Regional Tectonic Deformation Research Articles

Petroleum Geology of Solimoes Basin, Brazil

Solimoes basin is located at the northwestern portion of the South American continent, adjacent to a subandean pericratonic belt. It constitutes a Paleozoic intracratonic basin covering an area of about 600,000 km[sup 2] and displaying a maximum sedimentary thickness of 3400 m. The tectono-sedimentary evolution can be largely related with important events that occurred at the western border of the continent. The regional flexural subsidence and tectonic deformation observed are correlated to two major phases: Pre-Andean (Paleozoic) and Andean (Mesozoic and Cenozoic). In the Andean phase, compression with shear components, of Triassic age, known as the Jurua Tectonism, became very important for oil exploration of the basin. The stratigraphic conditions considered of greatest interest for petroleum exploration comprise Devonian gas and oil sourcing shales and Carboniferous sandstones reservoirs with an evaporitic seal. The main traps are anticlines related to reverse faults. In this context, Petrobras discovered Jurua gas field in 1978 and the Rio Urucu gas and oil field in 1986. These were the first fields discovered in Brazilian Paleozoic basins. Total petroleum fields discovered in Brazilian Paleozoic basins. Total petroleum reserves discovered represent 400 million bbl of equivalent oil. Present migration and accumulation call for previous accumulation in intrabasinalmore » Paleozoic highs and related stratigraphic traps. Late tectonism seems to have resulted in redistribution and/or escape of fluids into new traps. Less disturbed areas are expected to contain original accumulations.« less

An application of the slip model of brittle deformation to focal mechanism analysis in three different plate tectonic situations

The “slip model” of brittle deformation ( Reches, 1983) imposes a set of constraints on the angular relationships between faults, their striations on slickensides and the type and orientation of the deformation ellipsoid. In focal mechanisms of earthquakes, these relationships can be used to determine which of the two nodal planes is the fault plane, and then to obtain the strain ellipsoid of regional tectonic deformation controlling seismicity. In this approach it is convenient to use pitch/dip diagrams ( De Vicente, 1988) and P/T diagrams ( Angelier and Mechler, 1977), both developed for fault population analysis. In this paper three regions in different plate-tectonic situations are analysed using these methods. These regions are a zone of intracontinental seismicity in the eastern Peruvian Andes, a zone of continental compressional deformation in the Iranian Plateau, and the Fenwei Graben of China. Nodal planes of focal mechanisms, published in different papers about seismicity in these regions, have been used, the following conclusions being reached. In the analysed zone of the Peruvian Andes, three regional deformation types appear, causing fault movements that range from slightly reverse strike-slip to reverse strike-slip. In addition, exclusively at depths in excess of 18 km, there is a deformation of an uniaxial shortening type. In all of these deformations the horizontal shortening axis is oriented from N87° to N93°. In the Iranian Plateau two horizontal shortening directions appear, N151° and N61°, even showing faults that imply an uniaxial deformation ellipsoid the shortening axis of which is at N61°. In the Fenwei Graben the ellipsoid of regional deformation has a stretching principal axis at N145°/150° and a shortening axis at N55°/60°, the strike-slip movement along faults being as important as the normal one.

花こう岩の変形構造と貫入テクトニクス

Emplacement mechanism of granitic bodies is classified into forceful emplacement and permitted emplacement. Forceful emplacement is caused by buoyancy of granitic magma relative to the country rocks. This mechanism is called diapirism. Permitted emplacement is promoted by regional tectonic deformation or fault activities which provided space for their emplacement. In many cases, emplacement mechanism and tectonic setting of each granitic body can be clarified through its deformation features. The basic methods for structural studies on granites and some representative emplacement meachanisms are described. Future problems of deformation structures, emplacement mechanism, and tectonic setting of emplacement for Japanese granitic bodies are discussed.

