Francisco Anguita

The Canary Islands origin: a unifying model

Abstract 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 geochemical 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.

Tectonics and volcanism of Sierra Chichinautzin: extension at the front of the Central Trans-Mexican Volcanic Belt

Abstract Because of its recent activity and position at the southern magmatic front of the Trans-Mexican Volcanic Belt (TMVB), the Sierra Chichinautzin volcanic field (SCN) is a key area for the understanding of this controversial volcanic province. Volcanic activity has built more than 220 monogenetic volcanoes (shields, scoria cones, thick lava flows, and hydromagmatic structures) during the last 40,000 years, for a total volume of about 470 km 3 . The SCN basalts are geochemically similar to OIBs, while the intermediate and felsic volcanic rocks show a calc-alkaline trend and abundant evidence for magma mixing. The structural analysis of this volcanic field and surrounding areas has been based on field data, satellite images, and a method for detecting volcanic center alignments. The tectonic data, together with geophysical evidence, confirm active general N–S extensional conditions with a strike–slip component for the SCN area, the same structural setting that prevails in the rest of the Central TMVB. Extensional tectonics, a negative regional Bouger gravity anomaly, a low-velocity mantle, high heat flow, and shallow seismicity suggest a rift-type setting involving the upwelling of anomalous mantle beneath the Central TMVB. The combined petrological, structural and geophysical arguments support that the SCN volcanism is rift-related, and rule out processes involving the subduction of the Cocos plate, which casts further doubts on the standard subduction model for the TMVB volcanism.

Tharsis dome, Mars: New evidence for Noachian-Hesperian thick-skin and Amazonian thin-skin tectonics

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.

Roque Nublo caldera : a new stratocone caldera in Gran Canaria, Canary Islands

Abstract An inferred elliptic structure 5 × 4 km in diameter, has been identified as the caldera that brought the Roque Nublo Pliocene (5-3.5 Ma.) volcanic cycle in Gran Canaria to an end. This buried caldera, whose morphological expression is no longer evident, is now filled with almost 400 m of interbedded lacustrine sediments and nephelinite lava flows, ashes and tuffs. Mapping of the postcaldera sedimentary layers, which comprise debris-flow lobes, turbiditic sandstones and muds, has permitted the paleogeographic reconstruction of a saucerlike volcanic depression. This has been confirmed by a gravimetric survey, whose data clearly define a double gravity minimum in the area of the inferred caldera. Some postcaldera vents are located near the caldera rim, but most of them define a NW-SE line, across the caldera, which apparently marks an important tectonic lineation. The paucity of remains of Roque Nublo breccias in the caldera infilling suggests a powerful explosive terminal phase for the stratocone, after which the center of the residual edifice collapsed to form the caldera. The very early resumption of activity in the caldera is not a definite proof for a resurgent activity in the area. Whereas the Roque Nublo Formation was formed after a quiescent period of almost four million years on Gran Canaria, this event was soon followed by the third and last cycle of volcanic activity on the island.

Circular features in the Trans-Mexican Volcanic Belt

One hundred and ninety-one circular or elliptical features have been located on Landsat imagery of the Trans-Mexican Volcanic Belt (TMVB). The origin of most of these features is unknown. Nine have been recognized as collapse calderas (clearly visible on Landsat imagery) and studied in detail, while an equally small number have been tentatively identified as such but not thoroughly investigated. On the basis of the identification of at least five of the nine calderas through their detection on Landsat images, it is proposed that the present inventory is a reliable base to extend the census of the TMVB confirmed calderas, now clearly too small for a population of roughly 8000 volcanic centers, many of which emitted large volumes of felsic pyroclastic products.

Shear-induced folding in Arsia Mons aureole: Evidence for low-latitude martian glaciations

Folds up to 50 km across have been identified on Arsia Mons aureole. Tharsis Province, Mars. The structures, located on Mars for the first time, are close to Aganippe Fossa and other huge faults which have behaved as left-lateral shear zones and then as extensional features. A tectonic scheme is proposed to explain the folds as shear-induced structures. Folding reveals a layered sequence in the aureole, and that is taken as a definitive evidence for its deposition by ice.If at least some of the Tharsis volcanoes aureoles are basal moraines, their study is critical, as they could contain a record of Mars paleoclimatic fluctuations. Martian past frozen lakes or oceans have been proposed, and some sediments found on the northern plains could have been deposited on the bottom of those basins. If this is so, those formations should be layered sequences and could also bear the traces of tectonic stresses, detectable as folds on Viking imagery. Correlation of these two kinds of evidence seems a promising line to tackle the Martian paleoclimatic problem.

Arabia Terra, Mars: tectonic and palaeoclimatic evolution of a remarkable sector of martian lithosphere

A regional geologic study of Arabia Terra, a densely cratered area of Mars northern hemisphere, has revealed the individuality of this province. This is best expressed by an equatorial belt with a crater age distinctly younger as compared to the northern part of Arabia Terra and to Noachis Terra to the south. We interpret this as an incipient back-arc system provoked by the subduction of Mars lowlands under Arabia Terra during Noachian times. The regional fracture patterns are also best explained in this manner, making it unnecessary to appeal to a rotational instability of the planet, which is not supported by the palaeoclimatic indicators in the area. This model could be the first regional-scale confirmation of Sleeps (1994) hypothesis of a limited plate consumption as an explanation of the martian dichotomy.

Ovoid structures in the solar system: The coronae of Venus and Miranda, and the domes on the Earth

A comparison of Venus and Miranda coronae, and the Earth ovoidal structures, suggests that Venusian coronae, thermal structures associated with important compressional stress fields, could be compared to Archaean gneiss domes. Among Miranda coronae, Inverness has some characteristics not explained by either the ‘raiser’ or the ‘sinker’ models, and which deserve further investigation.