Details

Autor(en) / Beteiligte

• Evangelidis, Christos P

Titel

Three-dimensional traveltime tomography of Ascension Island and the Mendocino Triple Junction area

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2004

Beschreibungen/Notizen

• Seismic traveltime tomography was performed on two wide-angle three-dimensional datasets in the regions around Ascension Island and the Mendocino Triple Junction. Ascension Island forms the summit of a large volcanic edifice in the equatorial Atlantic. The experiment covers an area roughly 80 km x 70 km and consists of offshore shots received on land and at sea. The shallow extrusive part of the Ascension edifice reveals two main high velocity regions coinciding with the highest topography on land and a gravity maximum off the west coast of the island, respectively. These features are connected to a high velocity intrusive core that is created either within or on the top of oceanic Layer 3 and is interpreted as a possible relic magma chamber. The thickness of the surface low velocity region is similar to that observed at Hawaii, Jasper Seamount and Great Meteor Seamount, suggesting a similar process of edifice construction. There is no evidence for magmatic underplating beneath Ascension Island. There is no simple flexural model that explains the shape of the Moho beneath the island, perhaps because of the long lived volcanism and the proximity of the island to the Mid-Atlantic Ridge and the Ascension Fracture Zone. The second experiment collected wide-angle seismic data around the Mendocino Triple Junction in and offshore northern California, and covered an area roughly 280 km x 270 km. The results of a first arrival tomographic model are complemented by the results of a model based on Pg arrivals. In addition to the seismic tomography model, the hypocentre of earthquakes that occurred within the study area between 1977 and 2002 are relatively relocated using source-specific station terms and waveform cross correlation techniques. A unifying 3D picture of the Mendocino triple junction velocity structure is in agreement with most previously published 2D seismic studies. Apart from extending the major velocity features in three-dimensions, this study also reveals significant offline features that were not imaged before. High velocity bodies within the North America crust are possibly related to outliers of the Coastal Range ophiolites. The San Andreas fault cuts through the crust, separating the Pacific plate from North America. It passes onshore north of Point Delgada, with the King Range terrane being a part of the Pacific plate or a sliver in the boundary zone. Offshore, within the Vizcaino block, an oceanic high velocity body is observed within the upper crust, independent from the underlying lower crust. North of the Mendocino fault the subducting Gorda slab appears to be internally deformed as it approaches the triple junction in a series of dipping vertical velocity offsets, from northwest to southeast, and intra-slab seismicity in NE-SW lineations. The relocated Petrolia earthquake, in April 1992, is a Cascadia thrust event that triggers shallower events up-dip the thrust and within the North America accreted terranes, as well as events deep into the subducting slab and the subducting Moho. Finally, some limitations of the forward and inverse methods used in active source tomography have been also explored.