Continental Geology Research Articles

Diverse Styles of Lithospheric Dripping: Synthesizing Gravitational Instability Models, Continental Tectonics, and Geologic Observations

AbstractDensity instabilities in the lithosphere can founder gravitationally via viscous dripping and decoupling from overlying crust. The lithospheric dripping concept has been invoked across the globe, but the diversity of crustal effects, observable evidence, and tectonic settings involved in dripping remain underexplored. Here, we synthesize numerical and analogue modeling studies and geologic data from the literature, including all proposed lithospheric dripping events to‐date. We argue that two distinct styles of dripping can occur depending on crustal strength (relative to that of the mantle lithosphere). Near‐surface contraction and subsidence of strong crusts contrasts with near‐surface extension and uplift of weak crusts. We discuss these events in terms of tectonic setting, timing, size, and the main types of data associated with each event. We also find that lithospheric dripping is associated with a distinct suite of geological observations including sedimentological, structural, volcanic, and geophysical data, which can be used to distinguish strong crusts from weak crusts. We find 27 events for which lithospheric dripping is a key hypothesis, including 9 with clear evidence for strong‐crust dripping and 3 with clear evidence of weak‐crust dripping. We review emerging research methods have the potential to detect the signals of dripping in the geologic and geophysical record, and we suggest additional techniques in light of our strong‐crust versus weak‐crust framework. The diverse tectonic settings and inferred consequences of these lithospheric drips, if confirmed, would demand a shift in our understanding of continental geology to emphasize the role of vertical removal of continental lithosphere.

Dividing the South American continent to fit a Gondwana reconstruction: A model based on continental geology

The South American continental plate has been affected by intraplate deformation since the start of West Gondwana disruption in the Lower Cretaceous (about 140 Ma). That the present shape of South America is not precisely the same as its shape in reassembled Gondwana partly explains the imperfect fits of the conjugate margins of the South Atlantic proposed since the first reconstruction models of the early 20th century. Several attempts at defining subplates within South America have been published but not all take account of existing knowledge of its continental geology. Here a subdivision into eight rigid subplates is proposed, based primarily on geological and tectonic evidence. Our model is tested against three published models of a multi-subplate Africa, as re-shaped to the pre-breakup outline of that continent, by visual fitting and the use of piercing points. The South America blocks were rotated and the Euler poles calculated interactively in reconstruction software. All three proposed fits had overlapping block margins within South America, indicating post-breakup rifting, except for the Transbrasiliano lineament. This NNE-SSW crustal-scale shear zone was used as a boundary for seven of the eight blocks. It is clearly the main intraplate accommodation zone in South America and an important piercing point in relation to the Kandi lineament in West Africa. The other block boundaries correspond to narrow zones with sedimentary basins and/or dyke swarms that have developed since South Atlantic opening. Each fit required a different configuration of the South America subplates since the pre-rift disposition of the African subplates also differ from each other, contributing to the uncertainty in any detailed reassembly.

Full-plate modelling in pre-Jurassic time

AbstractA half-century has passed since the dawning of the plate tectonic revolution, and yet, with rare exception, palaeogeographic models of pre-Jurassic time are still constructed in a way more akin to Wegener's paradigm of continental drift. Historically, this was due to a series of problems – the near-complete absence of in situ oceanic lithosphere older than 200 Ma, a fragmentary history of the latitudinal drift of continents, unconstrained longitudes, unsettled geodynamic concepts and a lack of efficient plate modelling tools – which together precluded the construction of plate tectonic models. But over the course of the last five decades strategies have been developed to overcome these problems, and the first plate model for pre-Jurassic time was presented in 2002. Following on that pioneering work, but with a number of significant improvements (most notably longitude control), we here provide a recipe for the construction of full-plate models (including oceanic lithosphere) for pre-Jurassic time. In brief, our workflow begins with the erection of a traditional (or ‘Wegenerian’) continental rotation model, but then employs basic plate tectonic principles and continental geology to enable reconstruction of former plate boundaries, and thus the resurrection of lost oceanic lithosphere. Full-plate models can yield a range of testable predictions that can be used to critically evaluate them, but also novel information regarding long-term processes that we have few (or no) alternative means of investigating, thus providing exceptionally fertile ground for new exploration and discovery.

