Showing papers by "Sean C. Solomon published in 1976"

North American-Eurasian Plate Boundary in northeast Asia

Abstract: The intracontinental portion of the boundary between the North American and Eurasian plates can be identified ort the basis of seismicity, recent tectonics, and earthquake focal mechanisms. The simplest plate geometry that can explain these data involves a North American-Eurasian boundary that extends from the Nansen ridge through a broad zone of deformation in northeast Asia to the Sea of Okhotsk and thence southward through Sakhalin and Hokkaido to a triple junction in the Kuril-Japan trench. Such a configuration can account quantitatively for the slip vectors derived from earthquake mechanisms in Sakhalin and Hokkaido. On the basis of new slip vector data the North American-Eurasian angular velocity vector is revised only slightly from previous determinations. The intracontinental plate boundary is diffuse and may be controlled by ancient plate sutures. Deformation within about 10° of the rotation pole, which lies very near the boundary, cannot be modeled by rigid plate tectonics. These characteristics of intracontinental plate boundaries are related to the greater thickness and heterogeneity of continental lithosphere and to the influence of continents on the plate tectonic driving forces.

Intraplate stress as an indicator of plate tectonic driving forces

Abstract: To test driving force models for plate tectonics, the global intraplate stress fields predicted by various force systems are compared with the long-wavelength features of the observed stress field as determined by midplate earthquake mechanisms, in situ measurements, and stress-induced geologic structures. The calculated stresses are obtained by a finite difference solution to the equilibrium equations for thin elastic spherical shells in the membrane state of stress. Buoyancy forces at spreading centers and convergent plate boundaries and viscous drag at the base of the lithosphere are modeled as surface tractions applied to the shell. Drag is modeled as both a resistive and a driving force, and both symmetric and nonsymmetric forces at subduction zones are considered. The net driving push at spreading centers is found to be at least comparable in magnitude to other forces acting on the lithosphere and in particular is 0.7 to 1.5 times the net driving pull at convergence zones. Subducting lithosphere, which from seismic and thermal evidence has more potential energy available to drive plates than does a spreading center, thus converts relatively little of this energy to a net force acting on the surface plates. The drag coefficient at the base of the lithosphere may be greater by a factor of 3 to almost 10 beneath continents than beneath oceans without substantially affecting the fit between calculated and observed stress fields. Intraplate stresses calculated for models in which viscous drag at the base of the lithosphere acts in the direction of absolute plate velocity to drive plate motion are in much poorer agreement with observed stresses than are those calculated for models in which drag resists plate motions.

Geophysical constraints on radial and lateral temperature variations in the upper mantle

Abstract: Temperatures in the upper mantle may be inferred from measurements of physical properties which are strongly dependent on temperature and by identification of seismic discontinuities with phase transitions of measurable equilibrium boundaries. Temperature at the top of the low-velocity zone, equated with the onset of incipient melting, is generally 1100/sup 0/ +- 100/sup 0/C from a wide variety of evidence. The extensive melting detectable beneath mid-ocean ridges implies temperatures above 1250/sup 0/C in the shallow asthenosphere under spreading centers. Below a thermal boundary layer at the top of the asthenosphere, associated with convection on a scale secondary to plate motions, geotherms in the asthenosphere follow nearly adiabatic gradients and intersect the olivine-spinel transition at a temperature of 1300/sup 0/ +- 150/sup 0/C. The olivine--spinel transition is elevated about 100 km in subducted oceanic lithosphere, implying a 1000/sup 0/C temperature contrast between slab and normal mantle at 250 km depth. Systematic thermal differences between stable ocean basins and continental shields are necessary but are not likely to extend deeper than 200 km. Lateral temperature variations of perhaps 200/sup 0/C do persist deeper than 200 km and are probably associated with convective flow in the asthenosphere.

The Relationship Between Crustal Tectonics and Internal Evolution in the Moon and Mercury

Abstract: Faulting and volcanism on a planetary surface can be closely related to the thermal evolution of the planetary volume. Interior warming leads to global expansion, surface extensional tectonics and a crustal stress system that aids surface volcanism. Interior cooling leads to contraction, compressional tectonics and crustal stresses that act to shut off surface volcanism. Limits on the sign and magnitude of the change in volume of Mercury and the moon since the period of heavy bombardment on both bodies restrict possible early thermal histories. Mercury began hot throughout most of its volume, in order to have completed core-mantle differentiation in the first 0.6 b.y., and has cooled and contracted since. Solidification of an inner core in Mercury of up to 60% of the outer core radius is permitted by current limits on total contraction of the planet. The moon began with a 200–300 km thick outer shell at near melting conditions and a relatively cold deep interior, resulting in modest expansion for the first 2 b.y. and subsequent slight contraction. The duration of plains volcanism on a planet may be controlled by the length of the period of volumetric expansion and crustal extension in the planet's history. On this basis the youngest major units of plains volcanics on Mercury are predicted to be older than the youngest major mare units on the moon.