Mid-Cretaceous to Recent Plate Boundary Processes in the Southwest Pacific
Suzanne L. Baldwin, Geosciences Department, University of Arizona, Tucson, AZ 85721
Gordon S. Lister, Australian Crustal Research Center, Monash University, Melbourne 3168 Victoria, Australia
Since the advent of plate tectonics the southwest Pacific has provided a natural laboratory for the study of plate boundary processes. Breakthroughs in our understanding of plate boundary processes continue to be made in this region, including those resulting from recent technological developments (e.g., swath bathymetry, seismic tomography). A Penrose conference, "Mid-Cretaceous to Recent Plate Boundary Processes in the Southwest Pacific," was convened to assess how far we have come in our understanding of the tectonic evolution of the southwest Pacific and to identify and catalyze potential future research directions.
Held at the Wilderness Lodge, Arthur's Pass, South Island, New Zealand, March 2530, 1999, the conference brought together 45 geo-scientists from seven countries, including seven graduate students. Oral, poster, and discussion sessions addressed plate boundary processes such as subduction rollback, backarc basin evolution, initiation of subduction, the role of arc magmatism in crustal growth, transition from continental extension to seafloor spreading, and mountain building processes at a transpressional plate boundary.
Southwest Pacific Models and Reconstructions
The conference began with a summary of key issues with respect to tectonic reconstructions of the southwest Pacific stretching from Papua New Guinea and parts of Southeast Asia in the north, to Antarctica in the south, and east to the Tonga-Kermadec-New Zealand plate boundary. Rupert Sutherland provided an overview of the many outstanding problems throughout the region, emphasizing that the first-order geometry of the 13085 Ma plate boundaries is not well understood. Other questions concern the development of Cenozoic subduction zones and backarc basins north of New Zealand and the amount of plate motion taken up between East and West Antarctica during Gondwana break-up.
Robert Hall summarized Cenozoic plate reconstructions of Southeast Asia and the southwest Pacific, outlining three important periods of regional plate boundary reorganization, at 45, 25, and 5 Ma, which can be related to changes in plate motion. He presented evidence for major regional vertical axis plate rotations and stressed the importance of extension in a largely convergent setting during the regional tectonic development. Most subduction hinges have been retreating since 25 Ma (e.g., Tonga, New Hebrides, Izu-Bonin), accompanied by arc volcanism and marginal basin formation, in contrast to periods of hinge advance, which are accompanied by a reduction or cessation of volcanism (e.g., Kermadec, Mariana). Southeast Asia presents special reconstruction problems because so much oceanic lithosphere has been subducted, most of the marginal basins lack well-developed magnetic lineations, and there is evidence for major regional vertical axis rotations. Roll-back of the hinge of the retreating Indian-Australian lithosphere in the Banda arc may have led to the unusual 180° curvature of the arc.
Loren Kroenke discussed how major changes in Pacific absolute plate motions may have triggered tectonic events along the boundary such as the formation of lengthy new subduction zones (e.g., Melanesian arc at 43 Ma) and onset of rifting between Australia and Antarctica at ~95 Ma. He showed how the hot spot reference framework could be used to produce an absolute frame of reference, and then analyzed the polar wander path of the Pacific plate. While some tectonic events result from changes in Pacific absolute plate motion, the late Neogene collision of the Ontong Java Plateau with the northern margin of the Australian plate was used as an example of an event that caused a change in the Pacific plate motion. Dallas Abbott and others provided other examples of plateau and seamount interaction in the Tonga and Izu-Bonin trenches.
Recent work in the Ross Sea, South Tasman Sea, and along the West Antarctic margin now permits more accurate Cenozoic reconstructions of Australia-Pacific-Antarctica plate motion. Joann Stock focused on the seafloor spreading record around the New Zealand region used to constrain major plate boundary changes and reorganizations during and subsequent to Gondwana breakup. Subduction of the Phoenix-Pacific ridge beneath the Antarctic plate, which led to attachment of the Chatham Rise and Campbell Plateau to the Pacific plate, was compared to the breakup of the Farallon Plate and its effects on western North America. Posters with this session focused on specific aspects of plate motion associated with Australia-Pacific-Antarctica plate boundaries.
Computer animations presented by Robert Hall, Loren Kroenke, and Carmen Gaina illustrated how rapidly southwest Pacific plate boundaries change, identified regional events, and provided a frame-work for discussion of outstanding problems. Laurent Ailleres presented a strategy for an ongoing project to develop a four-dimensional (three-dimensional through time) geodynamic model of the northern part of the Australia-Papua New Guinea region.