Oceanic microplate formation records the onset of India–Eurasia collision

Mapping of seafloor tectonic fabric in the Indian Ocean, using high-resolution satellite-derived vertical gravity gradient data, reveals an extinct Pacific-style oceanic microplate (‘Mammerickx Microplate’) west of the Ninetyeast Ridge. It is one of the first Pacific-style microplates to be mapped outside the Pacific basin, suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern Pacific ridges. The microplate formed at the Indian–Antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south, whose conjugate is located north of the Kerguelen Plateau. Independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric, also seen in multibeam data. Magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o (∼47.3 Ma). Plate reorganizations can trigger ridge propagation and microplate development, and we propose that Mammerickx Microplate formation is linked with the India–Eurasia collision (initial ‘soft’ collision). The collision altered the stress regime at the Indian–Antarctic ridge, leading to a change in segmentation and ridge propagation from an establishing transform. Fast Indian–Antarctic spreading that preceded microplate formation, and Kerguelen Plume activity, may have facilitated ridge propagation via the production of thin and weak lithosphere; however both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event. Prior to the collision, the combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and ‘W’ shaped lineations that record back and forth ridge propagation. Microplate formation provides a precise means of dating the onset of the India–Eurasia collision, and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories.

The Basement Features Research of Junggar Basin, China: A Review

There are different understanding of long-standing and significant differences about the characteristic and age of the basement of Junggar Basin. And the existence of Precambrian crystalline basement and the nature of Paleozoic folded basement of the Junggar basin are under question. The research aims including the basin peripheral orogenic clastic zircon dating and the drilling samples. Those research playing a key role in rebuilding the tectonic framework, reconstructing the geological evolution of northern Xinjiang region; and provides the basis for geological research in the area of objective information, provides an important basis in the evolution of continental geology and dynamics; also available as a scientific theory and support to resource development, it is of great value in the exploration and evaluation of resources within the basin.

On Coral Reefs, Volcanoes, Gods, and Patriotic Geology; Or, James Dwight Dana Assembles the Pacific Basin

While dozens of naturalists had examined discrete Pacific environments prior to the 1830s, the American geologist James Dwight Dana was the first to hypothesize the underlying forces that created and unified this vast ocean basin as a whole. During his four-year journey with the U.S. Exploring Expedition (1838–1842), Dana developed a holistic view of geological systems throughout the Pacific, including those continental lands soon claimed by the United States as its Far West. But Dana's innovative work on Pacific geology and his extra-continental reading of the Far West changed in the 1850s. Like other American explorer-geologists who found cause for reifying a continental geology, Dana's work lost sight of the Pacific Basin and instead focused on the exceptional and spiritually preordained structure of American landforms.

Resurrecting vintage paper seismic records

Marine seismic data has been collected for several decades in both academia and industry. Early single channel acquisitions were often, if at all, recorded on magnetic media that are generally no longer supported for replay. Although seismic data over 10 years old has limited quality compared to modern acquisition, thousands of survey km of hard copy records remains world wide in areas with no recent coverage. In addition, many processed multi-channel datasets are now not available as digital files for a variety of reasons. Rising interest in continental margin geology in response to climate change priorities and the UNCLOS submission requirements of maritime states has created a demand to re-use these older records where the cost of new or re-acquisition is not pragmatic. Through creating digital SEG-Y files from images of these records they can be reprocessed and re-interpreted. This paper investigates the practicalities of paper seismic record re-use and measures the performance of a typical conversion option as an indicator of the validity of the concept. Examples of applications of the process illustrate what can be achieved and the limitations that exist so that users can make an informed choice of whether to resurrect their information.

Interpretation of aeromagnetic anomalies of the Sulu region, eastern China and implications for deep geology

By using data on the 1:100 000 aeromagnetic anomalies of the Sulu orogenic belt, we designed three simulated geotraverses, in which deep seismic reflection and other geophysical investigations have been completed. Based on the features of magnetism of the three profiles, and under the constraints of deep seismic reflection data, together with the magnetism of the core petrology at the Chinese Continental Scientific Drilling (CCSD) pilot-hole and areal geology, the three inversions of magnetic anomalies are carried out. The characteristics of terrane structure are presented: the rocks are mostly composed of eclogite, marble, and gneiss at the depth of 5 km. At the depth between 5 and 7 km under the surface, inverse magnetic bodies are mainly the ultra high pressure metamorphic (UHPM) rock slices containing a lot of coesite-bearing eclogite. At the depth between 7 km and the bottom of upper crust are the rocks of the gneiss, granite and granite diorite that underwent ultra high pressure metamorphic process. Middle crust (10–19 km) is mostly composed of UHPM gneiss and granite that intruded later. The rocks of acid and basic granulite dominate the lower crust. Based on the inversed results of the three simulated geotraverses, we know that the UHPM rock slices of the three profiles are dipping north, stacking each other and being uplifted to the earth’s surface, which may be the result of the North China craton’s subduction and exhumation in the Triassic.

