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The crustal dichotomy between the ancient highlands of the Southern Hemisphere and the young-appearing northern lowlands is a defining feature of Mars. In the Eastern Hemisphere the dichotomy boundary is marked by a prominent scarp and extensional and compressional tectonic features. Topographic data across the boundary returned from the Mars Global Surveyor indicate lithospheric flexure of the southern highlands. The topography of the boundary can be fit by a universal lithospheric deflection profile that corresponds to an elastic thickness of ~31-36 km. Flexure of the southern highlands may be due to late Noachian-early Hesperian vertical loading of the northern lowlands. Fracturing and normal faulting along the boundary may be in response to bending stresses, while thrust faulting may result from a combination of stresses due to flexure, erosion, and global contraction. Keywords: Mars, tectonics, flexure, dichotomy boundary, elastic thickness.

NEAR Shoemaker imaging of the asteroid Eros has revealed a variety of surface forms that probably reflect underlying structures. They range from global to local in scale. The most prominent positive topographic feature is a long ridge, Rahe Dorsum, which extends over 18 km and has the morphology of a thrust fault. Two large, ancient trough systems are also present. Grooves are ubiquitous, commonly occur in suborthogonal sets, and have linear or scalloped outlines. The grooves on Eros are similar in morphology and scale to those on other small bodies such as Gaspra, Ida, and Phobos and are likely to have a similar mode of origin, namely drainage of regolith into underlying fractures. Models that use groove dimensions as indicators of regolith thickness imply depths of up to several tens of meters. Many craters have been subjected to structural control, resulting in squared outlines. We find no evidence that specific groove sets are associated with individual impact events; however, evidence exists that preexisting structures have been reactivated by later impact activity. The large scale of Rahe Dorsum and many grooves on Eros's surface suggests that the asteroid is a largely coherent, but fractured body. Key Words: asteroids; Eros; impact processes; regoliths; asteroid surfaces; tectonics.

The link between back arc extensional tectonics and the emergence history of eclogite bearing units in the Cycladic islands in the Aegean Sea, in the island of Corsica and in the internal Betic Cordillera of the Alboran Sea, is examined. The tectonic style associated with the denudation of Mediterranean eclogites fits an active accretionary-wedge setting.

Uplifts flank three major grabens in central Greece. The geological background of the Aegean region and conceptual models used in rift flank uplift simulation are summarized. A data analysis procedure is outlined and is modeled using elastic and viscoelastic broken plate models. The elastic predictive modeling was found to fit well with observed flexural wavelengths and flanking seismicity, although it does not predict graben widths and inferred elastic layer thickness.

The tectonic evolution of the South Carpathians - Moesian platform area is obtained from depth-interpreted seismic sections of the Getic Depression foredeep, paleostress indicator data and outcrop- to regional-scale structure analysis. Particular attention is paid to basin formation processes and their evolution.

Late Miocene paleostress data from the Alpine-Carpathian-Pannonian region present evidence of a homogeneous E-W compressive stress field in the brittle upper crust. Compression occurred between 9 Ma and 5.3 Ma and was coeval with late-stage west-directed subduction of the European plate below the Eastern Carpathians. During Late Miocene soft collision, E-W compressive stress was transmitted from the eastern Carpathian plate boundary to the Pannonian Basin and into the Eastern Alps. The basic tectonic effects of the stress field on the area are discussed.

Thermobarometric investigation of upper mantle xenoliths of the Gibeon Kimberlite region reveal the prominent role of a kimberlite magma in carrying them to the Earth's surface. Majority of the coarse equant and mosaic-porphyroclastic xenoliths exhibit thermal equilibrium for the mantle, whereas the other coarse equant xenoliths portray an ongoing cooling state. A few porphyroclastic xenoliths show transient heating of the mantle, while the P-T values of some coarse equant xenoliths nears a geothermal gradient of 44 mW/sq m. Electron microprobe surveys enable analysis of tectonic elements.

Multi-system isotopic surveys of the Sesia-Lanzo Zone reveal different stages of the P-T-deformation evolution that are consistent with the established geotectonic Alpine orogeny framework. The Rb-Sr white mica and U-Pb sphene data restrict the age of eclogite facies metamorphism/deformation to 60-70 Ma and suggest that EMC recrystallization is behind sphene growth. The behavior is unlike similar lithologies in the Gneiss Minuti Complex and the isotopic data is unable to distinguish between two mechanical contiguity models. The results indicate at least three tectonic Alpine collision slices.