Crustal Growth, Breakup, and Dispersal
Richard Arculus provided an overview of the role of magmatism in crustal growth and disruption in the southwest Pacific. Contributions from supra-subduction zone environments and plumes, as well as the importance of accretion and obduction of oceanic plateaus on changes in plate boundary geometries and orientations were out-lined. A better understanding of mass fluxes through supra-subduction zones is needed. Distinctions in isotopic (Sr-Nd-Pb) compositions for the asthenospheric source of Pacific and Indian mid-ocean ridge basalts were summarized, and questions concerning discontinuities within the region were raised (e.g., in Vanuatu an Indian-type source is tapped in arc-backarc magmas only where the D'Entrecasteaux ridge-fracture zone is subducted).
Brian Taylor spoke on the mechanics of lithospheric extension during the transition from continental rifting to seafloor spreading in the Woodlark Basin. After ~200 km of continental extension is accommodated along low- and high-angle normal faults, seafloor spreading begins. Considerable along-strike variation in seafloor spreading initiation occurs from east to west; stepwise spreading nucleation begins within rheologically weak zones of continental lithosphere.
Posters presented thermochronologic data from the Papua New Guinea mobile belt that bear on the Neogene evolution of arc-continent collision, geomorphologic evidence for landslide-driven drainage network evolution during rapid surface uplift to form the Finisterre Mountains, and petrologic data from the Moresby Seamount in the western Woodlark Basin. Additional poster presentations provided an overview of ideas concerning the nature and timing of plate boundary processes in Papua New Guinea, and cautioned against the use of interpreting paleotectonics on the basis of clastic sedimentary compositions.
Phil Symonds and others presented results of a deep-seismic transect from the eastern Australian continental margin to the New Hebrides arc which imaged the major tectonic provinces of the south-west Pacific, including zones of extended continental crust, narrow ocean basins, volcanic arcs, and subduction zones. The transition from rifting and breakup to convergence in the Lord Howe Rise-Nor-folk region was the topic of another poster by Symonds et al.
Russell Korsch outlined the early Mesozoic continental sedimentary record from the Australian continental record (eastern margin of Gondwana), which set the stage for subsequent events. From Neoproterozoic to 95 Ma, a convergent margin existed along the eastern margin of Australia, but when did subduction cease on the Australian convergent margin, and did the youngest New Zealand Torlesse sediments form at the Australian convergent margin? Ian Duddy discussed mid-Cretaceous-late Miocene tectonothermal events related to uplift and erosion in southern and eastern Australia.
The tectonic evolution of New Caledonia was the focus of several talks (Sebatien Meffre, Christian Picard), posters (Baldwin et al., Cluzel et al., Rawling and Lister), and considerable discussion, as the different data sets for this small sliver of the Australian(?) continental crust were integrated. The tectonic evolution that began when Late Carboniferous to Jurassic basement terranes of New Caledonia originating in an arc-forearc setting were rifted away from Gondwana in Cretaceous time. In latest Eocene time, this continental sliver was overthrust first by an allochthonous basaltic nappe (3638 Ma) and subsequently by an ultramafic nappe (by 34 Ma). Obduction tempo-rally coincided with rapid exhumation of the high P-T terrane of northern New Caledonia (4034 Ma), which led Baldwin and others to propose that the high P-T terrane was not juxtaposed against the pre-Eocene terranes of New Caledonia until post-Oligocene time. The significance of repeated cycles of compressional and extensional deformation as it relates to exhumation of the New Caledonia high P-T terrane and high-pressure terranes in general was also discussed, and it was postulated that additional Gondwana fragments should exist east of New Caledonia.
Subduction Rollback and Mantle Convection
Lidia Lonergan described a subduction rollback model to explain the tectonic evolution of the Alboran Sea and Betic-Rif orocline. Although aspects of this model remain to be applied to areas of the southwest Pacific, tomographic and deep-focus earthquake studies may identify the orientation of tears in regions of rapid rollback, while paleomagnetic studies are required to gain insight into the degree of block rotations and tightly arcuate geometries.
Louis Moresi presented results of modeling used to examine the manner in which Earth's lithosphere is mobilized and subducted. The lithosphere was modeled as the cool thermal boundary layer of a convecting fluid with a strongly temperature dependent viscosity. Material properties of the fluid are strongly dependent on composition and strain history. Strain softening yield criteria are used to examine the conditions under which deformation is localized within the lithosphere.
