Along-strike Length Research Articles

Late Quaternary Surface Displacements on Accretionary Wedge Splay Faults in the Cascadia Subduction Zone: Implications for Megathrust Rupture

Because splay faults branch at a steep dip angle from the plate-boundary décollement in an accretionary wedge, their coseismic displacement can potentially result in larger tsunamis with distinct characteristics compared to megathrust-only fault ruptures, posing an enhanced hazard to coastal communities. Elsewhere, there is evidence of coseismic slip on splay faults during many of the largest subduction zone earthquakes, but our understanding of potentially active splay faults and their hazards at the Cascadia subduction zone remains limited. To identify the most recently active splay faults at Cascadia, we conduct stratigraphic and structural interpretations of near-surface deformation in the outer accretionary wedge for the ~400 km along-strike length of the landward vergence zone. We analyze recently acquired high-frequency sparker seismic data and crustal-scale multi-channel seismic data to examine the record of deformation in shallow slope basins and the upper ~1 km of the surrounding accreted sediments and to investigate linkages to deeper décollement structure. We present a new fault map for widest, most completely locked portion of Cascadia from 45 to 48°N latitude, which documents the distribution of faults that show clear evidence of recent late Quaternary activity. We find widespread evidence for active splay faulting up to 30 km landward of the deformation front, in what we define as the active domain, and diminished fault activity landward outside of this zone. The abundance of surface-deforming splay faults in the active outer wedge domain suggests Cascadia megathrust events may commonly host distributed shallow rupture on multiple splay faults located within 30 km of the deformation front.

Differences between the central Andean and Himalayan orogenic wedges: A matter of climate

The Central Andean and Himalayan orogenic belts provide an ideal natural experiment to test the potential role of climate in controlling orogeny. Approximately equal in age and along-strike length, both orogenic wedges are forming in plate-marginal convergent tectonic settings: The Andes in a retroarc setting and the Himalaya in a collisional setting against the Tibetan backstop. The Central Andes orogenic wedge is volumetrically and aerially nearly two times larger than the Himalayan orogenic wedge, despite the Himalaya having accommodated two to three times more tectonic shortening. The Himalaya exports at least four times more sediment owing to much greater erosion rates as signified by widespread Cenozoic metamorphic rocks and very young (<10 Ma) low-temperature thermochronologic ages. The Central Andes are thermochronologically old (mostly >20 Ma), have no exposures of Cenozoic metamorphic rocks, and are mantled by volcanic and sedimentary rocks, attesting to shallow, slow erosion. We conclude that greater intensity of the Indian Monsoon relative to the South American Monsoon since Oligocene time accounts for the differences in orogen size and characteristics. When viewed as an orogenic wedge that has developed largely after formation of the Tibetan orogenic collage, the Himalaya is neither the largest nor hottest among Earth's orogens.

Understanding the Caviahue-Copahue volcanic complex through kinematic solutions, paleotensors and analogue modelling

This work aims to study the interaction between two of the most representative structural systems controlling the Caviahue-Copahue volcanic complex. To achieve this objective, a structural analysis based on outcrop-scale fault-slip data field surveys and analogue models were carried out. The deformation regime acting on the studied area was characterized, and the associated paleostresses were obtained from the kinematic data inversion.The performed analysis in Caviahue-Copahue volcanic complex allowed to define two main sets of faults controlling the deformation of the area: NE-SW to ENE-WSW, and WNW-ESE to NW-SE, respectively. The first group comprises high-angle normal faults, resulting in a horst-and-graben setting with along-strike lengths up to 2 km. The second group shows strike-slip kinematics with a minor normal component. These NW-SE faults are related to the Miocene-Pliocene fissure-related volcanism and define the major caldera axis direction. In the geothermal area, the obtained paleostress orientation shows a consistent vertical σ1, denoting a local extensional regime. Regarding the geothermal field, the NE-SW extensional fault system is proposed as the main circulation pathways for hydrothermal fluids rising to the surface. The major NW-SE faults would act as barriers for this circulation.The set of analogous experiments was used to contrast the obtained local structural kinematic results. Two non-coaxial extensional events were established to achieve a local scale approach to the structural configuration observed in the area. Simulations aimed to understand the structural behavior of the superposition of non-coaxial extensive events; they allowed us to assess the role of the different fault sets surveyed in the field within the system. Particularly, our findings support that the NW-SE-oriented structures compartmentalize the subsequent NE-SW-oriented faults, acting on occasions as transfer zones.

