Resolving problems with the origin of Las Canadas Caldera (Tenerife, Canary Islands); Los Roques de Garcia Formation, part of a major debris avalanche or an in situ, stratified, edifice-building succession?

Las Canadas caldera complex, on Tenerife, Canary Islands, truncated the construction of Las Canadas edifice, a central composite volcanic complex formed after a main period of basaltic shield construction. The origin of the present Las Canadas caldera complex is still a matter of considerable debate between two contrasting hypotheses, vertical (caldera forming) or lateral (landslide) collapse. However, there is increasing evidence that a long history of explosive phonolitic volcanism, including several caldera episodes, characterized the construction of Las Canadas edifice. Los Roques de Garcia forms a large spur that divides the Las Canadas caldera complex into two morphological depressions. The sequence of rocks exposed along the spur consists of several formations that from base to top include: Los Roques de Garcia Formation, Los Azulejos Formation, and the lower part of the Ucanca Formation. Los Roques de Garcia Formation occupies the main part of Los Roques de Garcia spur and includes proximal facies of pyroclastic (Lower Member) and sedimentary (epiclastic) (Upper Member) deposits, predominantly breccias, all of which are intruded by a dense network of phonolitic dikes and necks. Pyroclastic deposits mostly correspond to coignimbrite lag breccias, lithic-rich ignimbrites, and minor surge and ash-fall beds. Epiclastic rocks mainly include poorly to well-stratified, proximal debrisflows breccias deposited in an alluvial-fan environment, with some interbedded pyroclastic and epiclastic sandstone and conglomerate units. The central sector of Los Roques de Garcia spur is highly fractured due to the movement of several normal faults, thus conferring a chaotic aspect to that zone. Strong hydrothermal alteration also has affected some sectors of Los Roques de Garcia spur, enhancing this chaotic appearance. New detailed mapping and stratigraphic logging have been crucial to interpreting the nature and stratigraphy of Los Roques de Garcia rocks, which represent two fundamental aspects for the interpretation of the origin and evolution of Las Canadas caldera. Los Roques de Garcia spur exhibits a stratigraphy that is concordant with the rest of Las Canadas caldera wall and corresponds to the lower part (Lower Group) of Las Canadas edifice, without having any relation to latter explosive episodes responsible for the deposition of its upper part (Upper Group). Lithological, sedimentological, and volcanological characteristics of Los Roques de Garcia rocks allow them to be interpreted either as a former Las Canadas intracaldera sequence or as the apron of an older stratovolcano, in contrast with previous interpretations, which have suggested that they correspond to the products of a major debris avalanche event that contributed to the formation of the present caldera.