Plate Kinematic Data Research Articles

High-resolution reconstructions and GPS estimates of India–Eurasia and India–Somalia plate motions: 20 Ma to the present

SUMMARY We reconstruct the movement of the India Plate relative to Eurasia at ≈1-Myr intervals from 20 Ma to the present from GPS site velocities and high-resolution sequences of rotations from the India–Somalia–Antarctic–Nubia–North America–Eurasia Plate circuit. The plate circuit rotations, which are all estimated using the same data fitting functions, magnetic reversal sampling points, calibrations for magnetic reversal outward displacement, and noise mitigation methods, include new India–Somalia rotations estimated from numerous Carlsberg and northern Central Indian ridge plate kinematic data and high-resolution rotations from the Southwest Indian Ridge that account for slow motion between the Nubia and Somalia plates. Our new rotations indicate that India–Somalia plate motion slowed down by 25–30 per cent from 19.7 to 12.5–11.1 Ma, but remained steady since at least 9.8 Ma and possibly 12.5 Ma. Our new India–Eurasia rotations predict a relatively simple plate motion history, consisting of NNE-directed interplate convergence since 19 Ma, a ≈50 per cent convergence rate decrease from 19.7 to 12.5–11.1 Ma, and steady or nearly steady plate motion since 12.5–11.1 Ma. Instantaneous convergence rates estimated with our new India–Eurasia GPS angular velocity are 16 per cent slower than our reconstructed plate kinematic convergence rates for times since 2.6 Ma, implying either a rapid, recent slowdown in the convergence rate or larger than expected errors in our geodetic and/or plate kinematic estimates. During an acceleration of seafloor faulting within the wide India–Capricorn oceanic boundary at 8–7.5 Ma, our new rotations indicate that the motions of the India Plate relative to Somalia and Eurasia remained steady. We infer that forces acting on the Capricorn rather than the India Plate were responsible for the accelerated seafloor deformation, in accord with a previous study. India–Eurasia displacements that are predicted with our new, well-constrained rotations are fit poorly by a recently proposed model that attributes the post-60-Ma slowdown in India–Eurasia convergence rates to the steady resistance of a strong lithospheric mantle below Tibet.

A geodynamic model of subduction evolution and slab detachment to explain Australian plate acceleration and deceleration during the latest Cretaceous–early Cenozoic

During the latest Cretaceous-early Cenozoic, the northern margin of the Australian plate was characterized by a large (4000 km wide) northto northeast-dipping subduction zone (New Guinea-Pocklington subduction zone) consuming a marginal basin. Geological and geophysical data imply that the subduction zone was active ca. 71-50 Ma, and suggest that it was responsible for plate acceleration from ~1.0 to ~7.3 cm/yr ca. 64-59 Ma, and plate deceleration from ~7.3 to ~0.3 cm/yr at 52-49 Ma. This paper presents a numerical model of buoyancy-driven subduction to test if the rates of Australian plate acceleration and deceleration can be ascribed to the progressive evolution of a subducting slab. The geodynamic model reproduces the first-order plate velocity evolution of the Australian plate, with a transient ~5 m.y. period of acceleration from 2 to 8 cm/yr during upper mantle slab lengthening, an ~5 m.y. period of rapid plate motion (~5-8 cm/yr), and a short, 3.9 m.y., period of plate deceleration, starting with a 2 cm/yr velocity drop during 3.1 m.y. of continental subduction and followed by ~0.8 m.y. of rapid deceleration (4 cm/yr velocity drop) during slab detachment. The geodynamic model demonstrates that plate velocity increases or decreases of ~4-6 cm/yr can occur over a period lasting < 1 m.y. to a few million years, comparable to what is observed for the latest Cretaceous-early Cenozoic evolution of the Australian plate. Such rates of plate acceleration and deceleration could be tested against plate kinematic data for other subduction settings on Earth.

