Kinematic and geodynamic evolution of the Western Tethys in a context of adjacent continents and ocean basins

Abstract

This thesis studies the links between plate tectonics and deep Earth processes during the breakup of northern Pangea from the Early Jurassic. To explore the major episodes of extensional and compressional tectonics during this break up, I establish a regional-scale plate tectonic model that links the kinematics of the Atlantic to the opening and closure of Western Tethys. My reconstructions include a revised history for the rifting of Greenland, Eurasia, Iberia and Africa away from North America and the early seafloor spreading that followed. I then combine this kinematic model for northern Pangea breakup with the history of rifting, spreading and subduction events in the western Tethys domain. Using recently published plate tectonic models for this region as a starting point, I critically assess conflicting end-member plate tectonic scenarios. I use mantle seismic tomography models to unravel the controversial subduction history of the Vardar Ocean and Alpine Tethys and to investigate the origin of an enigmatic high-velocity anomaly in the lower mantle beneath Africa. Geodynamic models with alternative plate kinematic histories and initial boundary conditions were built to link the revised history of subduction to a global models of mantle flow. To generate more robust plate reconstructions of the Mesozoic rifting between Greenland and North America, I developed models incorporating a quantitative description of continental deformation and margin restoration. To address major controversies about the duration of rifting, onset of seafloor spreading and the nature of the “transitional crust” in this area, I synthesised observations from seismic refraction experiments across Labrador Sea and Baffin Bay, carried out gravity inversion to map the crustal thickness within the conjugate rifted margins, and accounted for the addition of igneous material to the crust during and after rifting. Crust that underwent extension during rifting is restored to its pre-stretching location and the restored margins quantitatively reconstructed to compute a set of alternative total-fit reconstructions. The new results benefit from geophysical data not available to previous reconstruction studies, which show that magnetic lineations landward of chron A27 reflect intrusions into continental crust and are not oceanic magnetic anomalies. My reconstructions indicate the breakup process was diachronous and propagated from south to north between 88 and 61 Ma, and shows that there is no need for Mesozoic reconstructions to include additional plate boundaries within North America or Greenland as proposed previously. Opening of the central and north Atlantic rifts is intimately linked to the closure of western Tethys ocean basins between Africa and Eurasia. Little of the western Tethys is now preserved, and there are major controversies about the nature and location of plate boundaries and timing, location and polarity of subduction zones. However, subducted Tethys ocean remnants can still be imaged in the mantle, and an emerging methodology to unravel the evolution of such regions, as employed here, is to iteratively build plate models with closing topological plate boundaries, capturing the inferred spreading and subduction history; further insights can be gained by linking these plate kinematic reconstructions to mantle convection models. Geodynamic models provide a quantitative connection between kinematics and the deep mantle via predicting mantle structure derived from an imposed subduction history, which can then be compared to present-day mantle velocity structure imaged through seismic tomography. Correlating seismic tomography signatures with past subduction is less straightforward for Mesozoic western Tethys subduction than for more recent history; however, correlations between surface reconstructions and deep Earth structure suggest that mid-deep mantle seismic features under present day Northeast-Central and Northwest Africa-Arabia may correspond with the Mesozoic subduction systems in the Vardar Ocean, Alpine Tethys and Western Neotethys respectively. My analysis supports a scenario with intra-oceanic subduction of Vardar Ocean from Middle Jurassic to Early Cretaceous, and mid-Early Cretaceous initiation of oceanic subduction in the Ligurian-Piemont Ocean. Assessing the uncertainties in the tectonic model, I show that the choice of absolute reference frame can only partially account for the lateral offset between the reconstructed surface location of Vardar subduction and associated slab material interpreted in deep mantle, suggesting a role for additional mechanisms such as lateral drift of slab material. A complementary approach to explore Mesozoic Tethyan subduction involves building geodynamic models with different parameters and boundary conditions. Following this approach, I find that a model case with intra-oceanic subduction of the Vardar Ocean during the Middle Jurassic to Early Cretaceous, and Cretaceous subduction of Alpine Tethys Oceans, are better able to reconcile observed velocity anomalies than model cases without intra-oceanic subduction, or with uninterrupted subduction along the Eurasian margin since the Jurassic. My results also favour the inclusion of Neotethyan intra-oceanic subduction between Arabia and Eurasia. Westward lateral motion of Vardar slab material is observed in all model cases, further supporting the view that this mechanism may help to reconcile surface kinematics with fast seismic anomalies beneath Northwest Africa.