David L. Kimbrough

Overview of the Median Batholith, New Zealand: a new interpretation of the geology of the Median Tectonic Zone and adjacent rocks

This paper proposes an alternative high-order, non-genetic classification of the basement rocks of medial New Zealand. More than 90% of the rocks in the Median Tectonic Zone are plutonic and can be included in part of a newly defined Carboniferous to Early Cretaceous, ca 10,200 km2 composite regional batholith — the Median Batholith. The plutonic rocks of the batholith intrude the volcanic and sedimentary rocks of the Brook Street and Takaka Terranes (Eastern and Western Provinces, respectively). Emerging matches between the chronology of magmatism in the Median Batholith and batholiths in the Western Province also support probable in situ growth of most of the batholith. The internal and external contacts, and shape, of the batholith have been strongly modified by post-plutonic Cretaceous and Late Cenozoic tectonism, particularly within 50 km of the Alpine Fault. The Median Batholith represents a significant but previously little-recognised 250 Ma record of magmatism along the continental margin of South Gondwana, and invites comparison with other Cordilleran batholiths.

Lamprophyre dike intrusion and the age of the Alpine fault, New Zealand

The orientations of intrusive rocks from a carbonatitic lamprophyre dike swarm and the history of emplacement relative to country-rock schist structures are compatible with intrusion into tension fractures and Riedel shears formed during initiation of the dextral wrench system of the Alpine fault. New U-Pb and Rb-Sr dates indicate a late Oligocene-early Miocene time of intrusion which, in turn, suggests a mid-Tertiary history for propagation of the Alpine fault plate boundary through South Island, New Zealand.

Uranium‐lead zircon ages from the Median Tectonic Zone, New Zealand

Abstract The Median Tectonic Zone (MTZ) of New Zealand is a generally north trending belt of Mesozoic subduction‐related I‐type plutonic, volcanic, and sedimentary rocks in South Island and Stewart Island that separates Permian strata of the Eastern Province Brook Street Terrane from lower to mid‐Paleozoic Gondwana margin assemblages of the Western Province. High‐precision isotope dilution U/Pb ages of zircons from 30 rocks are reported. Pre‐digestion leaching of zircon in hydrofluoric acid yielded significantly more concordant residues by removing common Pb and dissolving more soluble high‐U domains that have been more affected by relatively recent Pb loss. The results show that MTZ magmatism ranges in age from at least Early Triassic to Early Cretaceous (247–131 Ma), with a pronounced gap in the Middle Jurassic. Triassic plutons tend to occur on the eastern side of the MTZ, and they intrude volcanic/sedimentary sequences of the MTZ in Nelson and eastern Fiordland. These sequences are in turn intruded by...

Paired plutonic belts in convergent margins and the development of high Sr/Y magmatism: Peninsular Ranges Batholith of Baja California and Median Batholith of New Zealand

Cretaceous plutons of the eastern Peninsular Ranges batholith and the Separation Point suite of New Zealand represent major fluxes of relatively high Na, Sr, and low Y heavy rare earth element (HREE) magmas. They have similarities to Archean trondhjemite-tonalite-granodiorite (TTG) granitoids and Cenozoic adakites, but their genesis in Phanerozoic subduction zone settings is controversial. The well-documented margin-normal asymmetry of the Peninsular Ranges batholith is similar to that observed in the Median batholith of New Zealand. In both areas, similar-sized belts of high Na, Al, Sr, and low Y (here termed HiSY, after high Sr/Y) diorite-tonalite-granodiorite plutons developed continentalward of, and 10-15 m.y. after, parallel belts of low Sr/Y (LoSY) gabbro-diorite-granite plutons representing 30-40 m.y. of convergent margin magmatism. In the Peninsular Ranges batholith, the HiSY La Posta suite ( 99-92 Ma) lies inboard of a western belt of LoSY plutons (130-104 Ma) over the 800 km length of the batholith. In New Zealand, plutons of the HiSY Separation Point suite (126-105 Ma) mostly lie inboard of the LoSY Median suite (mostly 170-128 Ma). Chemical and isotopic links between HiSY and LoSY belts indicate genetic relationships between the paired belts within each area. Comparative features from both margins support a model that involves underthrusting of the outboard LoSY arc base during shallowing subduction to a deeper, more continentalward position. The mafic arc base is then partially melted under high-pressure conditions, resulting in plagioclase-poor or absent garnet-bearing residual mineral assemblages that produce high Sr/Y partial melts. The La Posta plutons appear to represent mixtures of HiSY magmas and Paleozoic metasedimentary crust.

