Strike-slip Tectonic Regime Research Articles

A new insight into Pan-African tectonics in the East–West Gondwana collision zone by U–Pb zircon dating of granites from central Madagascar

The assembly of Gondwana was the result of a major collision orogen, the East African Orogen, between East and West Gondwana during Neoproterozoic times. Madagascar, which represents a fragment of East Gondwana, is located in a key area of this Pan-African orogen. Granites of unambiguous tectonic setting have been dated using the U–Pb zircon method in order to constrain the timing of orogenic events. The central part of Madagascar is characterized by syntectonic alkaline granitic sheets, referred to as “stratoid” granites. These are of both mantle and crustal derivation. Their U–Pb zircon ages are well defined between 627 and 633 Ma for both plutonic suites, regardless of either mainly mantle or crustally origin. It is not surprising that the crustally-derived suite contains inherited zircons in the 2.2–2.4 Ga range attesting to the existence of Lower Proterozoic crust in northern central Madagascar. The generation of huge amounts of granitic magma is regarded as the result of post-collision extension under a high heat flow regime. Therefore, an age between 700 and 650 Ma is inferred for the beginning of Gondwana assembly along the collision zone between central Madagascar and Kenya, i.e., in the central part of the East African Orogen. Following this, brittle fracturing of the stratoid granite series permitted the emplacement of the Ambatomiranty granitic dyke swarm at a minimum age of 560 Ma, in possible connection with a nearby shear belt. The strike–slip tectonic regime at ∼570–560 Ma is well known in southern Madagascar and in its Gondwana connections. This stage corresponds to intracontinental reworking and the final suturing of Gondwana.

Indosinian NW-trending shear zones within the Truong Son belt (Vietnam) 40Ar 39Ar Triassic ages and Cretaceous to Cenozoic overprints

Several NW-trending ductile and partly mylonitic shear zones cross the Truong Son belt of Central Vietnam, along the Song Ma and Song Ca valleys and, north of the Kontum block, from Da Nang to Aluoi and Khe Sanh. High-grade metamorphic rocks of amphibolite facies, showing a retrograde evolution and consisting of ortho- and paragneisses, metavolcanics, amphibolites, marbles and quartzites are exposed along these structures. They display a homogeneous deformation pattern characterized by a generally steeply dipping foliation and a near-horizontal to gently plunging mineral stretching lineation, indicating a strike-slip tectonic regime of deformation. Along the southernmost fault zone at least, various and consistent kinematic indicators, including SC structures, asymmetric tails of porphyroclasts, prove that the strain and metamorphism have been generated by a phase of dominantly non-coaxial deformation with a dextral sense of shear. 40Ar 39Ar dating, applied on the high-grade metamorphic rocks minerals occurring along these zones, provide plateau cooling ages, closely around 245 Ma, establishing that this event took place in the lowermost Triassic as an early phase of the Indosinian orogeny. The existence of Indosinian movements in Vietnam, as they have been defined by previous authors during the early century, is now accurately confirmed and this is the first insight in the occurrence of ductile strike-slip tectonics of Indosinian age along NW-SE fault zones. Well-expressed in Central Vietnam is a Cretaceous thermal and deformational overprint, marked by epimetamorphism, which took place between 90 and 120 Ma, as attested by low-temperature degassing ages. This Cretaceous event is not found further north in the Song Ma zone where younger ages appear as a result of the influence of Cenozoic shear movements along the Red River fault which displays 20–30 Ma ages. On the northern flank of Song Ca, in the Bu Khang-Phu Hoat core complex, comparable Oligocene-Lower Miocene ages (20–35 Ma), yielded by biotite and phlogopite, reflect a strong overprinting and attest to a rapid uplift of the basement.

Palaeostress analysis, a contribution to the understanding of basin tectonics and geodynamic evolution. Example of the Permian/Cenozoic tectonics of Great Britain and geodynamic implications in western Europe

Microtectonic analysis in association with tectonic-sedimentological observations enables us not only to define palaeostress tensors but also to date each of them. Such a study, carried out in central and northeastern England and in southernmost Wales has permitted us to point out several tectonic stages during the Permian-Cenozoic period. (1) Inferred from syn-sedimentary faulting in the Late Permian series, the Permian/Early Triassic tectonic regime is characterised by a NNW-SSE-oriented extension. (2) During the Late Triassic to the early Late Jurassic, an E-W- to ENE-WSW-oriented extension was acting as suggested by syn-sedimentary normal faulting in Keuper, Lias and Middle Dogger sediments and post-sedimentary normal faulting in Oxfordian sediments. (3) The following tectonic event probably started during the Malm and was particularly active during the Early Cretaceous. It was characterised by a N-S- to NNE-SSW-oriented extension and was associated with an E-W- to WNW-ESE-oriented strike-slip tectonic regime (transtension). (4) A locally observed NW-SE-oriented transpressional strike-slip tectonic regime is thought to correspond to the ‘Laramide’ inversion phase of the Middle Paleocene. (5) The later tectonic event is characterised by a N-S-oriented transpressional strike-slip regime and affects a Paleocene tholeiitic dyke (‘Cleveland dyke’). It is supposed to be synchronous with the so-called Eocene ‘Pyrenean’ to the Early Miocene ‘Helvetic’ compressional stages. These paleostress tensors are compared with others defined in France, the Benelux countries and Germany and are also collated to the tectonic evolution of oil basins assessed from geophysical analyses. These comparisons raise discussions about the paleostress distribution within the geodynamic evolution of the West European shelf.

