Alan P. M. Vaughan

Terrane Processes at the Margins of Gondwana

The process of terrane accretion is vital to the understanding of the formation of continental crust. Accretionary orogens affect over half of the globe and have a distinctively different evolution to Wilson-type orogens. It is increasingly evident that accretionary orogenesis has played a significant role in the formation of the continents. The Pacific-margin of Gondwana preserves a major orogenic belt, termed here the ‘Australides’, which was an active site of terrane accretion from Neoproterozoic to Late Mesozoic times, and comparable in scale to the Rockies from Mexico to Alaska, or the Variscan-Appalachian orogeny. The New Zealand sector of this orogenic belt was one of the birthplaces of terrane theory and the Australide orogeny overall continues to be an important testing ground for terrane studies. This volume summarizes the history and principles of terrane theory and presents 16 new works that review and synthesize the current state of knowledge for the Gondwana margin, from Australia through New Zealand and Antarctica to South America, examining the evolution of the whole Gondwana margin through time.

The eastern Palmer Land shear zone: a new terrane accretion model for the Mesozoic development of the Antarctic Peninsula

A major ductile fault zone, the eastern Palmer Land shear zone, has been identified east of the spine of the southern Antarctic Peninsula. This shear zone separates newly identified geological domains, and indicates that during Late Jurassic terrane accretion and collision, two and possibly three separate terranes collided, resulting in the Palmer Land orogeny. The orogeny is best developed in eastern Palmer Land and eastern Ellsworth Land. There, shallow-marine sedimentary rocks of the Latady Formation, and a metamorphic and igneous basement complex of possible Lower Palaeozoic to pre-Early Jurassic age, are thrust and folded. This forms an arcuate, east-directed, foreland, fold and thrust belt up to 100 km wide and 750 km long, parallel to the axis of the Antarctic Peninsula. The newly identified Antarctic Peninsula domains include: (1) a parautochthonous Eastern Domain that represents part of the margin of the Gondwana continent, comparable to the Western Province of New Zealand, the Ross Province superterrane of Marie Byrd Land, the Eastern Series of south-central Chile, the Pampa de Agnía and Tepuel rocks of north Patagonia, and the Cordillera Darwin rocks of Tierra del Fuego, (2) a suspect Central Domain that represents an allochthonous, microcontinental, magmatic arc terrane, comparable to the Median Tectonic Zone of New Zealand, the Amundsen Province superterrane of Marie Byrd Land, and Coastal Cordillera of north Chile and (3) a suspect Western Domain, with strong similarities to the Eastern Province of New Zealand, Western Series of south-central Chile, and Chonos metamorphic complex of north Patagonia, that represents either a subduction–accretion complex to the Central Domain, or another separate crustal fragment. Although an allochthonous terrane hypothesis for the Antarctic Peninsula remains to be fully tested, this has much in common with models for the New Zealand and South American parts of the Pacific margin of Gondwana. The identification of a potential allochthonous terrane–continent collision zone allows us to define the edge of the Gondwana continent in the Antarctic Peninsula sector of the supercontinent margin, which has implications for Mesozoic reconstructions of Gondwana.

Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica

U-Pb detrital zircon geochronology from the upper Cambrian to Devonian part of the Ellsworth Mountains succession, Antarctica, yields dominant late Mesoproterozoic and late Neoproterozoic–Cambrian age populations that are onsistent with a provenance from within Gondwana. Hf isotope compositions reveal a source predominantly within west Gondwana and identify a change in provenance up-stratigraphy that coincides with the change of sedimentation setting from active rift to passive margin, which has been independently determined by stratigraphic, structural, and geochemical arguments. For the Late Cambrian Frasier Ridge Formation, late Mesoproterozoic grains have positive eHf values, suggesting derivation from juvenile crust, and late Neoproterozoic–Cambrian grains have eHf values greater than –5, consistent with remelting of similar juvenile late Mesoproterozoic crust during the Pan African–Ross orogenies. Provenance during rifting was from proximal sources from within west Gondwana, most likely, southernmost Africa and basement to the Ellsworth-Whitmore Mountains block. At higher stratigraphic levels where deposition occurred along a passive margin, in the early Ordovician Mount Twiss Member and middle Devonian Mount Wyatt Earp Formation, late Neoproterozoic–Cambrian grains have eHf values less than –5; this means that early Mesoproterozoic–Archean crust was remelted to generate these zircons. Provenance was from a more expansive source region within west Gondwana, and probably included the Kaapvaal and Congo cratons of south and west Africa. Isolated outcrops of sedimentary rock of uncertain age at Mount Woollard and the Whitmore Mountains have detrital zircon signatures similar to the Frasier Ridge Formation, suggesting correlation with these Late Cambrian deposits. Sedimentary rock from the Stewart Hills contains some late Mesoproterozoic grains with lower eHf values than the previously mentioned samples. This suggests that the Stewart Hills sample has a provenance from within east Gondwana and was possibly deposited on the East Antarctic craton prior to the Ross orogeny and is not part of the displaced Ellsworth-Whitmore Mountains crustal block.

