Paul B. O'Sullivan

The importance of apatite composition and single-grain ages when interpreting fission track data from plutonic rocks: a case study from the Coast Ranges, British Columbia

Abstract Fission track results determined from granitic rocks exposed within the Coast Ranges of British Columbia indicate that significant variations in apatite chemistry may occur in small plutonic rock samples. To test ages determined earlier by Parrish using the population method of fission track dating, new apparent apatite ages were completed using the external detector method so that single-grain ages could be generated. In most cases, the new apatite ages were similar to the earlier ones (within ±2σ). However, in many cases, it was found that individual samples contained significant variations in single-grain apatite ages. Microprobe analyses showed the spread in ages were directly related to variations in apatite chemistry, such that chlorine-rich apatites retained older ages than fluorine-rich grains. Examination of the single-grain age data suggested that the spread in single-grain ages provided information which constrained the thermal history of the sample and that distributions of single-grain ages reflected the temperatures at which they resided during cooling. These results suggest that it is important to generate more than a few single-grain fission track ages when dating apatite from granitic rocks, since variations in the chlorine concentration in apatite grains may exist in kg-sized granitic rock samples. Therefore, it is no longer a valid assumption that each apatite grain separated from a small granite sample will have a similar chlorine concentration, and in future apatite fission track studies of granitic terrains it is important to recognize that the apatite composition might be heterogeneous when interpreting results.

Shaping the Australian crust over the last 300 million years: insights from fission track thermotectonic imaging and denudation studies of key terranes

Apatite fission track thermochronology is a well‐established tool for reconstructing the low‐temperature thermal and tectonic evolution of continental crust. The variation of fission track ages and distribution of fission track lengths are primarily controlled by cooling, which may be initiated by earth movements and consequent denudation at the Earths surface and/or by changes in the thermal regime. Using numerical forward‐modelling procedures these parameters can be matched with time‐temperature paths that enable thermal and tectonic processes to be mapped out in considerable detail. This study describes extensive Australian regional fission track datasets that have been modelled sequentially and inverted into time‐temperature solutions for visualisation as a series of time‐slice images depicting the cooling history of present‐day surface rocks during their passage through the upper crust. The data have also been combined with other datasets, including digital elevation and heat flow, to image the denudation history and the evolution of palaeotopography. These images provide an important new perspective on crustal processes and landscape evolution and show how important tectonic and denudation events over the last 300 million years can be visualised in time and space. The application of spatially integrated denudation‐rate chronology is also demonstrated for some key Australian terranes including the Lachlan and southern New England Orogens of southeastern Australia, Tasmania, the Gawler Craton, the Mt Isa Inlier, southwestern Australian crystalline terranes (including the Yilgarn Craton) and the Kimberley Block. This approach provides a readily accessible framework for quantifying the otherwise undetectable, timing and magnitude of long‐term crustal denudation in these terranes, for a part of the geological record previously largely unconstrained. Discrete episodes of enhanced denudation occurred principally in response to changes in drainage, base‐level changes and/or uplift/denudation related to far‐field effects resulting from intraplate stress or tectonism at plate margins. The tectonism was mainly associated with the history of continental breakup of the Gondwana Supercontinent from Late Palaeozoic time, although effects related to compression are also recorded in eastern Australia. The results also suggest that the magnitude of denudation of cratonic blocks has been significantly underestimated in previous studies, and that burial and exhumation are significant factors in the preservation of apparent ‘ancient’ features in the Australian landscape.