Middle Tertiary Extension Recorded by Lacustrine Fan-Delta Deposits, Plush Ranch Basin, Western Transverse Ranges, California

ABSTRACT The Plush Ranch Formation (upper Oligocene and lower Miocene) consists of more than 1800 m of nonmarine sedimentary and volcanic rocks that record the history of an extensional basin referred to here as the Plush Ranch basin. Distinctive depositional facies, provenance, and sediment transport directions along each basin margin suggest an asymmetric basin shape that is consistent with a half-graben origin. The northern basin margin consists of sandstone-dominated alluvial-plain deposits (0.1-1.5 m thick, normally graded, lenticular sandstone beds). Small deltaic sequences 1-2 m thick were formed where these alluvial systems flowed southward into a lake. Lenses of massive, boulder-rich granitic breccia that represent rockslide deposits derived from a nearby northern granitic provenance nterfinger with the alluvial-plain facies. In contrast to the northern margin, the southern basin margin is represented by coarse-grained fan-delta deposits. Matrix- and clast-supported lenticular conglomerate beds 0.2-5 m thick with interbedded trough-cross-bedded pebbly sandstone represent braided-stream and flood-flow and/or noncohesive debris-flow deposits of alluvial fans that drained a highland area to the south. The alluvial-fan deposits interfinger to the north with several types of subaqueous sediment-gravity-flow facies including turbidite sandstone beds and matrix-supported debris-flow conglomerate. Each of the basin-margin depositional systems grades basinward and to the east into lacustrine deposits that include organic-rich dark shale, evaporite, and limestone. The lacustrine deposits represent the central and eastern parts of the Plush Ranch basin, which received little coarse siliciclastic sediment. Basalt deposits that are at least 50 m thick in the west and thicken eastward are interbedded mainly with the lacustrine facies. The southern margin of the Plush Ranch basin formed along a north-dipping, normal-slip fault along which dip separation increased toward the southwest; the northern margin developed on the tilted hanging-wall block of this fault. This fault was later reactivated in post-middle Miocene time as the present left-lateral strike-slip Big Pine fault. The Plush Ranch is one of several extensional and transtensional basins that formed in southern California and western Arizona about 25-20 Ma as a response to the change from a convergent to a strikeslip tectonic regime along western North America.

Stress tensor determination in France and neighbouring regions

The tectonic behaviour of different seismic regions of France is inferred from the properties of the local stress tensor obtained from focal mechanisms of regional earthquakes. Polarities of P-wave arrivals for several earthquakes are used in the inversion of the stress tensor and individual focal mechanisms. The principal axes and the shape of the tensor are closely related to the forces which generate seismic ruptures on pre-existing faults. We analyse 142 earthquakes of magnitude above 3.7 which occurred between 1965 and 1989. Good results are obtained for some regions. The northern Alps and the southern end of the Upper Rhinegraben show a quasi-compressive, strike-slip tectonic regime, with a maximum stress-axis oriented NW-SE. The tectonic regime changes to an extensional strike-slip type in the Rhenish Massif and the Lower Rhinegraben, the direction of the minimum stress-axis being NNE-SSW. The central and western regions of France (Central and Armorican Massifs) exhibit an extensional tectonic regime combined with strike-slip faulting. The minimum stress-axis strikes in a NE-SW direction. A strongly non-homogeneous stress field seems to be present in the southwestern Alps, and no unique stress tensor could be obtained for that region. This problem is likely to be related to a rapid variation of the stress field in space which could be expected, given the arc-like geometry of the mountain range. The Pyrenean domain is unresolved due to lack of data and, moreover, existing focal mechanisms are globaly inconsistent. Nevertheless, those mechanisms which are well constrained suggest a NNW-SSE to NW-SE direction of maximum stress in the western part of the Pyrenees. Thus, the stress field in France is fairly homogeneous at a regional scale for most of the territory except for the highly deformed belts.

