Large-scale Tectonics Research Articles

Slip partitioning in the Sea of Marmara pull-apart determined from GPS velocity vectors

Dislocation modelling is used to examine the GPS velocity vectors for the Marmara Sea region. First, the vectors due to the known Anatolia/Eurasia rotation are reproduced by introducing structures that approximate the large-scale tectonics. Observed features of the smaller scale fault system in the Marmara region are then progressively included with slip amplitudes and directions adjusted to fit an 80-vector subset of the GPS data. The motion in the Marmara Sea region is partitioned with the faults that bound the north of the basin carrying more strike-slip motion than predicted from the Anatolia-Eurasia plate motion and faults to the south having a greater perpendicular component. Taken together however, there is no net opening across the Marmara Sea perpendicular to the overall trend of the boundary and thus deformation in the Marmara region results only from the pull-apart geometry of the North Anatolian fault. No extension related to the Aegean system is needed to explain the observations. The GPS results are consistent with motion over the last 5 Myr that has been determined from geological reconstructions.

Postmineralization Faults in the Beaufor Gold Deposit, Abitibi Greenstone Belt, Canada: Geometry, Origin, and Tectonic Implications for the Val-d’Or Mining District

The Beaufor mine is a mesothermal vein-type gold deposit hosted by the Bourlamaque pluton, a synvolcanic intrusion crosscutting Archean volcanic rocks of the Abitibi greenstone belt in the Val-d’Or mining district. The auriferous system is related to south-dipping reverse shear zones and consists of up to 2-m-wide sheared and extensional sulfide-rich quartz veins. The auriferous veins system is crosscut by two generations of faults, east-west-trending, south-dipping (~60°) brittle-ductile reverse faults (e.g., the Dike 01-Central fault system) and subvertical to steeply north-dipping brittle faults (e.g., the Beaufor fault system). The Dike 01- Central fault system presents structural fabrics, kinematic, and hydrothermal characteristics similar to those of auriferous shear zones. Both are interpreted as master faults and secondary structures, respectively, with the Dike 01-Central fault system acting as a channelway for the updip draining of ore fluids that were mainly trapped in low-angle synthetic shear zones. Incremental deformation and progressive decrease of bulk fluid pressure led to strain localization and faulting of auriferous quartz veins by the Dike 01-Central fault system. The Beaufor fault system crosscuts all structures of the Beaufor deposit. It consists of a series of oblique-reverse, dextral faults that lack evidence for pervasive hydrothermal alteration. The main structure of that system is the east-west-trending, north-dipping Beaufor fault, which is associated with northeast- southwest-trending synthetic (i.e., West shear faults) and north-south-trending antithetic faults (i.e., Post-ore faults). Fault zones of the Beaufor mine are reminiscent of large-scale tectonics in the Val-d’Or mining camp. Auriferous quartz-tourmaline vein deposits are commonly associated with east- west-trending high-angle reverse shear zones and hydrothermal events coeval with, or slightly postdating, peak metamorphism and regional deformation. Several fault zones of the Val-d’Or area preserve evidence for both reverse shearing and dextral strike-slip faulting. Structures recorded in the Beaufor mine suggest that reverse and dextral trancurrent faulting is genetically unrelated and probably resulted from sequential faulting rather than transpression.

Distribution of apparent magnetization for Asia

Magsat total field anomalies over Asia were used to construct an equivalent magnetization model, which represents the apparent magnetization distribution within an equivalent layer 40 km thick and correlates well with large-scale tectonics, for example, the Kazakhstan, Tarim, Yangtze, India, Sino-Korea and Indochina blocks. The basin, plain, sea basin, and islands are delineated by magnetization lows whereas the plateau and marine ridge correspond to magnetization highs. The boundary between Tibetan Plateau and India marked by a strong gradient along its length coincides with the Yarlung Zangbo River fault roughly. The Tanlu fault belt is the boundary between positive and negative anomalies. This boundary stretches in southwest direction and joins Sanjiang fault belt. The boundary between the Southeast China block and the Yangtze block is also clearly delineated by the magnetization anomalies. Generally, the magnetization boundaries are consistent with the collisional suture of blocks.

