Chris Powell memorial volume

Tectonophysics 375 (2003) 1 – 4 www.elsevier.com/locate/tecto

Preface

Chris Powell memorial volume During his long scientific career Chris McAulay Powell (1943 – 2001) made extensive contributions to the study of orogenic belts and tectonics on regional and global scales. The wide variety of topics and localities presented in this volume is a testament to his insatiable curiosity. The papers presented here are divided broadly into four sections to reflect different aspects of Chris’s work. As many of his colleagues will attest, Chris was no stranger to the cut and thrust of scientific debate and he relished the opportunity to engage in intense discussions about the latest interpretations and the overthrow of long accepted dogma. Fittingly, some of these papers presented here will challenge currently accepted interpretations and models. Some potentially may be quite controversial and will trigger extensive debate. That debate will be fitting tribute to Chris and his passion for science.

1. Orogeny and climate The first section contains three papers that embrace projects that had sparked Chris’s interest both early and late in his career. He was widely regarded as the expert on the Tasman orogenic zone of eastern Australia. Thus, Powell, Baillie and Vandenberg open the volume with a discussion of the development of the Melbourne Zone of the infamously complex and controversial Lachlan Fold Belt in southeastern Australia. The Melbourne Zone was previously known as the Melbourne Trough and thought to be a depositional entity confined between local greenstone belts. Through the detailed application of paleocurrent and sandstone modal composition data, they demonstrate that the sedimentary rocks of the Melbourne Zone 0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-1951(03)00329-9

from the Early Ordovician to early Devonian were in fact part of a larger continental margin open to the north and east. Keep continues the Lachlan theme with a description of physical analog modelling of the Tasman orogenic zone. She notes that a remarkable feature of the Lachlan orogenic belt is that structures are dominated by upright folds and high-angle reverse faults that verge eastward—toward the inferred centre of orogenic activity. This is contrary to other orogenic belts where vergence is toward the stable craton. These anomalous structures are duplicated with a physical model and are thought to be a result of varying strength gradients between oceanic and continental crustal blocks involved in the collision. The third paper of this section is by Zheng, Powell, Butcher and Cao and builds on an issue that had recently captivated Chris: using the terrestrial sedimentary deposits of central Asia to chart the tectonic evolution and environmental results of Himalayan mountain-building. The Taklimakan Desert is a probable source of the dust deposited in the Chinese Loess Plateau, thus understanding the evolution of this desert—especially the onset of aridity—has far reaching implications for understanding the recent climatic evolution of the Earth. Zheng et al. report a sedimentological investigation of the Tarim Basin that documents a change from distal fluvial to proximal debris flow facies interpreted as evidence for the uplift of the Tibetan Plateau. They also report the intercalation of siltstone beds with indications of an aeolian origin in the succession, suggesting that desertification of the Taklimakan—and by implication the modern climatic regime—may have already been established by the Early Pliocene.

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2. Regional geology and Rodinia The importance of detailed field geology and regional studies to underpin larger studies in global tectonics based on the use of techniques such as isotope geochronology and paleomagnetism was one of the hallmarks of Chris putting together the big picture. The second and third sections contain a series of papers that illustrate the style of projects that Chris considered vital in integrating a range of geological and analytical data to help piece together a global picture. Burchfiel, Nakov and Tzankov begin section two on regional geology with a controversial structural examination of the Mesta half-graben in southwestern Bulgaria. For the first time, they present evidence for low-angle extensional faulting in Paleogene time in southwestern Bulgaria. They further suggest that other nearby half-grabens with similar structures could indicate a broader region of extension lying above a series of west-dipping detachment faults that matches extensional features of a similar age in the Aegean region. As part of his wide-ranging studies in global tectonics, Chris became interested in the tectonic evolution of east Africa and Madagascar. One of his aims was for his group to look for a possible suture zone in Madagascar that represented the merging of East and West Gondwana to form the Gondwana supercontinent in the Early Cambrian. Chris was an enthusiastic participant in several expeditions to remote parts of Madagascar in the initial stages of these studies. Collins, Kro¨ner, Fitzsimons and Razakamanana present a study based on detrital zircon geochronology in an attempt to identify possible oceanic suture zones. The zircon age distributions from metasedimentary rocks in eastern Madagascar indicate potential derivation from the Dharwar craton of southern India. Likewise, the Tanzanian craton of East Africa is dismissed as a potential source. In contrast, previous similar work further to the west in Madagascar suggested an East African derivation. These results indicate the presence of two provenance fronts within eastern and central Madagascar and support the interpretation of the Betsimisaraka suture zone as a remnant of the former Mozambique Ocean. Reddy, Collins and Mruma provide a detailed spatial, kinematic and geochronological study of the

