The geometry, kinematics and rates of deformation within an en echelon normal fault segment boundary, central Italy

The geometry, kinematics and rates of deformation have been investigated wi thin an en echelon fault segment boundary between two left-stepping major n ormal faults in the central Apennines, Italy. Examination of faulted post-g lacial sediments and geomorphic features attributed to glacial retreat (18 ka) reveals that the two major faults dominate the recent slip, with rates that are 5-10 times greater than the other faults. At the centres of the ma jor faults, slip is parallel to the regional extension direction (NE-SW) de fined by focal mechanisms and borehole breakouts. Slip-vector azimuths defi ned by striations on faults in the stepover zone indicate NE-SW extension o n NW-SE faults, sub-parallel to the regional extension direction, together with significant and contemporaneous NW-SE extension, along the strike of t he Apennines, on ESE-WNW and ENE-WSW faults. Thus, distributed three-dimens ional strain occurs in the stepover between the two major bounding faults. Extension rates summed across the stepover are 0.68 mm/y parallel to the re gional extension, compared to similar to 2 mm/y at the centre of one of the major faults. Extension rates along-strike of the fault zone are 0.14 mm/y within the stepover and zero at the centres of the two major bounding faul ts. The above information is used to discuss how extension is partitioned b etween different structures during the growth and linkage of normal faults. (C) 2000 Elsevier Science Ltd. All rights reserved.