P. V. Crowhurst’s research while affiliated with University of Melbourne and other places

New 40Ar/39Ar, apatite fission track and (U–Th)/He data from the late Cretaceous indenting and buttressing margins of Ecuador have been combined with previous thermochronological studies to constrain the timing of syn- and post-accretionary tectonic events in the Ecuadorian Andes to within ±1 Ma. Our interpretations are more accurate than previous hypotheses because i) they are more sensitive to lower temperatures (<60 °C), ii) we directly compare data obtained from in-situ and detrital rocks, and iii) they are constrained by recently published palaeomagnetic, stratigraphic and geochronological data. The response of the buttressing Ecuadorian margin to the collision of the Caribbean Plateau and its overlying arc was diachronous. Exhumation occurred as an immediate response to collision at ∼75 Ma, south of S1°30′, whereas the northern region started to exhume at ∼65 Ma, suggesting that accretion may have been oblique. Elevated cooling and exhumation rates within specific massifs dispersed along the entire length of the Ecuadorian cordilleras, during 43–30 and 25–18 Ma, are attributed to i) an increase in convergence rates between the Farallon and South American plates during 42–37 Ma, and an increase in spreading rates in the southern Atlantic ocean, and ii) a change in the vector of the subducting plate, which changed from ESE to E at 25 Ma in response to fragmentation of the Farallon Plate. Previous suggestions that Eocene reactivation of the buttressing margin were driven by collision of the Macuchi Arc are shown to be incorrect. 40Ar/39Ar, zircon fission track, and apatite fission track and (U–Th)/He analyses from the Eastern Cordillera north of S1°30′ reveal well defined periods of rapid cooling and exhumation at 15 Ma, 9–7 Ma and 5.5–0 Ma. Apatite (U–Th)/He data reveals late Miocene-Recent cooling and exhumation (≤1.3 km) of the southern Eastern Cordillera by a lower quantity than that experienced to the north (≥3.5 km). These distinguishable differences in cooling and exhumation are attributed to the collision of the Carnegie Ridge with the northern SOAM Plate at 15 Ma, and the subsequent subduction of high topography along the ridge at ∼5 Ma, which reactivated the serpentinised Campanian suture via dextral transcurrent displacement.

Single-grain (U-Th)/He ages from two profiles were used to reconstruct the post-Permian tectonic-thermal history of basement rocks in Heimefrontfjella, East Antarctica. The (U-Th)/He ages from one sample collected below the late Carboniferous/Early Permian sedimentary cover rocks indicate Jurassic-Early Cretaceous basement paleotemperatures of ~40°-60°C due to post-Permian burial. Combined apatite fission track and (U-Th)/He analyses from samples of a profile in Sivorgfjella suggest a period of flexural-related tilting after ~87 Ma. The timing was further constrained using forward and inverse models of the (U-Th)/He data. Model results indicate a Cenozoic phase of relatively rapid cooling from ~40°C to surface temperatures. As the driving mechanism, we propose flexural isostatic rebound due to glacial load during the development of the intracontinental ice sheet in the hinterland of the Heimefrontfjella region.

Apatite fission-track (FT) and single grain (U–Th)/He ages from four vertical profiles in central Dronning Maud Land (East Antarctica) range from 312 ± 20 Ma to 135 ± 11 Ma and 304 ± 28 Ma to 104 ± 8 Ma, respectively. The combined age data allows to discriminate between undisturbed cooled (due to exhumation) and thermally overprinted crustal blocks. Profiles at the Zwieselhöhe and the Conradgebirge revealed unusual apatite FT vs. elevation relationships and (U–Th)/He ages older than the corresponding central apatite FT ages, possibly providing evidence for a Jurassic thermal overprint. Most probably Jurassic magmatism and associated advective heating led to total annealing of the apatite fission-tracks but helium only partially diffused. The model developed in this paper suggests that the (U–Th)/He ages from the Zwieselhöhe and Conradgebirge profiles are in part relicts of the pre-Jurassic cooling history.

While initial studies showed a high degree of consistency between AFTA and apatite (U Th)/He dating, an increasing number of studies are reporting apatite (U Th)/He ages which are older than expected on the basis of apatite fission track data from the same region. We present data from a range of geological settings to document a systematic discrepancy between the two systems, which becomes more pronounced in samples with older fission track ages (except for samples with very low uranium contents). Results from a granite pebble and enclosing volcanogenic sandstone provide a well-controlled test case in which independent constraints are available on the underlying thermal history. These demonstrate that the progressive discrepancy between the two techniques arises not from anomalous fission track annealing behaviour, as has been suggested, but as a result of a change in the He retention properties of apatite. This change appears to be linked to the degree of accumulated radiation damage within the crystal lattice, although underlying mechanisms remain unclear. We suggest detailed experiments should be performed to delineate and quantify the processes responsible. Until this is achieved, we suggest that apatite (U Th)/He studies should also incorporate apatite fission track data, as well as other low temperature indicators, in order to monitor the (U Th)/He system response and to guard against the anomalous behaviour described here.

