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The Magdalena fan is an apparently beheaded submarine depocenter that has fi gured prominently in reconstructions of middle to late Miocene Pacifi c-North American plate interactions. The deposit accumulated rapidly at the base of the continental slope on top of newly formed oceanic crust of the Magdalena microplate from 14.5 to 13 Ma. Subduction of this crust ceased as the Pacifi c-Magdalena spreading center encountered the trench. The widely accepted two-phase kinematic model for the formation of the Gulf of California holds that ~300 km of dextral shear between the Pacifi c and North American plates occurred along faults west of Baja California prior to the onset of dextral-transtensional shearing in the gulf ca. 6 Ma. We measured 1796 detrital zircon U-Pb ages from 65 samples in an effort to characterize the provenance of the fan, determine its source region, and defi ne the cumulative dextral slip along faults offshore of southwestern Baja California. Zircons from the fan are dominantly 120-65 Ma with subordinate 15-35 Ma grains. Excellent matches to the fan can be obtained by mixing Magdalena shelf strata and/or adding detritus from the west-draining portion of the Los Cabos block. The same cannot be accomplished with zircons from the east-draining portion of the Los Cabos block and mainland Mexico. Our results favor a western Baja source region for the fan and suggest that cumulative dextral slip along faults west of Baja was <150 km, much less than previously believed. We propose that the fan was fed by erosional denudation of the Magdalena shelf produced by increased mantle buoyancy due to the ridge-trench juxtaposition. The fan's source was cut off when faults west of Baja California began to accommodate transtensional shearing and form rift basins that captured detritus that previously reached the trench.

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In Westland‐Nelson provinces of New Zealand, high‐grade metamorphic and granitic basement rocks showing mylonitic ductile deformation are juxtaposed beneath low‐grade metasedimentary rocks and undeformed granites by uplift on low‐angle detachment faults. Several metamorphic core complexes analogous to those described from western North America are recognized. In the Paparoa Range, basement rocks include late Precambrian(?) paragneiss and granitic rocks of both Paleozoic and Cretaceous ages. Cover rocks include Ordovician turbidites, Paleozoic and Cretaceous granites, and mid‐Cretaceous breccia‐conglomerates. Brittle deformation and hydrothermal alteration (silica, chlorite, hematite, carbonate ± fluorite, uranium) characteristic of the detachment zone are also superimposed on uppermost lower‐plate mylonites. Kinematic indicators in the mylonitic rocks including composite S‐C febrics indicate that the detachment faults on the northeast and southwest sides of the Paparoa Core Complex had opposite senses of shear, with cover rocks on both sides moving away from the metamorphic core. Ductile deformation postdates several 114±18 Ma granitic plutons but by 108 Ma had ceased to affect at least some of the rocks currently exposed. Mylonitic rocks were uplifted to the surface and eroded into evolving half‐grabens by 105–100 Ma. Uplifted basement yields K‐Ar dates as young as 88 Ma, and tilting of the graben sediments indicates detachment continued well into the Late Cretaceous, when both cover and basement were intruded by alkali lamprophyre dykes. The Nelson‐Westland core complexes occur within an Early Cretaceous granitic province characterized by relatively radiogenic strontium. The boundary of this province, the NW trend to mid‐Cretaceous half‐grabens, the NNE trend of stretching lineations in mylonitic rocks, and the ESE trend of late Cretaceous lamprophyre dykes indicate that regional extension was maintained in a NNE direction for much of the Cretaceous. This regional extension may be part of an “extension corridor” which traversed the entire Gondwana continental margin from NE Queensland, Australia, to Marie Byrd Land, Antarctica. Extension preceded opening of the Tasman Sea between New Zealand and Australia at approximately 84 Ma and closely followed long‐lived compression on the Pacific convergent margin of Gondwana. The presence of core complexes in western New Zealand contrasts with the Australian margin to the Tasman Sea and lends support to simple shear models of continental rifting.

The eastern Peninsular Ranges batholith is dominated by voluminous La Posta-type tonalite-granodiorite intrusions that compose half of the magmatic arc at present erosion level. Zircon U-Pb and hornblende 40 Ar/ 39 Ar results from these intrusions indicate that they were emplaced in a remarkably narrow interval (99-92 Ma) that closely followed cessation of west-directed compression of the arc system. Emplacement of the La Posta suite coincided with a major pulse of coarse-grained sediment into the adjacent forearc basin in early Cenomanian to middle Turonian time. Paleontologic control, and plutonic age and detrital zircon U-Pb data demonstrate the virtual absence of a time lag between magma emplacement and sedimentary response. The tight linkage between magmatism, arc exhumation, and sediment delivery to the forearc indicates that development of major erosional topography in the arc was driven by thermal and mechanical effects associated with large-volume batholith emplacement.

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