Loren W. Kroenke

Structure and deformation of north and central Malaita, Solomon Islands: tectonic implications for the Ontong Java Plateau-Solomon arc collision, and for the fate of oceanic plateaus

Abstract The island of Malaita, Solomon Islands, represents the obducted southern margin of the Ontong Java Plateau (OJP). The basement of Malaita formed during the first and possibly largest plateau-building magmatic event at ∼122 ± 3 Ma. It subsequently drifted passively northwards amassing a 1–2 km thickness of pelagic sediment overburden. A major change in OJP tectonics occurred during the Eocene, possibly initiated by the OJP passing over the Samoan or Raratongan hotspot. Extension facilitated increased sedimentation and basin formation (e.g., the Faufaumela basin) and provided readily available deep-crustal pathways for alkali basalt and subsequent Oligocene alnoite magmas, with related hydrothermal activity producing limited Ag + Pb mineralisation. Eocene to Mid-Miocene sediments record the input of arc-derived turbiditic volcaniclastic sediment indicating the relative closeness of the OJP to the Solomon arc. The initial collision of the OJP and Solomon arc at 25-20 Ma was of a ‘soft docking’ variety and did not result in major compressive deformation on Malaita. South-directed subduction of the Pacific Plate briefly ceased at this time but resumed intermittently on a local scale from ∼15 Ma. Subduction of the Australian Plate beneath the Solomon arc commenced at ∼8-7 Ma. Increased coupling between the Solomon arc and the OJP led to the gradual emergence of the OJP at 6-5 through to 4 Ma. The most intense period of compressive to transpressive deformation recorded on Malaita is stratigraphically bracketed at between 4 and 2 Ma, resulting in estimated crustal shortening of between 24 and 46%, and the inclusion of between 1 and 4 km of basement OJP basalts within the larger anticlines. Basement and cover sequences are deformed together in a coherent geometry and there are no major decollement surfaces; the large asymmetrical fold structures of Malaita are likely to be the tip regions of blind thrusts with detachment surfaces between 1 and 4 km beneath the cover sequence. Mid-Pliocene deformation records the detachment of the upper parts of the OJP, with initial material movement direction towards the northeast and later obduction of an upper allochthonous block of the OJP southwestwards over the Solomon arc. A model is presented whereby an upper 5–10-km-thick flake of the OJP is obducted over the Solomon arc to form the Malaita

Toward a self‐consistent, high‐resolution absolute plate motion model for the Pacific

The hot spot hypothesis postulates that linear volcanic trails form as lithospheric plates move relative to stationary or slowly moving plumes. Given geometry and ages from several trails, one can reconstruct absolute plate motions (APM) that provide valuable information about past and present tectonism, paleogeography, and volcanism. Most APM models have been designed by fitting small circles to coeval volcanic chain segments and determining stage rotation poles, opening angles, and time intervals. Unlike relative plate motion (RPM) models, such APM models suffer from oversimplicity, self-inconsistencies, inadequate fits to data, and lack of rigorous uncertainty estimates; in addition, they work only for fixed hot spots. Newer methods are now available that overcome many of these limitations. We present a technique that provides high-resolution APM models derived from stationary or moving hot spots (given prescribed paths). The simplest model assumes stationary hot spots, and an example of such a model is presented. Observations of geometry and chronology on the Pacific plate appear well explained by this type of model. Because it is a one-plate model, it does not discriminate between hot spot drift or true polar wander as explanations for inferred paleolatitudes from the Emperor chain. Whether there was significant relative motion within the hot spots under the Pacific plate during the last ∼70 m.y. is difficult to quantify, given the paucity and geological uncertainty of age determinations. Evidence in support of plume drift appears limited to the period before the 47 Ma Hawaii-Emperor Bend and, apart from the direct paleolatitude determinations, may have been somewhat exaggerated.

Intermediate-wavelength (400–600 km), South Pacific geoidal undulations: their relationship to linear volcanic chains

Abstract Intermediate-wavelength (400–600 km) geoidal undulations observed in SEASAT data from the South Pacific appear to be: (1) much more pronounced and extensive than the short-wavelengh lineations of Haxby and Weissel (which start, and are confined to within 1500 km of the EPR); (2) copolar, aligned roughly parallel to the absolute motion of the Pacific plate; (3) continuous across transform faults or fracture zones; and (4) surmounted at their crests, in places, by linear volcanic seamount chains formed since 43 Ma. Whereas the short-wavelength lineations could be caused by shallow gravitational instabilities forming in the uppermost mantle just below the base of the lithosphere, we think that the intermediate-wavelength lineations result from dynamic, elastic deformation of the lithosphere caused by well-developed, secondary convection rolls extending much deeper into the upper mantle. Because individual hotspot volcanic chains invariably coincide with crestal highs of discrete positive geoid lineations, the location of the hotspots is probably controlled by the rolls.

