Tectonic history of the terrestrial planets

It is impossible in a single brief summary to convey the full range of research results that have come from this project over the last 13 years. The sweep of subjects covered ranges widely over the broad areas of the thermal and tectonic evolution of the terrestrial planets. A full list of all publications supported by this grant is presented. The list includes 48 published journal articles, 2 papers currently in press, 3 chapters of books, 4 M.I.T. theses, 1 technical report, and 107 published abstracts and extended abstracts. All of these publications were submitted separately to NASA at the time of publication or submission

Secular cooling of Earth as a source of intraplate stress

The once popular idea that changes in planetary volume play an important role in terrestrial orogeny and tectonics was generally discarded with the acceptance of plate tectonics. It is nonetheless likely that the Earth has been steadily cooling over the past 3-4 billion years, and the global contraction that accompanied such cooling would have led to a secular decrease in the radius of curvature of the plates. The implications of this global cooling and contraction are explored here for the intraplate stress field and the evolution of continental plates

The tectonics of Venus: An overview

While the Pioneer Venus altimeter, Earth-based radar observatories, and the Venera 15-16 orbital imaging radars provided views of large-scale tectonic features on Venus at ever-increasing resolution, the radar images from Magellan constitute an improvement in resolution of at least an order of magnitude over the best previously available. A summary of early Magellan observations of tectonic features on Venus was published, but data available at that time were restricted to the first month of mapping and represented only about 15 percent of the surface of the planet. Magellan images and altimetry are now available for more than 95 percent of the Venus surface. Thus a more global perspective may be taken on the styles and distribution of lithospheric deformation on Venus and their implications for the tectonic history of the planet

Transform fault earthquakes in the North Atlantic: Source mechanisms and depth of faulting

The centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern Mid-Atlantic Ridge were determined from an inversion of long-period body waveforms. The earthquakes occurred on the Gibbs, Oceanographer, Hayes, Kane, 15 deg 20 min, and Vema transforms. The depth extent of faulting during each earthquake was estimated from the centroid depth and the fault width. The source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes; slip vector azimuths agree to 2 to 3 deg of the local strike of the zone of active faulting. The only anomalies in mechanism were for two earthquakes near the western end of the Vema transform which occurred on significantly nonvertical fault planes. Secondary faulting, occurring either precursory to or near the end of the main episode of strike-slip rupture, was observed for 5 of the 12 earthquakes. For three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform. In all three cases, the site of secondary reverse faulting is near a compression jog in the current trace of the active transform fault zone. No evidence was found to support the conclusions of Engeln, Wiens, and Stein that oceanic transform faults in general are either hotter than expected from current thermal models or weaker than normal oceanic lithosphere

The gabbro-eclogite phase transition and the elevation of mountain belts on Venus

Among the four mountain belts surrounding Lakshmi Planum, Maxwell Montes is the highest and stands up to 11 km above the mean planetary radius and 7 km above Lakshmi Planum. The bulk composition and radioactive heat production of the crust on Venus, where measured, are similar to those of terrestrial tholeiitic basalt. Because the thickness of the low-density crust may be limited by the gabbro-garnet granulite-eclogite phase transitions, the 7-11 km maximum elevation of Maxwell Montes is difficult to understand except in the unlikely situation that the crust contains a large volume of magma. A possible explanation is that the base of the crust is not in phase equilibrium. It has been suggested that under completely dry conditions, the gabbro-eclogite phase transition takes place by solid-state diffusion and may require a geologically significant time to run to completion. Solid-state diffusion is a strongly temperature-dependent process. In this paper we solve the thermal evolution of the mountain belt to attempt to constrain the depth of the gabbro-eclogite transition and thus to assess this hypothesis quantitatively. The one-dimensional heat equation is solved numerically by a finite difference approximation. The deformation of the horizontally shortening crustal and mantle portions of the thermal boundary layer is assumed to occur by pure shear, and therefore the vertical velocity is given by the product of the horizontal strain rate and depth

