Выпуск 280
Автор(ы):Lardeaux J.-M., Pili E., Ricard Y., Sheppard S.M.F.
Издание:Journal Tectonophysics, 1997 г., 15 стр.
Lithospheric shear zones and mantle-crust connections

A crustal-scale ductile shear zone network in the Precambrian granulite-facies crust of Madagascar is examined to determine the nature of the connections between the mantle arid lower crust. Based on three independent data sets - field and satellite mapping, C- and O-isotope geochemistry and gravimetry - this crust is divided into three zones: (1) outside of shear zones; (2) minor shear zones that are <140 km long and 7 km wide; and (3) major shear zones mat are >350 km long (up to 1000 km) and 20-35 km wide. The mantle is uplifted by about 10 km beneath the major shear zones. The major shear zones are rooted in and are inferred to be controlled by the mantle; they directly tapped mantle-derived C02. The small-scale minor shear zones were controlled by crustal processes and focused crustally derived H20-rich±C02 fluids. The regular distribution of the shear zones on a crustal scale is in agreement with models of buckling of the continental lithosphere in a compressional context. The propagation of these mechanical instabilities promoted and channelled fluid flow. These major Pan-African shear zones thinned the crust and were reactivated during the subsequent drifting of Madagascar and opening of the Indian Ocean during Jurassic to Cretaceous times. They also controlled many of the brittle fault zones in the overlying sedimentary basins. Mantle-rooted large-scale shear zones are inferred to be a general feature of cratonic areas reactivated by shear zone systems.

ТематикаРегиональная геология
МеткиFluid, Granulites, Gravimetry, Lithosphere, Shear zone, Stable isotopes, Гравиметрия, Гранулиты, Литосфера, Разломные зоны, Флюиды
Выпуск 300
Автор(ы):Cloetingh S., Jan-Diederik van Wees, Ziegler P.A.
Издание:Journal Tectonophysics, 1998 г., 27 стр.
Mechanical controls on collision-related compressional intraplate deformation

Intraplate compressional features, such as inverted extensional basins, upthrust basement blocks and whole lithospheric folds, play an important role in the structural framework of many cratons. Although compressional intraplate deformation can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the development of the most important compressional intraplate structures. These can occur at distances of up to 1600 km from a collision front, both in the fore-arc (foreland) and back-arc (hinterland) positions with respect to the subduction system controlling the evolution of the corresponding orogen. Back-arc compression associated with island arcs and Andean-type orogens occurs during periods of increased convergence rates between the subducting and overriding plates. For the build-up of intraplate compressional stresses in fore-arc and foreland domains, four collision-related scenarios are envisaged: (1) during the initiation of a subduction zone along a passive margin or within an oceanic basin; (2) during subduction impediment caused by the arrival of more buoyant crust, such as an oceanic plateau or a microcontinent at a subduction zone; (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism, and convergence rates and directions; (4) during post-collisional over-thickening and uplift of an orogenic wedge. The build-up of collision-related compressional intraplate stresses is indicative for mechanical coupling between an orogenic wedge and its fore- and=or hinterland. Crustal-scale intraplate deformation reflects mechanical coupling at crustal levels whereas lithosphere-scale deformation indicates mechanical coupling at the level of the mantle-lithosphere, probably in response to collisional lithospheric over-thickening of the orogen, slab detachment and the development of a mantle back-stop. The intensity of collisional coupling between an orogen and its fore- and hinterland is temporally and spatially variable. This can be a function of oblique collision. However, the build-up of high pore fluid pressures in subducted sediments may also account for mechanical decoupling of an orogen and its fore- and=or hinterland. Processes governing mechanical coupling=decoupling of orogens and fore- and hinterlands are still poorly understood and require further research. Localization of collision-related compressional intraplate deformations is controlled by spatial and temporal strength variations of the lithosphere in which the thermal regime, the crustal thickness, the pattern of pre-existing crustal and mantle discontinuities, as well as sedimentary loads and their thermal blanketing effect play an important role. The stratigraphic record of collision-related intraplate compressional deformation can contribute to dating of orogenic activity affecting the respective plate margin.

МеткиCollision, Compression, Intraplate, Lithosphere, Rheology, Rifting, Коллизия, Литосфера, Рифтогенез
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