Deformation and metamorphism of mafic and ultramafic lithologiesduring mylonisis at the St. Paul transfom system, mid-atlantic ridge

Álden de Brito Adrião

Daniele Brunelli (UNIMORE) and Christophe Hémond

Co tutelage/double degree: UBO and Università Degli Studi di Modena e Reggio Emilia (UNIMORE, Italy)

The COLMEIA cruise (2013) on the St. Paul fracture zone system (Equatorial Atlantic Ocean), has revealed the presence of a large transpressive domain along the northern transform fault of the St. Paul system (Figure). This region presents one of the most complex magmatic and tectonic settings of the whole Mid Ocean Ridge system. The northern boundary is affected by the presence of the Sierra Leone hot spot, interacting with the axial ridge, the segments affected by the hot spot have swollen topography related to a large amount of volcanism and increased oceanic crust production. A sharp transition to a hot-spot-free region to the south occurs inside the multiple transform system. The southern domain presents the opposite characters with a low magma production accompanied by the presence of large Oceanic Core Complexes, corrugated dome-shaped regions of seafloor created by very large offset low angle normal faults at spreading ridges.

Mylonitic rocks have been sampled along the whole system. Their composition and structure attests for polybaric decompressive deformation ranging from granulitic to the green schist facies. Along with few undeformed occurrences their composition attests for the exhumation of deep lithospheric portions composed by mantle-derived rocks variably mixed with basaltic melts and deep crustal lithologies.

These rocks allow defining the deformation regimes in three different settings: the deep portions of a fracture zone, a detachment regime along axial bounding normal faults and a transpressive region affecting the lithospheric mantle and the deep oceanic crust. These cases are strongly significant for the definition of the mechanical behavior of the oceanic crust in terms of strain response to different thermal regimes and variable hydrating conditions.

The main goal of the project is to define the style of deformation of the upper mantle and deep crustal portions, define the relative position of brittle-plastic transition, and the T-stress profile under different tectonic and stress-distribution settings: strike-slip, extensive and compressive. Of particular interest will be exploring the compositional control on the viscosity of the mylonitic rocks that define the strength profile of the considered section.