A new model, partially based on the three most widely cited previous hypotheses, is proposed to explain the genesis of the Canary Islands. From the hotspot hypothesis it retains the notion that the islands originated from a thermal anomaly in the mantle. From the propagating fracture hypothesis it takes the critical role of regional fractures in the onset of magmatism. The uplifted block hypothesis contributes with the notion that the islands are in their present freeboard attitude due to the action of tectonic forces.The main drawbacks of the three preceding hypotheses are solved within this unifying approach: the thermal anomaly is an upper mantle residue from an old plume, and therefore it does not carry (or does it in a highly diluted form) the typical geophysical and geochernical plume signatures; the fractures are well developed on the continental and oceanic crust, but not in the extremely thick sedimentary pile between the Canary Islands and Africa; and the Canary Islands uplift took place through transpressive shears, and not by means of purely reverse faults. This unifying model, which integrates the thermal and tectonic histories of the lithosphere and the sublithospheric mantle, is considered to be a valid approach to a number of volcanic areas where, as has been highlighted in recent years, pure hotspot or pure fracture models are found wanting to explain oceanic or (less frequently) continental volcanic lines.

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apparent head of Chasma Boreale was interpreted as an impact crater. Then, the Viking images showed the crater A detailed geomorphological study of Chasma Boreale, a did not exist, and other questions were raised over the widely known feature of Mars' north polar cap, has been carried out for the first time, along with a quantitative paleohydro-feasibility of the subglacial river.logical model. It is concluded that the chasma was eroded Clifford (1980Clifford ( , 1987 has studied in detail the mechaby a flow whose discharge was on the order of magnitude of nisms for a catastrophic water release from the permafrost, 109 m sec ؊1 . This catastrophic flow is thought to have been looking for analogs on both Earth and Mars. His comparipreceded by a powerful sapping process, caused by a tectoni-son with Ravi Vallis, a box-headed channel on Mars equacally focused thermal event. Several lines of evidence indicate tor (1ЊS, 43Њ) is particularly compelling, because of the tectonic control in the distribution and present aspect of most angular, straight head and huge flow marks. His hypothesis of the polar troughs. This tectonic forcing probably played a of glaciovolcanic outburst water flows (jö kulhlaups of Icemajor role in the inception of not only Chasma Boreale but landic authors) also seems appropriate, since this mechaalso other polar reentrants as well.

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Abstract. A photogeological reconnaissance of Viking mosaics and images of the Tharsis dome has been carried out. Fifteen new areas of transcurrent faulting have been located which, together with other structures previously detected, support a model in which the Thaumasia Plateau, the southeastern part of the Tharsis dome, is proposed to be an independent lithospheric block that experienced buckling and thrust faulting in Late Noachian or Early Hesperian times as a result of an E-W directed compression. Evidence is presented that this stress field, rather than the Tharsis uplift, was decisive in the inception of Valles Marineris, which we consider a transtensive, dextral accident. The buckling spacing permits us, moreover, to tentatively reconstruct a Martian Hesperian lithosphere similar in elastic thickness to the mean present terrestrial oceanic lithosphere, thus supporting the possibility of a restricted lithospheric mobility in that period. Tharsis lithosphere was again subjected to shear stresses in Amazonian times, a period in which important accidents, such as strike-slip faults, wrinkle ridges, and straight and sigmoidal graben, were formed under a thin-skin tectonic regime, while the lithosphere as a mechanical unit had become too thick and strong to buckle. The possible causes of those stresses, and especially their relationships to a putative period of plate tectonics, are discussed.

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. We thus postulate that Chasma Australe originated in a catas trophic flood.The tectonic study of an area (roughly 20 million km2 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 linea ments can be explained by a stress field with a crI N100E in strike, the wrinkle ridges being reverse faults and the other lineaments di rect and strike-slip faults. The straight layout of parts of Chasma Australe almost 200 km long suggests that the chasm a was carved following a fracture network. The effectiveness of the erosional pro cess (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.