Estimates of elastic plate thicknesses beneath large volcanos on Venus

Megellan radar imaging and topography data are now available for a number of volcanos on Venus greater than 100 km in radius. These data can be examined to reveal evidence of the flexural response of the lithosphere to the volcanic load. On Earth, flexure beneath large hotspot volcanos results in an annual topographic moat that is partially to completely filled in by sedimentation and mass wasting from the volcano's flanks. On Venus, erosion and sediment deposition are considered to be negligible at the resolution of Magellan images. Thus, it may be possible to observe evidence of flexure by the ponding of recent volcanic flows in the moat. We also might expect to find topographic signals from unfilled moats surrounding large volcanos on Venus, although these signals may be partially obscured by regional topography. Also, in the absence of sedimentation, tectonic evidence of deformation around large volcanos should be evident except where buried by very young flows. We use analytic solutions in axisymmetric geometry for deflections and stresses resulting from loading of a plate overlying an inviscid fluid. Solutions for a set of disk loads are superimposed to obtain a solution for a conical volcano. The deflection of the lithosphere produces an annular depression or moat, the extent of which can be estimated by measuring the distance from the volcano's edge to the first zero crossing or to the peak of the flexural arch. Magellan altimetry data records (ARCDRs) from data cycle 1 are processed using the GMT mapping and graphics software to produce topographic contour maps of the volcanos. We then take topographic profiles that cut across the annular and ponded flows seen on the radar images. By comparing the locations of these flows to the predicted moat locations from a range of models, we estimate the elastic plate thickness that best fits the observations, together with the uncertainty in that estimate

Gravitational spreading of Danu, Freyja and Maxwell Montes, Venus

The potential energy of elevated terrain tends to drive the collapse of the topography. This process of gravitational spreading is likely to be more important on Venus than on Earth because the higher surface temperature weakens the crust. The highest topography on Venus is Ishtar Terra. The high plateau of Lakshmi Planum has an average elevation of 3 km above mean planetary radius, and is surrounded by mountain belts. Freyja, Danu, and Maxwell Montes rise, on average, an additional 3, 0.5, and 5 km above the plateau, respectively. Recent high resolution Magellan radar images of this area, east of approx. 330 deg E, reveal widespread evidence for gravity spreading. Some observational evidence is described for gravity spreading and the implications are discussed in terms of simple mechanical models. Several simple models predict that gravity spreading should be an important process on Venus. One difficulty in using remote observations to infer interior properties is that the observed features may not have formed in response to stresses which are still active. Several causes of surface topography are briefly examined

A test of the hypothesis that impact-induced fractures are preferred sites for later tectonic activity

Impact cratering has been an important process in the solar system. The cratering event is generally accompanied by faulting in adjacent terrain. Impact-induced faults are nearly ubiquitous over large areas on the terrestrial planets. The suggestion is made that these fault systems, particularly those associated with the largest impact features are preferred sites for later deformation in response to lithospheric stresses generated by other processes. The evidence is a perceived clustering of orientations of tectonic features either radial or concentric to the crater or basin in question. An opportunity exists to test this suggestion more directly on Earth. The terrestrial continents contain more than 100 known or probable impact craters, with associated geological structures mapped to varying levels of detail. Prime facie evidence for reactivation of crater-induced faults would be the occurrence of earthquakes on these faults in response to the intraplate stress field. Either an alignment of epicenters with mapped fault traces or fault plane solutions indicating slip on a plane approximately coincident with that inferred for a crater-induced fault would be sufficient to demonstrate such an association

Confronting the Peppercorn Settlement in Merger Litigation: An Empirical Analysis and a Proposal for Reform

