X-RAY AND MAGNETIC FABRICS OF THE CARBONERAS FAULT, SPAIN; COMPARISON WITH PERMEABILITY STRUCTURE

The left lateral Carboneras fault, part of the boundary between Africa and Iberia, accommodated ~40 km of offset. The fault zone is ~1 km wide with anastamosing layers of clay-rich gouge up to ~50 m thick. Permeability anisotropies up to three orders of magnitude have been reported for the gouge, indicating that elevated fluid pressure along the fault could be maintained, thereby weakening the fault. X-ray diffraction shows major mineral transformation from mica-rich protolith to clay-rich gouge, mainly composed of illite and chlorite, reflecting major fluid activity. Magnetic susceptibility similarly shows that the samples are dominated by these paramagnetic phases. X-ray goniometry and anisotropy of magnetic susceptibility show that clay fabrics are oblate, and that the shape factor of both methods is similar, while the intensity is greater with X-ray goniometry (P’ of ~1-1.3 vs. ~1.6-2.5). The attitude of X-ray fabrics is variable, often reflecting small-scale variations within clast-rich samples. This problem is overcome by AMS, due to a much larger sample volume. In the thickest gouge zones the minimum susceptibility axis parallels the pole to the fault plane and the maximum axis tracks the displacement direction (stretching lineation); notably, this relationship is not present in thinner fault zones. The gouge fabric is generally weak, and therefore the degree of preferred orientation is low, similar to studies of other fault zones. Preliminary comparisons of X-ray and magnetic fabrics and permeability anisotropy in the Carboneras fault suggest that only the planar element of these fabrics agrees, but that the maximum axes are perpendicular. Thus, the AMS fabric seems to track the horizontal displacement direction, while the permeability anisotropy reflects a fault-parallel, vertical component.