Michael Stefanick

Stress observations and driving force models for the South American Plate

Stress observations are compared to predictions of driving force models for the South American plate. Stress observations have the potential to give detailed information about the forces driving the plates, in that sets of forces with the same net torque can produce quite different stress patterns. For a two-dimensional (vertical section) model of a plate there are three conditions for dynamical equilibrium: the net forces and the net torque about any point must all be zero. These conditions are used to estimate the magnitudes of forces as a function of age, with ridge push and slab pull as active forces, and plate drag and slab resistance as passive forces. The results are consistent with earlier published models, and indicate that the primary balance for an oceanic plate is between slab pull and the resisting drag forces. We generalize the model for the possible forces driving the South American plate and compare with observed stress measurements and kinematics. The South American plate has a small slab pull component, amounting to about one-fifth the ridge push force, with the other components reasonably isolated geographically, Thus, the plate balance is largely between ridge push and plate drag. This gives an opportunity of resolving the effects of trench suction or collision at the subduction zones. Most remarkable is the nearly uniform E-W orientation of the regional stress field, SHmax directions, which is extensional in the high Andes; a second feature is the N-S orientations in the Amazon basin. The two-dimensional, horizontal stress patterns are compared for successive models of combinations of driving forces. Models with ridge push, slab pull at the Scotia and Caribbean arcs, and trench suction near the west coast, balanced by plate drag, produce similar stress patterns to those observed. The magnitude of a possible trench suction force is estimated using a model with an eddy between the Nazca slab and the overlying South American plate.

Flow models for back-arc spreading

Abstract A secondary flow model for back-arc spreading is developed in this paper that shows some of the characteristics of observed back-arc spreading. Back-arc spreading has formed marginal seas around the west and southwest rim of the Pacific. The episodic spreading and different directions of opening are not completely understood; however, there does appear to be a limited lifetime (

Paleozoic plate dynamics

Current plate motions can be accounted for by a balance of active forces, slab pull, ridge push, and, for continental plates, trench suction, with drag beneath the plate as a resistive force. If we assume that the same forces have acted through time, we can reconstruct plate motions from the geometry of past plate boundaries. Paleozoic reconstructions are made with paleomagnetic, tectonic, climatic, and biogeographic data, as no ocean floor remains. PALEOMAP reconstructions are used to estimate past plate speeds and to test simple dynami- cal models in order to determine which ranges of forces best accounts for the observations. Over the last 600 m.y., plate speeds averaged over 40- to 100-m.y. intervals show considerable variation; Gondwanas speed oscillates from 20 to 60 km/m.y. over a long timescale (200-400 m.y.) with considerable noise superposed. Over the Paleozoic Era motions for large continental regions average 28 krn/m.y.; force balance models based on present-day observations suggest that continental regions without a large attached slab would move 30 mrn/yr. The opening and closing of the ocean between Laurentia and Gondwana 560-400 Ma is used to test dynamical models and the parameter values assumed. In the late Precambrian, Laurentia rifted away from Gondwana. In the earliest Cambrian it was near 40oS; by Late Cambrian and Ordovician it had moved to the equator. During the Silurian and Devonian, Laurentia reversed direction and later collided with Gondwana at 40oS. In a model of the forces acting on the plates, slab pull, ridge push, and trench suction are assumed to balance plate drag. Only certain ranges of ridge-push and trench parameters can model both the opening and subsequent closing of the ocean. The dynamic models, with parameter values inferred from present rates, bracket the rates required by the reconstructions.

Venus coronae, craters, and chasmata

The distributions of Venus coronae and craters are related to chasmata, which are thought to be extensional zones. Coronae are almost twice as dense near the chasmata as a random set of the same size. Of the various types of coronae, the radial-concentric and multiple are even more highly concentrated near chasmata, whereas the concentric-caldera type are absent near the chasmata. Craters, to the first order, are randomly distributed, although when distributions are compared with random sets there is a deficit of about 15-20 craters close to the chasmata. The tectonized and embayed craters tend to be near the rift zones, and their distribution closely resembles that of the coronae. The higher proportion of tectonized, and especially embayed, craters within 5-10° of the arcs, is about the fraction that would be expected from modification of the full set of craters over a width of 100-200 km. Kolmogorov-Smirnov statistics are used to compare cumulative distributions for craters and coronae with a large random distribution and to compute probabilities. Craters and coronae fill disjoint regions that are more connected than regions of randomly assigned points. This suggests that the volcano-tectonic process creating coronae may be the same one destroying craters.

The forces driving the plates: constraints from kinematics and stress observations

Plate kinematics and stress observations are used to assess the nature and relative magnitudes of the forces driving the plates. Dynamical equilibrium for a purely oceanic plate determines the relative magnitudes of the active and responding forces, largely balancing drag against slab-pull. Continental plates, moving more slowly, have errors of about 30% of the estimated motions. A torque balance model is used to describe the evolution of plate dynamics over the Cenozoic Era for reconstructions of plate geometry and velocities. Torques have been fairly stable for the past 64 Ma; the misfit to the model systematically increases for earlier times, most likely due to errors in the locations of past convergent plate boundaries and velocities. Unlike kinematics, which integrates the forces acting on a plate, the stress field responds locally and can differentiate between models for forces. Stress models which incorporate the forces are compared with stress orientations for the North and South American plates.

Vertical extrapolation of Mars magnetic potentials

[1] Mars Global Surveyor (MGS) measured the most strongly magnetized crust in the heavily cratered southern hemisphere of Mars. Our analysis concentrates on the magnetic lineations or patterns centered near latitude 40� S, longitude 180� W, with a range of values ±40� , using a rotated Cartesian coordinate system. We downward continued the magnetic field measured at � 400 km elevation and very closely match the corresponding component measured during the aerobraking phase at altitudes extending down to � 100 km. Using the vertical component of the magnetic field alone, we construct a unique scalar potential and independently obtain from the derivatives of this scalar potential the x and y components of the field. These derived components agree very well with the observed horizontal components. This demonstrates the validity and utility of the method and the Cartesian approximation, and also it confirms the consistency of the MGS magnetic data set. A model constructed with just 8 vertical dipoles accounts for 80% of the variance of the scalar potential at 400 km over the region analyzed, but 14 dipoles can account for only 64% of the variance at 100 km. We also construct the vector potential, the curl of which generates the three components of the magnetic field. This more complicated description may contain more physical meaning than the scalar potential. The vector potential shows abrupt changes in direction over the analyzed region, suggesting either different stages of magnetization or local demagnetization. INDEX TERMS: 5440 Planetology: Solid Surface Planets: Magnetic fields and magnetism; 5420 Planetology: Solid Surface Planets: Impact phenomena (includes cratering); 6225 Planetology: Solar System Objects: Mars; KEYWORDS: magnetic, Mars, models