Southernmost North Research Articles

Plate coupling in the northern South Island and southernmost North Island, New Zealand, as illuminated by earthquake focal mechanisms

Subduction of the Pacific plate in the northern South Island and southernmost North Island of New Zealand is transitional, insofar as the crustal thickness of the Pacific plate increases significantly along strike in the northern South Island. Focal mechanisms of 145 events shallower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes. The stress regime in the subducted plate appears to be dominated by slab pull. T axes in both the upper and lower planes of the dipping seismic zone generally parallel the local dip of the zone, and the average azimuth of these T axes is rotated some 25° clockwise out of the direction of dip of the subducted plate. This can be related to the asymmetrical shape of the subducted slab. In contrast, the stress regime in the overlying plate appears to be dominated by subhorizontal compression. Low‐angle thrust events near the plate interface in Cook Strait and the southernmost North Island concentrate in two areas which may mark the updip and downdip edges of a locked region identified from Global Positioning System (GPS) observations. An absence of low‐angle thrust events near the plate interface in the northern South Island and the tendency of P axes of events in the subducted plate to become more horizontal suggest that plate coupling there is stronger than in the southernmost North Island. Differential coupling at the plate interface provides a viable mechanism for producing the large tectonic rotations seen in the northern South Island.

Reconnaissance Feasibility Study of Heavy-Mineral Suites in the Fine-Grained Matrix of Several Lithostratigraphic Terranes, Southern New Zealand

Nineteen samples of melange matrix and volcanogenic sandstone matrix were collected from the Maitai (including the Hawtel), Caples, and Torlesse terranes, South Island, and southernmost North Island, New Zealand, in an attempt to identify contrasting provenances for these lithostratigraphic units. Heavy minerals were concentrated employing water settling columns followed by high-speed centrifugation utilizing bromoform and methyl iodide. Semiquantitative scanning-electron-microscope and quantitative-microprobe investigations of heavy-mineral concentrates support conclusions of previous workers. (1) The Torlesse Complex is a richly quartzofeldspathic, at least partially multicycle, unit derived from an evolved, well-dissected continental margin or mature island arc. It contains a diverse suite of igneous, metamorphic, and sedimentary detritus including widespread traces of clastic zircon, rutile, biotite, barite, almandine, clinopyroxene, Ca-amphibole, staurolite, chromite, and epidote. (2) Melanges belong...

Microseismicity in the Tararua—Wairarapa area: depth-varying stresses and shallow seismicity in the southern North Island, New Zealand

Abstract Results from previously unpublished microearthquake field studies in the Tararua-Wairarapa area during 1971–72 demonstrate important differences between crustal seismicity shallower than 20 km and intense, regionally extensive, underlying seismicity in the 20–40 km depth range. As earlier observed in the Marlborough region, and more recently in the southernmost North Island, composite focal mechanisms in the Tararua-Wairarapa area indicate markedly different stress orientations above and below 20 km. Compressional transcurrerit faulting in the upper crust contrasts with extensional normal faulting at depths of 20–40 km. The layer 20–40 km deep appears to be far more seismically active than the upper crust and is interpreted to mark the uppermost part of the subducted Pacific plate. Accordingly, the data here provide valuable control for locating the plate interface, for modelling elastic deformation at a convergent plate boundary, and for assessing fault activity and shallow seismicity in the southern North Island.

Structural Trends in the Southernmost North American Precambrian, Oaxaca, Mexico

Structures observed in a 1000 km2 area of Grenville age Precambrian metasediments near Oaxaca in southern Mexico record the effects of a single orogenic event that produced cylindroidal folds about axes with an average bearing of N. 32° W. and plunge of 26° NW. The orientation of this structural grain appears to provide support for previous proposals (based on geochronology) that a structural belt of Grenville age rocks extends southward from Texas into the Oaxaca area. Further interpretations based on these trends suggest that the Pacific boundary of the Oaxacan Precambrian terrane resulted from continental fragmentation and that adjustments associated with opening of the Gulf of Mexico have not reoriented significantly the Oaxaca area.