Apatite fission-track thermochronology of the Sierras Pampeanas, central western Argentina: Implications for the mechanism of plateau uplift in the Andes

Over much of its length, the Andean orogen is characterized by a generally east-vergent geometry and a progressive eastward (cratonward) migration of individual arc-orogenic elements. A departure from this model occurs in the Sierras Pampeanas region of central western Argentina where a terrane of active basement uplifts is currently developing to the east of the main Cordillera. Apatite fission-track data from uplifted Precambrian and Phanerozoic basement rocks of the Sierras Pampeanas constrain the tectonic development of this terrane and indicate that deformation associated with exhumation may have propagated broadly westward since the late Miocene. Two pre-Andean cooling events—during the Carboniferous–Permian (ca. 300–280 Ma) and the early Jurassic–middle Jurassic (ca. 200–174 Ma)—have been identified. The onset of Andean deformation is represented by a cooling event during the late Paleocene–middle Eocene. This cooling was followed by a period of middle Miocene–late Miocene reheating, during foreland basin-style sedimentation. Exhumation, and possible westward migration of the exhumation “front” in the Sierras Pampeanas, commenced during the late Miocene-Pliocene to the east of the dominantly east-vergent Precordillera fold-and-thrust belt. The apparent convergence of deformation in these two terranes and the progressive closure of Miocene-Pliocene intermontane basins in the Sierras Pampeanas may reflect the early stages of Andean plateau uplift. The relative timing of plateau development along strike suggests that lateral thickening of the orogen is progressing southward at least from the latitude of central Bolivia (ca. 20°S). Furthermore, the time-space coincidence between basement uplift and flattening of the subducted slab beneath the Sierras Pampeanas suggests that a relationship exists between westward displacement of the terrane and the dynamics of plate interaction.

Fission track data from the Bathurst Batholith: Evidence for rapid mid‐Cretaceous uplift and erosion within the eastern highlands of Australia

Apatite fission track thermochronology reveals that uplift and erosion occurred during the mid‐Cretaceous within the Bathurst Batholith region of the eastern highlands, New South Wales. Apatite fission track ages from samples from the eastern flank of the highlands range between ca 73 and 139 Ma. The mean lengths of confined fission tracks for these samples are > 13 μm with standard deviations of the track length distributions between 1 and 2 μm. These data suggest that rocks exposed along the eastern flank of the highlands were nearly reset as the result of being subjected to palaeotemperatures in the range of approximately 100–110°C, prior to being cooled relatively quickly through to temperatures < 50°C in the mid‐Cretaceous at ca 90 Ma. In contrast, samples from the western flank of the highlands yield apparent apatite ages as old as 235 Ma and mean track lengths < 12.5 μm, with standard deviations between 1.8 and 3 μm. These old apatite ages and relatively short track lengths suggest that the rocks w...

Thermotectonic history of Mt Logan, Yukon Territory, Canada: implications of multiple episodes of middle to late Cenozoic denudation

Abstract The extreme relief of the St. Elias Mountains, including Mt. Logan, is suggestive of a young mountain belt. New apatite fission track data indicate that the Mt. Logan massif experience rapid low-temperature cooling during three distinct periods: during the middle Eocene, Middle Miocene and Pliocene. Each cooling episode, the magnitude of which varies due to temporal variations in recorded paleogeothermal gradients, can be tentatively linked to a tectonic cause. Eocene cooling may be due to re-equilibration of isotherms following normal faulting, caused by a combination of thermal weakening of the crust and a decrease in regional compressional stress, due to the ∼ 43 Ma change in relative motions between the Pacific and North American plates. The cause of Miocene cooling is problematic, and could reflect denudation in response to initial underplating of the Yakutat terrane, or a recorded change in heat flow unrelated to denudation. Pliocene cooling reflects erosion due to surface uplift that produced the spectacular present-day topography of the St. Elias Mountians. This surface uplift is probably related to the coeval significant change in relative motion between the North American and Pacific plates and/or resistance to subduction by the Yakutat terrane.