Structural control of gold mineralization at the Bousquet mine, Abitibi, Quebec

The Bousquet gold deposits are structurally controlled, disseminated and vein type lodes located within a 500 m wide anastomosing deformation zone. Ore is located within narrow zones of high strain surrounded by lozenge-shaped panels of less-deformed rock. Strain characteristics are those of the bulk inhomogeneous flattening style. Ore lenses are spatially related to highly sheared, fractured, and altered mafic and felsic volcanic and volcaniclastic rocks of contrasting rheologic properties. Deformation features can be ascribed to multistage progressive ductile → brittle deformation. Strain markers and kinematic indicators show that the principal displacement within the deformation zone was reverse faulting with a minor sinistral throw. A structural analysis demonstrates that the deformation responsible for the development of a pervasive regional foliation, brittle fractures, and oblique reverse faults can be attributed to a north–south compression.Metamorphic minerals such as andalusite, kyanite, garnet, biotite, chlorite, chloritoid, and calcic plagioclases indicate that upper greenschist metamorphism was attained locally within the ductile deformation zones. Subsequent pervasive retrograde alteration, including carbonatization and hydration of silicates to white mica and chlorite, suggests an important period of hydrothermal activity after peak metamorphism. Native gold is typically closely associated with pyrite and with these hydrothermal assemblages and was probably channelled into ductile and brittle structural zones prior to and after peak metamorphism.Two principal types of steeply dipping auriferous sulphide veins are present in the mine: foliation-oblique veins and foliation-parallel veins. Foliation-oblique veins occur within steeply dipping conjugate shear fractures spatially related to competent protoliths. The main set was emplaced during late stages of the regional tectonic deformation, after the initial development of a pervasive regional foliation and before the end of the progressive deformation. Foliation-parallel veins are located within openings created by decoupling schistosity laminae or by overriding of irregular surfaces such as fault planes and shear zones. These veins are relatively younger and less deformed than the foliation-oblique veins.Pervasive pyritic disseminations along foliation surfaces are earliest and synchronous with the development of foliation and probably continued throughout the progressive deformation. Early disseminated sulphides may also have been remobilized by pressure solution into later vein systems.

Historical seismicity near Chagos: a complex deformation zone in the equatorial Indian Ocean

Over the last twenty years, Chagos Bank has a seismicity rate disproportionate to its supposed intraplate location. Earthquake relocation also shows a high seismicity rate in pre-WWSSN time (1912–1963), with seven events located off of the Central Indian Ridge, including large events in 1912 ( M = 6.8) and 1944 ( M = 7.2). This study uses the moment variance technique, a systematic search for the mechanism which best fits P, PP, SH, Love and Rayleigh amplitudes, to determine the focal mechanisms of two pre-WWSSN earthquakes. A test with a recent event of known mechanism demonstrates that accurate focal parameter determination is possible even when only a few good records are available. Moment variance analysis shows a thrust faulting mechanism for the 1944 event, northeast of Chagos Bank near the Chagos-Laccadive ridge, and a strike-slip focal mechanism for a smaller 1957 event west of Chagos Bank. The 1944 event, one of the largest oceanic “intraplate” earthquakes known (moment 1.4 × 10 27 dyne-cm), indicates that the Chagos seismicity reflects not an isolated occurrence of normal faulting as previously thought, but rather regional tectonic deformation extending northeast of Chagos Bank and including thrust, normal and strike-slip events. This seismicity and previously studied seismicity near the Ninetyeast Ridge and Central Indian Basin suggest a broad zone of deformation stretching across the equatorial Indian Ocean. This zone contains all known magnitude seven oceanic “intraplate” earthquakes not associated with subduction zones or continental margins, suggesting that elsewhere such extensive deformation occurs only along plate boundaries. This study proposes that a slow, diffuse plate boundary extends east from the Central Indian Ridge to the Ninetyeast Ridge and north to the Sumatra Trench. A recent plate motion study confirms this boundary and suggests that it separates the Australian plate from a single Indo-Arabian plate.