Temporal histories of Cordilleran continental arcs: Testing models for magmatic episodicity

![][1] Magmatic activity in continental arcs is known to vary in a non-steady-state manner, with the mechanisms driving magmatic activity being a matter of ongoing discussion. Of particular importance is the question of what extent episodic magmatism in continental arcs is governed by external factors (e.g., plate motions) and internal factors (e.g., feedback processes in the upper plate). To test existing models for magmatic episodicity, which are mostly based on temporally and spatially limited records, this study uses large data sets of geochronological, geochemical, and plate kinematic data to document the Paleozoic to Mesozoic development of the North and South American Cordilleras in eight transects from British Columbia to Patagonia. The temporal distribution of U/Pb bedrock and detrital zircon ages, used as a proxy for timing of magmatic accretion, shows that some minima and maxima of zircon abundance are nearly synchronous for thousands of kilometers along the arc. Some age patterns are characterized by a periodicity of 50–80 Ma, suggesting a cyclic controlling mechanism. Other magmatic lulls or flare-ups find no equivalents in adjacent sectors, indicating that either discrete events or variable lag times may also be important in governing magmatic activity in continental arcs. Magma composition in Mexico, the Peninsular Ranges, and the Sierra Nevada varies episodically and proportionally with the temporal record of arc activity. During flare-up events, there is an increase in Sm/Yb, indicating deeper melting, and a decrease in eNdi, suggesting a higher degree of crustal assimilation. Geochemical scatter also increases during the initiation of flare-up events. Plate kinematic data provide a means of evaluating mantle heat input. The correlation between plate convergence rate and magmatic accretion varies for each sector, suggesting that different flare-ups or lulls likely reflect variable combinations of processes. [1]: /embed/graphic-1.gif

Comment on “Geochemistry of the Early Miocene volcanic succession of Northland, New Zealand, and implications for the evolution of subduction in the Southwest Pacific” by M.A. Booden, I.E.M. Smith, P.M. Black and J.L. Mauk

In a recent paper Booden et al. (2011) present new geochemical and petrological data of Early Miocene volcanics from the Northland region (Northland volcanic belt) in New Zealand, and interpret these data to support a particular regional tectonic model. This tectonic model involves Early Miocene westward subduction of Cretaceous Pacific oceanic lithosphere below the Northland volcanic belt and the authors interpret the volcanic belt as a continental magmatic arc. Although the new data are not in disagreement with such a tectonic model, they provide more support for an alternative interpretation that involves a northeast-dipping subduction zone. Furthermore, geometric and plate kinematic data show that the west-dipping subduction model is unviable, geological and geophysical data contradict the model, while geodynamic arguments indicate that the model is implausible. Here it will be shown that a subduction model, involving a northeast-dipping southwestward retreating slab (made of the Late Cretaceous–Paleocene South Loyalty backarc basin lithosphere) that subsequently detaches, is in agreement with the local geology, geophysics and geochemistry, is geometrically, kinematically and geodynamically viable, and fits within the regional Southwest Pacific tectonic framework.