Arcuate accretionary wedge formation at convex plate margin corners: results of sandbox analogue experiments

Indentation of a rigid box into a sand layer leads to accretion of sand into a wedge which has an arcuate shape in map-view. Curvature of the wedge is most pronounced at the corners of the indentor, where deformation has been investigated in detail in this study. Several features of the arcuate wedge in the corner region depend on the indentation angle, i.e. the angle between the indentation direction and the normal to the frontal face of the indentor: (a) The ratio between the widths of the lateral and frontal parts of the wedge increases linearly with increasing indentation angle. (b) Orogen-parallel extension (OPE) in the corner area is initially high and leads thus to large finite OPE in the firstly formed nappes, but decreases and attains a stable level during progressive convergence. Finite OPE in the corner region is smaller at larger indentation angles. (c) Displacement vectors of accreted material exhibit fanning around the arc, but in oblique indentation the spread of their orientation is smaller than the change of structural strike (nappe traces and fold axes). Displacement vectors trend mainly in such positions normal to structural strike where displacement is parallel to the indentation direction. The results from these experiments can be applied to natural arcuate fold–thrust belts if natural conditions do not differ much from experimental boundary conditions. Such an application may give information about the former plate movement direction across an arcuate plate margin. In the example of the Eastern Carpathian arc, a Tertiary plate-movement direction of approx. 125–130° was determined. This direction is in accordance with current models of large-scale tectonics of the Carpathian region, which indicates the applicability of the experimental results to natural examples.

Large-scale Hercynian West-directed tectonics in southeastern Sardinia (Italy)

Widespread Hercynian west-directed thrusting and west-facing folds, until now only reported in a limited area, are ubiquitous in the Sarrabus unit, southeastern Sardinia. Intense thrusting and imbrication in the higher tectonic levels of the Sarrabus unit potentially originated during west-directed thrusting of higher tectonic units, which have been eroded. The floor thrust of the Sarrabus unit, the Villasalto thrust, is a west- directed thrust that places the Sarrabus unit above the Gerrei and Meana Sardo units. East-west shortening in southeastern Sardinia followed north-south shortening in central Sardinia and internal deformation in the Castello Medusa-Riu Gruppa, Gerrei, Meana Sardo and Barbagia units. After restoration of movement along the Villasalto thrust, the Sarrabus unit is located east of the Gerrei and Meana Sardo unit. Therefore, it cannot be directly correlated with the Meana Sardo or the Barbagia units. © Elsevier, Paris

Roots of Etna volcano in faults of great earthquakes

Results from several seismic methods allow us to sketch the deep structure of Etna and its Ionian margin. Under Etna a volume of high velocity material is found in a structurally high position; the emplacement of this suggests spreading of the surrounding medium. Just offshore, down-to-the-east normal faults penetrate through the upper crust. The deeper crustal structure beneath appears upwarped from the basin towards Etna. Juxtaposed with the crust of Sicily, a thinner crust reaches from the Ionian Basin under Etna, and the mantle is upwarped. In such a structure, magma can then be viewed as a melted lens capping a mantle upwarp at shallow depth, rather than in an intracrustal chamber. This reduces the conflict between estimates of its volume from excess output of volatiles and short residence times. A link in time is indicated between volcanic and seismic activity at a large scale: over the millennium the reported ends of episodes of high output rates of magma are followed by the reported occurrences of magnitude 7 + earthquakes which caused destruction in southeastern Sicily. Several steep active normal fault have been imaged to a depth of 10 km the crust up to 30 km offshore of the cities of Catania and Augusta, which may be fault planes for such large earthquakes. They expand and prolongate the system of the Timpe faults on the eastern flank of Etna, thus linking large-scale tectonics offshore with the volcano. Etna developed together with normal faulting, upwarp, and spreading during the recent evolution of the former Ionian subduction. Activation of the material at depth at the lateral edge of the slab, by vertical motion with extension above, could produce the peculiar type of Etna magmatism.

Topography of large-scale watersheds: fractal texture and global drift. Application to the Mississippi basin

The topography of large-scale hydrographic basins is assumed to involve two components: a meso-scale isotropic fractal pattern and a polarized large-scale trend which is oriented by large-scale tectonics and supports the global drainage. An algorithm is developed which splits up topography into a fractional Brownian function and a trend gradient C. Focusing on the large scales a logarithmic profile and a rectangular planform are detected through either the relationship between the gradient C and the normalized elevation E or by the basin hypsometric curve and the inertia tensor of the basin area. This is successfully applied to the digital terrain model of eight large-scale sub-basins of the Mississippi. The profile belonging to the basin main valley is also logarithmic but its slope is twice smaller than that of the mean profile, denoting a trend surface with converging slopes towards the mid-valley. The scaling transition is investigated through angular distributions. This is synthesized by a ternary diagram involving the following poles according to increasing scales: a pure isotropic fractal F, a transient off-centred fractal O and a tilted tectonic plane T. The fractal parameters are the intensity σ at scale s = 1 km and the Hurst exponent H. Over a basin area H is constant while σ is proportional to the trend slope C whatever the elevation E. This implies a constant horizontal scale ξ f for each basin, around 30 km separating the fractal-dominated topography from the trend-dominated topography.