Western Fiordland orthogneiss: Early Cretaceous arc magmatism and granulite facies metamorphism, New Zealand

U-Pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks, here termed the Western Fiordland Orthogneiss (WFO), in Fiordland, southwest New Zealand, indicate synkinematic magmatic emplacement between ∼120 and 130 Ma ago. These rocks were previously interpreted as possibly being of Precambrian age. Initial Pb and Sr ratios are consistent with arc/subduction related magmagenesis with little or no involvement of ancient continental crust. Subsequent high pressure (>12 kb) metamorphism of the WFO may reflect a major collision event involving crustal thickening by overthrusting of a >15 km thick sequence. Metamorphism ceased ≤116 Ma ago based on206Pb/238U ages of zircon from a retrogressed granulite. U-Pb isotopic analysis of apatite, along with previously published Rb/Sr mineral ages, indicate that final uplift and cooling to <300–400° C was largely completed by ∼90 Ma. The average uplift rate during this period is inferred to have been in excess of 1 mm/yr.Unmetamorphosed gabbronorites of the Darran Complex in eastern Fiordland, inferred by some investigators to be the granulite protolith, yield concordant U/Pb zircon ages of 137±1 Ma. U-Pb ages of apatite, and previously published K/Ar mineral ages indicate that these rocks experienced a rapid and simple cooling history lasting only a few million years. The high-grade WFO and unmetamorphosed Darran Complex are now separated by a profound structural break. However, the ages and similarities in initial Pb and Sr isotopic ratios suggest that both suites are products of the same Early Cretaceous cycle of subduction-related magmatism. The timing of Early Cretaceous magmatism and metamorphism, collision and resultant crustal thickening, and subsequent great uplift and erosion in Fiordland has important implications for terrane accretion and the evolution of relative plate motions along the New Zealand segment of the Gondwana margin.

Ridge-trench interactions and the Neogene tectonic evolution of the Magdalena shelf and southern Gulf of California: Insights from detrital zircon U-Pb ages from the Magdalena fan and adjacent areas

ABSTRACTThe Magdalena fan is an apparently beheaded submarine depocenter that has fi g-ured prominently in reconstructions of mid-dle 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 kine-matic model for the formation of the Gulf of California holds that ~300 km of dextral shear between the Pacifi c and North Ameri-can plates occurred along faults west of Baja California prior to the onset of dextral-trans-tensional shearing in the gulf ca. 6 Ma. We measured 1796 detrital zircon U-Pb ages from 65 samples in an effort to character-ize the provenance of the fan, determine its source region, and defi ne the cumulative dextral slip along faults offshore of south-western Baja California. Zircons from the fan are dominantly 120–65 Ma with subor-dinate 15–35 Ma grains. Excellent matches to the fan can be obtained by mixing Magda-lena shelf strata and/or adding detritus from the west-draining portion of the Los Cabos block. The same cannot be accomplished with zir cons from the east-draining portion of the Los Cabos block and mainland Mex-ico. Our results favor a western Baja source region for the fan and suggest that cumula-tive 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 Califor-nia began to accommodate transtensional shearing and form rift basins that captured detritus that previously reached the trench.Keywords:

Geochemical evidence for the tectonic setting of the Coast Range ophiolite: A composite island arc–oceanic crust terrane in western California

The Middle to Late Jurassic age Coast Range ophiolite (CRO) of California contains two geochemically distinct volcanic rock associations that formed in different tectonic settings. Volcanic rocks from the southern CRO (Point Sal, Cuesta Ridge, Stanley Mountain, Llanada, Quinto Creek, and Del Puerto) and parts of the northern CRO (Healdsburg, Elder Creek) are similar to low-K tholeiites and calc-alkaline rocks of the island-arc suite. The thin volcanic sections of these ophiolite remnants suggest formation by intra-arc rifting. In contrast, volcanic rocks from Stonyford seamount and Paskenta in the northern CRO are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid-ocean ridge basalts or ocean-island tholeiites. These rocks are associated with Tithonian radiolarian cherts and may be part of the Franciscan Complex. Alternatively, they may represent a change in tectonic setting within the CRO during the Late Jurassic. Regardless, the CRO as currently conceived cannot be considered a single terrane with one mode of origin.

