to NNW-SSE pure tectonic extension. Outside the pull-apart structure, the stress tensor illustrates a pure strike slip stress regime with a minimum stress axis that varies from NW-SE to NNW-SSE, and maximum stress axis that varies from ENE-WSW to NE-SW. The pull-apart northwestern corner develops, at the end stage, a circular to elliptic basic intrusion known as the Al Ji'lani layered gabbro intrusion. The intrusion is located in the releasing stepover's corner and characterized by inward dipping layers, suggesting an evolution genetically related to the pull-apart growth. Field data and faults kinematic analysis reveal that the Al Ji'lani structure is an ancient Ediacaran–Cryogenian gabbro intrusion developed in pull-apart corner, widely recognized as preferential crustal weakness zone in recent systems of fault-related-volcanism. The results support a predominant relationship between brittle tectonic and volcanism rather than shearing as it was previously accepted.

Intrusion and fracture styles of some mineralized porphyry systems of the southwestern Pacific and their relationship to plate interactions

Porphyry copper systems in Papua New Guinea and contiguous areas reveal distinctive intrusion geometries and fracture patterns. These distinctive characteristics are in turn related to their time of formation and to the tectonic province in which they occur.Porphyry systems of Miocene-Oligocene age occur in the mobile belt of Papua New Guinea, the Admiralty Islands, and on New Britain. In these older deposits, intrusion and fracture styles are indicative of porphyry bodies emplaced into settings produced and dominated by strike-slip stress regimes. The resulting geometrical distribution of fracture and intrusion patterns fixes the orientation of ambient principal stresses during their evolution. Derived principal stress directions appear to correlate with known and inferred directions and styles of coeval plate convergence.Younger porphyry systems of Pliocene and Pleistocene age occur on both mainland New Guinea and in some of the volcanic terrain of nearby islands. On mainland New Guinea, they occur scattered along the Aure tectonic zone some 250 km inland from the northern edge of New Guinea. In contrast with the older porphyry centers, the Plio-Pleistocene intrusion-fracture patterns are suggestive of forcefully emplaced porphyry intrusions into tectonically relaxed crustal blocks. Concomittant differential elevation of these young systems is an integral component of their tectonic evolution and is viewed as a consequence of high convergence rates between the Australian and Pacific Plates.