Pacific Plate motion and undulations in geoid and bathymetry

Abstract Previous studies have shown that the Pacific geoid and gravity fields exhibit lineated anomalies, trending approximately in the direction of absolute plate motion over the underlying mantle. Because the undulations obliquely cross fracture zones they have often been attributed a convective origin. Recently, lithospheric boudinage caused by diffuse extension has been proposed as a possible mechanism. We have examined the undulations in the free-air anomalies, geoid and bathymetry over a portion of the Pacific Plate to determine quantitatively how the undulations are related to plate motion. We compare the observed data to an axisymmetric, sinusoidal undulation defined in an arbitrary frame of reference; in particular, we seek the north pole of this reference frame that maximizes the correlation between data and model. Poles that are close to the Pacific hotspot pole represent copolar undulations possibly related to plate motion. The distance between the best-fitting poles and the hotspot pole is determined as a function of undulation wavelength and reveals several minima (with distance

Reconciling late Neogene Pacific absolute and relative plate motion changes

New models of Pacific absolute plate motion relative to hot spots and models of relative plate motion involving the Pacific plate all agree there was a significant change in the late Neogene (Chron 3A, ∼5.89 Ma), reflecting a more northerly absolute motion than previously determined. As Pacific absolute plate motion became slightly more northerly, left-stepping transform segments came under compression. Some left-stepping segments became microplates with clockwise rotation; others show clear evidence of compressional deformation. Conversely, right-stepping transforms came under tension, and many developed intratransform spreading centers or show similar evidence for transform magmatism. Several large left-stepping transform offsets represent portions of the Nazca plate protruding into the Pacific and as such act as obstacles to the more northerly Pacific absolute plate motion. We suggest these obstructions act to enhance the generally tensile equatorial Pacific stress regime caused by distant slab pull. As a consequence, the greater French Polynesia region has experienced diffuse volcanism that increased considerably following the Chron 3A plate motion change. We propose that since the Hawaii-Emperor Bend time, the distant Pacific slab pull and the friction between the large, buoyant Ontong Java plateau and the northern margin of the Australia plate have produced episodes of increased tensile stresses in the equatorial Pacific. We believe these stress excursions are responsible for much of the intraplate volcanism observed in a wide triangular region from Samoa to the East Pacific and Pacific-Antarctic Rises.

The record of Ontong Java Plateau: Main results of ODP Leg 130

The drilling campaign of ODP Leg 130 on Ontong Java Plateau resulted in the recovery of complete Neogene sections at several depths, providing materials for detailed biostratigraphic and paleoceanographic studies in the western equatorial Pacific. The acquisition of extensive logging records and high-resolution physical-property data allow detailed correlation from hole to hole and from site to site and provide the basis for a paleoceanographic interpretation of acoustic reflectors. We drilled 16 holes at 5 sites on the north-eastern flank of the plateau (Sites 803 through 807). All sites are close to the equator, at water depths ranging from 2,500 m to 3,900 m. Sites 803 and 807 penetrated into basement (26 m and 149 m, respectively). The K/T boundary was recovered at both of these sites. Neogene sedimentation rates decrease with depth, as expected, but this decrease is much greater than calculated from carbonate content, under the assumption that carbonate dissolution is the sole cause of the decrease. At any one site, sedimentation rates vary by a factor of more than two, with a striking maximum in the latest Miocene to early Pliocene, and strong minima in late early to early middle Miocene and in the Pleistocene. Many acoustic reflectors correlate between sites, within the limits of stratigraphic resolution. This suggests paleoceanographic events as a cause, generating changes in physical properties of sediments at the time of deposition. Many of the reflectors occur at carbonate reduction events (CREs). Some apparently are the product of diagenetic enhancement of property changes, as, for example, within the ooze/chalk transition (which is diachronous). The interval corresponding to the Cretaceous/Tertiary (K/T) transition in the area is characterized by the presence of a deep CCD. The sequence at one site is calcareous; that at the other, is not. The fact that the two K/T sections recovered occur in sequences with major hiatuses suggests special conditions for preservation during the transition. We propose early cementation caused by high silicate concentrations in an ocean with greatly reduced productivity. The basalt cored at Sites 803 and 807 is predominantly aphyric to sparsely olivine or plagioclase phyric; the last flows are Albian to Aptian in age. At Site 807, pillow lavas buried sediments. One very thick flow (∼28 m) was penetrated here, possibly a flood basalt, indicative of massive outpourings on Ontong Java Plateau during the middle Cretaceous.

Motion of the Ontong Java Plateau in the hot-spot frame of reference: 122 Ma-present

Abstract A new model of Pacific absolute plate motion between 140 and 0 Ma, generated in the fixed hot-spot frame of reference, has been used to track palaeogeographic positions of the Ontong Java Plateau (OJP) from the time (c. 122 Ma) and location (c. 43°S) of its formation to its present location north of the Solomon Islands. The resulting OJP seafloor flow-line suggests that changes in Pacific plate motion, passage over hot spots and Pacific Rim tectonism all have influenced the continuing structural development and deformation of the plateau. Satellite-derived gravity, bathymetry and Rayleigh-wave tomography potentially reveal the structural fabric of the OJP and adjoining Nauru Basin, including the orientation of probable fracture zones, location of possible relict spreading centres and the presence of a thick lithospheric root, as well as possible later hot-spot-related modification of the fabric. The most recent phase of OJP deformation, which began about 6 Ma, accelerated at 2.6 Ma and continues today, has resulted in the uplift of the islands of Malaita and Santa Isabel, and the formation of the Malaita Anticlinorium, with slip along the old fracture zones possibly triggering submarine canyon formation on the NE side of the OJP. This collision-related deformation also is probably responsible for the ongoing uplift and tilting of the islands of Nauru and Banaba NE of the OJP high plateau.