Isotopic ages from the Nelson region of South Island New Zealand: crustal structure and definition of the Median Tectonic Zone

Abstract Plutonic rocks in the Rotoroa Complex and Drumduan Terrane of South Island, New Zealand yield zircon U/Pb dates of 156 and 142 Ma, respectively, that are interpreted as crystallization ages. Hornblende and biotite 40 Ar/ 39 Ar dates of 140-130 Ma from the Rotoroa represent either emplacement ages, cooling ages or a metamorphic resetting event. These two units crop out between the Brook Street Terrane and the Separation Point Batholith and lack any clear affinity with tectonostratigraphic terranes of the New Zealand Western or Eastern provinces. The Rotoroa Complex and Drumduan Terrane are interpreted as part of a series of dismembered Mesozoic volcanic-plutonic arc complexes that are sandwiched between terranes of the Western and Eastern provinces, occupying a structural position here referred to as the Median Tectonic Zone (MTZ). Correlative units in Fiordland on the opposite side of the Alpine Fault include the Mackay Intrusives, Darran Complex, Largs Terrane, Lochburn Formation and the Halfway Peak Gabbro. Farther south on Stewart Island the Anglern Complex and Paterson Group are part of the same structural belt. The MTZ is an extension of the original concept of the Median Tectonic Line put forth by Landis and Coombs (1967). Dismemberment and juxtaposition of arc magmatic assemblages in the MTZ with Western and Eastern Province terranes is related to large-scale transcurrent faulting in the Early Cretaceous. Its essential features as a regional tectonostratigraphic terrane were established by ~ 117 Ma as indicated by stitching of the Rotoroa Complex to the Takaka Terrane (Western Province) by the Separation Point Batholith (117-114 Ma). The Echinus Granite yields a 310 Ma U/Pb zircon crystallization age that suggests the granite and associated gneisses are part of the Western Province which may constrain the position of the western margin of the MTZ near Nelson City.

U-Pb geochronology of mid-Paleozoic plutonism in western New Zealand: Implications for S-type granite generation and growth of the east Gondwana margin

New U-Pb isotope-dilution-thermal ionization mass spectrometry (ID-TIMS) ages (371–305 Ma) for 30 granitic plutons along the New Zealand sector of the East Gondwanan active margin reveal a highly episodic emplacement history and crustal growth pattern. The Late Devonian-late Carboniferous ages also establish specific links with both the mostly older, Lachlan and the mostly younger, New England fold belts of eastern Australia. Dated plutons are representative of two S- and I-type suite pairs, the volumetrically predominant Karamea-Paringa (371–360 Ma) and minor Ridge-Tobin (355–342 Ma) pulses, as well as sporadic Foulwind Suite A-type granites (350–305 Ma). Emplacement of the bulk of the dominant ∼3400 km 2 Karamea Suite S-type granite-granodiorite plutons within a 2.11 Ma interval is explained by major and intimate intrusion of mantle-derived magma into largely metasedimentary crust during intra-arc extension of previously overthickened crust. Transient emplacement rates were thus of similar magnitude as some young ignimbrite flare-ups and an order of magnitude greater than long-term averages for Mesozoic-Cenozoic cordilleran batholiths of the western Americas. Extension likely was terminated abruptly by resumption of convergence, possibly associated with amalgamation of the Buller and Takaka terranes, between 368 and 355 Ma. Significant crustal growth occurred during generation of the two S-type suites, where mantle basalt contributed mass, and heat for rapid melting, during transient intra- or backarc extensional episodes. In contrast, the I-type suites were dominated by partial melting of meta-igneous crust, and they are relatively small in volume. The Karamea S-type suite shares striking similarities in terms of age, composition, and extensional tectonic setting with S-type granites of the Melbourne terrane of the Lachlan fold belt. Both regions may have formed in a backarc position with respect to the Late Devonian-early Carboniferous subduction zone in the New England fold belt. Foulwind Suite A-type magmatism in New Zealand overlaps in age with the widespread 320–285 Ma A- and I-type magmatism in the northern New England fold belt. The likely continuation of the New England subduction system must have subsequently been removed from outboard of the New Zealand region after 320–285 Ma magmatism, and prior to Triassic accretion of a Permian oceanic arc terrane to the New Zealand margin.

Multi-Stage Origin of the Coast Range Ophiolite, California: Implications for the Life Cycle of Supra-Subduction Zone Ophiolites

The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the fore-arc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation. Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks with arc tholeiitic or (more rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB dikes) to represent the collision of an active spreading ridge. Subsequent reorganization of relative plate motions led to sinistral transpression, along with renewed subduction and accretion of the Franciscan Complex. The latter event resulted in uplift and exhumation of the ophiolite by the process of accretionary uplift.