New aerogeophysical view of the Antarctic Peninsula: More pieces, less puzzle

New airborne geophysical data reveal subglacial imprints of crustal growth of the Antarctic Peninsula by Mesozoic arc magmatism and terrane accretion along the paleo-Pacific margin of Gondwana. Potential field signatures indicate that the Antarctic Peninsula batholith is a composite magmatic arc terrane comprising two distinct arcs, separated by a >1500 km-long suture zone, similar to the Peninsular Ranges batholith in southern and Baja California. Aeromagnetic, aerogravity and geological data suggest that a mafic Early Cretaceous western arc was juxtaposed against a more felsic eastern arc which, in mid-Cretaceous times, was intruded by highly magnetic tonalitic/granodioritic plutons of island arc affinity. Suturing of the two arcs against the Gondwana margin caused the mid-Cretaceous Palmer Land orogenic event. Convergence and suturing may have been driven by two subduction zones or, alternatively, by a decrease in slab dip, leading to an inboard migration of the arc, as in California.

Circum-Pacific mid-Cretaceous deformation and uplift: A superplume-related event?

Evidence of short-lived episodes of mid-Cretaceous deformation, metamorphism, uplift, and hiatus in sedimentation is widespread in the Lower Cretaceous rocks that bordered the Cretaceous Pacific basin. I present a model in which these coeval but widely spaced events are linked to increased ridge-push force at subduction zones triggered by large-scale upwelling of mantle associated with the mid-Cretaceous superplume. This model can account for mid-Cretaceous terrane accretion and ophiolite obduction events and can explain brief compressional phases in predominantly extensional orogenies as recognized in Alaska, British Columbia, California, western South America, and West Antarctica.

Geodynamic evolution of the Antarctic Peninsula during Mesozoic times and its bearing on Weddell Sea history

Abstract This review of the tectonic evolution of the Antarctic Peninsula during Mesozoic times highlights four main events; (1) Late Triassic-Late Jurassic extension, (2) Late Jurassic-Early Cretaceous dextral transpression, (3) Early Cretaceous extension and (4) mid-Cretaceous compression. Magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in Early Jurassic and Late Jurassic-Early Cretaceous times. The second of these breaks corresponded to the first compressional event. There was no apparent hiatus in the magmatic record during the mid-Cretaceous compressional event, although there was a significant change in the pattern of sedimentation in the Larsen basin on the eastern margin of the Weddell Sea at about that time. The tectonic evolution of the peninsula is compared to, and puts some constraints on, existing Weddell Sea models. The Late Triassic-Late Jurassic arc extension correlates with initial rifting in the Weddell Sea region during sinistral motion between East and West Gondwana. However, there is no known record of large-scale pre-Mid-Jurassic transcurrent deformation in the Antarctic Peninsula that would have been consistent with rotation of West Antarctic crustal blocks in the initial rifting period. The peninsula-wide Late Jurassic-Early Cretaceous compressional event may correlate with the major change in Gondwana plate motions from E-W to N-S (African reference frame) separation of East and West Gondwana. This change probably resulted in formation of an ocean-continent boundary along the northern margin of the Weddell Sea embayment and initial seafloor spreading. Geological data do not seem to support subduction of southwestern proto-Weddell Sea oceanic lithosphere beneath the eastern margin of the peninsula at that time. Cretaceous arc extension was coeval with the initial seafloor spreading phase in the Weddell Sea. Mid-Cretaceous arc compression, linked to a global increase in ocean floor spreading rates and a superplume event, correlates with a change from NE-SW to NW-SE spreading in the Weddell Sea.