Thermal history of Canadian Williston basin from apatite fission-track thermochronology—implications for petroleum systems and geodynamic history

Abstract Apatite fission track (AFT) thermochronology has been applied to a composite depth profile of Precambrian basement rocks underlying the Phanerozoic Canadian Williston Basin. Thermal histories derived from the AFT data record cycles of heating and cooling which follow the pattern of regional burial history, but which also indicate major temporal and geographic variations in the timing and degree of maximum Phanerozoic temperatures. These variations in the thermal history were not previously recognised from organic maturity indicators and subsidence models. Specifically, our study suggests a late Paleozoic heat flow anomaly with a geographic extent closer to that of Middle Devonian–Carboniferous Kaskaskia subsidence patterns than to that of the Williston Basin proper. This thermal anomaly has both economic and geodynamic significance. The recognition that potential Upper Cambrian–Lower Ordovician petroleum source rocks became fully mature during the late Paleozoic distinguishes that petroleum system from others that entered the main hydrocarbon generation stage in latest Cretaceous and Paleogene time. The late Paleozoic heat flow anomaly suggested from the AFT data implies a geodynamic coupling between inelastic Kaskaskia subsidence and previously inferred late Paleozoic lithospheric weakening. While the temporally varying heat flow model is preferred, the lack of independent constraints on the maximum thickness of upper Paleozoic strata precludes the outright rejection of the previous constant heat flow model. The AFT data provide important new constraints on the evolution of the epicratonic Williston Basin and its geodynamic models.

Climate and vegetation in southeastern Australia respond to Southern Hemisphere insolation forcing in the late Pliocene–early Pleistocene

Terrestrial climate responses to orbital forcing during the late Pliocene–early Pleistocene are poorly understood, particularly in the Southern Hemisphere, but are important for determination of the timing of regional climate evolution early in the history of the glaciated Quaternary world. We present a pollen record from southeastern Australia that shows marked cyclic change over some 280,000 yr straddling the Pliocene-Pleistocene boundary. Rainforest communities responded to climate forcing primarily within the precession and eccentricity bands, suggesting that major vegetation changes were driven directly by summer insolation, rather than by obliquity-dominated glacial cycles.

Multiple postorogenic denudation events: An example from the eastern Lachlan fold belt, Australia

Fission-track results from 40 surface samples from the eastern part of the Lachlan fold belt, Australia, suggest that two distinct episodes of rapid kilometre-scale denudation have occurred since the middle Carboniferous, when deformation within the fold belt is believed to have ceased. The first episode, during the Early Triassic, was possibly in response to the Hunter-Bowen orogeny, which affected the New England fold belt and the Sydney-Bowen basin, but the effects of which have previously not been recognized within the Lachlan fold belt. The second episode occurred during the middle Cretaceous, possibly in response to the onset of continental extension in the Tasman Sea ca. 96 Ma. Uplift at this time resulted in kilometre-scale denudation over much of the southeastern highlands of Australia and may have been caused by underplating inward of the rift. These results indicate that the Lachlan fold belt has remained tectonically active long after the last recognized deformational event in the region, and highlight the importance of fission-track data in elucidating the posttectonic histories of orogens previously undetected due to a lack of stratigraphic and structural crosscutting relationships.