Detailed chemical surveys of phlogopite/biotite in diverse tectonic lavas provide a database to understand degassing-related variations in the ferric-ferrous ratios, O/H2O contents, and delta-D values. Cation/anion-based normalization techniques accurately unravel mineral formulae of most of the cations/anions, but seem unable to provide realistic estimates of Fe3+/Fe(super T) ratios. Analysis of the phenocrysts in arc-related lavas of western Mexico and eastern California reveals the highest ferric-ferrous ratios pointing to high magmatic oxygen fugacities.

A tectonic survey of the Variscan collisional belt within the southeastern Bohemian Massif reveals thick-skinned thrusting of hinterland units leading to an inverted metamorphic zonation that contrasts significantly with the thin-skinned foreland units. An investigation of 40 Ar/39 Ar hornblende/muscovite plateau ages of hanging wall units shows complete restoration of intracrystalline argon isotopic systems during Late Variscan thrusting. A rapid exhumation/cooling process combined with thrusting along crystal-scale ramps leads to systematic decrease in Variscan mineral ages.

The plutonic Socorro Complex of the Precambrian Rio Paraiba do Sul Shear Belt displays magmatic, metamorphic and tectonic features that unmask crosscutting relationships between emplacement and the regional ductile shear zones. U/Pb zircon analysis disclose the temporal overlap in magmatic and tectonic processes. Main igneous activity occurred between 650 and 595 million years ago. The origin of calc-alkaline granites of the Ribeira Belt is attributed to partial melting of the lower crust and a bit of the lithospheric mantle during thickening of the transpressional plate margins.

The use of stress indicators, such as borehole breakouts, faults and earthquake focal mechanisms, show the existing stress trend in the Portuguese mainland territory. The stress pattern shows a northwest-southeast trend of the maximum horizontal stress direction (SHmax). A geodynamical model links this rotating compressive stress to the collision of the Eurasian and the African plates during the late Miocene. This collision resulted in the westward expulsion of the Iberian microplate since Pleistocene and an enhanced SHmax in the Atlantic continental margin.

- Mafic dykes from the Oksfjord-Langfjord area were intruded at different stages during the prolonged tectonomagmatic evolution of the Seiland Igneous Province. Field relations, petrography and geo-chemistry indicate the presence of four dyke generations with alkali basalt composition and one generation of alkaline lamprophyres. The entire dyke suite has geochemical signatures consistent with formation in a within-plate geotectonic environment. Trace elements indicate that the alkali basalt dykes have OIB (ocean island basalt) affinities and we suggest a sublithospheric mantle source. The data support a rift-related origin for the Seiland Igneous Province. Longevity of magmatism in the Seiland Igneous Province (300 m.y.) is difficult to explain within a conventional mantle plume framework. Instead, it is proposed that the intermittent magmatism in the province was predominantly permissive and controlled by lithosphere structure.

Detrital clinopyroxenes occur abundantly in several greywacke formations in the Southern Uplands, both as single grains and within andesite lithociasts. Compositionally, these pyroxenes fall into two main groups. Those from the oldest unit, the early Caradoc Galdenoch Formation, were derived from a simple tholeiitic volcanic arc. In contrast, the pyroxenes in the younger Portpatrick (late Caradoc) and Glenlee (Ashgill) Fonnations and the Gala Group (early Llandovery), show initial strongly calc-alkaline affinities, followed by a more complex calc-alkaline to transitional (calc-alkaline to tholeiitic) volcanic arc signature. The latter may be due to a mixture of calc-alkaline and tholeiitic components. A presently exposed source for the andesites has not yet been established, although the Ballantrae Complex is a potential candidate for a tholeiitic component.

The Belingwe greenstone belt in Zimbabwe has a thick tholeiitic basalt and komatiite sequence above the shallow water shelf rocks, and the nature of the contact plays an important role in the tectonic setting of greenstone belts. The tholeiitic basalt and komatiite together form the Mberengwa allochthon. The distribution of the rock types indicate that the tectonic history indicated by the Belingwe belt may be found all over central Zimbabwe.

Preliminary results from a three-dimensional, finite-element computer model of the dynamics of compressional orogens suggest that the deformation of the brittle crust is the superposition of two modes of deformation: a forced mode dictated by preexisting structures or plate-boundary geometry, and a natural mode resulting from the nonlinear elastoplastic rheology of the crust that may depend on the choice of a particular failure criterion. Natural modes of deformation exist in both the vertical (thrust planes) and horizontal (shear zones) directions, and there is a large degree of coupling between them (thrust planes accommodate vertical and horizontal shear). The obliqueness of convergence affects the geometry of thrusting; therefore, two-dimensional plane-strain models may be inappropriate to model orogens in which the direction of compression is oblique to the strike of the orogenic belt.

