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This article was downloaded by: [University of Queensland] On: 12 September 2008 Access details: Access Details: [subscription number 778575344] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Australian Journal of Earth Sciences Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t716100753 Geochronology of the Australian Cenozoic: a history of tectonic and igneous activity, weathering, erosion, and sedimentation* P. M. Vasconcelos a; K. M. Knesel a; B. E. Cohen a; J. A. Heim a a Earth Sciences, University of Queensland, Australia Online Publication Date: 01 January 2008 To cite this Article Vasconcelos, P. M., Knesel, K. M., Cohen, B. E. and Heim, J. A.(2008)'Geochronology of the Australian Cenozoic: a history of tectonic and igneous activity, weathering, erosion, and sedimentation*',Australian Journal of Earth Sciences,55:6,865 — 914 To link to this Article: DOI: 10.1080/08120090802120120 URL: http://dx.doi.org/10.1080/08120090802120120 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Australian Journal of Earth Sciences (2008) 55, (865 – 914) Geochronology of the Australian Cenozoic: a history of tectonic and igneous activity, weathering, erosion, and sedimentation* P. M. VASCONCELOS{, K. M. KNESEL, B. E. COHEN AND J. A. HEIM Earth Sciences, University of Queensland, Qld 4072, Australia. The development and application of geochronological tools suitable for dating Cenozoic rocks and processes have been instrumental to our understanding of the modern history of Australia. Geochronology reveals a dynamic continent that traced a long and rapid trajectory from a position adjacent to Antarctica in the early Cenozoic to its present position near the tropics. The average travel velocity along this path is revealed by the age of hotspot volcanoes, derived by the K–Ar method, and is complemented by measured geomagnetic pole positions on dated igneous rocks and sedimentary deposits. K–Ar dating of volcanic rocks also provided constraints on rates of landscape evolution before and after volcanism and the timing and pattern of dispersion of life—including human inhabitation. K– Ar geochronological results reveal a history of faunal and floral evolution suggestive of a continent undergoing progressive cooling and dehydration with a few brief warm and humid excursions. In contrast, 40Ar/39Ar, SHRIMP U–Pb, fission-track thermochronology, luminescence techniques, and cosmogenic-isotope methods have played relatively minor roles in reconstructing the chronology of Cenozoic volcanism in Australia. Integrated application of these techniques will be critical to providing more precise constraints on the volcanic history of the continent and its climatic and biological evolution. While Cenozoic volcanism, uplift, and denudation were active along the eastern and southeastern margins, a significant part of Australia west of the Tasman Line remained relatively quiescent. The history of this part of the continent is marked by slow and subdued uplift and subsidence, with subtle displacements along major continental structures, and occasional invasion by shallow seas. Despite the general absence of Cenozoic igneous rocks west of the Tasman Line, Australia (east and west) is blanketed by Cenozoic sedimentary covers and weathering profiles. If we consider weathering as a fourth rock-forming process (in addition to igneous, metamorphic and sedimentary), Australia has one of the most complete Cenozoic rock covers of any continent. Retrieving information recorded in these weathering profiles is essential for unravelling its Cenozoic history. Paleomagnetic studies, calibrated d18O curves, and weathering geochronology by K–Ar, 40Ar/39Ar, and (U–Th)/He provide insights into the imprint of climatic events and tectonic processes and illustrate the importance of erosion and weathering to the formation or enrichment of ore and mineral deposits. Except for diamondiferous lamproites of Western Australia and sapphire-rich volcanic rocks in eastern Australia, all other Cenozoic ore and mineral deposits in Australia are related to weathering and erosion. The widespread weathered blanket in Australia suggests low Cenozoic erosion rates. Numerical constraints on chronology and erosion rates are derived from the cooling and denudation histories retrieved from apatite and zircon fission-track and, more recently, (U–Th–Sm)/He thermochronology and cosmogenic Downloaded By: [University of Queensland] At: 05:12 12 September 2008 isotope studies. Geochronological studies of veneers of sediments, lake and cave deposits, marine carbonates, organic matter and groundwaters provide information on sediment provenance, subtle tectonic movements, and the Australian Cenozoic climatic history. These studies reveal a continent sensitive to global climatic cycles and subject to active, but subtle, tectonism and erosion. This record shows that Australia suffered periods of extreme aridity during cyclical glaciation at high latitudes and precise dating of carbonate sediments and speleothems reveals the exact timing and duration of these glacial and interglacial periods. Cosmogenic isotopes also provide constraints on the age and migration paths of Australia’s limited and finite groundwater resources. Lastly, age information extracted from surficial deposits reveals a protracted history of human occupation. KEY WORDS: Australia, geochronology, magmatism, weathering, erosion, sedimentation, climate. *Appendices 1 and 2 [indicated by an asterisk (*) in the text and listed at the end of the paper] are Supplementary Papers; copies may be obtained from the Geological Society of Australia’s website (5http://www.gsa.org.au4) or from the National Library of Australia’s Pandora archive (5http://nla.gov.au/nla.arc-251944). {Corresponding author: [email protected] ISSN 0812-0099 print/ISSN 1440-0952 online Ó 2008 Geological Society of Australia DOI: 10.1080/08120090802120120 866 P. M. Vasconcelos et al. INTRODUCTION trajectory from high-latitude zones (70–358S) in the Cretaceous to its present position at mid to low latitudes The interplay between endogenous tectonic and igneous (43–108S) (Figure 1). Geochronological techniques rele- activity, and exogenous weathering, erosion, deposi- vant to the study of the Australian Cenozoic are, tional, and diagenetic processes has shaped the Austra- therefore, those techniques suitable for dating igneous lian landscape. In turn, the superimposed effects of activity; uplift and fault movement; and weathering, these processes through time provide the record erosion, depositional, and diagenetic processes. Some of that permits tracing Australia’s geological, tectonic, these techniques (e.g. K–Ar, Rb–Sr, U–Pb) are based on geomorphological and climatic histories during its absolute clocks (i.e. the radioactive decay of a parent Downloaded By: [University of Queensland] At: 05:12 12 September 2008 Figure 1 Cenozoic plate recon- structions for Australia and Ant- arctica (after Duncan & McDougall 1989a), showing the position of Australia at high lati- tudes for much of the Early Cen- ozoic, before fast seafloor spreading between the two conti- nents, commencing around 55 Ma, led to rapid migration of Australia into tropical and subtropical latitudes. Geochronology of the Australian Cenozoic 867 isotope into a daughter product), while other techniques namic processes and constraints on climatic events that are based on relative methods that require external shaped the Australian continent in the more recent past calibration [e.g. paleomagnetic dating, thermolumines- (Wellman & McDougall 1974a; Johnson 1989). Perhaps cence (TL), optical emission spectroscopy (OES)]. for these reasons, volcanic rocks are the most intensely Australian geoscientists have played a leading role in dated Cenozoic features in Australia, with nearly 1300 the development, refinement and application of geochron- geochronological analyses undertaken over the last 40 ological tools suitable for unravelling Australian and years (Appendix 1*). Most of the geochronological global Cenozoic histories. These include early improve- record is derived from K–Ar and, to a lesser extent, ments in K–Ar dating (McDougall 1961, 1966); the seminal 40Ar/39Ar and fission-track dating; Rb–Sr, U–Pb, cosmo- development of sensitive high-resolution ion microprobe genic isotopes, 14C and TL have been only sporadically (SHRIMP) U–Pb methodologies (Compston et al. 1984; employed in dating Australian Cenozoic volcanic units. Ireland et al. 2008); methodological advances in 40Ar/39Ar dating (McDougall 1974; McDougall