Geologically current plate motions

We describe best-fitting angular velocities and MORVEL, a new closure-enforced set of angular velocities for the geologically current motions of 25 tectonic plates that collectively occupy 97 per cent of Earth's surface. Seafloor spreading rates and fault azimuths are used to determine the motions of 19 plates bordered by mid-ocean ridges, including all the major plates. Six smaller plates with little or no connection to the mid-ocean ridges are linked to MORVEL with GPS station velocities and azimuthal data. By design, almost no kinematic information is exchanged between the geologically determined and geodetically constrained subsets of the global circuit—MORVEL thus averages motion over geological intervals for all the major plates. Plate geometry changes relative to NUVEL-1A include the incorporation of Nubia, Lwandle and Somalia plates for the former Africa plate, Capricorn, Australia and Macquarie plates for the former Australia plate, and Sur and South America plates for the former South America plate. MORVEL also includes Amur, Philippine Sea, Sundaland and Yangtze plates, making it more useful than NUVEL-1A for studies of deformation in Asia and the western Pacific. Seafloor spreading rates are estimated over the past 0.78 Myr for intermediate and fast spreading centres and since 3.16 Ma for slow and ultraslow spreading centres. Rates are adjusted downward by 0.6–2.6 mm yr−1 to compensate for the several kilometre width of magnetic reversal zones. Nearly all the NUVEL-1A angular velocities differ significantly from the MORVEL angular velocities. The many new data, revised plate geometries, and correction for outward displacement thus significantly modify our knowledge of geologically current plate motions. MORVEL indicates significantly slower 0.78-Myr-average motion across the Nazca–Antarctic and Nazca–Pacific boundaries than does NUVEL-1A, consistent with a progressive slowdown in the eastward component of Nazca plate motion since 3.16 Ma. It also indicates that motions across the Caribbean–North America and Caribbean–South America plate boundaries are twice as fast as given by NUVEL-1A. Summed, least-squares differences between angular velocities estimated from GPS and those for MORVEL, NUVEL-1 and NUVEL-1A are, respectively, 260 per cent larger for NUVEL-1 and 50 per cent larger for NUVEL-1A than for MORVEL, suggesting that MORVEL more accurately describes historically current plate motions. Significant differences between geological and GPS estimates of Nazca plate motion and Arabia–Eurasia and India–Eurasia motion are reduced but not eliminated when using MORVEL instead of NUVEL-1A, possibly indicating that changes have occurred in those plate motions since 3.16 Ma. The MORVEL and GPS estimates of Pacific–North America plate motion in western North America differ by only 2.6 ± 1.7 mm yr−1, ≈25 per cent smaller than for NUVEL-1A. The remaining difference for this plate pair, assuming there are no unrecognized systematic errors and no measurable change in Pacific–North America motion over the past 1–3 Myr, indicates deformation of one or more plates in the global circuit. Tests for closure of six three-plate circuits indicate that two, Pacific–Cocos–Nazca and Sur–Nubia–Antarctic, fail closure, with respective linear velocities of non-closure of 14 ± 5 and 3 ± 1 mm yr−1 (95 per cent confidence limits) at their triple junctions. We conclude that the rigid plate approximation continues to be tremendously useful, but—absent any unrecognized systematic errors—the plates deform measurably, possibly by thermal contraction and wide plate boundaries with deformation rates near or beneath the level of noise in plate kinematic data.

Convergent plate margin dynamics: New perspectives from structural geology, geophysics and geodynamic modelling

Convergent plate margins occur when two adjoining tectonic plates come together to form either a subduction zone, where at least one of the converging plates is oceanic and plunges beneath the other into the mantle, or a collision zone, where two continents or a continent and a magmatic arc collide. Convergent plate margins are arguably the most complicated and dynamic plate boundaries on Earth and have been the subject of many investigations and discussions since the advent of plate tectonic theory. This paper provides a historical background and a review of the development of geological and geodynamic theories on convergent plate margins. Furthermore, it discusses some of the recent advances that have been made in the fields of structural geology, geophysics and geodynamics, which are fundamental to our understanding of this phenomenon. These include: (1) the finding that plates and plate boundaries move at comparable velocities across the globe; (2) the emerging consensus that subducted slabs are between two to three orders of magnitude stronger than the ambient upper mantle; (3) the importance of lateral slab edges, slab tearing and toroidal mantle flow patterns for the evolution of subduction zones; and (4) clear evidence from mantle tomography that slabs can penetrate into the lower mantle. Still, many first-order problems regarding the geodynamic processes that operate at convergent margins remain to be solved. These include subduction zone initiation and the time of inception of plate tectonics, and with it convergent plate margins, on Earth. Fundamental problems in orogenesis include the mechanism that initiated Andean mountain building at the South American subduction zone, the potential episodicity of mountain building with multiple cycles of shortening and extension, and the principal driving force behind the construction of massive mountain belts such as the Himalayas–Tibet and the Andes. Fundamental questions in subduction dynamics regard the partitioning of subduction into a trench and plate component, and the distribution of energy dissipation in the system. In seismic imaging, challenges include improving resolution at mid to lower mantle depth in order to properly understand the fate of slabs, and better constraining the 3-D flow-related anisotropic structure in the surrounding mantle. Future insights into such fundamental problems and into the regional and global dynamics of convergent plate margins will likely be obtained from integrating spatio-temporal data, structural geological data, geophysical data and plate kinematic data into plate tectonic reconstructions and three-dimensional geodynamic models of progressive deformation.