NGA-Sub source and path database

We describe source and path attributes of the Next Generation of Ground-Motion Attenuation for Subduction zones (NGA-Sub) project relational database. The database contains 991 earthquakes between 1937 and 2016 that meet quality assurance standards and have assigned event types (mostly interface or intraslab). Data curation emphasized large magnitudes: 73% of events have M &gt; 5 and 13 events have M &gt; 8. Event attributes in the database can be broadly divided into those related to the moment tensor, those related to rupture surface geometry (referred to as finite-fault parameters), and categorizations by type (e.g. interface, intraslab) and sequence classification (mainshock, aftershock). Earthquake information and moment tensor parameters compiled from earthquake catalogs include event date and origin time, hypocenter location, seismic moment, and the strike, dip, and rake angles of nodal planes. Finite-fault parameters include along-strike length, down-dip width, and depth to top-of-rupture for one or more rectangles used to parameterize rupture surface geometry. These are modified from models in literature where available, and otherwise are simulated using procedures customized for subduction earthquakes. Rupture distance and other metrics are computed using finite-fault representations of sources, and (as applicable) are partitioned into forearc and backarc components. Forearc and backarc regions are defined based on volcano locations, and are assigned to events and sites.

Automated detection and location of tectonic tremor along the entire Cascadia margin from 2005 to 2011

We have constructed an automated routine to identify prominent bursts of tectonic tremor and locate their source region during time periods of raised amplitude in the tremor passband. This approach characterizes 62 episodes of tectonic tremor between 2005 and 2011, with tremor epicenters forming a narrow band spanning the entire length of the Cascadia Subduction Zone. We find a range of along-strike lengths in individual episodes, but the length appears proportional to both duration and geodetic moment, consistent with proposed scaling laws for slow earthquake phenomena. Examination of individual episodes in detail reveals intriguing updip–downdip migration patterns, including slow updip migration during initiation and repetitive downdip migration between different episodes. The broader catalog of tremor episodes refines the inferences from earlier work that episodic tremor and slip are segmented along-strike and correlated with apparent seismogenic zone segmentation in most cases. The overall band of tremor is offset ∼50 km from the downdip edge of interseismic coupling along the central and northern parts of the subduction zone. Along the southern part of the subduction zone, it is adjacent to this boundary, suggesting that the locked and transition zones may be more closely linked in southern Cascadia.

Magmatic storage conditions, decompression rate, and incipient caldera collapse of the 1902 eruption of Santa Maria Volcano, Guatemala

