Effective elastic thickness of the lithosphere along the Easter Seamount Chain

Bathymetry and gravity data collected during Legs 5, 6, and 7 of the 1993 GLORIA Expedition and the recently released 2-min altimetry-derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the Easter Seamount Chain (ESC). Forward modeling, admittance, and coherence methods yield consistent results. With the exception of the eastern and western ends of the ESC the effective elastic thickness along the chain is similar to 1-4 km. The thin elastic thickness for the majority of the ESC seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the Nazca plate seafloor age. The elastic thickness southeast of the Nazca fracture zone is similar to 6 km, apparently because of the seafloor age discontinuity across the fracture zone. The elastic thickness near the San Felix Island, at the eastern end of the ESC, is even greater (similar to 11 km), which is compatible with the estimated seafloor age at the time of loading. A slight increase in the effective elastic thickness of the far western part of the ESC suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center. These findings combined with published geochemistry support a hotspot origin for the ESC, complicated by large-scale plate boundary reorganizations and channeling of plume material to the East Pacific Rise

Chronology and Geochemistry of Lavas from the Nazca Ridge and Easter Seamount Chain: an ∼30 Myr Hotspot Record

The Easter Seamount Chain and Nazca Ridge are two of the most conspicuous volcanic features on the Nazca plate. Many questions about their nature and origin have remained unresolved because of a lack of geochronological and geochemical data for large portions of both chains. New ⁴⁰Ar⁻³⁹Ar incremental heating age determinations for dredged rocks from volcanoes east of Salas y Gomez Island show that, with very few exceptions, ages increase steadily to the east from 1·4 to 30 Ma, confirming that the two chains are parts of the same hotspot trail and indicating a hotspot location near Salas y Gomez rather than beneath Easter Island some 400 km farther west. Most of the volcanoes appear to have been erupted onto seafloor that was 5–13 Myr old, and no systematic variation in seafloor age at the time of seamount formation is apparent. At about 23 Ma, the formation of the Nazca Ridge ceased and that of the Easter Seamount Chain began, corresponding to a change in the direction of motion of the Nazca plate. Most of the studied rocks are moderately alkalic to transitional basalts. Their geochemical characteristics suggest that they represent relatively small mean amounts of partial melting initiating in garnet-bearing mantle and ending in the spinel facies. Nd–Sr–Pb isotopic compositions are within the range of values previously observed for volcanoes of the Easter Seamount Chain, west of Easter Island; moreover, most of our data cluster in a rather small part of this range [e.g. ε[subscript Nd](t) is between +6·0 and +4·0]. The results indicate that the mantle source has consisted of the same two principal components, a C/FOZO-type component and a high-ε[subscript Nd], incompatible-element-depleted Pacific mid-ocean ridge basalt-source-type component, since at least 30 Ma. The lack of any geochemical gradient along the chain east of Salas y Gomez implies that no systematic change over time has occurred in the proportions of these end-members.Keywords: Nazca Ridge, Pb-Nd-Sr isotopic ratios, Salas y Gomez Island, Easter Seamount Chain, ⁴⁰Ar⁻³⁹Ar geochronolog

Improved X-ray detection and particle identification with avalanche photodiodes

Avalanche photodiodes are commonly used as detectors for low energy x-rays. In this work we report on a fitting technique used to account for different detector responses resulting from photo absorption in the various APD layers. The use of this technique results in an improvement of the energy resolution at 8.2 keV by up to a factor of 2, and corrects the timing information by up to 25 ns to account for space dependent electron drift time. In addition, this waveform analysis is used for particle identification, e.g. to distinguish between x-rays and MeV electrons in our experiment.Comment: 6 pages, 6 figure