Shareholder litigation challenging corporate mergers is ubiquitous, with the likelihood of a shareholder suit exceeding 90%. The value of this litigation, however, is questionable. The vast majority of merger cases settle for nothing more than supplemental disclosures in the merger proxy statement. The attorneys that bring these lawsuits are compensated for their efforts with a court-awarded fee. This leads critics to charge that merger litigation benefits only the lawyers who bring the claims, not the shareholders they represent. In response, defenders of merger litigation argue that the lawsuits serve a useful oversight function and that the improved disclosures that result are beneficial to shareholders. This Article offers a new approach to assessing the value of these claims by empirically testing the relationship between merger litigation and shareholder voting on the merger. If the supplemental disclosures produced by the settlement of merger litigation are valuable, they should affect shareholder voting behavior. Specifically, supplemental disclosures that are, in effect, “compelled” by settlement should produce new and unfavorable information about the merger and lead to a lower percentage of shares voted in favor of it. Applying this hypothesis to a hand-collected sample of 453 large public company mergers from 2005-2012, we find no such effect. We find no significant evidence that disclosure-only settlements affect shareholder voting. These findings warrant a reconsideration of Delaware merger law. Specifically, under current law, supplemental disclosures are viewed by courts as providing a substantial benefit to the shareholder class. In turn, this substantial benefit entitles the plaintiffs’ lawyers to an award of attorneys’ fees. Our evidence suggests that this legal analysis is misguided and that supplemental disclosures do not in fact constitute a substantial benefit. As a result, and in light of the substantial costs generated by public company merger litigation, we argue that courts should reject disclosure settlements as a basis for attorney fee awards. Our approach responds to critiques of merger litigation as excessive and frivolous by reducing the incentive for plaintiffs’ lawyers to bring weak cases, but it would have an additional benefit. Current practice drags state court judges into the task of indirectly promulgating disclosure standards in connection with the approval of fee awards. We argue, instead, for a more efficient specialization between state and federal courts in the regulation of mergers: public company merger disclosure should be policed by the federal securities laws while state corporate law focuses on substantive fairness

Confronting the Peppercorn Settlement in Merger Litigation: An Empirical Analysis and a Proposal for Reform

Shareholder litigation challenging corporate mergers is ubiquitous, with the likelihood of a shareholder suit exceeding 90%. The value of this litigation, however, is questionable. The vast majority of merger cases settle for nothing more than supplemental disclosures in the merger proxy statement. The attorneys that bring these lawsuits are compensated for their efforts with a court-awarded fee. This leads critics to charge that merger litigation benefits only the lawyers who bring the claims, not the shareholders they represent. In response, defenders of merger litigation argue that the lawsuits serve a useful oversight function and that the improved disclosures that result are beneficial to shareholders. This Article offers a new approach to assessing the value of these claims by empirically testing the relationship between merger litigation and shareholder voting on the merger. If the supplemental disclosures produced by the settlement of merger litigation are valuable, they should affect shareholder voting behavior. Specifically, supplemental disclosures that are, in effect, “compelled” by settlement should produce new and unfavorable information about the merger and lead to a lower percentage of shares voted in favor of it. Applying this hypothesis to a hand-collected sample of 453 large public company mergers from 2005-2012, we find no such effect. We find no significant evidence that disclosure-only settlements affect shareholder voting. These findings warrant a reconsideration of Delaware merger law. Specifically, under current law, supplemental disclosures are viewed by courts as providing a substantial benefit to the shareholder class. In turn, this substantial benefit entitles the plaintiffs’ lawyers to an award of attorneys’ fees. Our evidence suggests that this legal analysis is misguided and that supplemental disclosures do not in fact constitute a substantial benefit. As a result, and in light of the substantial costs generated by public company merger litigation, we argue that courts should reject disclosure settlements as a basis for attorney fee awards. Our approach responds to critiques of merger litigation as excessive and frivolous by reducing the incentive for plaintiffs’ lawyers to bring weak cases, but it would have an additional benefit. Current practice drags state court judges into the task of indirectly promulgating disclosure standards in connection with the approval of fee awards. We argue, instead, for a more efficient specialization between state and federal courts in the regulation of mergers: public company merger disclosure should be policed by the federal securities laws while state corporate law focuses on substantive fairness