The potential role of non‐Newtonian rheology in the resurfacing of Venus

Several analyses of the impact cratering record on Venus have concluded that 80% of Venus was resurfaced sometime in the last 300 m.y.–700 m.y. The cratering record further indicates that the resurfacing took place over a 30 m.y. ‐ 100 m.y. interval. Since this period, there has been little large scale tectonic activity on Venus. This study proposes that such behavior may result from the interaction of phase change induced avalanches with a non‐Newtonian lithosphere. The mechanism proposed here suggests that because of non‐Newtonian rheology, the Venusian lithosphere becomes highly mobile during brief periods when avalanches occur. Afterwards, due to the reduction in stress levels, the lithosphere becomes rigid and large scale tectonics ceases.

Rock deformation: Ductile and brittle

Rock deformation studies are conducted to constrain the non‐elastic behavior of rocks subjected to non‐hydrostatic stresses in various geological and geophysical situations including long term creep responsible for large scale tectonics such as mantle convection and brittle failure related to earthquakes. Significant advancements made in these areas during the last quadra‐annual period are reviewed. In ductile deformation, important progress has been made toward understanding rheological properties under deep mantle conditions. These studies provide important insights into the dynamics of the Earth's deep interior. In addition, significant progress has been made toward understanding the role of chemical environment on rheology and the influence of microstructures such as grain‐size. In the brittle field, important advances were made in the observation of the spatio‐temporal evolution of faulting. The complex interplay of microcracking, crystal plasticity, diffusive mass transfer and pore collapse in controlling the brittle‐plastic transition under crustal conditions have been elucidated. Recent investigations demonstrate the importance of fluid flow on frictional strength. New measurements have clarified the influence of normal stress, temperature and mineralogy on friction constitutive behavior. Some of these experimental observations are being incorporated into crustal faulting models that provide physical insights into the dynamics of the earthquake cycle.

Postglacial land uplift patterns of south Sweden and the Baltic Sea region

ABSTRACTComparison of the land uplift pattern for the last 10,300 years, shown by studies of raised shorelines of the Baltic Ice Lake, with the pattern of present‐day land uplift of Fennoscandia, shows that significant regional changes of uplift pattern have taken place. Some of these changes seem to be related to a halt in ice retreat during the Younger Dryas cold stage. It is also probable that some observed anomalies in the present uplift are not of glacio‐isostatic origin, but are possibly related to structures in the lower lithosphere and upper mantle or large scale tectonics.

Crustal and upper mantle electrical conductivity structures in West Africa: Geodynamic implications

Measurements of the electrical and magnetic fields at the surface of the earth have been carried out in West Africa in the period range 10–10,000 s. In this area, the method of magnetotelluric (MT) sounding finds its application in a variety of geological problems such as the deep shape of sedimentary basins, the geometry of orogenic belts and craton deep structures. The present article is an attempt to correlate the conductivity model with large scale tectonics. MT surveys have revealed the presence of several areas where electrical resistivity is abnormally low at crustal and upper mantle depths and the existence of lateral electrical conductivity inhomogeneities within the lithosphere. Subsurface electrical conductivity distribution is in good relation with the geological features, and its variation associated with lithological and structural changes. A major discontinuity found at a depth of several hundred kilometers in the lithosphere separates two structural blocks, the eastern one, with higher resistivities, being the West African craton. The lithosphere derived from this work has a maximum thickness of 300 km under the Senegalo-Mauritanian basin and more than 400 km beneath the craton. The electrical asthenosphere is certainly not “well-developed” under the craton. A sketch of electric structures across West Africa is presented and a geodynamic interpretation is proposed to explain the present structural pattern of the southern segment of the Mauritanides belt.

