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New bathymetric and magnetic anomaly data from the Phoenix Ridge, Antarctica, show that extinction of all three remaining segments occurred at the time of magnetic chron C2A (3.3 ± 0.2 Ma), synchronous with a ridge-trench collision south of the Hero Fracture Zone. This implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision. Spreading rates slowed abruptly at the time of chron C4 (7.8 ± 0.3 Ma), probably as a result of extinction of the West Scotia Ridge, which would have led to an increase in slip rate and transpressional stress across the Shackleton Fracture Zone. Spectacular, highrelief ridges flanking the extinct spreading center, mapped for the first time using multibeam swath bathymetry, are interpreted as a consequence of a reduction in spreading rate, involving a temporary magma oversupply immediately prior to extinction.

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We have studied the morphology and the morphometric relations between alluvial fans and drainage basins in a bajada system including more than 20 coalescent fans developed since the Late Pleistocene as a result of the recent uplift of the Sierra Nevada (Betic Cordillera, Spain). Three allocycles of tectonic origin were recognised across which there is a clear evolution from debris flow to sheet flow dominated fans, in connection with a decrease in the volume of fines available in the source areas. The larger volume of accommodation space created by higher tectonic subsidence in the northern sector favoured vertical accumulation of sediment, with the appearance of less elongated fans. In the rest of the system, where subsidence is less, more elongated fans appear, with the development of incised channels and irregular distribution of sediment in depositional lobes. This involves the appearance of markedly asymmetric transverse profiles, as well as lengthy recurrence between the sedimentation events on particular sectors of the fans, where headward-eroding gullies develop. The lithology of the source area, where intensely fractured rocks are found, is responsible for an important sediment supply and a significant degree of clast sorting from the source area. Consequently, a weak longitudinal trend in particle size can be recognised, which influences the predominance of longitudinal constant-slope profiles and anomalous relations between both fan area and fan slope and their drainage areas. Recent intense tectonic activity has caused the appearance of abnormally low slopes in upper sectors of some catchments, where mass flows are trapped, and the fans present a subsequently higher proportion of sheet flows. Recent piracy phenomena can be recognised in some of the drainage basins, which indicate rejuvenation of the source area, in which case the fan presents a rapid increase in the proportion of mass flows with the development of a segmented longitudinal profile. Due to the recent nature of the processes, the morphometric relations in the fan fed by the basin affected by capture do not coincide with the other fans in the area. 0 1997 Elsevier Science B.V.