Phase equilibria experiments and analysis of natural pumice and phenocryst compositions indicate the 1902 Santa Maria dacite was stored at ~140–170MPa and 840–850°C prior to eruption. H2O-saturated, cold-seal experiments conducted in vessels with an intrinsic log fO2 of NNO+1±0.5 show that the natural phase assemblage (melt+plagioclase+amphibole+orthopyroxene+Fe–Ti oxides+apatite) is stable from approximately 115–140MPa at temperatures below ~825°C, to ~840–860°C at 150MPa, to >850 and <875°C at 200MPa. Natural plagioclase phenocrysts have rim compositions that range from An40–An45; this range of compositions defines a ~20°C band that intersects the stable phase assemblage at ~150MPa and 850°C. Plagioclase hosted melt inclusions were analyzed using FTIR and commonly contain <5wt.% H2O, which corresponds to a pressure of ~170MPa at 850°C, under pure H2O saturated conditions. Amphibole geothermobarometry (Ridolfi et al., 2010) applied to experimental samples suggest two populations of amphiboles, phenocrysts grown during the experiments and inherited xenocrysts, but the pressure–temperature conditions returned by the geothermobarometer are routinely >50MPa and >50°C greater than experimental run conditions; precise estimates of magmatic conditions based solely upon amphibole composition are likely inaccurate. The experimental results and analysis of natural crystals suggest that although the natural amphiboles likely record a broad range of magmatic conditions, only the lower bounds of that range reflect pre-eruptive storage conditions. Comparison of Santa Maria microlite abundances with decompression experiments examining other silicic systems from the literature suggests that the 1902 dacite decompressed at the rate of ~0.005 to 0.01MPa/s during the eruption. Applying the decompression rate with the previously described eruption rate of approximately 2–3×108kg/s (Williams and Self, 1983; Carey and Sparks, 1986) to the conduit model CONFLOW reveals that the eruption conduit was dike-like with an along-strike length >1km. Despite depositing ~20km3 of dacite tephra (equivalent to ~8.5km3 magma), the 1902 eruption did not form an obvious caldera. This work suggests that collapse of the dike-like conduit terminated the eruption, preventing full caldera collapse.

Rapid estimation of fault rupture extent using envelopes of acceleration

We present a new strategy to estimate the geometry of a rupture on a finite fault for rapid reporting of seismic intensity. We use envelope attenuation relationships which were presented by Huo et al. (Acta Seismol Sin 16:519–525, 1994). An important base of this work is the fault finiteness theory. We propose a new model to simulate high-frequency motions from earthquakes with large rupture dimension. The envelope of high-frequency ground motion from a large earthquake can be expressed as a root-mean-squared combination of envelope functions from smaller earthquakes. We use simulated envelopes of ground acceleration to estimate the direction and along-strike length of a rupture. Using the Wenchuan and Jiji (Chi-Chi) earthquake dataset, we parameterize the fault geometry with an epicenter, a fault strike, and along-strike rupture lengths. So this methodology seems quite appropriate for the rapid reporting systems of seismic intensity.

The formation of mountain range curvature by gravitational spreading

This paper presents a mechanism by which mountain ranges can form curved range-fronts. Gravitational spreading of mountain ranges that have been thrust onto rigid lowlands will result in the formation of curvature, provided that enough gravity-driven flow occurs to dominate the shape of the topography. Whether this mechanism can operate during the lifetime of a given mountain range depends upon the viscosity of the range, the square of the along-strike length of the range, and the cube of the elevation of the range. The curvature of the southern edge of the Tibetan Plateau is consistent with formation by gravitational spreading provided that the viscosity is similar to that previously estimated using other, independent, methods. The low elevation and young age of the Zagros mountains mean that large-scale curvature has not had time to develop. The short along-strike extent and possibly low viscosity of the Sulaiman Ranges in Pakistan may have allowed the ranges to form their distinctive arcuate shape. The formation of range-front curvature plays an important role in controlling the tectonic evolution of the interiors of the ranges, with arc-parallel extension becoming progressively more important as range-front curvature develops.

Rapid source characterization of the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake

On March 11th, 2011, a moment magnitude 9.0 earthquake struck off the coast of northeast Honshu, Japan, generating what may well turn out to be the most costly natural disaster ever. In the hours following the event, the U.S. Geological Survey National Earthquake Information Center led a rapid response to characterize the earthquake in terms of its location, size, faulting source, shaking and slip distributions, and population exposure, in order to place the disaster in a framework necessary for timely humanitarian response. As part of this effort, fast finite-fault inversions using globally distributed body- and surface-wave data were used to estimate the slip distribution of the earthquake rupture. Models generated within 7 hours of the earthquake origin time indicated that the event ruptured a fault up to 300 km long, roughly centered on the earthquake hypocenter, and involved peak slips of 20 m or more. Updates since this preliminary solution improve the details of this inversion solution and thus our understanding of the rupture process. However, significant observations such as the up-dip nature of rupture propagation and the along-strike length of faulting did not significantly change, demonstrating the usefulness of rapid source characterization for understanding the first order characteristics of major earthquakes.

