The Wellington coast between Paekakariki and Pencarrow Head is underlain mostly by Late Triassic Torlesse Terrane strata (Rakaia Subterrane). The principal rock types are fine-medium-grained greywacke and argillite that have been metamorphosed to prehnite-pumpellyite facies. These strata were deposited by turbidity currents and mass-flow processes. Diamictites containing greywacke, chert, basalt, and limestone clasts in an argillite matrix are rare and originated as subaqueous debris flows. Lithofacies relationships and sedimentary structures suggest that the sediments were deposited on a series of overlapping fans in a base-ofslope slope-apron environment.Melange zones, consisting of isolated blocks or lenses of greywacke in a sheared argillite matrix, are subparallel to bedding within the turbidites. Individual structures vary from compressional to extensional and brittle to ductile; stratal disruption resulted from bedding-parallel extension. The melange zones represent decollements and, at least locally, are at the tops of coherent sedimentary sequences.Most of the strata strike NNE, dip steeply, and face west. Shear planes subparallel to bedding are pervasive. Megascopic and some larger folds plunge gently to moderately parallel to the regional strike of bedding; some map-scale folds plunge steeply oblique to bedding. West of Wellington, the strata are folded into steeply plunging, map-scale upright and inverted anticlines and synclines.The structural history of the Wellington area Torlesse is based on cleavage-fold relationships and similarity of fold styles, and is consistent with deformation in an accretionary prism over a west-dipping subduction zone: (1) formation of melange beneath the toe of an accretionary prism; (2) underplating to the base of the accretionary prism and tectonic thickening; (3) subhorizontal asymmetric folding and cleavage development; (4) imbrication, stacking, and coaxial rotation of strata to subvertical; and (5) folding due to layerparallel oblique-slip. Subsequent events less clearly related to deformation within the accretionary-prism area are Early Cretaceous shearing and mineralisation, and Cenozoic rightslip folding and faulting. G92058
Stratigraphic and structural information from the Castaneda Hills-Signal area of west-central Arizona, between the Buckskin-Rawhide metamorphic core complex and the Colorado Plateau, shows that the first event associated with formation of the highly extended terranes (HET) in this region was upwarping in the area of the metamorphic core complex ca. 27 Ma. Basalt of lithospheric-mantle derivation was erupted about 19 Ma, signaling the beginning of volcanic activity. After this time, and probably before 13 Ma, extension characteristic of the HET occurred and was manifested by mylonitization of lower-plate rocks, detachment faulting, (probably) listric faulting and strong tilting of blocks in the upper plate, mineralization, and silicic volcanism of lower-crust derivation. HET tectonism evolved into basin-range (BR) tectonism, accompanied by the eruption of mantle-derived basalts during the period 13-12 Ma; however, by about 8 Ma tectonism ceased altogether, and volcanism was restricted to emplacement of minor volumes of megacryst-bearing basalt derived from the asthenospheric mantle.The data do not support domino or rolling-hinge models for upper-plate deformation, or simple-shear models for the crustal-scale evolution of the HET. Instead, they suggest a thermally driven, dominantly pure-shear mechanism. We propose a thermotectonic model driven by an asthenospheric upwelling that heats up the crust and applies divergent stresses to it, causing HET-type tectonism. With time, the upwelling migrates northeast, allowing the crust to gradually cool and become atectonic, while the crust of the nearby Colorado Plateau heats up and begins to rift.
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