Downward Continued Seismic Analysis of Axial Upper Crustal Structure at the Central Lau Spreading Center Propagating Tip
Abstract
In the Lau Basin, crust formed at the Eastern Lau Spreading Center is currently being rifted apart by the southward propagating tip of the Central Lau Spreading Center (CLSC). Densely spaced MultiChannel Seismic refraction and reflection data enables improved determination of the along- and across-axis shallow crustal structure of the southern ~140 km of the CLSC down to a depth of ~1-1.2 km. The objective is to characterize crustal structure associated with 1) the segmentation of an axial magma chamber (AMC) and 2) a propagating rift. We employ the Synthetic On Bottom Experiment (SOBE) downward continuation technique to increase the number of usable first arrival picks within the data allowing the creation of a continuous 2-D upper crustal tomographic model while also improving vertical resolution in the shallow crust. Ultimately, our updated velocities will be used to reprocess the reflection sections in order to more accurately characterize the distribution of crustal reflectors to assess the processes associated with the determined structure. Our new tomographic models show a dramatic change in the thickness of layer 2 as the tip of the propagator is approached. Based on shot gather analysis and tomography, crustal layer 2A (< 3 km/s) maintains a relatively constant thickness of ~150-250 m along the ~140 km of the CLSC immediately north of the propagating rift, showing some local variation in structure associated with the segmentation of the underlying AMC. Layer 2A transitions to a thickness of 500-600 m (including uppermost velocities < 2 km/s) starting about 15 km north of the bathymetrically defined ridge tip. Likewise, layer 2B (> 4-5 km/s) is imaged over a thickness of at least 500 m for the majority of the CLSC, but has reduced velocities or starts deeper near the propagator. Based on the southward along-axis transition from steep to shallow velocity gradient representing the layer 2A/2B transition, our results reflect a change in the accretionary mechanism toward the propagating tip associated with the loss of a steady state AMC. One interpretation of this change is from distinct layers of pillow lavas above sheeted dykes to a region with thicker pillow lavas and a more diffuse pillow/dyke boundary. Geologic interpretation of the velocity structure, however, depends on the chosen velocities for each layer and transition, the implications of which will be discussed. Reflection results to date support our initial inference, showing that both the AMC reflector and the layer 2A reflector disappear about 18 and 11 km, respectively, from the bathymetric signature of the propagating tip. Although seismic velocities south of the termination of these reflectors are considered characteristic of extrusive volcanic material, the character of the refracted arrival changes significantly south of these reflectors along and across the propagating tip, indicating a distinct upper crustal structure, perhaps consisting of aged and thickened layer 2A created previously by the ELSC. Additionally, across-axis models are employed to visualize the lateral transitions from crustal structures formed at the two spreading centers, across the trace of the propagator pseudofaults.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T53E..05H
- Keywords:
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- 3001 MARINE GEOLOGY AND GEOPHYSICS / Back-arc basin processes;
- 3025 MARINE GEOLOGY AND GEOPHYSICS / Marine seismics;
- 3035 MARINE GEOLOGY AND GEOPHYSICS / Midocean ridge processes