Investigation of lithospheric flexure at continental margins by sedimentary loading has been carried out using elastic-plate theory. The geometry of mature marginal basins fits the deflection produced by loading of an infinite plate in which the flexural rigidity of the continental lithosphere is larger than that of oceanic lithosphere. Decoupling of a previously locked passive margin by reactivation of margin faults may cause the lithospheric bending behavior of the margin to change from that of an infinite plate to that of a semi-infinite plate, with a resultant increase in deflection of the marginal basin and possible subduction initiation.

The interpretation of prograded gravel as an indicator of tectonic uplift, that is, 'syntectonic' deposition, is not necessarily correct. Increases in sediment supply induced by tectonic uplifting did not control the progradation of gravel in the Pine Valley basin, an actively subsiding half graben in northeastern Nevada. Rather, the distribution of lithofacies in the Pine Valley basin is primarily controlled by the uneven distribution of subsidence within the basin. Differences in subsidence along the axis of the basin indicate a strong inverse relationship between the long-term subsidence rate since late Miocene time and the distance of gravel transport out into the basin. Differential progradation of alluvial gravel into the basin from various drainages along the footwall is not related to uplift or lithology of the source area, size of the catchment area, or climate. In addition, the relatively fixed location of the topographic low within the basin through time, as indicated by the distribution of lacustrine deposits and the position of the modern axial drainage, suggests that a balance between basin subsidence and sediment supply to the basin has been generally maintained through time. Thus, if subsidence rates are a measure of tectonic activity, gravel progradation may record times of reduced, not increased, tectonism.

The uniform southwesterly trend of fast-traveling split SKS waves that traverse the upper mantle beneath the Yellowstone swell provides good evidence for both southwesterly motion of North America over a relatively stable deep Earth interior, and young strain accommodation within the Yellowstone swell mantle via dislocation creep of olivine. These results contrast with the many SKS splits recorded across the western United States, the splitting behavior of which often is very complex at individual sites, and the orientations of which, as a set, cannot be attributed simply to plate motion or to small-scale convection beneath North America lithosphere. These data suggest that, away from Yellowstone, (1) mantle fabric is complex, and (2) North America motion is accommodated by strain at depths greater than those typically associated with asthenosphere. Keywords: asthenosphere, anisotropy, plate motion, western United States.

The deformation of the Wandel Sea Basin of eastern North Greenland and the West Spitsbergen Fold Belt are the result of Late Cretaceous-Palaeocene Greenland-Barents Sea/Svalbard convergence following the opening of the Labrador Sea-Baffin Bay opening. The Tertiary rocks of the Wandel Sea Basin are undeformed while the Late Palaeozoic-Mesozoic are weakly folded and thrust faulted. Thrust fault movements in North Greenland ceased before Mid-Palaeocene time when the dextral motion separating Greenland and Svalbard began. The shortening was orthogonal to the continental margin and a transpressional origin for the deformation of the Wandel Sea basin is not confirmed. Keywords: Compression tectonics, Cretaceous, Wandel Sea, Arctic.