A mid-Cretaceous age for the Palmer Land event, Antarctic Peninsula: implications for terrane accretion timing and Gondwana palaeolatitudes

New structural and age data suggest that West Gondwana may have been at lower palaeolatitudes than previously interpreted from Albian sequences in Gondwana marginal suspect terranes. The Palmer Land event, which juxtaposed Mesozoic terranes on the Gondwana margin, deformed granitoids in the southern Antarctic Peninsula. U–Pb SHRIMP dating of zircons from a microgranite dyke yields a crystallization age of 106.9± 1.1 Ma. This result and re-interpretation of the structural position of another granite pluton date the Palmer Land event, and probable terrane collision, as late Early Cretaceous, and not latest Jurassic as formerly interpreted.

Ophiolite obduction pulses as a proxy indicator of superplume events

Abstract A major new synthesis of ophiolite geochronology and map of global Phanerozoic distribution indicates that previously noted pulses of ophiolite obduction can be linked to superplume-related tectonism. A marked cyclicity is evident in obduction ages obtained from minerals in ophiolite metamorphic soles and obduction-related minor intrusions. This episodicity is in phase with periods of predominantly uniform polarity of the geomagnetic field, formation of massive carbon-rich deposits, sea-level highstands, and formation of large flood basalt provinces; all generally considered to be superplume proxy indicators. A key to interpreting ophiolite obduction as a further proxy is the mid-Cretaceous, superplume-associated, ocean-margin compressional deformation. During this event, thermal rejuvenation and increased buoyancy of ocean lithosphere caused arc–terrane collision, marginal-basin shoaling, back-arc basin closure and ophiolite obduction. Convergent margins were placed in compression with increased coupling between subducting and overriding plates resulting in major ocean margin deformation. Being directly datable and tectonically and petrologically distinctive makes ophiolite assemblages of considerable use for identifying superplume events.

Relics of a complex triple junction in the Weddell Sea embayment, Antarctica

Abstract Interpretation of an airborne magnetic data compilation containing a key, new survey, together with re-tracked satellite gravity data from the Weddell Sea embayment (WSE), West Antarctica, suggests Rift–Rift–Rift triple junction formation at the onset of Gondwana breakup in the Early Middle Jurassic. A complex system of northwest–southeast rifts was active contemporaneously with an east–west trending rift. This rift activity led to northward separation of the Falkland Plateau, and formation of the Weddell Sea by sea floor spreading. Atypically, the Jurassic passive margin of Gondwana shows evidence for coeval extension in two directions and a large volume of interpreted magmatic material. This is consistent with initial doming above a mantle plume and we suggest that this resulted in the formation of a triple junction. Magnetic anomalies indicate a series of faults perpendicular to igneous intrusions and extrusions with outlines that range in shape from lozenges to parallel ridges. They show remarkably good spatial correlation with free air gravity anomalies, even in areas of sea ice. We base a structural elements map and timing sequence for the events in the WSE during early Gondwana breakup on anomaly cross-cutting relationships.

Mid-Cretaceous ductile deformation on the Eastern Palmer Land Shear Zone, Antarctica, and implications for timing of Mesozoic terrane collision

Ar–Ar dating of high-strain ductile mylonites of the Eastern Palmer Land Shear Zone in the southern Antarctic Peninsula indicates that reverse movement on the shear zone occurred in late Early Cretaceous times (Albian), and not latest Jurassic times as previously supposed. The Eastern Palmer Land Shear Zone forms a major tectonic boundary, separating suspect arc terranes from rocks of Gondwana continental affinity. The dated mylonites are developed in Lower Jurassic plutonic rocks at Mount Sullivan, eastern Palmer Land, and form part of a zone of ductile reverse deformation up to 25 km wide. Biotite from a fine-grained mafic mylonite yields an Ar–Ar cooling age of 102.8 ± 3.3 Ma. Movement of this age on the Eastern Palmer Land Shear Zone is coeval with circum-Pacific deformation, possibly related to a mantle superplume event, and provides support for allochthonous-terrane models for the Antarctic Peninsula with accretion in post-Early Cretaceous times.