Thoughts on studies of China continental geology and tectonics

Abstract As an important part of the global continents, China continent has long been situated in the peculiar tectonic position and experienced extremely complicated activities, which resulted in the regional unique characteristics for China continent on the global common geological background. These characteristics contain abundant information regarding scientific key issues of modern geological frontier. Thus, China continent can be a natural laboratory and excellent arena for the modern geosciences. The modern earth sciences have started entering the era featured with earth systematic science and beyond plate tectonics. How to take the regional advantage and exploit the treasure resource to participate the new theoretical and methodological creation is a historic opportunity and great challenge we are facing. This paper generalizes research priorities in four fields on China continental geology and tectonics for discussion. They are China continental tectonics and dynamics, Mesozoic-Cenozoic crustal d...

Ongoing compression across South American plate: observations, numerical modelling and some implications for petroleum geology

Abstract The South American plate is now in horizontal compression and shortening. This is shown by stress data compilations, intraplate stress field numerical models and space-based geodetic results. Consistent with the compressive scenario, analyses of leak-off and hydraulic fracturing data indicate that the maximum principal stress is horizontal for most Brazilian basins. The observed compression/shortening is probably mostly due to the convergence of the South American and the Nazca plates and the divergence of the South American and the African plates. Plate-wide deformation related to the Andean tectonics has been evidenced by analyses of integrated visualizations of available plate-scale information on tectonics, continental geology, topography/bathymetry, seismicity, stresses, active deformation, residual isostatic anomalies, fission track analyses, and seismically derived Moho depths and P and S wave velocity anomalies. Here, some results of these analyses are presented and some implications of the ongoing compression for petroleum exploitation and exploration are discussed. A conceptual model for the plate-wide deformation is presented and numerically tested using elasto-plastic rheologies. The model states that in response to the compression, the lithosphere as a whole (or only the crust if thermal gradients are high enough) tends to fold and fracture. This tendency is stronger during peaks of the Andean orogeneses. The forming antiforms are responsible for uplift along the erosional basin borders, whereas the forming synforms are sites of continental sedimentation, at basin centres. The denudation of sedimentary covers promotes the exhumation of deeper and deeper rocks, cropping out at the foot of retreating scarps. Consequently, the erosional borders of the basins form local topography highs with respect to the adjacent basement. In exploration, neotectonics is usually simply disregarded, at least with regard to the exploratory activities developed in ‘passive’ margin basins. Nevertheless, we infer that neotectonics has an important role in the distribution and preservation of petroleum accumulations, since: (i) accumulations are ephemeral in a geological time scale, being strongly dependent on the seals’s fine geometry and biodegredation; (ii) a strong positive correlation exists between permeability anisotropy and maximum horizontal stress; (iii) the source rocks of the most important Subandean and intraplate petroleum systems are still in the oil generation window; and (iv) the disruption of ‘kitchens’ of generation is a positive factor to primary migration. This inference seems to be confirmed, since the most important South American petroleum accumulations are found along the actively deforming border between the South American and Caribbean plates. Indeed, the most important accumulations in marginal basins are found in the southeastern Brazilian margin (the Campos basin), which has been deformed the most with respect to other margins during the Cenozoic, and continues to be the most seismically active.

