Raphaël Pik

Modes of rifting in magma-rich settings: Tectono-magmatic evolution of Central Afar†

Recent research in Afar (northern Ethiopia) has largely focused on the formation of the present-day ocean-continent transition at active segments (e.g. Manda Hararo). However, the Oligo-Miocene history of extension, from the onset of rifting at ~25 Ma to the eruption of the massive Stratoid flood basalts at ~ 4 Ma, remains poorly constrained. Here, we present new structural data and radiometric dating from Central Afar, obtained along a zone stretching from the undeformed Oligocene Ethiopian plateau to the Manda Hararo and TatAle active volcanic segments. Basaltic and rhyolitic formations were mapped in two key areas corresponding to the proximal and distal parts of a half-rift. We present a balanced composite cross-section of Central Afar, reconstructed using our new data and previously-published geophysical data on the crustal structure. Our main findings are: 1) Extension during the Mio-Pliocene corresponds to a ‘wide rift’ style of rifting. 2) The lower crust has been underplated/intruded and re-thickened during rifting by magmatic injection. 3) Our restoration points to the existence of mid-crustal shear zones that have helped to distribute extension in the upper crust and to localize extension at depth in a necking zone. Moreover, we suggest that there is a close relationship between the location of a shear zone and the underplated/intruded material. In magma-rich environments such as Central Afar, break-up should be achieved once the initial continental crust has been completely replaced by the newly, magmatically accreted crust. Consequently, and particularly in Afar, crustal thickness is not necessarily indicative of break-up but instead reflects differences in tectono-magmatic regimes.

Rift-to-collision transition recorded by tectono-thermal evolution of the northern Pyrenees

The impact of rift-related processes on tectonic and thermal evolution of collisional orogens is poorly documented. Here, we study the northern Pyrenees, a region that has preserved a geological record of the transition from rifting to collision. Using modeling of new low-temperature thermochronological data, including fission track and (U-Th)/He on apatite and zircon, we propose a temporal reconstruction of the inversion of the European rifted margin. Our data confirm that rifting and related cooling started in the Late Paleozoic-Triassic. Throughout the Jurassic and Early Cretaceous the European margin recorded slow heating during postrift subdsidence. Modeling of thermochronological data allows distinguishing subsidence and denudation controlled by south dipping normal faults in granitic massifs that reflect a second episode of crustal thinning at 130–110 Ma. Following onset of convergence at 83 Ma, shortening accumulated into the weak and hot Albian-Cenomanian rift basins floored by both hyperextended continental crust and exhumed subcontinental mantle. The lack of cooling during this initial stage of convergence is explained by the persistence of a high geothermal gradient. The onset of exhumation-related cooling is recognized in the whole Pyrenean region at 50–35 Ma. This timing reveals that the main phase of mountain building started when hyperextended rift basins closed and collision between proximal domains of the rifted margin occurred.

Miocene detachment faulting predating EPR propagation: Southern Baja California

At the southern tip of the Baja California peninsula, we characterize the onshore structures and kinematics associated with crustal necking leading up to the Pliocene breakup and early East Pacific Rise seafloor spreading. From a combination of tectonic field investigations, K-Ar and cosmogenic isotope dating and geomorphology, we propose that the Los Cabos block represents the exhumed footwall of a major detachment fault. This north trending detachment fault is marked by a conspicuous low-dipping brittle-ductile shear zone showing a finite displacement with top to the SE ending to the ESE. This major feature is associated with fluid circulations which led to rejuvenation of the deformed Cretaceous magmatic rocks at a maximum of 17.5u2009Ma. The detachment footwall displays kilometer-scale corrugations controlling the present-day drainage pattern. This major detachment is synchronous with the development of the San Jose del Cabo Basin where syntectonic sedimentation took place from the middle Miocene to probably the early Pliocene. We propose that this seaward dipping detachment fault accommodates the proximal crustal necking of the Baja California passive margin, which predates the onset of formation of the East Pacific Rise spreading axis in the Cabo-Puerto Vallarta segment. Our data illustrate an apparent anticlockwise rotation of the stretching direction in Baja California Sur from ~17u2009Ma to the Pliocene.