Climatic and topographic changes since the Miocene influenced the diversification and biogeography of the tent tortoise (Psammobates tentorius) species complex in Southern Africa

Background: Climatic and topographic changes function as key drivers in shaping genetic structure and cladogenic radiation in many organisms. Southern Africa has an exceptionally diverse tortoise fauna, harbouring one-third of the world’s tortoise genera. The distribution of Psammobates tentorius (Kuhl, 1820) covers two of the 25 biodiversity hotspots in the world, the Succulent Karoo and Cape Floristic Region. The highly diverged P. tentorius represents an excellent model species for exploring biogeographic and radiation patterns of reptiles in Southern Africa. Results: We investigated genetic structure and radiation patterns against temporal and spatial dimensions since the Miocene in the Psammobates tentorius species complex, using multiple types of DNA markers and niche modelling analyses. Cladogenesis in P. tentorius started in the late Miocene (11.63–5.33 Ma) when populations dispersed from north to south to form two geographically isolated groups. The northern group diverged into a clade north of the Orange River (OR), followed by the splitting of the group south of the OR into a western and an interior clade. The latter divergence corresponded to the intensifcation of the cold Benguela current, which caused western aridifcation and rainfall seasonality. In the south, tectonic uplift and subsequent exhumation, together with climatic fuctuations seemed responsible for radiations among the four southern clades since the late Miocene. We found that each clade occurred in a habitat shaped by diferent climatic parameters, and that the niches difered substantially among the clades of the northern group but were similar among clades of the southern group. Conclusion: Climatic shifts, and biome and geographic changes were possibly the three major driving forces shaping cladogenesis and genetic structure in Southern African tortoise species. Our results revealed that the cladogenesis of the P. tentorius species complex was probably shaped by environmental cooling, biome shifts and topographic uplift in Southern Africa since the late Miocene. The Last Glacial Maximum (LGM) may have impacted the distribution of P. tentorius substantially. We found the taxonomic diversify of the P. tentorius species complex to be highest in the Greater Cape Floristic Region. All seven clades discovered warrant conservation attention, particularly Ptt-B–Ptr, Ptt-A and Pv-

Rifting above a mantle plume: structural analysis of Iceland and analogue modelling

PosterInternational audienc

Rifting above a mantle plume: structural analysis of Iceland and analogue modelling

PosterInternational audienc

The Ratio Between Magma Supply and Lithospheric Stretching Rates Controls the Architecture of Continental and Oceanic Rifts.

Poste

Magmatism during rifting controls the polarity of tilted blocks

Poste

Lithospheric strength controls transform zone structure

International audienceOceanic transform zones have often been regarded as plate boundaries. The origin of their structural variability is poorly constrained. A simple observation indicates that the transform zone is narrow and linear when the offset is large; while it is wide with a complex faulting pattern in the case of a small offset. On the other hand, for a given offset, large structural differences exist between transform zones located on the fast-spreading SouthEast Pacific Rise and on the slowspreading Mid-Atlantic ridge. In general, the transform zones in slow-spreading environments are linear with a simple fault pattern, while in fast-spreading systems they are wide with a complex pattern of deformation. We perform small-scale analogue modelling to constrain the influence of lithospheric strength on the development of deformation above a transform boundary. The models are made up of sand and silicone putty as analogues of the brittle layer and the viscous layer of the lithosphere, respectively. Two plastic sheets coming from shifted gashes form a setup of two diverging discontinuities connected by a transform boundary. The rheological layering and strength of the model were modified using different shapes of the viscous layer placed on the transform boundary. Above the divergent discontinuities, the faulting pattern is always formed by parallel normal faults. When no viscous layer is placed on the transform boundary (strong discontinuity), the deformed zone is narrow and has few linear faults. By adding a narrow and thin viscous layer, the deformed zone becomes wider with a complex faulting pattern formed by oblique-slip faults on the limits and by pure strike-slip faults in the inner part. These latter strike-slip faults trend oblique to the transform boundary. When a viscous layer with a wide lateral extent overlays the transform discontinuity (weak strength), the faulting is dominated by obliquely normal faults extending over a wide zone, and the strike-slip is restricted to the inner part of the deformed zone. Therefore, the mechanical strength of the small scale-model controls the shape of the deformed zone and the deformation partitioning. These results were applied to 24 oceanic transforms zones: we point out that the spreading rate and the transform offset are the two dominant parameters controlling the deformation pattern. These two factors directly control the lithospheric strength at the transform boundary. However, the distance to the nearest hotspot, which may generate warmer thermal conditions even in slow-spreading environments, should modify this result

The topography of North Namibian Margin during the Meso-Cenozoic

Poster 541