Ongoing compression across intraplate south America: observations and some implications for petroleum exploitation and exploration

Stress data compilations, intraplate stress field numerical models and space-based geodetic results show that the South American plate is now in horizontal compression and shortening. Plate wide deformation related to the Andean tectonics has been put in evidence by analyses of integrated visualizations of plate-scale information on tectonics, continental geology, topography/bathymetry, seismicity, stresses, active deformation, residual isostatic anomalies, fission track analyses, and seismically derived Moho depths and P and S wave velocity anomalies. Here, a few results of these analyses are presented and some implications of the ongoing compression for petroleum exploitation and exploration are discussed. A conceptual model for the plate-wide deformation is presented. The model states that in response to the compression, the lithosphere as a whole (or only the crust if thermal gradients are high enough) tends to buckle. The resulting antiforms are responsible for uplift along erosional borders of the basins, whereas the resulting synforms are sites of continental sedimentation, at basin centers. The denudation of sedimentary covers promotes the exhumation of increasingly deeper rocks and the adiabatic decompression, facilitating fusion and hence the observed Upper Cretaceous and Tertiary alkali-magmatism. The basement low topography adjacent to the retreating scarps is frequently associated with large Neogene basins (e.g. the Pantanal). The tendency to buckle is controlled by the previous lithospheric/crustal structure. The perception of ongoing compression opens up new perspectives for petroleum exploitation and exploration. The knowledge of the stress field is crucial for secondary recovery, hydraulic fracturing, fracture reservoir characterization and well design. About 82% of the total reserves are found along the actively deforming border between the South American and the Caribbean plates. The most important intraplate accumulations (about 5% of the total reserves) are found in the Southeastern Brazilian margin, which has been deformed the most with respect to other margins during the Cenozoic, and continues to be the most seismically active. These facts suggest that the actual role of the active tectonics on the petroleum accumulation patterns may have been underestimated.

Cenozoic Tectonism in the Central Basin and Range: Motion of the Sierran-Great Valley Block

According to geologic reconstructions, the motion of the Sierran-Great Valley block with respect to the Colorado Plateau was mainly westerly at more than 20 mm/yr from 16 to 10 Ma, changing to northwest or NNW since 8 to 10 Ma, at an average rate of 15 mm/yr. These kinematics are consistent with two other independent methods of determining the position of the block since 20 Ma–reconstructions based on paleomagnetic data from range blocks that bound the Basin and Range on the west, and a revised history of Pacific-North America plate motion based on a global plate circuit (Atwater and Stock, 1998, this issue). The plate-tectonic reconstruction shows a change to more northerly motion between the Pacific and North American plates at ∼8 Ma, in concert with the motion of the Sierran-Great Valley block. Moreover, the northeast limit of extant oceanic crust (as indicated by the reconstruction of the continental geology) tracks closely with the southwest limit of extant continental crust (as indicated by the positions of oceanic plates) since 20 Ma. The coordination between plate motions and the intraplate geology suggests that plate-boundary forces strongly influenced deformation within the continent.

Evolution of the Mariana Convergent Plate Margin System

The Mariana convergent plate margin system of the western Pacific provides opportunities for studying the tectonic and geochemical processes of intraoceanic plate subduction without the added complexities of continental geology. The system's relative geologic simplicity and the well‐exposed sections of lithosphere in each of its tectonic provinces permit in situ examination of processes critical to understanding subduction tectonics. Its general history provides analogs to ancient convergent margin terranes exposed on land and helps to explain the chemical mass balance in convergent plate margins. The Mariana convergent margin's long history of sequential formation of volcanic arcs and extensional back arc basins has created a series of volcanic arcs at the eastern edge of the Philippine Sea plate. The trenchward edge of the overriding plate has a relatively sparse sediment cover. Rocks outcropping on the trench's inner slope are typical of the early formed suprasubduction zone's lithosphere and have been subjected to various processes related to its tectonic history. Pervasive forearc faulting has exposed crust and upper mantle lithosphere. Many large serpentinized peridotite seamounts are within 100 km of the trench axis. From these we can learn the history of regional metamorphism and observe and sample active venting of slab fluids. Ocean drilling recovered suprasubduction zone lava sequences erupted since the Eocene that suggest that the forearc region remains volcanologically dynamic. Seismic studies and seafloor mapping show evidence of deformation throughout forearc evolution. Large portions of uplifted southern forearc are exposed at the larger islands. Active volcanoes at the base of the eastern boundary fault of the Mariana Trough vary in size and composition along strike and record regional differences in source composition. Their locations along strike of the arc are controlled in part by cross‐arc structures that also facilitate formation of submarine volcano chains extending from the base of the fault westward into the back arc basin. The western boundary is the West Mariana Ridge, the western portion of the volcanic arc active prior to formation of the Mariana Trough. The trough evolved in a two‐stage extension process of rifting and subsequent seafloor spreading. The back arc basin varies along strike from rifted arc lithosphere with scattered volcanoes but no real spreading center in the north to a complex mid‐ocean‐type spreading center south of 20°N. The change from initial rifting to true seafloor spreading is also evident across the Mariana Trough from rifted topography near the West Mariana Ridge to spreading ridges in the central to eastern basin south of 20°N. This morphologic change indicates an early stage of extension with basin‐and‐range‐type topography predominant and volcanism restricted to fissure eruptions along fault block boundaries. The spreading ridges and abyssal hill morphology evolved later as new lithosphere was generated at elongate volcanic ridges located in the center of rift valleys. The center of extension intersects the active volcanic front differently at either end of the Mariana Trough. In the north, extension is by rifting of arc lithosphere where it intersects the arc. In the south a major strike‐slip fault extends from the trench axis across the forearc, through the volcanic arc, and into the back arc basin. Arc magmas apparently leak along this fault zone into the forearc and the back arc spreading center. The complexity of interrelated tectonism and magmatism in this convergent margin is daunting, but studies of arc systems such as this provide the best hope of interpreting many of the exposed terranes accreted to continents. Comparison of subaerial terranes with recent studies of intraoceanic convergent margins will add to our understanding of plate interactions and of the evolution of the volcanic arcs and extensional back arc basins generated within such environments.