The low sensitivity of apatite fission track (AFT) thermochronometry at temperatures less than w60 8C suggests that AFT data sets from the Andean Cordilleras may have frequently failed to identify specific periods after 9 Ma when cooling rates were high. Forward modeling of (U–Th)/He apatite age data obtained from the juxtaposed Paleozoic–Mesozoic Alao, Loja, and Salado terranes in the northern Cordillera Real, Ecuador, has improved the resolution of previous AFT thermal histories for the past 9 My. The Alao and Loja terranes form a coherent, structural block that resided at temperatures greater than 70–80 8C until w3.3–2.8 Ma and then cooled rapidly to less than 40 8C at rates of O15 8C/My. Intraterrane variations in the cooling and exhumation histories in the Salado terrane suggest that nonterrane-bounding faults played a significant role during its Pliocene–Recent evolution. The Salado terrane preserves an older history that reveals elevated cooling rates during 22–19 and 18–15 Ma. Subsequently, the terrane cooled rapidly from greater than 90 8C to less than 40 8C during 11–8 and 5.5–3.5 Ma at rates of O8 8C/My. Vertical reactivation of the Llanganates fault, which separates the Salado and Loja terranes, during the Pliocene–Recent coincides with the main stages of formation of the juxtaposed Interandean Depression, which provides further constraints on the growth phases of the depression and the Cordillera.

The low sensitivity of apatite fission track (AFT) thermochronometry at temperatures less than ∼60 °C suggests that AFT data sets from the Andean Cordilleras may have frequently failed to identify specific periods after 9 Ma when cooling rates were high. Forward modeling of (U–Th)/He apatite age data obtained from the juxtaposed Paleozoic–Mesozoic Alao, Loja, and Salado terranes in the northern Cordillera Real, Ecuador, has improved the resolution of previous AFT thermal histories for the past 9 My. The Alao and Loja terranes form a coherent, structural block that resided at temperatures greater than 70–80 °C until ∼3.3–2.8 Ma and then cooled rapidly to less than 40 °C at rates of >15 °C/My. Intraterrane variations in the cooling and exhumation histories in the Salado terrane suggest that nonterrane-bounding faults played a significant role during its Pliocene–Recent evolution. The Salado terrane preserves an older history that reveals elevated cooling rates during 22–19 and 18–15 Ma. Subsequently, the terrane cooled rapidly from greater than 90 °C to less than 40 °C during 11–8 and 5.5–3.5 Ma at rates of >8 °C/My. Vertical reactivation of the Llanganates fault, which separates the Salado and Loja terranes, during the Pliocene–Recent coincides with the main stages of formation of the juxtaposed Interandean Depression, which provides further constraints on the growth phases of the depression and the Cordillera.

New U–Pb zircon ages and Sr–Nd isotopic data for Triassic igneous and metamorphic rocks from northern New Guinea help constrain models of the evolution of Australia's northern and eastern margin. These data provide further evidence for an Early to Late Triassic volcanic arc in northern New Guinea, interpreted to have been part of a continuous magmatic belt along the Gondwana margin, through South America, Antarctica, New Zealand, the New England Fold Belt, New Guinea and into southeast Asia. The Early to Late Triassic volcanic arc in northern New Guinea intrudes high-grade metamorphic rocks probably resulting from Late Permian to Early Triassic (ca 260–240 Ma) orogenesis, as recorded in the New England Fold Belt. Late Triassic magmatism in New Guinea (ca 220 Ma) is related to coeval extension and rifting as a precursor to Jurassic breakup of the Gondwana margin. In general, mantle-like Sr–Nd isotopic compositions of mafic Palaeozoic to Tertiary granitoids appear to rule out the presence of a North Australian-type Proterozoic basement under the New Guinea Mobile Belt. Parts of northern New Guinea may have a continental or transitional basement whereas adjacent areas are underlain by oceanic crust. It is proposed that the post-breakup margin comprised promontories of extended Proterozoic–Palaeozoic continental crust separated by embayments of oceanic crust, analogous to Australia's North West Shelf. Inferred movement to the south of an accretionary prism through the Triassic is consistent with subduction to the south-southwest beneath northeast Australia generating arc-related magmatism in New Guinea and the New England Fold Belt.

Oceanic hotspot activity, generating large oceanic igneous plateau provinces, plate rearrangements and the generation of new spreading centers since at least 90 Ma have formed large structural, thickness and density heterogeneities in the approaching and subducting oceanic crust offshore NW South America (SOAM). Various oceanic allochthonous terranes comprise western Ecuador and the relatively thick and buoyant Carnegie Ridge is being subducted. We present 40Ar/39Ar, fission track (FT) and (U-Th/He) data from i) the Eastern Cordillera and the Amotape Complex, which define the palaeo-continental margin, ii) the Western Cordillera, which is built upon allochthonous, oceanic crust and iii) a tectonic mélange at the ocean-continent suture. 40Ar/39Ar ages and FT data from exotic, Triassic blocks within the ocean-continent suture record elevated cooling rates of