Chris Powell memorial volume
Tectonophysics (2003) v. 375, p. 1 – 4
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 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.
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, Kröner, 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 ~3000 Ma basement, protolith emplacement ~2700
Ma, and metamorphic rim growth ~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.
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 proto- Andean 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 ~1100 Ma Laurentian paleomagnetic poles suggesting the occurrence
of a largescale magmatic event. The results for the Nankoweap Formation indicate a ~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 ~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 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 pre-Rodinia 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 supercontinent formed by a series of continental
collisions involving most continents around ~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
f1400 and 1100 Ma. Neither is there evidence for a ~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. Kröner 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 São
Francisco-Congo, Kalahari and Paraná 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.
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