Geometrically coherent continuous deformation in the volume surrounding a seismically imaged normal fault-array

We calculated an apparent dip attribute, which was used to ascertain the spatial distribution of fault-related continuous deformation. The vertical component of displacement calculated from the continuous deformation acts to “fill-in” missing displacement in the fault-throw profile. This result shows that apparently complex 3D patterns of continuous strain in the volumes surrounding the fault-array developed as part of a single, geometrically coherent fault-array. However, if this component of continuous deformation was not added to the throw profile, the fault-array could have been misinterpreted as a series of isolated fault segments with coincidental overlaps. This technique permits the analysis of continuous deformation structures, which are up to an order of magnitude smaller than previously described. In the study area, these structures are interpreted as small fault-propagation folds, forming in a shale-dominated cover sequence. The fault-propagation folds above the upper tip line of the mapped fault-array bifurcate upwards from the fault surface into three coherent lobes and resemble secondary fault segments. The near-constant along-strike length of the region of continuous deformation throughout the syn-rift sequence implies that the length of the fault-array was established at an early stage in its growth, prior to the establishment of a seismically-visible fault surface.

Orographic precipitation isotopic ratios in stratified atmospheric flows: Implications for paleoelevation studies

An idealized, fully nonlinear atmospheric model was used to evaluate the effects of stratified atmospheric flows on orographic precipitation isotopic ratios, with the aim of improving the foundation for interpreting proxy records used for paleoelevation studies. Orographic precipitation isotopic ratios can be understood in terms of two nondimensional parameters: the flow parameter Nh / U , where N is the moist Brunt-Vaisala frequency, a measure of atmospheric stratification, h is the large-scale relief, and U is the upstream horizontal wind speed perpendicular to the orogen; and the horizontal aspect ratio β, the ratio of the along-strike length of the orogen to its across-strike width. Existing Rayleigh condensation models for paleoaltimetry studies are only strictly applicable when Nh / U 0 much of the time. High values of both Nh / U and β favor precipitation upstream of the terrain, leading to isotopic ratios that are more depleted than predicted by the Rayleigh model. High Nh / U and low β favor low-level deflection of winds around the terrain, which substantially reduces lifting and condensation, leading to precipitation isotopic ratios in that are less depleted than predicted by the simple Rayleigh model. Changes in climate or the horizontal terrain aspect ratio can change precipitation isotopic ratios at least as much as changes in surface elevation, and surface uplift need not always affect precipitation isotopic ratios, suggesting that proxy records must be interpreted within a broad context of climate variability and landscape evolution.

Analysis of the 9 September 1998 Mw 5.6 Mercure earthquake sequence (Southern Apennines, Italy): A multidisciplinary approach

The Mercure earthquake ( Mw 5.6) of September 9, 1998 and the associated aftershocks occurred in a small Pleistocene–Holocene continental basin of the Southern Apennines, in a region of low instrumental and moderate historical seismicity. Seismological, photogeological and field survey data were analyzed and integrated in order to identify the likely seismogenic structure, to depict its 3-D geometry and kinematics and to provide further constraints to the seismogenic potential of the rupture processes in the study area. The mainshock occurred at the NW edge of the seismic sequence (40.03°N and 15.95°) at a depth of 10.5 ± 1.5 km. The aftershocks volume was determined from the relocation of about 200 events (1.1 = Ml = 3.9) registered by local networks from September 10 to October 12, 1998. The relocation procedure was based on choosing P and S waves for all the events and the definition of ten 9-layers velocity models appropriate for the different stations. The kinematics of the seismogenic deformation was defined through the computation of 36 well-constrained focal mechanisms. The seismological and geological stress tensors were determined through inversion of focal mechanisms and fault slip data. Both of them resulted in the tensional type, with ENE–WSW and NE–SW trending σ 3 axis, respectively. The map and the section distribution of the aftershocks sequence depicts an average NW–SE striking and 60° SW-dipping seismogenic volume. Most of the events (80%) were located at depths between 3 and 8 km in the footwall of the Mercure basin (MBB) boundary fault but along the possible down-dip continuation of a previously unidentified, N120°E striking and WSW-dipping, Holocene normal fault alignment, which extends from Castello Seluci to Piana Perretti and Timpa della Manca (CPST fault). A small percentage of events (10%) were located at depths between 10 and 12 km where the CPST seismogenic fault may detach. The reconstructed rupture area ( RA) of the Mercure 1998 earthquake has an along-strike length ( L) of about 9 km and a down-dip width ( W) of about 9 km, yielding a total area of approximately 81 km 2. On the other hand, the L and W dimension of the entire individual seismogenic structure identified as responsible for the earthquake, e.g. the CPST fault, are about 19 and 12 km, respectively, with a consequent RA of about 230 km 2. This may imply a maximum magnitude ( Mw) equal to 6.3 which lead us to compare the Mercure area, in terms of seismogenic hazard, to the adjacent Pollino-Castrovillari area where strong paleoseismological events are documented.

