Michael L. Curtis

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.

Late Palaeozoic to Mesozoic structural evolution of the Falkland Islands: a displaced segment of the Cape Fold Belt

The Falkland Islands lie on a displaced crustal block presently forming part of the South American plate. The islands possess two roughly orthogonal structural grains, the relative chronology of which is here unequivocally established for the first time. D1 structures form a southerly verging, Permo-Triassic age fold belt, striking E–W to WNW–ESE, which represents a displaced segment of the Gondwanian orogenic belt. The strike swing from E–W to WNW–ESE is coincident with a decrease in deformation intensity toward the west. On West Falkland, D1 folds are superimposed by the early Mesozoic D2, NE–SW-trending, Hornby Mountains anticline, producing localized kilometre-scale Type I and III fold interference patterns. The Hornby Mountains anticline is interpreted to be the result of dextral transpressive reactivation of a pre-existing NE–SW basement fault. If the Falkland Islands are rotated 180°, the structure of the D1 fold belt, and tectono-sedimentary features of the Permian age Lafonian Supergroup, display a remarkable correlation with the structural style and tectonic evolution of the eastern Cape Fold Belt of South Africa. Our data, therefore support a 180° tectonic rotation of the Falkland Islands and reaffirm their pre-Gondwana break-up position adjacent to the southeast coast of South Africa. In such a reconstruction the Falkland Islands constrain the easterly extension and eventual lateral termination of the Cape Fold Belt.

U–Pb zircon (SHRIMP) ages for the Lebombo rhyolites, South Africa: refining the duration of Karoo volcanism

U–Pb SHRIMP ages are reported for three rhyolite flows from the Lebombo rift region of the Karoo volcanic province. Two flows are interbedded with the Sabie River Basalt Formation and a third sample is from the overlying rhyolitic Jozini Formation. The interbedded rhyolites yield ages of 182.0 ± 2.1 and 179.9 ± 1.8 Ma, whilst the overlying Jozini Formation rhyolite yields an age of 182.1 ± 2.9 Ma. Combined with existing 40Ar/39Ar geochronology, the new SHRIMP data fine-tunes the chronology of the Karoo volcanic province and indicates the 12 km succession of volcanic rocks in the Lebombo rift were erupted in 1–2 million years and lends considerable support to the links between the Pleinsbachian–Toarcian extinction event and the global environmental impact of Karoo volcanism.

Tectonic history of the Ellsworth Mountains, West Antarctica: Reconciling a Gondwana enigma

Early Paleozoic orogenesis has been recognized along the southern African (Saldanian orogeny) and East Antarctic (Ross orogeny) sectors of the Gondwana paleo- Pacific margin. However, the absence of a contemporaneous orogenic event in the Ellsworth Mountains of West Antarctica, once a contiguous part of this margin, has resulted in their geology being considered enigmatic. In this contribution, widespread, detailed structural studies from all stratigraphic levels of the Ellsworth Mountains allow their tectonic evolution to be reassessed. Geochemical, stratigraphic, and structural data indicate that the Middle to Upper Cambrian Heritage Group developed in a continental rift basin. Multiple observations of the crucial end-Cambrian contact between this rift sequence and the overlying Crashsite Group reveal the contact to be regionally conformable; there are localized outcrop-scale unconformities. Furthermore, structural continuity can be demonstrated across this key boundary, thus precluding the possibility of an end-Cambrian orogenic event. The entire stratigraphic succession was affected by two post-Permian phases of deformation. D1 structures are locally developed and superimposed by the main dextral transpressive D2 Permian−Triassic Gondwanian deformation event. D2 is succeeded by an episode of extension orthogonal to orogenic strike. Recent geochronological data on the early Paleozoic evolution of the Cape fold belt reveal a strong tectonostratigraphic correlation with the Ellsworth Mountains, indicating that this sector of the Gondwana paleo-Pacific margin was affected by Middle to Late Cambrian rifting. A new tectonic model is proposed for the southwestern paleo-Pacific margin of Gondwana that accounts for the presence of rifting along a margin otherwise dominated by active subduction.

