Basal sediment accretion along the south central Chile margin and implications for megathrust processes and great earthquakes
Abstract
The south-central Chile margin regularly produces great earthquakes such as the 1960 Mw 9.5 Valdivia event that ruptured an ~1,000 km x 100 km area and generated the largest event on record. Great earthquakes occur here along a smooth, broad, homogeneous plate interface that accumulates enough strain to generate 10s of m of slip; however, the processes causing such conditions remain poorly understood.
In 2017 we acquired ~5,000 km of seismic reflection data on the R/V Langseth using a 15-km, 1212-channel streamer and 6,600 in3 airgun source to examine structure and tectonics along the 2010 Mw 8.8 Maule and Valdivia ruptures. Processing removed multiples and noise to image structures within the upper plate and plate boundary. Along the Valdivia rupture area we find structures consistent with mostly basal rather than frontal accretion. Our lines show an unusually shallow decollement that allows 80 - 90% of the 1.5 - 3.0 km of trench turbidites to bypass the frontal wedge and subduct beneath the lower slope. On Line 28, which runs across the region of highest coseismic slip (10 - 40 m), a 100-500 m thick section of interpreted underthrust sediment is bounded by a fault above and the active decollement below between 10 to 25 km from the deformation front. It appears detached from the underthrust section and accreted to the upper plate. Directly overlying it are folded and disrupted packages of locally continuous reflectors that are consistent with a 25-km wide accretionary wedge of older underplated sediment formed seaward of the upper plate basement and buried by up to 1.5 km of deformed slope cover. Recent sediment underplating on Line 28 has removed < ¼ of the underthrust section, which may be typical in the 1960 and 2010 rupture areas because deeper images of the margin show the decollement > 1 km above the top of ocean crust. Unlike Coulomb wedges that form by frontal accretion, basal accretion can steepen slopes above critical taper, allowing coseismic slip with little internal deformation and enhancing slip to the wedge toe. Furthermore, the shallow decollement implies that subducting sediments may have a high frictional coefficient that resists forming deep detachments at the deformation front and during underplating. Large earthquakes here may thus result from a strong decollement that forms a broad, smooth and homogeneous megathrust.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T31C..03B
- Keywords:
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- 7230 Seismicity and tectonics;
- SEISMOLOGY;
- 8158 Plate motions: present and recent;
- TECTONOPHYSICS;
- 8170 Subduction zone processes;
- TECTONOPHYSICS;
- 8488 Volcanic hazards and risks;
- VOLCANOLOGY