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Varias zonas de fallas continentales registran la influencia de estructuras preexistentes en su desarrollo y comportamiento. Aquí usamos ejemplos de Nueva Zelanda y California del sur para mostrar cómo las fallas normales antiguas localizan la deformación e influyen el desarrollo de fallas activas en dos límites de placas transpresivas. La falla Alpina en Nueva Zelanda es una falla transforme transpresiva dextral que acomoda 60-90% del movimiento relativo entre las placas Australiana y del Pacífico. Su segmento más meridional es inusual porque está particionado, con movimiento de deslizamiento lateral acomodado en la falla principal y el acortamiento acomodado en fallas inversas separadas. La partición está controlada por la reactivación de dos conjuntos de fallas normales preexistentes. El primer conjunto se formó durante el Cretácico Superior cuando Nueva Zelanda se separó de Australia y Antártica. La segunda ocurrió durante el Eoceno-Oligoceno, cuando se formaron una serie de cuencas transtensionales en Fiordland. Después de que comenzó la transpresión en el Mioceno, dos orientaciones de fallas normales (rumbo NE y N) se reactivaron como fallas de deslizamiento lateral e inversas, respectivamente. Una consecuencia de esta reactivación fue la exposición de la sección más profunda a nivel mundial de la corteza de un arco continental. En California del sur, las montañas de San Gabriel se encuentran entre las trazas activas de la Falla de San Andrés (SAF) y la falla de Sierra Madre-Cucamonga (SCFZ). SAF es parte de un sistema transpresivo en que las fallas de deslizamiento lateral dextrales e inversas acomodan el movimiento relativo entre las placas del Pacífico y Norteamericana. SCFZ muestra movimientos inversos y oblicuos. Su pared colgante conserva fallas normales del Mioceno temprano que se reactivaron como fallas sinistrales y oblicuas-inversas después del inicio de la transpresión en el Mioceno medio. La reactivación de las fallas normales explica por qué los terremotos en la región exhiben movimientos complejos, incluyendo aquellos que son antitéticos al movimiento en SAF. Los resultados de este estudio nos permiten determinar el origen de patrones complejos de movimientos de fallas en dos límites de placas transformes continentales y explican por qué algunas fallas normales preexistentes se reactivaron en transpresión mientras que otras no. 

Abstract We present microbeam major- and trace-element data from 14 monzodiorites collected from the Malaspina Pluton (Fiordland, New Zealand) with the goal of evaluating processes involved in the production of andesites in lower arc crust. We focus on relict igneous assemblages consisting of plagioclase and amphibole with lesser amounts of clinopyroxene, orthopyroxene, biotite and quartz. These relict igneous assemblages are heterogeneously preserved in the lower crust within sheeted intrusions that display hypersolidus fabrics defined by alignment of unstrained plagioclase and amphibole. Trace-element data from relict igneous amphiboles in these rocks reveal two distinct groups: one relatively enriched in high field strength element concentrations and one relatively depleted. The enriched amphibole group has Zr values in the range of ∼25–110 ppm, Nb values of ∼5–32 ppm, and Th values up to 2·4 ppm. The depleted group, in contrast, shows Zr values <35 ppm and Nb values <0·25 ppm, and Th is generally below the level of detection. Amphibole crystallization temperatures calculated from major elements range from ∼960 to 830 °C for all samples in the pluton; however, we do not observe significant differences in the range of crystallization temperatures between enriched (∼960–840 °C) and depleted groups (∼940–830 °C). Bulk-rock Sr and Nd isotopes are also remarkably homogeneous and show no apparent difference between enriched (εNdi = 0·1 to –0·1; 87Sr/86Sri = 0·70420–0·70413) and depleted groups (εNdi = 0·3 to –0·4; 87Sr/86Sri = 0·70424–0·70411). Calculated amphibole-equilibrium melt compositions using chemometric equations indicate that melts were highly fractionated (molar Mg# <50), andesitic to dacitic in composition, and were much more evolved than bulk lower continental crust or primitive basalts and andesites predicted to have formed from hydrous melting of mantle-wedge peridotite beneath an arc. We suggest that melts originated from a common, isotopically homogeneous source beneath the Malaspina Pluton, and differences between enriched and depleted trace-element groups reflect varying contributions from subducted sediment-derived melt and sediment-derived fluid, respectively. Our data demonstrate that andesites and dacites were the dominant melts that intruded the lower crust, and their compositions mirror middle and upper bulk-continental crust estimates. Continental crust-like geochemical signatures were acquired in the source region from interaction between hydrous mantle-wedge melts and recycled subducted sediment rather than assimilation and/or remelting of pre-existing lower continental crust. 