Gondwana breakup and plate kinematics: Business as usual

A tectonic model of the Weddell Sea is built by composing a simple circuit with optimized rotations describing the growth of the South Atlantic and SW Indian oceans. The model independently and accurately reproduces the consensus elements of the Weddell Sea's spreading record and continental margins, and offers solutions to remaining controversies there. At their present resolutions, plate kinematic data from the South Atlantic and SW Indian oceans and Weddell Sea rule against the proposed, but controversial, independent movements of small plates during Gondwana breakup that have been attributed to the presence or impact of a mantle plume. Hence, although supercontinent breakup here was accompanied by extraordinary excess volcanism, there is no indication from plate kinematics that the causes of that volcanism provided a unique driving mechanism for it.

Geothermal regime and geodynamics of the North Pacific Ocean

The distribution of heat flow in the North Pacific Ocean has been examined, and a map of geothermal and geomagnetic fields for the Bering Sea as it is known today has been made. Reliable data are lacking regarding the time of origin for features of oceanic and continental genesis in the Bering Sea, which is an obstacle to the study of geodynamic processes in the North Pacific. Heat flow data were used to yield numerical estimates for the age of seafloor features in the Bering Sea: the Kamchatka Basin (21 Ma), Shirshov Ridge (95 Ma for the northern part and 33 Ma for the southern), the Aleutian Basin (70 Ma), Vitus Rise (44 Ma), Bowers Ridge (30 Ma), and Bowers Basin (40 Ma). These age estimates are corroborated by combined geological, geophysical, and plate kinematic data. A thermochemical model of global mantle convection has been developed in order to perform a numerical simulation of the thermal process involved in the generation of extended regional features in the North Pacific (the Emperor Fracture Zone, Chinook Trough, etc.). The modeling suggests a plume-tectonic origin for these features, yielding the optimal model for the tectonic evolution of the North Pacific. An integrated geological and geothermal analysis leads to the conclusion that the northern and southern parts of the Shirshov Ridge are different, not only in geologic age, but also in tectonic structure. The northern part is of imbricated-thrust terrane origin, while the southern part is of ensimatic island-arc origin, similar to that of Bowers Ridge. The seafloor of the Aleutian Basin is an outlier of the Upper Cretaceous Kula plate where, in the Vitus Rise area, backarc spreading processes originated during Eocene time. The terminating phase of activity in the Bering Sea began about 21 Ma by spreading in the older seafloor of the Kamchatka Basin. We developed plate-tectonic reconstructions of evolution for the North Pacific for the times 21, 33, 40, and 70 Ma in the hotspot system based on age estimates for the seafloor features derived from heat flow data and modeling of the thermal generation of regional faults, as well as on an analysis of geomagnetic, tectonic, and geological data.

Motion between the Indian, Capricorn and Somalian plates since 20 Ma: implications for the timing and magnitude of distributed lithospheric deformation in the equatorial Indian ocean

