Untangling the faults: Using 3D data at the Galicia margin to determine faulting history

The western Iberian margin, due to its limited post-rift sedimentary cover and limited volcanic activity, has provided significant data to aid the formulation of models of continental extension and breakup. Structural elements characteristic of highly extended post-breakup continental margins include: rotated faults blocks and associated syn-kinematic sedimentary wedges, low angle detachment faults, exhumed and serpentinised continental mantle and allochthonous blocks. In addition, the extension as measured by heave on faults that have been imaged in seismic data is not enough to account for the stretching or subsidence associated with this extension: there is an extension discrepancy.

The mechanisms postulated to account for these characteristic structures and the extension discrepancy include: depth-dependent thinning, sequential ocean-ward faulting and polyphase faulting. These mechanisms have resulted in a range of, often complex, evolutionary models that are 2D representations of an inherently three dimensional process.

A new, ~680 km2 3D seismic survey (the largest academic one of its kind) consisting of 800 inline (12.5m bin spacing) and 5000 crosslines has provided high resolution images of the edge of the continental crust. This data captures the 3D nature of extension and breakup as the northern Atlantic continental margins ‘unzipped’ northwards from the middle Jurassic to early Cretaceous.

Detailed interpretation of the deformation seen within the rotated fault blocks and their corresponding syn-kinematic sediments are presented and reveal a complex structural history. Changes in the style and relative ages of the dominant faulting imaged within the rotated fault blocks vary along the strike, revealing spatial and temporal variations in the accommodation of strain. More recent (steeper faults) are seen to dissect large blocks and cut earlier faulting. Steep antithetic faults seem to suggest structural collapse within discrete segments of some blocks. Fault linkage and the reactivation of earlier phases of faulting are essential characteristics of the progressive deformation. Furthermore the interaction between intra-block faults and a low angle detachment (the S reflector) demonstrates the complex patterns generated by the rifting process. This interaction is investigated using maps of the S reflector amplitudes showing the major fault intersections.

Further work is introduced including: untangling the fault movements both spatially and temporally, 2D restoration of the internal deformation of the individual blocks and restoration of blocks on the intra-block faults and the detachment fault. This will enable various breakup mechanisms to be tested and new ones developed. Such mechanisms are essential in the development of accurate heat and fluid flow models. Of particular interest from this work are the possible conduits for fluid flow in the heavily interconnected fault network.