Erika Labrin

Diachronous late-stage exhumation across the western Alpine arc: constraints from apatite fission-track thermochronology between the Pelvoux and Dora-Maira Massifs

We present new apatite fission-track (AFT) data from the central western Alps that confirm the synchronicity and high cooling rates during Latest Miocene–Pliocene final exhumation of the External Crystalline Massifs but also provide evidence for diachronous Neogene evolution along and across the internal arc. To the SE of the Pelvoux Massif, across the front of the internal arc (Penninic Frontal Thrust), the jump in AFT ages (c. 22 Ma) and in final cooling rates is significantly larger than further north. This difference results from reversal of movement along a major Oligocene thrust. In its hanging wall, the western Briançonnais Zone provides a mean AFT age of c. 27 Ma, which is older than further north. Early cooling in the southern Briançonnais Zone would result from rapid erosion of the compressional fan structure built during the Oligocene. Across the entire Briançonnais and Piémont nappe stack, with the exception of the Dora-Maira Massif, AFT ages young eastward and span the entire Miocene, a period during which this structure underwent extension. Further north a reverse gradient with ages younging northwestward has been described, prompting the question of the asymmetry of the internal western Alpine arc during its late-stage tectonic and morphological evolution.

Late Mesozoic and Cenozoic thermotectonic history of the Mexican Pacific margin (18 to 25°N): new insight from apatite and zircon fission-track analysis of coastal and offshore plutonic rocks

Abstract Apatite and zircon fission-track dating was used to constrain cooling histories on granitic samples taken from the offshore Acapulco trench batholith and onshore Manzanillo and Puerto Vallarta batholiths located along the southwestern active margin of Mexico, and from the La Paz batholith of southern Baja California. The apatite data indicate that many of the samples in the Manzanillo, Acapulco trench batholiths cooled rapidly below 60°C shortly after emplacement between 70 and 55 Ma. The La Paz batholith and the sample NM-20-08 of the Acapulco trench batholith experienced an older and slower cooling across the apatite partial annealing zone. The zircon fission-track ages obtained from two samples of the Puerto Vallarta batholith indicate a rapid cooling from 250 to 110°C between 54 and 50 Ma. This non-coeval cooling of batholiths at two different depths is probably due to uplift and erosion associated with the Laramide event in southwestern Mexico. The three samples from the Puerto Vallarta batholith suggest a younger evolution across low temperatures with two flat stages located near the base of the apatite partial annealing zone and above it. The first flat stage suggests that the denudation or rock uplift rate was reduced at a depth corresponding to the base of the apatite partial annealing zone. The younger single apatite ages of the Puerto Vallarta batholith, and the last common cooling of the other batholiths indicate the mild thermal influences of both the Plio-Pleistocene Trans-Mexican Volcanic Belt and the opening of the Gulf of California.

Fission Track Dating and Provenience of Archaeological Obsidian Artefacts in Colombia and Ecuador

We dated by the fission track method eighteen samples of obsidian glass. Of these, seventeen were artefacts collected in prehispanic archaeological sites from Ecuador and Colombia and one comes from a secondary obsidian source located in southern Colombia. When our data are compared to (i) the fission track ages of obsidian from volcanic sources and (ii) the PIXE chemical composition of the same artefacts and of samples from obsidian sources, eight discrete age/composition groups appear.