Reconstruction and break-out model for the Falkland Islands within Gondwana

The Falkland Islands are one segment of the Permo-Triassic Gondwanian Fold Belt that was displaced during the fragmentation of Gondwana. Palaeomagnetic, structural and palaeocurrent data, reviewed in this paper, provide convincing evidence that the Falkland Islands rotated from an original position off southeast Africa to their present position off South America during break-up. The rotation mechanism and trajectory are less certain but are an essential component of any plausible Gondwana break-up model. The Falkland Islands possess two roughly orthogonal structural grains. D1 structures form a southerly verging fold belt that correlates with the main folding in the eastern part of the Cape Fold Belt following relocation of the islands. D1 folds were overprinted by the Early Mesozoic D2, northeast-southwest trending, Hornby Mountains Anticline producing localised kilometrescale Type I and III fold interference patterns. It is suggested here that the D2 structures may represent a break-out structure related to a dextral transtensional shear couple that may have existed between East and West Gondwana during the initial stages of break-up. Clockwise rotation of the Falkland Islands Block (FIB) could have taken place along a series of east-west faults (e.g. the Gastre Fault Zone) in Patagonia as southern South America moved towards the Pacific during Middle Jurassic times. Contemporaneous Pacific-ward motion of southern South America during rotation of the FIB would have avoided collision with the Falkland Island Block as it docked. On a geological timescale, the break-out of the FIB and associated movements of other Gondwana fragments were rapid events, which appear to correlate with the major magmatic pulse at ca 183 Ma, related to a mantle plume beneath Africa and Antarctica. If this is correct, then doming above a large mantle plume in the South Atlantic region may have helped formation and rotation of Gondwana microplates, with rotation occurring above a viscously deforming, hotter-than-normal, substratum in a transtensional setting.

Middle Cambrian rift-related volcanism in the Ellsworth Mountains, Antarctica: tectonic implications for the palaeo-Pacific margin of Gondwana

The Ellsworth Mountains of West Antarctica represent part of a displaced terrane once situated along the palaeo-Pacific margin of Gondwana, prior to supercontinent break-up, adjacent to South Africa and the Weddell Sea coast of East Antarctica. Middle Cambrian sedimentary rocks of the southern Ellsworth Mountains host locally thick volcanic and subvolcanic rocks forming five igneous centres. Geochemically, most of the igneous samples are mafic, with a subordinate suite of evolved types. The mafic suite is geochemically varied, ranging from MORB (mid-ocean ridge basalt)-like compositions to shoshonitic and lamprophyric (e.g. LaN/YbN = 0.95 to 15.2), with eNdi values ranging from +5.2 to −2.0, correlating with Ti/Y. They are interpreted as representing melts derived from more than one mantle source, with the MORB-like rocks being derived from a depleted mantle source, and the more enriched compositions representing partial melting of lithospheric mantle. Silicic rocks contain melt contributions from Late Proterozoic crust, which is inferred to form the basement of the Ellsworth Mountains. We interpret these igneous rocks as having been formed in a continental rift environment, with MORB-like basalts erupted near the rift axis, and melts from lithospheric mantle emplaced on the rift shoulder. Such an interpretation is consistent with the sedimentary host-rock palaeogeography and contemporaneous structures. This Middle Cambrian rift event is correlated spatially and temporally with rift-related sedimentary rocks in South Africa. It is currently unclear what rifted off the southern African–Weddell Sea sector of the Gondwana palaeo-Pacific margin at that time.

Tectonic and magmatic patterns in the Jutulstraumen rift (?) region, East Antarctica, as imaged by high-resolution aeromagnetic data

The Jutulstraumen ice stream in western Dronning Maud Land may conceal a Jurassic continental rift. Delineating the geometry and the magmatic patterns of this inferred glaciated rift in East Antarctica is important to improve our understanding of the regional tectonic and magmatic processes associated with Gondwana break-up. A high-resolution aeromagnetic survey provides new insights over the largely buried tectonic and magmatic patterns of the Jutulstraumen area. Prominent NE-SW oriented aeromagnetic trends are detected over the Jutulstraumen. These trends delineate major inherited structural boundaries, active in Grenvillian (about 1.1 Ga) and Pan-African times (about 500 Ma), which appear to strongly control the location of the later Jurassic rift. The postulated eastern flank of the rift is marked by a broad positive anomaly over H. U. Sverdrupfjella. Buried Grenvillian age rocks may be the source of the long-wavelength anomaly. However, the higher frequency components correlate with granitoids of late Pan-African age. The inferred western flank of the rift features short-wavelength anomalies over the Borgmassivet and Ahlmannryggen areas, indicating a considerably greater extent of mid-Proterozoic tholeiitic sills than apparent in outcrop. In contrast, aeromagnetic signatures suggest that alkaline plutons, which relate to Jurassic rifting, are restricted to outcrop areas along the eastern rift flank. The prominent magnetic low over the Jutulstraumen indicates either a largely amagmatic rift, or perhaps subglacial sediments within the rift basin.