Fault Parameters of the Nisqually Earthquake Determined from Moment Tensor Solutions and the Surface Deformation from GPS and InSAR

The magnitude 6.8 Nisqually earthquake occurred on 28 February 2001 at a depth of 50-60 km within the subducting Juan de Fuca plate. The fault parameters are estimated from moment tensor solutions and by comparing the surface displacements from Global Positioning System (GPS) data and from satellite interferometry, with predictions from elastic deformation models. The simple deformation model calculates the coseismic surface displacements caused by an earthquake on an extensional rectangular fault in a uniform half-space. Continuous GPS stations within 200 km of the epicenter resolved horizontal displacements as great as 9 mm (210°) and vertical displacements as great as 13 mm of subsidence near the epicenter. The near-vertical displacements were also determined from a differential interferogram created from synthetic aperture radar (InSAR) data from RADARSAT , the only satellite data available for the event. A maximum vertical subsidence of approximately 20 mm is observed 30 km east of the epicenter. The GPS, InSAR, and moment tensor solutions provide a consistent solution for the rupture parameters of the Nisqually earthquake. The fault has a strike of 180° and a dip of 20° and is centered at 47.10° N and 122.67° W (4 km east and 6 km south of the epicenter) or has a strike of 360° and a dip of 70° and is centered at 47.10° N and 122.69° W (2 km east and 6 km south). The trade-off between fault area and rupture displacement is not resolved by our data, but a good fit is found with the main rupture having an area of 230 km2 and an along-strike length of 23 km and downdip width of 10 km. The rupture area is centered at a depth of 51 km with a scalar moment of 2.0 × 1019 N m and, with the given area, an average rupture displacement of 1.4 m. To refine the interpretation of the GPS and InSAR data, a 3D heterogeneous numerical model was generated having realistic shear moduli structure, a 3D model of the subducting slab, and a spherical Earth. The results are similar to those from the half-space model, but there is significant refinement with a deeper rupture center at 60 km. Manuscript received 9 April 2003.

Bending-related faulting and mantle serpentinization at the Middle America trench.

The dehydration of subducting oceanic crust and upper mantle has been inferred both to promote the partial melting leading to arc magmatism and to induce intraslab intermediate-depth earthquakes, at depths of 50-300 km. Yet there is still no consensus about how slab hydration occurs or where and how much chemically bound water is stored within the crust and mantle of the incoming plate. Here we document that bending-related faulting of the incoming plate at the Middle America trench creates a pervasive tectonic fabric that cuts across the crust, penetrating deep into the mantle. Faulting is active across the entire ocean trench slope, promoting hydration of the cold crust and upper mantle surrounding these deep active faults. The along-strike length and depth of penetration of these faults are also similar to the dimensions of the rupture area of intermediate-depth earthquakes.