Uplift across Long Valley Caldera, California

The results of levelings along U.S. Highway 395 between Lee Vining and Toms Place, California, in 1957, 1975, 1980, 1982, and 1983 define the development of a dome‐shaped uplift across the Long Valley caldera. These levelings indicate that Toms Place remained virtually invariant with respect to Lee Vining during the full interval 1957–1983, implying that the end points of the survey route lie beyond the range of the developing dome. No significant uplift occurred before 1975, but by October 1980 the uplift near Casa Diablo Hot Springs reached 0.25 m and the half width of the dome was of the order of 25 km. The uplift of Casa Diablo Hot Springs increased to 0.35 m by August 1982 and to about 0.40 m by August 1983 without any appreciable increase in the lateral extent of the dome. The August 1982 to August 1983 uplift apparently occurred before February 1983 with little or no uplift during the period February–August 1983. The 1975–1983 uplift is attributed to the injection of 0.19 km3 of magma into a spherical chamber at a depth of 10 km beneath the southern quadrant of the ancient resurgent dome within the caldera. Examination of the full vertical control record through the Long Valley area indicates that the 1975–1983 uplift is unique within the period 1905–1983. Although comparisons of the results of a 1932 leveling against a 1914 datum disclose a 1914–1932 down‐to‐the‐northwest tilt of about 10 µrad over the reach between Toms Place and Lee Vining, this tilt (if real) is more reasonably attributed to regional tectonic deformation than it is to the movement of magma at depth.

Gwna Melange, Upper Precambrian Olistostromal Sequence, North Wales, United Kingdom: ABSTRACT

The Gwna melange of north Wales is a sequence of pebbly mudstones and broken formations interstratified with undisturbed volcaniclastic units. The individual clasts, generally lying in a matrix of arenaceous mudstone, locally range up to a kilometer in outcrop. The clasts include graywackes, arenites, quartzites, limestones, basalts, cherts, and volcaniclastics. The melange has been frequently cited as the type melange indicative of a tectonic origin. Recent mapping, petrography, and sedimentologic analysis have produced evidence for a sedimentary (olistostromal) origin for most of the melange. Sedimentary structures present include clastic dikes, resedimentation and soft-sediment injection features, and dewatering cleavages in various stages of development. The upper contact of the melange is at least partly a sedimentary-erosional contact with overlying undisturbed beds of composition similar to that of the melange. Certain larger clasts (up to 50 m) of graywacke exhibit oriented tectonic kink folds. These large clasts are in sedimentary contact with the melange, and may be explained best as undisturbed sedimentary-slide units (olistoli hs) that were folded by later regional tectonic deformation. Smaller clasts within the melange responded to these regional forces mainly by continued movement, or shearing, within their ductile matrices. This olistostromal sequence possibly represents an unstable continental margin associated with a previously postulated late Precambrian subduction zone. End_of_Article - Last_Page 523------------

Microearthquakes in and near Long Valley, California

Sixteen portable seismograph stations were deployed in the vicinity of the Long Valley geothermal area, California, from April 27 to June 2, 1973. Only minor microearthquake activity was detected in the Long Valley caldera, but a high level of activity was detected to the south and east of the caldera. The abrupt spatial seismicity decrease at the southern boundary of the caldera suggests that the caldera is either structurally less competent than the surrounding crust or is at a junction of different regional tectonic deformation trends. No significant attenuation or delays occurred for either local P or S waves that traversed the caldera.

A re-assessment of the Upper Mississippi valley lead isotope data

Corrections for systematic bias, and a better regression method, are tested on the lead isotope data of Heyl, Delevaux, Zartman and Brock (1966). The new calculations suggest the time of mineralization in the Upper Mississippi zinc-lead district is more likely to lie in the range Mid Devonian (370 my) to Upper Pennsylvanian (280 my) on any reasonable assumption for the age of the basement. It is argued from the isotopic trends that the ore lead in this district was simply derived by radiogenic addition to lead originally derived from rocks of basement age, although the data do not exclude residence in the Lower Paleozoic sediments as an intermediate step. An isotopically different magmatic component is not required. Correlation with the geology suggests that genesis of the deposits probably was related to regional tectonic deformation in Middle to Late Paleozoic time.