Abstract We present a data set of >1500 in situ O-Hf-U-Pb zircon isotope analyses that document the existence of a concealed Rodinian lithospheric keel beneath continental Zealandia. The new data reveal the presence of a distinct isotopic domain of Paleozoic–Mesozoic plutonic rocks that contain zircon characterized by anomalously low δ18O values (median = +4.1‰) and radiogenic εHf(t) (median = +6.1). The scale (>10,000 km2) and time span (>>250 m.y.) over which plutonic rocks with this anomalously low-δ18O signature were emplaced appear unique in a global context, especially for magmas generated and emplaced along a continental margin. Calculated crustal-residence ages (depleted mantle model, TDM) for this low-δ18O isotope domain range from 1300 to 500 Ma and are interpreted to represent melting of a Precambrian lithospheric keel that was formed and subsequently hydrothermally altered during Rodinian assembly and rifting. Recognition of a concealed Precambrian lithosphere beneath Zealandia and the uniqueness of the pervasive low-δ18O isotope domain link Zealandia to South China, providing a novel test of specific hypotheses of continental block arrangements within Rodinia. 

Interpretation of geochronological and petrological data from partially-melted granulite is challenging. However, integration of multiple chronometers and mineral assemblage diagrams (MAD) can be used to estimate the nature and duration of processes. Excellent lower-crustal exposures of garnet granulite from the Malaspina Pluton, Fiordland New Zealand provide an ideal place to employ this kitchen sink approach. We use zircon U-Pb ages from LA-ICPMS, SHRIMP-RG, and CA-TIMS, garnet Lu-Hf and Sm-Nd ages, and MAD in order to evaluate local partial melting vs. melt injection, equilibrium volumes, P-T conditions, and the duration of lower crustal thermal events. Host diorite (H), garnet-clinopyroxene reaction zones (GRZ), coarse garnet selvages, and tonalite veins provide a record of intrusion and granulite facies partial melting. Zircon U-Pb ages range from 123 to 107 Ma (all); LA-ICPMS ages contain the entire range; CA-TIMS ages range from 118.30±0.13 to 115.7±0.18 Ma; and SHRIMP-RG ages range from 121.4±2 to 109.8±1.8 Ma. The latter two techniques are interpreted to indicate primary igneous crystallization from ~119 to ~116 Ma and the youngest ~110 Ma ages are interpreted as metamorphic zircon growth. Garnet ages for ~1 cm grains are ~113 Ma (Lu-Hf & Sm-Nd) and record metamorphic growth, and <0.3 mm grains with Sm-Nd ages from 113 to 104 Ma reflect high temperature intracrystalline diffusion and isotopic closure during cooling to amphibolite facies. Zircon trace-element compositions indicate 2 distinct crystallization trends reflecting evolution of primary magma batches. MAD indicate that garnet was not in equilibrium with sampled rock compositions. Instead, garnet shows apparent equilibrium with a modeled mixture of the GRZ and the H and grew in equilibrium with an effective bulk composition that shifted toward the leucosome. This would produce the observed increase in garnet grossular content. We conclude that: Malaspina rocks from Crooked Arm preserve evidence for 2 igneous layers which evolved as discrete magmas, igneous crystallization lasted 2 to 3 m.y., granulite metamorphism peaked ~ 3 m.y. after intrusion, metamorphism lasted ≥3 m.y., cooling occurred at ~20°C/m.y., and granulite minerals equilibrated with a mixture of solid phases and melt at ~14 kbar and 920°C (based on garnet compositions and MAD).