Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data

A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr(-1) (refs 1-3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the approximately 1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting.

Modelled palaeolatitudes for the Louisville hot spot and the Ontong Java Plateau

Formation of the Ontong Java Plateau (OJP), a large igneous province in the western Pacific, has been attributed to a rising plume head in the initial stage of the Louisville hot spot, approximately 120–125 Ma ago. However, the Neal et al. plate reconstruction suggests that the plateau formed approximately 9° north of the current location of this hot spot at 51°S. The magnetization of the plateau’s basement records a palaeolatitude of approximately 25°S which further increases the discrepancy with the plume-head model. Modelling the motion of the Louisville hot spot for the last 120 Ma yields a possible southward motion of up to about 6°. True polar wander (TPW) models also shift the predicted palaeolatitudes of the plateau farther north. Taking into account both hot-spot motion and TPW, formation of the OJP by the Louisville not spot remains a possibility.

Cenozoic climate change as a possible cause for the rise of the Andes

Causal links between the rise of a large mountain range and climate have often been considered to work in one direction, with significant uplift provoking climate change. Here we propose a mechanism by which Cenozoic climate change could have caused the rise of the Andes. Based on considerations of the force balance in the South American lithosphere, we suggest that the height of, and tectonics in, the Andes are strongly controlled both by shear stresses along the plate interface in the subduction zone and by buoyancy stress contrasts between the trench and highlands, and shear stresses in the subduction zone depend on the amount of subducted sediments. We propose that the dynamics of subduction and mountain-building in this region are controlled by the processes of erosion and sediment deposition, and ultimately climate. In central South America, climate-controlled sediment starvation would then cause high shear stress, focusing the plate boundary stresses that support the high Andes.

The Emperor Seamounts: southward motion of the Hawaiian hotspot plume in Earth's mantle

The Hawaiian-Emperor hotspot track has a prominent bend, which has served as the basis for the theory that the Hawaiian hotspot, fixed in the deep mantle, traced a change in plate motion. However, paleomagnetic and radiometric age data from samples recovered by ocean drilling define an age-progressive paleolatitude history, indicating that the Emperor Seamount trend was principally formed by the rapid motion (over 40 millimeters per year) of the Hawaiian hotspot plume during Late Cretaceous to early-Tertiary times (81 to 47 million years ago). Evidence for motion of the Hawaiian plume affects models of mantle convection and plate tectonics, changing our understanding of terrestrial dynamics.

The magnetic sense and its use in long-distance navigation by animals

True navigation by animals is likely to depend on events occurring in the individual cells that detect magnetic fields. Minimum thresholds of detection, perception and 'interpretation' of magnetic field stimuli must be met if animals are to use a magnetic sense to navigate. Recent technological advances in animal tracking devices now make it possible to test predictions from models of navigation based on the use of variations in magnetic intensity.

Simultaneous Rupture Along Two Conjugate Planes of the Wharton Basin Earthquake

Analysis of broadband teleseismic data shows that the 18 June 2000 Wharton Basin earthquake, a moment magnitude 7.8 intraplate event in the region of diffuse deformation separating the Indian and Australian plates, consisted of two subevents that simultaneously ruptured two near-conjugate planes. This mode of rupture accommodates shortening by a mechanism different from that previously known elsewhere in the region. The larger subevent occurred on a fossil fracture zone, with a relatively high stress drop of about 20 megapascals, showing that large stresses can accumulate in regions of distributed deformation.