Joseph F. Engeln

Block rotation and continental extension in Afar: A comparison to oceanic microplate systems

The reorganization of oceanic spreading centers separating major plates often appears to occur by a process in which discrete microplates form and evolve by rift propagation. To see whether such microplate behavior has implications for continental rifting, we investigate the application of a microplate model to the Afar region at the Nubia-Somalia-Arabia triple junction. Studies of marine magnetic anomalies, volcanic ages, bathymetry, and seismicity suggest that the westward propagating Gulf of Aden spreading center has propagated into eastern Afar within the past 2 m.y., causing rifting and extension within the continent. We derive constraints on the extension history from the geometry and timing of rift formation and from paleomagnetic data indicating that Pliocene to Pleistocene age rocks have undergone a clockwise rotation of ∼11°. We suggest that the history of rifting, the rotation, and several other features of the regional geology can be described by combining features of an oceanic microplate model and the concept of rift localization previously proposed for Afar. In this scenario, motion occurring on several rifts within an extensional zone preceding the propagating spreading center is gradually transferred to a single rift. While motion is transferred, the overlap region between the growing and dying rifts acts as one or more microplates or blocks that rotate relative to the surrounding major plates. The rifting history and rotations in eastern Afar are thus related to the rift propagation and localization that occurs as the plate boundary evolves. Provided the constraints we use are appropriate, our model better describes the regional kinematics than alternative block models including one based on “bookshelf” faulting. If the tectonics of Afar are typical for continental breakup, they have interesting implications for the geometry of passive margins. In particular, asymmetric rifted margins can be produced if the final location of the rift axis is not at the center of the zone of initially disrupted lithosphere. Additionally, if the rate of rift propagation and the rate and location of rift localization are not uniform, then along-axis structural variations will result.

Tectonics of the Easter plate

Abstract We present a new model for the Easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges. We use the distribution of earthquakes, eleven new focal mechanisms, and existing bathymetric and magnetic data to describe the tectonics of this area. Both the Easter-Nazca and Easter-Pacific Euler poles are sufficiently close to the Easter plate to cause rapid changes in rates and directions of motion along the boundaries. The east and west boundaries are propagating and dying ridges; the southwest boundary is a slow-spreading ridge and the northern boundary is a complex zone of convergent and transform motion. The Easter plate may reflect the tectonics of rift propagation on a large scale, where rigid plate tectonics requires boundary reorientation. We use simple schematic models to illustrate the general features and processes which occur at plates resulting from large-scale rift propagation.

Physical response of a karst drainage basin to flood pulses : example of the Devil's Icebox cave system (Missouri, USA)

Abstract In karst aquifers, water moves from the recharge area (sinkhole plains and swallets) to the discharge area (springs), traveling kilometers through the groundwater system in a period of hours to days. Transit times through karst aquifers are a function of the conduit geometry and connectedness, intensity and duration of the recharge event, and antecedent soil moisture. Often many of these factors are unknown or difficult to quantify. Therefore, predicting the response of a karst basin to recharge is difficult. Numerous researchers have attempted to understand the response of a karst basin, but a good understanding of whether the response is dependent on local features or regional effects is currently lacking. From April 1994 to May 1995, flood pulse hydrographs from a karst aquifer with well-developed and well-documented conduits (Devils Icebox cave system) were obtained from a gaging station near the spring of the karst basin. Data were also collected from within the conduit system in an attempt to determine whether flow was locally controlled by constrictions in the conduits. Based on an application of Bernoullis equation, analyses of the changes in kinetic head and potential head over time indicated local control during storm events. The observed sediment patterns and water level variations also support localized flow control during storm events. A numerical model of the constrictions was tested and reproduced the responses observed at the spring during initial periods of storm events. The model illustrated that the constricted flow was very sensitive to recharge. It also illustrated the transition from local control due to constriction to regional controls due to the aquifer matrix.

Estimation of intraplate strain accumulation in the New Madrid seismic zone from repeat GPS surveys

The New Madrid seismic zone (NMSZ), a region within the eastern portion of the North American plate where great earthquakes occurred in 1811–1812, provides a type example of large earthquakes within a relatively stable continental interior. Assessing the scale and causes of deformation in such regions, as well estimation of the recurrence interval for the large earthquakes within them, has long been recognized as an important but challenging issue, for which the advent of space-based geodesy provides an important new tool. Here we report initial results from two Global Positioning System (GPS) surveys of a geodetic network spanning the New Madrid seismic zone. Because the expected strain signal is small, the data were analyzed in three different ways. First, site velocities were estimated and analyzed for spatial gradients parallel and perpendicular to the trend of the major strike-slip faults inferred from seismicity. This analysis suggests, but does not require, a few millimeters per year of right-lateral shear across the southern portion of the seismic zone, consistent with the earthquake focal mechanisms. Second, we divided the network into near-field subnetworks and found possibly significant shear associated with approximately E-W shortening within the southern NMSZ and possible N-S shortening across the left step connecting the presumed strike-slip faults. Third, we compared the strain observed along individual baselines to that predicted from the results of earlier near-field surveys which combined GPS and triangulation data. In the southern part of the seismic zone, we found general agreement with the earlier study, although our data suggest lower strain rates. Hence after only 2 years of measurements, the GPS data suggest but do not yet require detectable strain accumulation. However, given that great earthquakes occur in the area and that our results are reasonably consistent with the results of other geodetic studies using different data and with earthquake mechanisms, it seems likely that we are seeing strain accumulation in the southern NMSZ. A simple interpretation of the data as showing about 3 to 5 mm yr−1 of right-lateral fault-parallel motion implies a 1000 to 3300-year recurrence for great earthquakes with 5 to 10-m slip, assuming all the accumulating strain is released seismically. This recurrence interval may be an underestimate, if the observed strains are largely postseismic effects of the 1811–1812 earthquakes, a possibility suggested by the observation that plate-wide data show less motion across the NMSZ. Future surveys should resolve this issue as the longer measurement spans yield more precise velocity estimates.

Slow subduction of old lithosphere in the lesser antilles

Abstract The Lesser Antilles subduction zone is an extreme case of the subduction of old (~ 90 m.y.) lithosphere at a slow (~ 2 cm/y) convergence rate. Focal mechanisms of the largest earthquakes in the area have been obtained using body and surface wave data. During the time period (1950–1978) studied the subduction seismicity appears to represent primarily intraplate rather than interplate deformation. All three large (magnitude seven) earthquakes were from intraplate normal faults; no large thrust faulting earthquakes and few small ones occurred. These observations suggest that the plate boundary is largely decoupled, that subduction is at least partially aseismic, and that the downgoing slab is in a state of extension.

Tectonics of the Nazca-Antarctic plate boundary

Abstract We have constructed a new bathymetric chart of part of the Chile transform system, based mainly on an R/V “Endeavor” survey from 100°W to its intersection with the East Ridge of the Juan Fernandez microplate at 34°30′S, 109°15′W. A generally continuous lineated trend can be followed through the entire region, with the transform valley being relatively narrow and well-defined from 109°W to approximately 104°30′W. The fracture zone then widens to the east, with at least two probable en echelon offsets to the south at 104° and 102°W. Six new strike-slip mechanisms along the Chile Transform and one normal fault mechanism near the northern end of the Chile Rise, inverted together with other plate motion data from the eastern portion of the boundary, produce a new best fit Euler pole for the Nazca-Antarctic plate pair, providing tighter constraints on the relative plate motions.