Neotectonics of Africa and the Indian Ocean: Development of the geoidal low

The western, trans-Africa lobe of the Indian Ocean geoidal low correlates with Neogene tectonic features, such as the east Africa rift system and the Congo river basin. The opposite, eastern flank of the Indocean geoidal low is indented westward by the geoidal high associated with Sundaland (Indonesia), believed to have been displaced westward into its present location since mid-Tertiary times. The Indocean low thus seems to be slowly mobile, and to have acquired its present shape, elbowed at the Equator pointing west, as a result of tectonic processes continuing at present. We have sought lithosphere motion accounting for the large-scale tectonics and for the changes in the geoidal low. The constraints suggest that Africa and the western part of the Indian Ocean basin have been experiencing slow westward drift in Neogene times. This has resulted in the development of the mass-deficiency to the east, expressed as the Indian Ocean geoidal low, in the Africa meridional rifting, and in seafloor spreading as material upwells from the asthenosphere in response to the developing hydrostatic deficiency. The motion is consistent with the seafloor magnetic record, and would explain the dextral displacement at the south boundary of the Eurasian lithosphere segment.

Far-field tectonics associated with a large impact basin: applications to Caloris on Mercury and Imbrium on the Moon

Lithospheric readjustments after the formation of a large impact crater have been computed. The models predict tectonic perturbations over a major portion of the surface of the planet. The weight of the ejecta and the mechanical perturbations in the crater area give rise to membrane stresses. If the planetary lithosphere is in compression, the direction of maximum compression after the impact is perpendicular to a meridian (in a referential where the basin would be at the North pole). On Mercury the preferential orientation of compressive scarps radial to Caloris is explained in this manner. For a planet in extension grabens perpendicular and parallel to a meridian can appear successively as the mechanical state of the crater area is modified. On the Moon this result is consistent with the large-scale tectonics associated with Imbrium.

A seismotectonic study of the 1908 Messina earthquake

Abstract A major earthquake occurred in the Straits of Messina in Southern Italy on December 28, 1908. The shock became known in literature as the “Messina earthquake”. The present paper deals mainly with a re-interpretation of the source mechanism of this shock using modern interpretation techniques. The following parameters are discussed: Epicentre, hypocentre, origin-time, macroseismic intensity, body and surface wave magnitudes, fault-plane solution, seismic moments derived from body and surface waves, residual displacements and tsunami values. The parameters are compared with the tectonic setting of Southern Italy. As a result of the large magnitude (M = 7) and the 30 km long fault extension of this shock, the Messina earthquake reflects the large scale tectonics of Southern Italy more significantly than any other earthquake from which seismograms exist. The focal mechanism studies show a good agreement between the orientation of the shear-plane of the earthquake and the tectonic features of a fault-line which is known from geological data, the so-called Comiso-Catania-Messina-S. Eufemia fault. The rupture of the shallow earthquake was associated with normal faulting. According to the normal faulting mechanism, the earthquake resulted from tensional forces acting approximately perpendicular to the strike direction of the Straits of Messina. Geodetic measurements which were performed shortly before and after the earthquake indicate an apparent asymmetry in the volumes of uplift and subsidence. It is suggestive that there exists a strong net subsidence of material displaced with the earthquake faulting. The normal faulting mechanism of the Messina earthquake is in tectonic accordance with the uprise of olivine basalt in the volcanism observed at Etna. However, the normal faulting is contrary to the postulation of a subducting oceanic plate giving rise to the Calabrian-Sicilian arc-like feature, as this hypothesis would require mainly a compressional zone in the region of the Straits of Messina.

Systematics of large-scale tectonics and age progressions in Alpine and Circum-Pacific blueschist belts

The early Alpine tectonics and relatively high-pressure metamorphic parageneses of the Helvetic, Pennine and Sesia—Lanzo realms are compared with analogous circumpacific terranes in western California, southwestern Japan, west-central Chile, and southern Alaska. Petrotectonic relationships appear to be compatible with a process involving syntectonic recrystallization and pervasive deformation in an imbricated subduction zone for each of these regions. Successive underthrusting of younger portions of the downgoing low-density material, followed by decoupling and rise towards the surface may account for the systematic decrease in both age and metamorphic intensity proceeding away from the old plate junction in these terranes. The environs of the present Aleutian trench serve as a modern analogue. Several other circumpacific blueschist belts (i.e., New Zealand, New Caledonia and eastern Papua) exhibit possible departures of timing, metamorphic polarity or direction of tectonic transport from that appropriate to the imbricated convergent plate-junction model presented in this paper, but the overall metamorphic sequences and tectonic settings are nevertheless strikingly similar. Many relatively high-pressure belts seem to be characterized by the occurrence of a major post-metamorphic transcurrent fault. This phenomenon suggests that blueschist belts, which must be exhumed rapidly in order to preserve the relatively high-pressure, low-temperature mineral assemblages, typically are obliterated through higher-temperature recrystallization unless a profound change from convergent to conservative plate motion occurs. Variation in convergence rate is yet another important factor, more rapid underflow promoting generation of blueschistic metamorphic parageneses, slower underflow favoring shallow decoupling and buoyant return towards the earth's surface of the successive underplatings. Other things being equal, a steeper angle of underflow would provide lower temperatures at a given depth for the subducted slab, thus promoting glaucophane schist-type metamorphism.