SUMMARY Approximately 2200 magnetic anomaly crossings and 800 fracture zone crossings flanking the Carlsberg ridge and Central Indian ridge are used to estimate the rotations of the Indian and Capricorn plates relative to the Somalian Plate for 20 distinct points in time since 20 Ma. The data are further used to place limits on the locations of the northern edge of the rigid Capricorn Plate and of the southern edge of the rigid Indian Plate along the Central Indian ridge. Data south of and including fracture zone N (the fracture zone immediately south of the Vema fracture zone), which intersects the Central Indian ridge near 10 ◦ S, are well fit assuming rigid Capricorn and Somalian plates, while data north of fracture zone N are not, in agreement with prior results. Data north of fracture zone H, which intersects the Central Indian ridge near 3.2 ◦ S, are well fit assuming rigid Indian and Somalian plates, while data south of and including fracture zone H are not, resulting in a smaller rigid Indian Plate and a wider diffuse oceanic plate boundary than found before. The data are consistent with Capricorn‐Somalia motion about a fixed pole since ≈8 Ma, but require rotation about a pole 15 ◦ farther away from the Central Indian ridge from 20 to ≈8 Ma. The post-8-Ma pole also indicates Capricorn‐ Somalia displacement directions that are 7 ◦ clockwise of those indicated by the pre-8-Ma stage pole. In contrast, India‐Somalia anomaly and fracture crossings are well fit by a single fixed pole of rotation for the past 20 Ma. India‐Somalia motion has changed little during the past 20 Myr. Nonetheless, astronomically calibrated ages for reversals younger than 12.9 Ma allow resolution of the following small but significant changes in spreading rate: India‐Somalia spreading slowed from 31 to 28 mm yr −1 near 7.9 Ma and later sped up to 31 mm yr −1 near 3.6 Ma; Capricorn‐Somalia spreading slowed from 40 to 36 mm yr −1 near 11.0 Ma, later sped up to 38 mm yr −1 near 5.1 Ma and further sped up to 40 mm yr −1 near 2.6 Ma. The motion between the Indian and Capricorn plates is estimated by differencing India‐Somalia and Capricorn‐Somalia rotations, which differ significantly for all 20 pairs of reconstructions. India has rotated relative to the Capricorn Plate since at least ≈20 Ma. If about a pole located near 4 ◦ S, 75 ◦ E, the rate of rotation was slow, 0.11 ◦ ± 0.01 ◦ Myr −1 (95 per cent confidence limits), from 20 to 8 Ma, but increased to 0.28 ◦ ± 0.01 ◦ Myr −1 (95 per cent confidence limits) at ≈8 Ma. The onset of more rapid rotation coincides, within uncertainty, with the inferred onset at 7‐8 Ma of widespread thrust faulting in the Central Indian basin, and with the hypothesized attainment of maximum elevation and initiation of collapse of the Tibetan plateau at ≈8 Ma. The plate kinematic data are consistent with steady India‐Capricorn motion since 8 Ma and provide no evidence for previously hypothesized episodic motions during that interval. The convergence since 8 Ma between the Indian and Capricorn plates significantly exceeds (by 13 to 20 km) the convergence estimated from three north‐south marine seismic profiles in the Central Indian basin. Where and how the additional convergence was accommodated is unclear.

Tectonic forcings of Maastrichtian ocean‐climate evolution

A global compilation of deep‐sea isotopic records suggests that Maastrichtian ocean‐climate evolution was technically driven. During the early Maastrichtian the Atlantic intermediate‐deep ocean was isolated from the Pacific, Indian, and Southern Oceans; deep water formed in the high‐latitude North Atlantic and North Pacific. At the early/late Maastrichtian boundary a major reorganization of oceanic circulation patterns occurred, resulting in the development of a thermohaline circulation system similar to that of the modern oceans. A combination of isotopic and plate kinematic data suggests that this event was triggered by the final breaching of tectonic sills in the South Atlantic and the initiation of north‐south flow of intermediate and deep water in the Atlantic. The onset of Laramide tectonism during the mid Maastrichtian led to the concurrent draining of major epicontinental seaways. Together, these events caused cooling, increased latitudinal temperature gradients, increased ventilation of the deep ocean, and affected a range of marine biota.