Continental dynamics research goals set

The need for long‐range scientific research goals in the field of continental dynamics was the impetus behind a report released by DOSECC (Drilling, Observation, and Sampling of the Earth's Continental Crust) Incorporated and the IRIS (Incorporated Research Institutions for Seismology) Consortium last fall.The idea for “A National Program for Research in Continental Dynamics” originated in the Workshop on Future Research Direction in Continental Dynamics held in Chandler, Ariz., March 4–7, 1989. The workshop was sponsored by the National Science Foundation, the U.S. Geological Survey, and the Department of Energy. The report is based on discussions by ten different panels at the workshop, who emphasized a focus on basic research in continental geology with attention to acquiring field data.

Archean plate tectonics: what can be learned from continental geology?

Some basic questions about Archean plate tectonics can be addressed by examining accretionary Archean margins, in particular fault zones with significant strike-slip components on the Canadian Shield. (1) Were the oceanic plates typically rigid like modern plates? Yes. Significant lateral viscosity contrasts in the lithosphere between plates and plate boundaries are required for major strike-slip faults to exist. Conversely, strike-slip faults are a kinematic consequence of rigid plates. (2) Did large oceanic plates exist in the Archean? Probably. First, the length and offset of the longest preserved segments of Archean faults are similar to modern examples such as in Alaska. Less directly, the duration of a period with a consistent sense of strike slip at a point on the continental side of an accretionary margin should be related to the time that a typical oceanic plate remains outboard of the margin. This time varies proportionally with size of typical ocean plates and inversely with their velocity. The duration of an example of persistent strike slip on the Canadian Shield is comparable to that of Cenozoic examples. (3) Did old oceanic crust and hence moderate plate velocities occur in the Archean? Perhaps. Paleomagnetic poles are the most direct line of evidence, but they usually relate to continental blocks. The duration of consistent strike-slip motion, preserved alkalic seamounts which record eruption on old oceanic crust, and the duration of ocean basins are potential indirect indications. Overall, the hotter mantle does not appear to have had a great effect on Archean plate motions. Thus, the geometry and rate of plate tectonics are strongly influenced by the lithosphere.

Mantle Plumes and Continental Tectonics

Mantle plumes and plate tectonics, the result of two distinct modes of convection within the Earth, operate largely independently. Although plumes are secondary in terms of heat transport, they have probably played an important role in continental geology. A new plume starts with a large spherical head that can cause uplift and flood basalt volcanism, and may be responsible for regional-scale metamorphism or crustal melting and varying amounts of crustal extension. Plume heads are followed by narrow tails that give rise to the familiar hot-spot tracks. The cumulative effect of processes associated with tail volcanism may also significantly affect continental crust.