Out-of-plane motion of a thrust sheet during along-strike propagation of a thrust ramp: a distinct-element approach

Using a 3D distinct-element mechanical modeling program, we examined the nature and magnitude of out-of-plane motion during along-strike propagation of a simple thrust ramp and attendant fault-related folding. Our numerical model was composed of about 83,000 spherical elastic particles that were bonded together to create an initial solid numerical rock mass, capable of macroscopic fracture and faulting in response to displacement loading. The algorithim is fully dynamic and as such allowed for the storage and sudden release of elastic strain energy in the form of seismic events. We induced progressive along-strike fault-growth and attendant non-cylindrical fold development by varying the along-strike friction within a seeded, planar zone of weakness within the aggregate, a zone that was to become the fault. The result was a well-developed, plunging ramp anticline that grew in amplitude and along-strike length as the macroscopic fault propagated from lower to higher friction regions. Lateral fault propagation was relatively rapid compared with shortening, with a ratio of thrust displacement-to-propagation length ( K) of 6.5%. Incremental motion of the particles reflected episodic slip at both the laterally propagating fault-tip and along the existing fault. These episodes were characterized by local clusters of outward radiating velocity vectors that propagated at seismic velocities, marking the model equivalents of earthquakes. The finite displacement vectors in the hanging wall were approximately parallel to the far-field transport direction, with a strike-parallel component that ranged from one to two orders of magnitude less than the strike-perpendicular component. Furthermore, about two-thirds of the particles had strike-parallel motion in the direction of fault propagation and fold plunge. Because the far-field, shortening boundary conditions were uniformly perpendicular to fault strike, this component of motion in the plunge direction was apparently due to a topographically-induced, plunge-parallel shear stress. The along-strike motion involved slip on the main fault as well as transient strike slip on minor vertical, sometimes conjugate zones, apparently induced by the increased topographic load.

Mid-Tertiary rifting and magmatism in the Traill Ø region, East Greenland

A number of planar normal faults displace Tertiary basaltic intrusions in the Traill Ø region of East Greenland. This 'post-magmatic' extension occurs over an along-strike length of &gt;350 km and over an across-strike width of &gt;100 km with fault displacements and fault-block rotations indicating that (3 increases in magnitude eastwards from near unity to β ≈ 1.1. The stretching direction was approximately E-W, orthogonal to the strike of major faults in the region, most of which are reactivated Mesozoic structures. Two periods of Tertiary basaltic magmatism are recognized in the area. 40 Ar– 39 Ar step heating suggests that the first event, which gave rise to tholeiitic sills with a composite exposed thickness of c. 1 km, occurred at c. 54 Ma, and the second, which formed smaller volumes of alkaline dykes and two large syenite complexes, occurred at c. 36 Ma. Field observations indicate that 'post-magmatic' extension occurred after 54 Ma and possibly continued after 36 Ma. The tholeiitic intrusions are probably related to a stretching event (β ≤ 1.1) prior to the onset of sea-floor spreading in the Norway Basin, whilst the alkaline basalts and 'post-magmatic' rifting are associated with the separation of the Jan Mayen continental block from the East Greenland margin during the Eocene-Oligocene. Both tholeiitic and alkaline basalts formed by the partial melting of garnet-spinel peridotite with greater degrees of partial melting (8–16%) for the earlier tholeiitic group. Rifting in East Greenland ceased with the development of an oceanic ridge between Jan Mayen and East Greenland in late Oligocene time. Subsequent minor compression, probably the result of ridge-push, is suggested by the development of broad wavelength, low amplitude folds in the Traill Ø region.

