From a detailed survey and sampling study of corrugated massifs north of the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge, we demonstrate that their surfaces are low-angle detachment fault planes, as proposed but not previously verified. Spreadingdirection-parallel striations on the massifs occur at wavelengths from kilometers to centimeters. Oriented drill-core samples from the striated surfaces are dominated by fault rocks with low-angle shear planes and highly deformed greenschist facies assemblages that include talc, chlorite, tremolite, and serpentine. Deformation was very localized and occurred in the brittle regime; no evidence is seen for ductile deformation of the footwall. Synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient.
[1] The External Betic-Rif arc, which lies between the converging African and Iberian plates, is one of the tightest orogenic arcs on Earth. It is a thin-skinned fold and thrust belt formed in Miocene time around the periphery of the Alborán Domain, an older contractional orogen that underwent extensional collapse coevally with the formation of the thrust belt. Restoration of four sections across the thrust belt, together with kinematic and paleomagnetic analysis, allows a reconstruction of the prethrusting geometry of the Alborán Domain, and the identification of the following processes that contributed to the formation of the arc: (1) The Alborán Domain moved some 250 km westward relative to Iberia and Africa during the Miocene. This initiated the two limbs of the arc on its NW and SW margins, closing to the WSW in the region of Cherafat in northern Morocco. The overall convergence direction on the Iberian side of the arc was between 310°and 295°, and on the African side it was between 235°and 215°. The difference in convergence direction between the two sectors was primarily a result of the relative motion between Africa and Iberia. (2) Extensional collapse of the Alborán Domain during the Miocene modified the geometry of the western end of the arc: the Internal Rif rotated anticlockwise to form the present north trending sector of the arc, and additional components of displacement produced by extension were transferred into the external thrust belt along a series of strike-slip faults and shear zones. These allowed the limbs of the arc to rotate and extend, tightening the arc, and creating variations in the amounts and directions of shortening around the arc. The Betic sector of the arc rotated clockwise by 25°during this process, and the southern Rif rotated anticlockwise by $55°. (3) Oblique convergence on the two limbs of the arc, dextral in the Betics and sinistral in the southern Rif, resulted in strongly noncoaxial deformation. This had three related effects: (1) large rotations of individual thrust sheets resulted from the oblique propagation of thrusts away from the thrust front, followed by pinning and rotation as the thrust sheets peeled off, (2) continued oblique convergence resulted in distributed shear, particularly in the rear of the thrust wedge, causing rotation of stacks of thrust sheets on the scale of a few tens of kilometers, and (3) distributed shear in the orogen resulted in the rotation of folds as they amplified, the hinges migrating through the rock body, and rotating at a slower rate than the rock. These rotations were substantially larger than the bulk rotations of the limbs of the arc, and they strongly modified the orientations of folds, thrust traces, and the structural indicators of fault slip directions.
Abstract. High-resolution, deep-towed side-scan sonar data are used to characterize faulting and variations in tectonic strain along a segment of the slow spreading Mid-Atlantic Ridge near 29øN. Sonar data allow us to identify individual fault scarps, to measure fault widths and spacing, and to calculate horizontal fault displacements (heave) and tectonic strain. We find that over long periods of time (> 1 Myr on average), tectonic strain is -10% on average and does not vary significantly along axis. There is a marked asymmetry in tectonic strain that appears to be linked to asymmetric accretion along the whole segment, indicated by -50% lower tectonic strain on the east flank than on the west flank. These variations in tectonic strain do not correlate directly with changes in fault spacing and heave. Fault spacing and heave increase from the center of the segment toward the end (inside comer) on the west flank and from the outside to the inside comer across the axis. These parameters remain relatively constant along the segment on the east flank and across the axis at the segment center. Tectonic strain appears to be decoupled from magmatic accretion at timescales > 1 Myr, as the decrease in magma supply from the segment center toward the end (inferred from variations in crustal thickness along the axis) is not correlated with a complementary increase in tectonic strain. Instead, tectonic strain remains relatively constant along the axis at-7% on the east flank and at-15% on the west flank. These results indicate that variations in fault development and geometry may reflect spatial differences in the rheology of the lithosphere and not changes in tectonic strain or magma supply along axis.
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