Paleoproterozoic Usagaran orogenic belt abutting the Tanzania craton. This belt contains the Isimani Suite which includes MORB derived eclogite-facies rocks that indicate formation in a subduction system around 2000 Ma—the oldest reported examples of subduction-related eclogite facies rocks. As such, these rocks provide a valuable insight into the evolution of tectonic processes from a hotter early Earth. Through detailed structural and geochronological analysis, Reddy et al. provide valuable constraints on the structural evolution of the Isimani Suite. Their observations are contrary to existing tectonic models for the region and they develop alternatives. Johnson, Cutten, Muhongo and De Waele present a third East African-themed paper with a geochronological and petrological study of the Western Granulites of the Mozambique Belt in central Tanzania. The results build on other recent work that the Eastern and Western Granulites formed in two unrelated tectonic events as evidenced by different geochronological and pressure/temperature histories. While the Eastern Granulites have predominantly Neoproterozoic geochronological affinity, in the Western Granulties Johnson, Cutten, Muhango and De Waele demonstrate xenocrystic evidence of f 3000 Ma basement, protolith emplacement f 2700 Ma, and metamorphic rim growth f 2600 Ma, suggesting that the central domain of the belt may represent a terrane unrelated to the Tanzanian craton. Chris was at the heart of the debate regarding the hypothesised late Proterozoic supercontinent Rodinia through his leadership of the Tectonics Special Research Centre (funded by the Australian Research Council and hosted at University of Western Australia, Curtin University of Technology and University of Texas at Austin). The next couple of papers reflect this interest and the importance of regional geology in piecing together the larger supercontinental picture. Moving from Africa to the Siberian craton, Vernikovsky, Vernikovskaya, Kotov, Sal’nikova and Kovach present a geochemical and geochronological study of the Yenisey Ridge belt on the western margin of that craton and the implications for correlation with other complexes marginal to the craton. Siberia is believed to have been a constituent part of Rodinia and the three Neoproterozoic events Vernikovsky et al. document may be related to the tectonic evolution of this supercontinent.

Preface

Furthering the Rodinian context, Loewy, Connelly, Dalziel and Gower complete the regional geology section with a whole rock Pb and U –Pb geochronology study that tests—and rejects—a previously proposed correlation between northeastern Laurentia and Amazonia. Instead they propose a correlation of Amazonia with the central and southern Appalachians and a match between the Arequipa-Antofalla Basement (a Proterozoic block along the protoAndean margin of Amazonia) and the Kalahari craton. Along with a previous correlation of southeastern Laurentia with the Kalahari they propose that Amazonia collided with this combined margin at around 1000 Ma.

3. Paleomagnetism Chris recognised that paleomagnetism is a crucial element in reconstructing ancient supercontinents and worked hard at integrating such data with regional geology. The papers of the third section focus on such applications of paleomagnetism. Weil, Geissman, Heizler and Van der Voo present paleomagnetic and geochronological data from the Unkar Group Cardenas Basalt and Nankoweap Formation of the Grand Canyon Supergroup to further improve the Laurentian Proterozoic paleomagnetic database. The results for the Cardenas Basalt is consistent with similar f 1100 Ma Laurentian paleomagnetic poles suggesting the occurrence of a largescale magmatic event. The results for the Nankoweap Formation indicate a f 200 Ma age difference providing evidence for a third major unconformity in the Grand Canyon succession. Pisarevsky and Natapov revise the hypothesised Mesoproterozoic connections between Siberia and Laurentia. On the basis of recent geochronological and paleomagnetic data they reject some previous models and present a new reconstruction at f 1050– 1000 Ma. While the similarity of Apparent Polar Wander Paths from both cratons indicate that both Siberia and Laurentia could have been part of Rodinia, Pisarevsky and Natapov argue that they were not directly contiguous and an unknown continental block lay in between. Finally in this section, Jones, Bates, Li, Corner and Hodgkinson present new paleomagnetic data from the

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Borgmassivet intrusions in the Ahlmannryggen region of Dronning Maud Land in Antarctica. The results coincide well with well-established data from the Umkondo Large Igneous Province and further supports other geological evidence that the Grunehogna craton was part of the Kalahari craton of southern Africa around 1100 Ma.