Tectonic Evolution of the Easter Microplate

The plate tectonic history of the Easter microplate has been reconstructed by “closing” the microplate in a series of steps using the Pacific-Nazca magnetic anomalies north and south of the microplate and the NUVEL 1 global plate motion model. After each step, the Easter microplate was rotated rigidly to match the Nazca and Pacific anomalies. Gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation, respectively, and show that rigid plate motions are insufficient to explain the complete tectonic evolution. Analysis of the magnetic anomaly data was guided by contoured SeaMARC II, Sea Beam, and 3.5-kHz bathymetry data and a lineament map derived from SeaMARC II side scan and Sea Beam bathymetry data. The patterns of lineaments and bathymetric structures suggest that rotational deformation of the Nazca plate is the general mechanism that accommodates the space problems arising from transfer of the Nazca plate to the microplate and rapid rotation of the microplate against the Nazca plate. Similar but smaller amounts of deformation are predicted along the southern boundary of the microplate. Prior to the origin of the microplate, the East Pacific Rise (EPR) was offset in at least two places according to the older magnetic anomalies, yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets. The magnetic, bathymetry, and satellite altimetry data indicate that the microplate initially formed at (or perhaps southeast of) Easter Island near a left-lateral offset of the EPR sometime between anomaly 3 and 3\u27. The East Rift started propagating north from the present location of Easter Island at ∼4.5 Ma, which is ∼1.5 m.y. earlier than previously proposed. However, the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of Easter Island. The geometry of the microplate changes very rapidly during its evolution. At the initial stages of development, the microplate resembles a large propagating rift system, suggesting that deformation may have been occurring throughout most of its interior up to about 2.47 Ma. At this time, the length to width of overlap ratio of the two rifts reaches a value of 3, the northward propagation slows down, the curved opening of the Southwest Rift becomes well established, and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate. At this time, the offset distance between the two overlapped rifts starts to increase. Some time after 2.47 Ma and before 1 Ma, the East Rift starts propagating northwestward, probably in response to the microplate rotation, and continues up until present. Also during this time period, the East Rift breaks into a series of northward propagating rifts, each propagating into the microplate interior, thereby transferring lithosphere from the microplate to the Nazca plate and reducing the total growth rate of the microplate

Recent Pacific-Easter-Nazca Plate Motions

Instantaneous relative plate motions have been calculated for the Pacific, Easter and Nazca plates by inverting spreading rates since the Brunhes/Matuyama reversal boundary (obtained from modeling 39 magnetic anomaly profiles across the divergent boundaries of all three plates), along with 10 transform azimuths (obtained from recent SeaMARC II, GLORIA and Sea Beam data) and 20 published seismic slip vectors. The rates along the Nazca-Pacific and Nazca-Easter spreading axes increase to the south. The rates along the Pacific-Easter spreading axis decrease to the south. Along ∼2400 km of the southern Nazca-Pacific plate boundary where spreading rates range from 145 to 160 km there are no Nazca-Pacific transform faults where spreading axes are offset. Instead, the offsets are accommodated by microplates, propagating rifts, or overlapping spreading centers. The origin of the Easter microplate cannot be attributed solely to fast spreading rates along the preexisting Nazca-Pacific boundary because the fastest seafloor spreading is to the south of the microplate. The Nazca-Pacific Euler vector (0–0.73 Ma) from this study has a slower angular velocity and lies outside the confidence ellipse of the Minster and Jordan RM2 Euler vector (0–3.0 Ma). It also lies outside of the confidence ellipse of the DeMets et al. NUVEL-1 Euler vector (0–3.0 Ma) but has approximately the same angular velocity. Our preferred Euler vector describing the absolute motion of the Easter microplate is near the center of the microplate with an angular velocity of about 15°m.y., making it a fast ‘spinning\u27 plate. Oblique convergence is predicted along the proposed Nazca-Easter and Pacific-Easter transform segments of the proposed northern and southern triple junctions, respectively. The similarity between the best fitting Euler vector for all three plate pairs and the Euler vectors derived by the three-plate closure condition suggests the microplate interior is behaving mostly rigidly. Reduced chi-squared values and F-ratio tests support this finding. However, comparison of the predicted motion vectors with the observed structures interpreted to be microplate boundaries indicates that deformation must be occurring over a broad area along the northern microplate boundary. This deformation is suspected to be a direct consequence of the large-scale rift propagation and rapid microplate rotation