Mountain-Building Processes at a Transpressional Plate Boundary
Kevin Furlong presented geophysical data which suggest that the lithospheric structure beneath Fiordland is significantly different from that beneath the Southern Alps, despite similarities in present-day plate motion vectors. This difference may result from differences in the rheology of the Australian and Pacific lithosphere and plate boundary geometry prior to the onset of transpression. An enigmatic zone of deep earthquakes beneath Fiordland may mark a possible tear in the subducting slab.
John Beavan summarized the present-day motion between the Australia and Pacific plates using GPS data to show that the present-day relative velocity is ~9.7 mm/yr and has been stable for the past 3 m.y. Fred Davey presented seismic profiles across the Pacific-Antarctica plate boundary in New Zealand which image the crustal structure of the westward-subducting Pacific plate in the North Island, the continental collisional orogen in the central South Island, the transition zones in between, and oblique northeastward subduction of Tasman Sea crust beneath Fiordland.
The final poster session of the conference presented results of ongoing studies of the Australia-Pacific plate boundary in New Zealand, including geophysical, structural, and thermochronologic constraints on the evolution of the Southern Alps, geomorphologic constraints on the evolution of the Wairarapa fold-and-thrust belt of the North Island, and structural controls on hydrothermal fluid flow in the Taupo Volcanic Zone. Recent structural studies document the kinematics of distributed ductile deformation at mid-crustal levels (Little et al.) and brittle-ductile shear band structures at higher structural levels within the inferred late Cenozoic brittle-ductile transition zone (Ilg and Little), which together suggest that a significant part of obliquely convergent Pacific-Australia plate boundary deformation in the Southern Alps is accommodated east of the Alpine fault.
New Zealand's pre-Alpine history was the focus of several poster presentations, which presented new thermochronologic and structural data that bear on the timing of shear zone development in central Otago (Forster and Lister), and on Late Cretaceous core complex formation related to opening of the Tasman Sea (Dunlap et al). Other posters examined the response of the lower crust to changes in plate boundary conditions, batholith emplacement in Fiordland, a comparison of the Mesozoic Kula-North America plate boundary evolution with that of the Cenozoic Alpine fault system, and attempts to track the Late Carboniferous to Neogene evolution of New Zealand plate boundaries.
At least 75% of the Australia-Pacific plate boundary motion is accommodated along the narrow high-strain zone associated with the Alpine fault, the rest is distributed across a 150200-km-wide zone east of the Alpine fault; Jarg Pettinga and John Bradshaw led two field trips to examine aspects of the plate boundary. During the first trip, participants viewed the remarkably uniform sandstones and mud-stones of the Torlesse terrane and their schistose equivalents, working westward to the edge of the Pacific plate at the Alpine fault. Other subjects were structures on the Australian plate related to crustal extension and separation of the New Zealand microcontinent from Gondwana, including an inverted rift basin (Paparoa anticline) and the lower plate mylonites and upper plate breccias of the Cretaceous Paparoa core complex. On the second field trip, participants examined the effects of recent deformation (young folds and developing shear zones and evidence for neotectonic uplift) related to movement on the Australia-Pacific plate boundary.
Toward the end of the conference, the group, spearheaded by Barry Drummond, began construction of a space-time plot of the southwest Pacific. Suggestions regarding how to improve this database were made and Russell Korsch outlined a database system that could be used for this purpose. Plans were made for the group to reconvene in 2001 in New Caledonia. Those interested in receiving information on the second southwest Pacific plate boundary conference should contact Christian Picard. For more information, including the abstract volume, please see our Web site for this Penrose Conference.
We appreciate support for the conference from the Geological Society of America, the National Science Foundation (Earth Sciences program in Tectonics and Ocean Sciences program in Marine Geology and Geophysics), and the Specialist Group in Tectonics and Structural Geology of the Geological Society of Australia. We thank meeting coordinators Lois Elms (Western Experience, Inc) and Megan Hough (Australian Crustal Research Centre) and the management and staff of the Wilderness Lodge for their help. John Bradshaw and Jarg Pettinga led the group on two superb field trips and provided an accompanying field guide. This report benefited from reviews by Paul Fitzgerald and Brian Monteleone. We take responsibility for any misperceptions. Finally, we are most grateful to the participants for providing stimulating discussions before, during, and after the conference.
Joann M. Stock