Benjamin Guillaume

Horizontal subduction zones, convergence velocity and the building of the Andes.

We discuss the relationships between Andean shortening, plate velocities at the trench, and slab geometry beneath South America. Although some correlation exists between the convergence velocity and the westward motion of South America on the one hand, and the shortening of the continental plate on the other hand, plate kinematics neither gives a satisfactory explanation to the Andean segmentation in general, nor explains the development of the Bolivian orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab segments below South America, arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive. We argue that the existence of horizontal slab segments, below the Central Andes during Eocene-Oligocene times, and below Peru and North-Central Chile since Pliocene, resulted (1) in the shortening of the continental plate interiors at a large distance from the trench, (2) in stronger interplate coupling and ultimately, (3) in a decrease of the trenchward velocity of the oceanic plate. Present-day horizontal slab segments may thus explain the diminution of the convergence velocity between the Nazca and South American plates since Late Miocene.

Spreading pulses of the Tyrrhenian Sea during the narrowing of the Calabrian slab

The opening of the Tyrrhenian Sea has been punctuated by short-lived episodes of oceanic accretion on separate small backarc basins during early Pliocene (Vavilov basin) and early Pleistocene (Marsili basin) time. These spreading pulses are related to slab rollback and are synchronous with the reduction of the subduction zone width during the formation of the narrow Calabrian arc. Using laboratory models, we investigated the long-term and transient effects of the reduction of slab width on the subduction kinematics. We found that the abrupt reduction in slab width results in a pulse of acceleration of the trench retreat velocity, as the balance between driving and resisting forces acting on the slab is temporarily modified. Our findings also show that the time scale and amplitude of spreading observed in the Tyrrhenian Sea can be experimentally fitted if the scaled viscosity of the uppermost part of the mantle ranges between 10 19 and 10 20 Pa s.

Can eustatic charts go beyond first order? Insights from the Permian–Triassic

First-order variations of eustatic charts (200–400 m.y.) are in agreement with our understanding of the geodynamic processes that control sea level. By extrapolation, second-order (10–100 m.y.) and third-order (1–10 m.y.) variations are also thought to follow the same rules. However, this assumption may be jeopardized by a closer examination of the Permian–Triassic example, for which climatic and tectonic eustasy fails to explain the variations of the eustatic charts. During this period, eustatic charts peak down to their lowermost Phanerozoic values and display second-order variations at rates of up to 3 m/m.y., which is inconsistent with the expected eustatic signal during the early fragmentation of the Pangean supercontinent and the late Paleozoic melting of ice sheets. Here, we review the possible mechanisms that could explain the apparent sea-level variations. Some of them do modify the eustatic sea level (ESL). In particular, dynamic deflections of Earth’s surface above subduction zones and their locations with respect to continents appear to have been the primary controls of absolute sea level as the Pangean supercontinent formed and broke up. Other mechanisms instead only locally or regionally produced vertical ground motions, either uplifting continents or tilting the margins where the control points were located. We show that (1) the thermal uplift associated with supercontinent insulation and (2) the dynamic uplift associated with the emplacement of a superplume both give rates of sea-level change in the range of long-term changes of ESL. We also show that (3) the dynamic tilt of continental margins not only produces apparent sea-level changes, but it also modifies the absolute sea level, which in turn may end up in the paradoxical situation wherein fingerprints of ESL drop are found in the geological record during actual ESL rise. We conclude that second-order absolute sea-level changes may remain elusive for some time.