A review of geological constraints on the pre-break-up position of the Ellsworth Mountains within Gondwana: implications for Weddell Sea evolution

Abstract It has long since been recognised that the Ellsworth-Whitmore mountains (EWM) crustal block possesses an anomalous structural and stratigraphic history relative to its neighbouring West Antarctic crustal blocks, and the Transantarctic Mountains. This has led to uncertainties in the original pre-break-up position of the EWM within Gondwana. Positions vary from along the East Antarctic margin, west of the Pensacola Mountains, to within the Natal embayment region between Africa and Antarctica. The original position of the EWM within Gondwana has important implications, as its subsequent transposition has to be accounted for during the tectonic evolution of the Weddell Sea region. Several geological features have been identified within the EWM as potential constraints on Gondwana reconstructions. These include a Grenvillian age basement devoid of mineral reset ages; an apparently continuous stratigraphic succession from Cambrian to Permian times; Middle-Upper Cambrian extension-related volcanic rocks; no Ross age deformation; and a dextral transpressive component to the Early Mesozoic Gondwanide deformation. Based on a consideration of these key geological features, and comparisons between the Ellsworth Mountains and the palaeo-Pacific margins of Gondwana, we conclude that the EWM displays geological affinities with both the Antarctic and South African margins, and that it was located outboard of both. A prerequisite of this conclusion is that rotation and translation of the EWM must be included in models of early Weddell Sea tectonic evolution.

Late Cambrian stratigraphy of the Heritage Range, Ellsworth Mountains: implications for basin evolution

Deposition of the Upper Cambrian succession of the Ellsworth Mountains was influenced by major, episodic tectonically-driven changes to the depositional basin geometry. We subdivide the succession into four stratigraphical sequences based on the recognition of three sequence-bounding unconformities. The upper part of Sequence 1 is composed of the laterally equivalent Liberty Hills, Springer Peak and Frazier Ridge formations, a siliciclastic fluvial to marine deltaic association displaying NW-directed palaeocurrents. A switch in the position of the Late Cambrian depocentre from the north-west to the south coincided with cessation of terrigenous clastic deposition and accumulation of Sequence 2, the limestones of the Minaret Formation. Previously unreported talus breccias from the Independence Hills provide important clues to basin configuration at this time. A brief period of emergence of the Minaret Formation is inferred, prior to rapid subsidence and disconformable deposition of Sequence 3 (the ‘transition beds’) in outer-inner shelf environments. Localized intra-basinal uplift occurred prior to the deposition of Sequence 4 (the lower Crashsite Group), the base of which is locally an erosive unconformity, with a correlative conformity exposed elsewhere. We interpret the Upper Cambrian succession as representing the ‘rift-drift’ transition from initial rifting (preceded by Middle Cambrian volcanism) to thermal subsidence along the South African sector of the palaeo-Pacific margin of Gondwana.

Cenozoic tectonic history of the South Georgia microcontinent and potential as a barrier to Pacific-Atlantic through flow

Cenozoic opening of the central Scotia Sea involved the tectonic translation of crustal blocks to form the North Scotia Ridge, which today is a major topographic constriction to the flow of the deep Antarctic Circumpolar Current that keeps Antarctica thermally isolated from warmer ocean waters. How this ridge developed and whether it was a topographic barrier in the past are unknown. To address this we investigated the Cenozoic history of the South Georgia microcontinental block, the exposed part of the ridge. Detrital zircon U-Pb geochronology data confirm that the Cretaceous succession of turbidites exposed on South Georgia was stratigraphically connected to the Rocas Verdes backarc basin, part of the South America plate. Apatite thermochronometry results show that South Georgia had remained connected to South America until ca. 45–40 Ma; both record a distinct rapid cooling event at that time. Subsequent separation from South America was accompanied by kilometer-scale reburial until inversion ca. 10 Ma, coeval with the cessation of spreading at the West Scotia Ridge and collision between the South Georgia block and the Northeast Georgia Rise. Our results show that the South Georgia microcontinental block could not have been an emergent feature from ca. 40 Ma until 10 Ma.