Fault Reactivation and Structural Inversion Physical Models Analyzed with X-Ray Scanner and Seismic Examples: ABSTRACT

Factors controlling the reactivation by compression of preexisting cohensionless faults and especially normal faults are discussed. Some faults will have been reactivated by compression or a new failure will have occurred, depending on their orientation, their coefficient of sliding friction, and the state of stress. The optimum conditions for reactivation are fault planes whose orientation is closer to the failure plane for isotropic rocks with the same mechanical properties as the preexisting fault plane, i.e., subvertical preexisting fault striking oblique to the main regional stress axis {delta}{sup 1} in strike slip tectonic regime ({delta}{sup 1} and {delta}{sup 3} horizontal) and low dipping preexisting fault in compressive tectonic regime ({delta}{sub 1} and {delta}{sup 2} horizontal). Reverse, strike-slip reverse, or strike-slip movement is excpected along the fault plane depending on its orientation relative to the regional stress field. Weakly inverted graben in an intracratonic area demonstrates these principles. The orientation of a fault plane relative to the regional stress field and the amount of shortening will control whether no reactivation or reactivation with reverse or oblique slip movements is to be expected. During the inversion process, rocks of the hanging walls of the main preexisting normal faults were folded and uplifted. Seismicmore » profiles across the eastern Sunda platform in Indonesia display both large- and small-scale uninverted and inverted grabens in different stages of evolution, together with less common features, such as the inversion of an eroded syncline and low-dipping weak zone reactivation During the reactivation by compression of preexisting normal faults, flexuring and folding of the sediment cover generally occurred before the reactivation of the fault plane.« less

Late Jurassic—Early Cretaceous strike-slip deformation in the Nordenskjöld Formation of Graham Land

Upper Jurassic–Lower Cretaceous anoxic mudstones and air-fall ashes of the Nordenskjöld Formation are exposed on the eastern coast of Graham Land. Deformation of the strata has a long history spanning dewatering and lithification and was probably produced in a strike-slip tectonic regime. Available evidence suggests the onset of deformation in the region was during Tithonian times. The strike-slip deformation provides further evidence of a plate boundary along the eastern margin of the peninsula during the break-up of Gondwana and the movement of the crustal blocks of West Antarctica. It may also be related to a change in the spreading history of the Weddell Sea region and be the cause of a major facies change from fine anoxic to coarse clastic sedimentation.

Possible Strike-Slip Faulting in the Southern California Borderland: Reply

The Eocene Poway Conglomerate on San Miguel and San Nicolas Islands and age-equivalent turbidite sands on Santa Rosa Island appear to be parts of a bathyal fan deposit with an apex and source lying east of the halfway point between Santa Rosa and San Nicolas Islands. Palinspastic reconstruction along a proposed East Santa Cruz Basin fault system with 120 to 160 km of right slip aligns this bathyal point source with the compositionally equivalent Eocene river and shallow marine gravels of the type Poway Conglomerate in the San Diego area. Paleocurrent indicators in the Miocene San Onofre Breccia on Santa Cruz Island and the geometry of schist-bearing strata in the subsurface of the Santa Barbara Channel can be explained by right slip of the southern part of Santa Cruz Island from the southeast along the same fault system. Basement rock types in the borderland place no constraints on the proposed reconstruction, although they do constrain models involving large-scale westerly drift. The ridge and basin bathymetry of the central borderland is consistent with a strike-slip tectonic regime.

Possible Strike-Slip Faulting in the Southern California Borderland

Research Article| February 01, 1974 Possible Strike-Slip Faulting in the Southern California Borderland D. G. Howell; D. G. Howell 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar C. J. Stuart; C. J. Stuart 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar J. P. Platt; J. P. Platt 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar D. J. Hill D. J. Hill 1Department of Geological Sciences, University of California, Santa Barbara, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar Geology (1974) 2 (2): 93–98. https://doi.org/10.1130/0091-7613(1974)2<93:PSFITS>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation D. G. Howell, C. J. Stuart, J. P. Platt, D. J. Hill; Possible Strike-Slip Faulting in the Southern California Borderland. Geology 1974;; 2 (2): 93–98. doi: https://doi.org/10.1130/0091-7613(1974)2<93:PSFITS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract The Eocene Poway Conglomerate on San Miguel and San Nicolas Islands and age-equivalent turbidite sands on Santa Rosa Island appear to be parts of a bathyal fan deposit with an apex and source lying east of the halfway point between Santa Rosa and San Nicolas Islands. Palinspastic reconstruction along a proposed East Santa Cruz Basin fault system with 120 to 160 km of right slip aligns this bathyal point source with the compositionally equivalent Eocene river and shallow marine gravels of the type Poway Conglomerate in the San Diego area. Paleocurrent indicators in the Miocene San Onofre Breccia on Santa Cruz Island and the geometry of schist-bearing strata in the subsurface of the Santa Barbara Channel can be explained by right slip of the southern part of Santa Cruz Island from the southeast along the same fault system. Basement rock types in the borderland place no constraints on the proposed reconstruction, although they do constrain models involving large-scale westerly drift. The ridge and basin bathymetry of the central borderland is consistent with a strike-slip tectonic regime. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.