Donna M. Jurdy

Subducted lithosphere, hotspots, and the geoid

Abstract The earths largest positive geoid height anomalies are associated with subduction zones and hotspots. Although the correlation with subduction has been noted for many years, the correlation with hotspots is fully evident only when the subduction-related geoid highs are removed from the observed field. Using the assumption that subducted lithospheric slabs are uncompensated and are thermally re-equilibrated with the asthenosphere at the maximum depth of earthquakes, the expected geoid anomaly over subduction zones is calculated. This field provides a satis-factory fit to the observed circum-Pacific high. Subtraction of this predicted anomaly leaves a residual field which is correlated, at greater than the 99% confidence level, with the distribution of hotspots. Broad residual geoid highs occur over the central Pacific and the Africa/eastern Atlantic regions, the same areas where the hotspots are concentrated. The mass anomalies associated with hotspots and subducted slabs apparently control the location of the earths spin axis.

Seismic imaging of deep crust

We introduce here an integral two‐dimensional (2-D) scheme for the processing of deep crustal reflection profiles. This approach, in which migration occurs before stacking, is tailored to the unique character of the data in which nonvertically propagating energy is as important as vertically propagating energy. Since reflector depths range beyond 30 km, the horizontal displacement of reflections which occurs in migration can be as large as reflector depths; under these circumstances, the common‐midpoint (CMP) stack is inadequate. In our scheme, each common‐source trace gather is transformed into a set of traces (beams) corresponding to set of different incidence angles. A correction for wavefront curvature similar to the normal moveout (NMO) correction yields traces (focused beams) which are focused at image points along the direction of arrival. While the method is equivalent to the Kirchhoff integral migration method, and therefore to any complete continuation method, it gives rise to an intermediate da...

True polar wander

Abstract A re-evaluation of the existence of true polar wander (TPW) since the Late Cretaceous and a comparison among the various approaches are made using updated paleomagnetic, hotspot and relative motion datasets. Previous attempts to determine the existence of TPW had resulted in different conclusions: comparison of hotspot locations and paleomagnetic poles required significant pole motion, although lithospheric plate displacement analysis yielded insignificant motion. However, these earlier determinations cannot be directly compared to find the reason for the discrepancies, because each used different datasets. For this study the different approaches are applied to a single updated model with three alternative relative motions of East and West Antarctica. Although the results are model-dependent, in general there was not significant motion of the pole relative to the lithosphere (1–5°) since the early Tertiary, but a large motion (10–12°) relative to the hotspot framework. It is unlikely that errors in the determinations could account for this disagreement: the A95 of the plate reconstruction is about 3°, the uncertainty in Antarctica motion is estimated to no larger than 3°, and cumulative errors in the relative plate motions may also amount to 3°. Only if all these errors are present in the maximum estimated amount, and in the same direction, could they account for the 10–12° gap between the two approaches. This conclusion of pole motion relative to the hotspots, but not the lithosphere, may indicate an independent shift of the mesosphere relative to the lithosphere (or “mantle roll” of Hargraves and Duncan).

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 (

Was the Midcontinent Rift part of a successful seafloor-spreading episode?

The ~1.1 Ga Midcontinent Rift (MCR), the 3000 km long largely buried feature causing the largest gravity and magnetic anomaly within the North American craton, is traditionally considered a failed rift formed by isolated midplate volcanism and extension. We propose instead that the MCR formed as part of the rifting of Amazonia (Precambrian northeast South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established. A cusp in Laurentias apparent polar wander path near the onset of MCR volcanism, recorded by the MCRs volcanic rocks, likely reflects the rifting. This scenario is suggested by analogy with younger rifts elsewhere and consistent with the MCRs extension to northwest Alabama along the East Continent Gravity High, southern Appalachian rocks having Amazonian affinities, and recent identification of contemporaneous large igneous provinces in Amazonia.

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.

Pacific plate reconstructions and uncertainties

Abstract The Pacific plate is positioned into the hotspot reference frame, first directly relative to the Pacific hotspots, and then indirectly relative to the African hotspots by using seafloor spreading data. The reconstructions are repeated for 10, 15, 37, 48, and 61 Ma. The errors associated with each link for both approaches are expressed by covariance matrices which can be combined to yield the overall error in the case of the relative plate motion circuit. Assuming the hotspots are fixed, and that the reconstructions record the true relative positions of the plates, the reconstructions should be equivalent within their uncertainties. In fact, however, there is agreement within the uncertainties only for 10 Ma, with the misfit increasing with age for the earlier reconstructions. These misfits can he accounted for by three alternative explanations: large hotspot location uncertainties, a shift in time of the Pacific group of hotspots relative to the African group, or an additional plate boundary in the relative plate motion circuit, which we assume to be in Antarctica. By examining each of these possibilities in turn, we conclude that the uncertainties in individual hotspot locations required to bring the reconstructions into agreement are unreasonably large, and that the data available at present do not allow us to distinguish which of the remaining two possibilities may be the primary source of reconstruction errors.