Tectonics and magma genesis in the northern North Fiji Basin

Abstract The North Fiji Basin is a complex back-arc basin in which active seafloor spreading is occurring along two north-south ridge systems. Lithospheric extension is also taking place along segments of the east-west trending Fiji Fracture Zone and South Pandora Ridge. The Fiji Fracture Zone is clearly a transform fault but the South Pandora Ridge is more difficult to interpret: some characteristics are consistent with a slow-spreading ridge whereas others suggest a transform fault. Basalts recovered on the South Pandora Ridge are enriched in high field strength and large ion lithophile elements and many of them resemble ocean island tholeiites or transitional alkali basalts. The South Pandora Ridge is an enigmatic tectonic feature and the geochemistry, morphology and seismicity are best explained in terms of a model involving slow-spreading ridge segments within a broad transform domain. At its southern end, the central ridge of the North Fiji Basin has a bathymetric profile normally associated with fast-spreading, steady-state, mid-ocean ridges, and the basalts recovered on this part of the ridge are moderately evolved normal mid-ocean ridge basalts (MORB). In contrast, the northern end of the ridge has a complex topography dominated by a series of deep linear troughs and grabens, and basalts recovered from the floors of these troughs vary widely in chemistry from normal MORBs to transitional basalts that are comparatively enriched in large ion lithophile and high field strength elements. The central ridge appears to be propagating northwards into old North Fiji Basin crust and the northern extension of the ridge is an incipient spreading locus in the very earliest stages of development. In areas of the basin where fast-spreading, large-scale mantle upwelling leads to extensive melting of asthenosphere-derived, depleted mantle, MORB-like compositions dominate the erupted rocks. In contrast, areas of slow or incipient spreading and other tectonic settings where a transient thermal disturvance occurs are characterized by basaltic magmatism that shows relative enrichment in large ion lithophile and high field strength minor and trace elements. Mixing of MORB and enriched end member components, either at source or as magmas, gives rise to a spectrum of compositions that can be related to a range of tectonic environments in the back-arc setting.

The possible reflection of mantle discontinuities in Pacific geoid and bathymetry

Geoid anomalies over the Pacific plate show lineated undulations approximately oriented in the direction of absolute plate motion. Conventional spectral analyses have revealed a broad range of dominant wavelengths, but as filtering is direction-dependent, results are difficult to interpret. Here, we present a new approach designed to quantify the correlation between the geoid undulations and Pacific plate motion. We calculate the spatial correlation between geoid lineations and arbitrarily oriented, axisymmetric sinusoidal undulations of given wavelength. The pole ( i.e., the orientation) of the sinusoid which maximizes the correlation is determined. The distance between this pole and the Pacific hotspot pole is calculated and represents a measure of how well a geoid lineation of a given wavelength aligns with Pacific plate motion. This distance varies with undulation wavelength and has discrete minima for several wavebands. These minima, moreover, occur at wavelengths whose values are remarkably close to the depths of seismically inferred mantle discontinuities. A similar analysis of available bathymetry corroborates these findings. If these correlations are significant, they suggest that geoid (and bathymetry) undulations are influenced by mantle structure and thus are likely to reflect mantle dynamics.

Mantle plume heads and the initiation of plate tectonic reorganizations

We present results of numerical simulations which examine the plausibility of a reorganization of plate motions brought about by the interaction of a hot, low-viscosity plume head with the underside of a tectonic plate. A numerical model of highly viscous fluid flow driven by thermal buoyancy is employed to examine the interaction of mantle plumes with surface plates in a two-dimensional cartesian geometry. Plate-like boundary conditions applied to the upper surface consist of a large active plate with fixed speed plus a smaller passive plate whose speed and direction of motion are permitted to react to tractions induced by the underlying mantle flow (driven by the active plate). The passive plate is also subject to applied end-loads intended to simulate incipient subduction. The mantle plume is initiated with a hot patch on the lower boundary, and viscosity varies with temperature following an Arrhenius rheology. The calculations, performed with fixed aspect ratio (4 × 1) and fixed Rayleigh number (Ra = 106, where Ra refers to the strength of the starting plume), were found to depend on three parameters: the strength of the viscosity variation, the size of the applied end-load, and the speed of the active plate. These calculations indicate that plate motions can be sharply influenced by the lubricating effects of mantle plume heads even to the point of causing a reversal in the direction of plate motion.