4. Synthesis Chris had the most notable ability to absorb and synthesise an enormous variety of geological data, develop a hypothesis with a global perspective and then recognise the key details and crucial absences within that hypothesis. This ability will be sorely missed as the debate about Rodinia continues. This section contains a series of review and hypothesis papers that honour this talent for synthesis and speculation. Meert and Torsvik begin with a review of the current status of paleomagnetic reconstructions in the critical 500– 1100 Ma interval and discuss challenges to the original concepts of Rodinia such as the SWEAT and AUSWUS models. New data require an entirely different fit of eastern and western Gondwana within the geological framework of one supercontinent breaking up (evidenced by Neoproterozoic – early Paleozoic rift-margins surrounding Laurentia) and another forming (evidenced by similar-aged collisional belts within Gondwana). Meert and Torsvik acknowledge the difficulties in interpreting often enigmatic geological data in developing continental reconstruction models and conclude that although there is evidence for a Rodinia supercontinent, only loose constraints can be placed upon reconstructions. Pesonen et al. look further back in time to examine the paleomagnetic evidence for Rodinia and preRodinia supercontinents in the 1000– 2450 Ma interval. The oldest is the Neoarchean Kenorland consisting of the Laurentia, Baltica, Australia and the Kalahari cratons. The second supercontinent is Hudsonland/Columbia and is interpreted on paleomagnetic data to have consisted of Laurentia, Baltica, Ukraine, Amazonia and Australia and perhaps also Siberia, North China and Kalahari from 1830 to 1500– 1250 Ma. The youngest is the Rodinia super-

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continent formed by a series of continental collisions involving most continents around f 1100– 1000 Ma. Pesonen et al. also note the amalgamation styles of each of the three supercontinents is different, possibly reflecting changes in the tectonic processes through time as mantle thermal conditions and cratonic size and thickness evolved. However, paleomagnetic evidence alone does not make a supercontinent and the next paper demonstrates the complex difficulties in matching paleomagnetic models with field and geochronological data. Kro¨ner and Cordani provide an extensive review of late Mesoproterozoic and early Neoproterozoic orogenic belts across South America, eastern, central and southern Africa, Madagascar, south India and Sri Lanka. In East Africa and Madagascar they demonstrate that there is little evidence for significant production of continental crust between f 1400 and 1100 Ma. Neither is there evidence for a f 1000 Ma high-grade metamorphic event in the Mozambique belt or the Sunsas mobile belt that has previously been correlated with the North American Grenville belt. All Mesoproterozoic terrains appear to have formed within extensional structures and any observed compressional deformation is related to the younger Gondwana amalgamation events. Subduction-related active margin processes during Gondwana amalgamation is also cited as the cause of extensive calc-alkaline granitoid magmatism between 840 and 600 Ma. Kro¨ner and Cordani conclude that the location of these Neoproterozoic magmatic arcs indicates that a large ocean domain separated the Rodinian core (Laurentia, Amazonia, Baltica and West Africa) from continental masses in the southern hemisphere such as the Sa˜o Francisco-Congo, Kalahari and Parana´ blocks.

Running alongside the Rodinia debate is an intense controversy regarding the discovery that Neoproterozoic glaciogenic material was deposited at low paleolatitudes—a theory popularly known as ‘‘Snowball Earth’’ that has enormous implications for our understanding of the evolution of the Earth’s climate and biosphere. In the final paper of the volume, Evans provides a detailed compilation of the global distribution of glaciogenic deposits throughout geologic history and develops an intriguing argument that there was a fundamental change in the style of glaciation on the Earth coincident with the Neoproterozoic – Cambrian transition. The editors would like to thank all the reviewers who contributed their time and talents to improving ˚ ha¨ll, K. Ansdell, G. Axen, T. this special issue: K. A Blenkinsop, P. Bowden, J. Bowler, B. Butler, R. Caby, P. Cawood, B. Collins, R. Cox, J. Crowell, M. de Wit, E. Derbyshire, D. Dinter, J. Dodson, R. Enkin, S. Harlan, C. Hauzenberger, M. Idnurm, A. Khramov, C. Klootwijk, C. MacNiocaill, J. Miller, B. Miller, A. Mo¨ller, J. Murphy, D. Papanikolaou, J. Parrish, S. Pisarevsky, V. Ramos, T. Rivers, R. Stern, R. Tosdal, R. Van der Voo, G. Young. Keith N. Sircombe * Michael W. McElhinny Tectonics Special Research Centre, School of Earth and Geographical Sciences, University of Western Australia, M004, 35 Stirling Highway, Crawley, WA 6009, Australia E-mail address: [email protected]

* Corresponding author. Tel.: +61-8-9380-7871; fax: +61-89380-7848.