Structure and geology of the continental shelf of the Laptev Sea, Eastern Russian Arctic

The Laptev Sea is of great significance for studying the processes of the initial breakup of continents. It is the southern termination of the Gakkel spreading ridge and thus the location of structural features resulting from a continental margin/spreading ridge intersection. The present-day understanding of the Laptev Shelf geology is based on the Russian multichannel seismic reflection data and extrapolation of the terrestrial geology. Geologic and plate-kinematic data are used to constrain the interpretation of the seismic reflection data. The Laptev Rift System consists of several deep subsided rifts and high standing blocks of the basement. From west to east these are: the West Laptev and South Laptev rift basins, Ust' Lena Rift, East Laptev and Stolbovoi horsts, Bel'kov-Svyatoi Nos and Anisin rifts. The central and eastern parts of the shelf have the greatest contrasts in the gravity field ranging from −60 mGal over the rifts to 50 mGal over the horsts. The rifts contain up to five seismic stratigraphic units bounded by clear regional reflectors and underlain by folded heterogeneous basement. They are suggested to be Late Cretaceous to Holocene in age and reflect different stages of spreading ridge/continental margin interaction. The estimated total thickness of the rift-related sediments varies between 4 and 8–10 km while the sedimentary cover on the uplifts is significantly reduced and generally does not exceed 1–2 km. An eastward decrease of the total thickness of the sedimentary sections from about 10 km in the South Laptev Basin to 4–5 km in the Bel'kov-Svyatoi Nos Rift and the simplicity of the entire rift structure may indicate a rejuvenation of the rifts in the same direction. The entire rift system is covered by the uppermost seismic unit, which probably reflects a deceleration of the rifting during the last reorganization of the North American/Eurasian plate interaction since about 2 Ma.

The Tertiary dynamics of the northern Eastern Alps (Austria): changing palaeostresses in a collisional plate boundary

Six Tertiary deviatoric palaeostress tensor groups from 165 stations in the Calcareous Alps describe the upper crustal dynamics of the leading edge of the Adriatic plate during protracted continental collision with the European lower plate. Palaeostress changes are correlated to the kinematic evolution of the plate boundary and to independently derived plate kinematic data of the Alpine-Carpathian-Pannonian area. Each palaeostress direction is defined by 32 to 89 individual tensors which constrain: (1) Late Eocene to ?Oligocene NW-directed compression during thrusting and dextral shearing on WNW-striking faults; NW-directed compression along the northern margin of the Adriatic plate is correlated to its N-directed translation combined with a counterclockwise rotation. (2a) ?Late Eocene to Early Miocene N-directed compression during N-directed thrusting of Penninic and Helvetic units and the Molasse; (2b) Early to Middle Miocene N-directed strike-slip compression during the onset of eastward lateral extrusion; the change from NW- to N-directed compression is interpreted to result from strain partitioning at the Periadriatic fault which has formed the new Adriatic plate boundary since the Oligocene. Dextral shearing on this fault accommodated continued anticlockwise rotation and decoupled the northern Eastern Alps from the rotational component. N-directed compression in the Calcareous Alps paralleled the N-directed translation of the Adriatic plate. (3) Middle Miocene NE-directed compression of thrust- and strike-slip type during lateral extrusion; NE-directed compression in the Calcareous Alps resulted from the drag of the eastward extruding central Eastern Alps. Sinistral shear stress was transmitted across the ENE-trending Salzach-Ennstal fault. NE-oriented σ 1-axes include 45° with the E-W-striking zone of distributed sinistral shear in the Calcareous Alps. (4) Middle Miocene E-directed extension associated with orogen-parallel normal faulting of the central Eastern Alps; E-directed extension paralleled the direction of mass transfer towards the Pannonian Basin during lateral extrusion. Normal faulting was induced by reduced lateral confinement due to removal of material east of the Alps. On the large scale, eastward motion was enabled by the E-directed retreat of the Eastern Carpathian subduction zone. (5) Late Miocene E-directed compression; orogen parallel compression obstructed lateral extrusion. The stress change from extension to E-directed compression was caused by the end of subduction retreat in the Eastern Carpathians due to dynamic changes in the subduction zone. (6) N-directed extension; Late- to post-Miocene extension paralleled recent topographic slopes in the foothill of the Eastern Alps. Palaeostresses changes correspond to distinct phases in the kinematic evolution of the northern Eastern Alps and to the systematic formation and reactivation of major fault zones. WNW-striking faults changed deformation styles from dextral shear to dextral transpression, reverse faulting and finally to sinistral shear. The Early to Middle Miocene Salzach-Ennstal fault which formed the northern boundary of the extruded central Eastern Alps changed from sinistral transpression to sinistral shear, sinistral transtension and finally to dextral shear.