Tectonic setting of Western Pacific marginal basins

Global kinematics as well as magnetic anomalies of marginal basins, with continental geology and paleomagnetic data as additional constraints, are used to present a set of reconstructions of the Western Pacific marginal basins between 56 Ma and the Present at key periods (56, 43, 32, 20, 12 and 3 Ma). Our model accounts for the rapid motion of “exotic terranes” along the whole of the Western Pacific convergent zone. Marginal basins appear to open in a great variety of tectonic settings, the two extreme examples being the Mariana trough where trench suction may be the predominant driving force and the South China Sea where intracontinental deformation appears to be the major mechanism. The study of marginal basins is a 3-D problem which must take into account the whole tectonic context (subduction related tectonics in cross-section and upper plate deformation in map view) and not only a 2-D problem (the classical trench-arc-back-arc problem).

Research in exploration geoscience: The AMIRA model

The paper summarises the exploration related R&D that has been managed by AMIRA/APIRA and gives details of the complex, market oriented process used to develop new projects.The process is illustrated by three case studies taken from the more than 70 exploration projects completed.The Gold Deposits of North Queensland Project conducted by James Cook University studied the various styles of gold mineralization. The Rock Magnetism project undertaken at CSIRO Division of Exploration Geoscience allowed both general and specific research themes to be undertaken in parallel with confidential proprietary investigations on sponsors' drill core. The Palaeogeographic Mapping project undertaken at the BMR Division of Continental Geology provided sponsors with palaeogeographic maps including interpretation and structural information of 70 time slices.Some of the benefits of the AMIRA/APIRA approach are discussed as well as some of the difficulties which arise in ensuring companies obtain appropriate benefits from their sponsorship.

Blast induced subsidence in the craters of nuclear tests over coral : Burton, D E; Swift, R P; Glen, H D; Bryan, J B In: Research and Engineering Applications in Rock Masses (paper to the 26th US Symposium on Rock Mechanics, Rapid City, 26–28 June 1985) V2, P999–1006. Publ Rotterdam: A. A. Balkema, 1985

The craters from high-yield nuclear tests at the Pacific Proving Grounds are very broad and shallow in comparison with the bowl-shaped craters formed in continental rock at the Nevada Test Site and elsewhere. Attempts to account for the differences quantitatively have been generally unsatisfactory. We have for the first time successfully modeled the Koa Event, a representative coral-atoll test. On the basis of plausible assumptions about the geology and about the constitutive relations for coral, we have shown that the size and shape of the Koa crater can be accounted for by subsidence and liquefaction phenomena. If future studies confirm these assumptions, it will mean that some scaling formulas based on data from the Pacific will have to be revised to avoid overestimating weapons effects in continental geology. 9 refs., 5 figs.

Late Cretaceous-Cenozoic tectonic development of the southwest pacific region

A new model of the plate tectonic development of the southwest Pacific integrates the continental geology of New Zealand with the age structure of the surrounding oceanic crust revealed previously from magnetic anomaly lineations. The model differs from previous ones in that the onland geology of New Zealand is used to constrain the tectonic development in two important ways: (1) the modern Australia-Pacific plate boundary did not transect the New Zealand sector until 23 million years ago; (2) there has been a total of only c.500 km of dextral displacement on the plate boundary through New Zealand. The model is described with reference to a series of paleotectonic maps drawn to represent the setting at the times of anomalies 32, 24, 7 and 5. Novel interpretations based on these reconstructions include the following. During the late Cretaceous the Campbell Fault was a continental transform fault between the Tasman Sea spreading centre and the Bounty Rift, its 330 km of dextral displacement accompanying 25° of counterclockwise rotation of a Campbell Plateau block. When the eastward-propagating Southeast Indian Ridge broke into the south Tasman Sea 57 million years ago, it continued to propagate directly into southern South Island, and generated late Eocene-Oligocene continental rifting through western New Zealand. At the same time, back-arc spreading in the Norfolk Basin caused continental rifting along a similar trend in western North Island. The Australia-Pacific plate boundary originated as a transform fault between the Southeast Indian Ridge and the pre-existing Kermadec Trench. It developed in response to the sudden cessation of spreading on the segment of the Southeast Indian Ridge east of a fracture zone here named Fracture Zone Z. Two implications arise from this model regarding the integrity of Antarctica: 1. (1) during the Cretaceous, Marie Byrd Land was probably no more than 200 km northwest of its present position; 2. (2) the proposition made in some earlier plate tectonic reconstructions, that if the Alpine Fault did not form until after the late Eocene there must be a late Cretaceous-early Tertiary plate boundary within Antarctica, is most probably valid.