Diffuse continental deformation: length scales, rates and metamorphic evolution

In contrast with the oceanic portions of the plates, continents may deform hundreds of kilometres away from any plate boundary. Calculations that treat the continents as continuous media suggest that the across-strike length scale of deformation associated with a convergent boundary is proportional to the along-strike length of the boundary, the constant of proportionality depending on the rheology of the lithosphere. The dependence on convergent boundary length implies that, for fixed velocity of convergence, strain rates decrease with increasing length of the orogen. The dependence on rheology is less straightforward, but it appears from laboratory determination of flow laws that the scale length of compressional deformation decreases as the fraction of the lithospheric strength supported by friction on faults increases; that fraction depends on the thermal profile of the continental lithosphere and will change during the evolution of an orogenic belt. The thermal development of a diffusely deforming compressional belt is nearly independent of its strain history unless the strain rates are less than about 10 -15 s -1 . If erosion terminates the heating phase of such an orogenic belt, the dominant control on the metamorphic history is the heat supply to the continental lithosphere. In contrast, if extension of the thickened crust ends metamorphism, peak metamorphic conditions depend only on the rheological properties of the lithosphere. Metamorphism in crust that is subject to major extension following compressional orogenesis should be nearly independent of the initial thermal conditions of the crust, and may be distinguished from metamorphism terminated by erosion by a final stage of isobaric cooling from temperatures close to the maximum experienced.

Plate tectonics synthesis: The displacements between Australia, New Zealand, and Antarctica since the Late Cretaceous

A comprehensive study of the last 75 m.y. of plate tectonics history has been undertaken for the region south of 30°S in the South Pacific, Southeast Indian Ocean and the Tasman Sea. Some aspects of plate boundary evolution have been clarified by our compilation and examination of available marine geophysical data. Reidentification of magnetic lineations in the southern Tasman basin shows that the controversial interval of subduction of Tasman basin crust along the east Australian margin that was previously proposed is no longer necessary. A comparison of Cenozoic magnetic lineations from both sides of the easternmost spreading segment of southeast Indian ridge indicates that a portion of the Indian plate younger than anomaly 10 (32 m.y.B.P.) is missing. We suggest that the missing crust was either subducted beneath or captured by the Pacific plate. Older lineations on the Indian plate out to about anomaly 21 have greater along-strike lengths than their counterparts on the Antarctic plate. The difference is due to an interval of crustal accretion at the Indian—Pacific plate boundary in the Early to Middle Tertiary. In the South Pacific, the Antarctic plate may not have extended northeast of the Eltanin fracture zone system prior to anomaly 29 (69 m.y.B.P.). Subduction of oceanic lithosphere was probably occurring beneath the Antarctic peninsula and eastern Ellsworth Land parts of the Antarctic plate at that time. Between anomaly-29 time and a major reorganization of plate boundaries in the Late Oligocene, plate interactions occurred in the central South Pacific between the Pacific, Farallon, Antarctic, Aluk and possibly a fifth plate. Spreading rate calculations for the Early Oligocene indicate that a simple three-plate system involving the Pacific, Farallon and Antarctic plates is difficult to maintain unless highly asymmetric spreading occurred at the Farallon—Antarctic boundary in the Early Oligocene. Further to the southeast in the Bellingshausen basin, spreading occurred along segments of the Antarctic—Aluk plate boundary beginning at about anomaly-29 time. Collisions of segments of this boundary with the trench along the Antarctic peninsula occurred in the Early and Middle Tertiary and resulted in the total disappearance of portions of Aluk plate. The collisions brought Antarctic—Aluk ridge segments into contact with Antarctic—Aluk trench segments with resulting stabilization of the Antarctic continental margin. From the magnetic lineations mapped in the southern oceans, we calculated new finite rotations which determine the relative positions of the Australian, New Zealand, and Antarctic continental fragments at 5–10 m.y. intervals during the last 75 m.y. For the New Zealand region, finite rotations calculated from magnetic lineations in the Tasman Sea and southwestern Pacific indicate that three plates were active during the Late Cretaceous to Paleocene. Previous workers have proposed that one of the three plate boundaries occurred southward between East and West Antarctica approximately along the trend of the Transantarctic mountains. As no direct geologic or geophysical evidence in Antarctica supports this proposal and since sea-floor spreading magnetic evidence shows that an active plate boundary passed through the New Zealand region after the Paleocene, we prefer to carry the required plate boundary northward. The Indian—Pacific poles of relative motion were always close to this boundary during the Cenozoic and thus corresponding evidence for plate interactions in New Zealand continental geology is generally variable and sometimes subtle.