Structural signature of tectonic processes in the Calabrian Arc, southern Italy: Evidence from the oceanic‐derived Diamante‐Terranova unit

The Diamante‐Terranova unit is an oceanic‐derived element within the thrust and nappe structure of the northern sector of the Calabrian Arc. Its metamorphic and structural signature reveals a long history of SE dipping subduction and collision‐related deformation of an oceanic lithospheric section underlying the narrow Diamante‐Terranova basin. The deformation history of the Diamante‐Terranova unit may be related to three main tectonic phases. The first one is characterized by a structural association which includes features that developed during a progressive deformation related to the evolution of a deep‐seated shear zone. Kinematic indicators characterizing subduction‐related structures clearly show a top‐to‐the‐northwest sense of overthrust shear. The second tectonic phase that can be recognized in the area is characterized by structures that may be related to the onset of shear deformation induced by a postcollisional northeastward motion of the overriding plate. The third tectonic phase is recorded by brittle features related to the latest tectonic events affecting the Calabrian Arc during the opening of the Tyrrhenian Sea. Shear sense indicators and available radiometric and stratigraphic information, together with plate kinematic data, allowed us to infer the time evolution of this sector of the Calabrian Arc and to assess that the Diamante‐Terranova unit cannot be considered as an “Eoalpine” element since its deformation, related to subduction‐collision processes, occurred prior to the end of the Cretaceous.

Neogene tectonic reconstruction of the Adelbert-Finisterre-New Britain collision, northern Papua New Guinea

The Finisterre terrane is presently colliding with the Australian continent in northern Papua New Guinea. The terrane is exposed in two distinct blocks — the Adelbert block and the Finisterre block. A clastic sedimentary sequence in the suture zone provides constraints on the age of initial collision for each block and on paleotectonic reconstructions of New Guinea. Provenance shifts within the Finisterre block sedimentary section date the collision of that block at 3.0–3.7 Ma. Analysis of the Adelbert block sedimentary section has not revealed similar provenance shifts. This absence of temporal trends in source area may be caused by oblique collision of the Adelbert block with an allochthonous terrane composed of oceanic crust. These factors would likely lead to a “soft” collision involving little uplift and hence little disruption of the pre-collisional sedimentation patterns. In contrast, the Finisterre block collided orthogonally with terranes composed of continental crust. This “hard” collision has led to rapid uplift and significant modification of the pre-collisional depositional system. A deep water basin existed between the Adelbert block and the continent in the Late Pliocene. Deep marine sediments deposited in this basin were subsequently overthrust by older lithologies of the Adelbert block. When combined with geochemical, seismic and plate kinematic data published by other workers, these data suggest collision of the eastern portion of the Adelbert block in the Middle to Late Pliocene. The western portion of the Adelbert block probably collided in the latest Miocene. Many tectonic reconstructions of northern Papua New Guinea have favored collision of the Finisterre terrane over a doubly-subducting Solomon Sea Plate in an extension of the modern tectonic configuration of the Solomon Sea. The presence of continentally-derived sediment in the Finisterre accretionary wedge casts doubt on this scenario. The trench of a southward-dipping subduction zone would be likely to block continentally-derived sediment from reaching the Finisterre accretionary wedge. The Maramuni Arc, the igneous association usually attributed to the hypothesized southward-dipping subduction zone, appears to have erupted on allochthonous terranes rather than on autochthonous crust. These observations suggest that collision of the Adelbert block and most of the Finisterre block occurred above a single, northward-dipping subduction zone. The double subduction present in the Solomon Sea probably never extended more than 200 km west of its present location.