Relation between large scale tectonics and level tectonics in the Ruhr carboniferous structure. In German : Drozdzewski, G 5F, 1T, 18R. Z. Deutsch. Geol. Ges. V124, N1, 1973, P177–189

Relation between large scale tectonics and level tectonics in the Ruhr carboniferous structure. In German : Drozdzewski, G 5F, 1T, 18R. Z. Deutsch. Geol. Ges. V124, N1, 1973, P177–189

Influence of Eustatic Sea-Level Changes in Oil and Gas Accumulations in Appalachian Basin: ABSTRACT

Regional stratigraphic studies indicate a minimum of 23 eustatic sea-level fluctuations in the Appalachian region from New York to Alabama. A eustatic fluctuation is interpreted if the stratigraphic and sedimentologic records on two or more sides of the Appalachian basin show evidence of a similar simultaneous shift in relative sea level within the limits of temporal resolution by fossils, intertonguing facies, or bentonite chronology. Simultaneous sea-level shifts affecting different lobes of ancient delta complexes built into the Appalachian basin from eastern sources also are considered eustatic. For analysis of a hypothetical single basin, a eustatic sea-level change is one which affects the entire basin. The cause may be large-scale tectonics of the continental area ontaining the basin, or a true sea-level shift related to glaciation or rate of sea-floor spreading. Timing on the sea-level variation curve is related closely to ages of strata with hydrocarbon production in the Appalachian basin. Changes in sea level result in shifting of sand deposition along shorelines, solution porosity in carbonate rocks exposed along basin margins, or modifications of reef growth. The clearest relations to hydrocarbon production are in the well-explored oil and gas fields in the Devonian and Silurian, where the sea-level shifts can be used to explain permeability distribution. Superposing sea-level shifts onto sedimentary tectonics in a basin of known shape allows prediction of exploration trends. The largest fluctuations of sea level are at the base of the Sauk sequence (Cambrian transgression), the Owl Creek discontinuity (base of the Middle Ordovician), the Wallbridge discontinuity (end of the Early Devonian), and the discontinuity at the base of the Absaroka sequence (beginning of Pennsylvanian). Fluctuations associated with the Wallbridge discontinuity are related to deposition of the Oriskany Sandstone, which is the largest Appalachian gas producer. Lesser sea-level changes are related to other production, notably oil and gas from the Upper Devonian fields which were the birthplace of the American petroleum industry, and gas from the Silurian Newburg sandstones. The eustatic sea-level curve from one basin such as the Appalachian area should be compared with other basins to identify worldwide patterns and to help to focus petroleum exploration in distant basins. The major level drops at the end of the Early Ordovician, end of the Early Devonian, and the beginning of the Pennsylvanian seem established. The sea-level drop in the Appalachians at the end of the Ordovician appears related to continental glaciation centered in the African Sahara. The rise in sea level at the end of the Precambrian is possibly a result of an increase in rate of sea-floor spreading as the proto-Atlantic Ocean opened. Recognition of true global changes should permit more precise intercontinental correlations because eustatic sea-level change is not related to distribut on of faunal provinces or local tectonic processes. End_of_Article - Last_Page 2216------------

Beziehungen zwischen Großtektonik und Stockwerktektonik im Ruhrkarbon

Beziehungen zwischen Großtektonik und Stockwerktektonik im Ruhrkarbon

Structure of the low-velocity channel and its relationship to large-scale tectonics

Structure of the low-velocity channel and its relationship to large-scale tectonics

Morphological aspects of the lunar crust

The most important factors for the interpretation of the lunar surface features are viscosity of original magma and thickness of the original crust. Morphological features first revealed by the US Moon rocket Ranger VII suggest that the original magma in Mare Nubium is basic and characterized by its rich gas content, and that the magnetic differentiation took place at the time of crustal formation, and craters and other surface features were molded by processes quiet and not catastrophic in nature. Morphological characteristics directly corresponding to petrological properties of magma are considered in some detail for large-scale tectonics.