Cretaceous to Recent extension in the Bering Strait region, Alaska

A key issue presented by the geology of northern Alaska concerns the demise of the Brooks Range going west toward the Bering Strait region. The main Brookian tectonic and stratigraphic elements continue into the Russian Far East, but the thick crustal root and high elevations that define the modern physiographic Brooks Range die out approaching the Bering and Chukchi shelves, which form an unusually broad area of submerged continental crust. Structural, geochronologic, and apatite fission-track data indicate that at least three episodes of extension may have affected the crust beneath the Bering Strait region, in the middle to Late Cretaceous, Eocene-early Oligocene, and Pliocene(?)-Recent. This extension may explain the present thinner crust of the region, the formation of extensive continental shelves, and the dismemberment and southward translation of tectonic elements as they are traced from the Brooks Range toward Russia. Evidence for these events is recorded within a gently tilted 10- to 15-km thick crustal section exposed on the western Seward Peninsula. The earliest episode is documented at high structural levels by the postcollision exhumation history of blueschists. Structural data indicate exhumation was accomplished in part by thinning of the crust during north-south extension bracketed between 120 and 90 Ma by 40Ar/39Ar and U-Pb ages. The Kigluaik Mountains gneiss dome rose through the crust during the later stages of this extension at 91 Ma. Similar gneiss domes occur within a broad, discontinuous belt of Cretaceous magmatism linking interior Alaska with northeast Russia; mantle-derived melts within this belt likely heated the crust and facilitated extension. Apatite fission-track ages indicate cooling below ≈120–85°C occurred sometime between 100 and 70 Ma, and the area subsequently resided at shallow crustal depths (<3–4 km) until the present. This suggests that denudation of deep levels of the crust by erosion and/or tectonism was mostly completed by the Late Cretaceous and thus that the present-day thinner crust of the Bering Strait region developed primarily in the Cretaceous. Regional tilting and at least several more kilometers of local erosion followed in Eocene-early Oligocene time as documented by fission-track ages from deeper levels of the crustal section exposed in the Kigluaik Mountains. This event is generally coeval with development of the offshore transtensional Hope and Norton Basins which flank the Seward Peninsula to the north and south. Modern seismicity, active normal faults, and basin-range topography document Pliocene(?) to Recent north–south extension across the region. Fission-track data indicate that exhumation during this period was quite limited, less than 2–3 km. This inferred history of protracted extension in the Bering Strait region stands in sharp contrast to well-documented Cretaceous and Tertiary north–south shortening in interior Alaska. This contrast in tectonic histories suggests a model in which contraction and westward extrusion of crustal fragments from interior Alaska by strike-slip faulting were accommodated by north–south extension in the Bering Strait region. This resulted in the counterclockwise rotation of extruded crustal blocks, the extensional thinning of the western part of the Brooks Range crustal root, and the formation of transtensional basins and unusually broad continental shelves between Alaska and Russia.

Multiple Phases of Tertiary Uplift and Erosion in the Arctic National Wildlife Refuge, Alaska, Revealed by Apatite Fission Track Analysis

The northeastern Brooks Range (NEBR) in the Arctic National Wildlife Refuge of Alaska (ANWR) is a complex Mesozoic to Cenozoic northward-verging fold-and-thrust belt. Shortening in the upper crust occurred through the duplexing of thrust sheets and formation of associated fault-bend folds. Regional deformation in the NEBR has extended farther to the north than elsewhere in the Brooks Range and is among the youngest in the region. Apatite fission track analyses (AFTATM) of Permian to Paleogene clastic rock sequences in four areas in the NEBR document the northward younging of uplift and erosion attending the duplexing of thrust sheets. AFTA data on the Permian to Albian Bathtub Ridge section indicate rapid (<3-5 m.y.) cooling through 110-60°C and a minimum of 2 km of uplift and erosion during the Paleocene at about 62 Ma, assuming reasonable geothermal gradients of about 20-30°C/km. This was followed by a second cooling phase which occurred after the Paleocene (<50 Ma). AFTA data from Albian sedimentary rocks at Arctic Creek suggest slower (<5-10 m.y.) cooling through 110-60°C due to uplift and erosion during the late Eocene-middle Oligocene between ^sim40 and 30 Ma. AFTA data on Neocomian to Eocene sandstones, exposed along the Canning River west of the Sadlerochit Mountains, and Upper Cretaceous to Paleocene sandstones, exposed at Sabbath Creek near the Jago River east of the Sadlerochit Mountains, suggest two phases of rapid (<3-5 m.y.) cooling due to uplift and erosion at about 45 and about 23 Ma. In contrast, AFTA data on Late Cretaceous to Tertiary sediments from the Arctic coastal plain show that these rocks have not been subjected to temperatures greater than about 60°C for longer than 1 m.y. since their deposition. Their fission track ages reflect the thermal, uplift, and erosion histories of their provenance terranes (i.e., the NEBR). Therefore, four major regional cooling (uplift and erosion) phases are recognized (^sim62 Ma, ^sim45 Ma, ^sim 0-30 Ma, and ^sim23 Ma). These phases progressively young to the north across a 100-km transect in the NEBR and are interpreted as periods of major compression, effectively dating periods of thrusting in the NEBR and constraining the timing of formation of possible oil-bearing structures on the coastal plain.