The Arizpe and Bacanuchi Quadrangles provide a geologic history representative of the north-central part of Sonora, where lithologies are dominated by late Mesozoic and Cenozoic igneous rocks. In this study, new geologic mapping, [sup.40]Ar/[sup.39]Ar dating, and geochemical analyses have been combined to provide a stratigraphic framework for this area. Ten lithostratigraphic units and several igneous and tectonic events can be recognized. The oldest outcropping rocks are Lower Cretaceous strata of the Bisbee Group, which along with the Picacho conglomerate record a middle Cretaceous compressive tectonic event and associated sedimentation. Laramide igneous activity is widespread and represented by (1) highly altered andesitic flows and volcaniclastic rocks (Arroyo Alcaparros andesitic rocks) of late Campanian to Maastrichtian age, (2) less altered andesitic and dacitic flows (Cerro Las Jarillas volcanic rocks) of late Paleocene age, and the intrusive bodies of (3) Sierra El Manzanal granodiorite and (4) Rancho Vaqueria quartz monzonite. The Sierra El Manzanal granodiorite was emplaced at ca. 68 Ma on the basis of a [sup.40]Ar/[sup.39]Ar biotite age (67.97 [+ or -] 0.19 Ma) and cooled relatively rapidly according to less precise [sup.40]Ar/[sup.39]Ar hornblende and K-feldspar ages from the same sample (64.8 [+ or -] 1.0 Ma and 62.8 [+ or-] 0.3 Ma, respectively). The Cerro Las Jarillas volcanic rocks are slightly younger ([sup.40]Ar/[sup.39]Ar biotite age of 58.67 [+ or -] 0.17 Ma). The Rancho Vaqueria quartz monzonite was emplaced at ca. 57 Ma ([sup.40]Ar/[sup.39]Ar biotite age of 56.73 [+ or -] 0.14 Ma and a less precise [sup.40]Ar/[sup.39]Ar hornblende age of 55.0 [+ or -] 0.7 Ma); a protracted cooling history of this pluton is indicated by the age spectrum of K-feldspar from the same sample. A probable magmatic lull and denudation seem to have occurred between middle and late Eocene time and probably until the early Oligocene. Subsequently, rhyolitic to mafic volcanism began close to late Oligocene time and lasted until the early Miocene. Felsic volcanism is represented by the Cerro Cebadehuachi volcanic rocks, from which [sup.40]Ar/[sup.39]Ar hornblende ages of 27.25 [+ or -] 0.09 and 27.32 [+ or -] 0.06 Ma and a biotite age of 26.97 [+ or -] 0.06 Ma were obtained at three different localities. The Mesa Pedregosa volcanic rocks represent the transition to younger, mafic volcanic activity that occurred during the late Oligocene, as indicated by a sanidine [sup.40]Ar/[sup.39]Ar age of 25.48 [+ or -] 0.05 Ma. This late Oligocene and early Miocene magmatism was paired by two episodes of extensional deformation. The first phase is characterized by northwest-striking normal faults and folds, which expose the deepest structural levels of the area, and by the related basin fill, the Bacanuchi conglomerate. The second phase is represented by north-striking normal faults and by the syntectonic basin fill, the Arizpe conglomerate. Basaltic andesite volcanic flows at the base of the Arizpe conglomerate yielded [sup.40]Ar/[sup.39]Ar (whole-rock) ages of 23.52 [+ or -] 0.17 and 21 [+ or -] 0.20 Ma. The extensional deformation (27 to 23 Ma) in the study area is coeval with the development of metamorphic core complexes in neighboring areas of Sonora and with the onset of extension in southern Sonora. The mafic volcanic rocks and clastic sedimentary units associated with this extension resemble the basin fills that in other parts of Sonora are assigned to the Baucarit Formation. Geochemical information from samples representing each of the igneous events displayed high-K calc-alkalic and mostly metaluminous compositions. The older units including the Arroyo Alcaparros andesitic rocks, the Cerro Las Jarillas volcanic rocks, the Sierra El Manzanal granodiorite, and the Rancho Vaqueria quartz monzonite are characterized by steep chondrite-normalized REE (rare earth element) slopes and generally well-developed negative Eu anomalies, suggesting garnet and plagioclase removal in the source. The younger igneous events including the Cerro Cebadehuachi and Mesa Pedregosa volcanic rocks, and the basaltic flows associated with the Arizpe conglomerate, showed basin-shaped REE slopes with no Eu anomalies, suggesting clinopyroxene or amphibole fractionation. Keywords: Cretaceous, geochronology, magmatism, Sonora Mexico, tectonic evolution, Tertiary.

Chasma Australe, 500 km long and up to 80 km wide, is the most remarkable of the martian south pole erosional reentrants carved in the polar layered deposits. We have interpreted Chasma Australe erosional and depositional features as evidence for a flood origin, which we have reconstructed using a modified Manning equation. The main characteristics of the flow are a water velocity between 30 and 50 m [s.sup.-1] and discharge values between 7 x [10.sup.8] and 3 x [10.sup.9] [m.sup.3] [s.sup.-1], very near to MGS data-based estimations for martian outflow channels (D. E. Smith et al. 1998, Science 279, 1686-1692). We thus postulate that Chasma Australe originated in a catastrophic flood. The tectonic study of an area (roughly 20 million [km.sup.2] in size) around Mars' south pole included the measurement and projection in rose diagrams of more than 300 lineaments, of which 85 were wrinkle ridges and the rest straight scarps. The whole set of lineaments can be explained by a stress field with a [[Sigma].sub.1] N10 [degrees] E in strike, the wrinkle ridges being reverse faults and the other lineaments direct and strike-slip faults. The straight layout of parts of Chasma Australe almost 200 km long suggests that the chasma was carved following a fracture network. The effectiveness of the erosional process (the canyon is up to 1000 m deep) leads us to suspect that this carving was preceded by a sapping period. Endogenetic and exogenetic processes would thus have contributed to the origin of this landform. Key Words: Mars; tectonics; climate; poles.

Many candidate alignments of terrestrial craters have been proposed and keenly pursued by science journalists and planetary scientists. Rampino and Volk identified a potential alignment of eight crater-like structures across Kansas and Missouri. John Luczaj has found that the ages of the Rampino and Vol string of structures could not be the same and that an impact origin would be impossible. He argues that the craters originated volcanically and that the alignment is tectonic.