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Coastal deformation and great subduction earthquakes, Isla Santa Maria, Chile (37 degrees S)

Daniel Melnick, Bodo Bookhagen, Helmut P. Echtler and Manfred R. Strecker
Coastal deformation and great subduction earthquakes, Isla Santa Maria, Chile (37 degrees S)
Geological Society of America Bulletin (December 2006) 118 (11-12): 1463-1480

Abstract

Isla Santa Maria at the active margin of south-central Chile is the result of earthquake-related uplift and deformation in the forearc since at least late Pleistocene time. Field mapping, dating of key depositional horizons, and analysis of seismic-reflection profiles reveal ongoing deformation in this sector of the Chilean forearc. The 30 km (super 2) island is located approximately 12 km above the interplate seismogenic zone and 75 km landward of the trench. It is situated near the southern termination of the Concepcion earthquake rupture segment, where Charles Darwin measured 3 m of coseismic uplift during a M > 8 megathrust earthquake in 1835. Permanent postearthquake deformation from this earthquake and an earlier event in 1751 is registered by emerged, landward-tilted abrasion surfaces. Uplift at approximately 2 m/k.y. and tilting at approximately 0.025 degrees /k.y. of the island have been fairly constant throughout the late Quaternary and have resulted in emergence of the island above sea level approximately 31 k.y. ago. The island is composed of a late Pleistocene upper, tilted surface with two asymmetric tilt domains, and Holocene lowlands characterized by uplifted and tilted strandlines. Industry offshore seismic-reflection profiles covering an area of approximately 1800 km (super 2) and crustal seismicity reveal active reverse-fault cored anticlines surrounding Isla Santa Maria; the principal fault apparently roots in the plate-interface thrust. These reverse faults in the upper plate result from inversion of Late Cretaceous to early Pliocene normal faults and rift structure of the Arauco forearc basin. Positive inversion of these inherited structures started between 3.6 and 2.5 Ma and resulted in continuous shortening rates of approximately 0.8 mm/yr. The seismic-reflection profiles show that the asymmetric tilt domains and progressive syntectonic sedimentation are linked to the position of the island in the forelimbs of two converging anticlines, whereas their backlimbs have been removed by cliff retreat. The 2 m uplift contour of the 1835 earthquake is parallel to the strike of active faults and antiforms in the Arauco-Concepcion region. The close relation among the asymmetric uplift and tilt of the island, modern deformation patterns, and reverse faults rooted in the plate interface suggests that slip on the plate interface thrust influences, localizes, and segments surface deformation during large interplate earthquakes. Furthermore, the link between positive inversion of pre-existing structures, uplift, and tilt patterns in the forearc emphasizes the importance of inherited structural fabrics in guiding plate-boundary deformation.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 118
Serial Issue: 11-12
Title: Coastal deformation and great subduction earthquakes, Isla Santa Maria, Chile (37 degrees S)
Affiliation: GeoForschungsZentrum Potsdam, Potsdam, Federal Republic of Germany
Pages: 1463-1480
Published: 200612
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 72
Accession Number: 2006-091386
Categories: SeismologySolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Repository Item 2006176
Illustration Description: illus. incl. 1 table, geol. sketch maps
S37°49'60" - S36°15'00", W75°30'00" - W72°45'00"
Secondary Affiliation: University of California at Santa Barbara, USA, United StatesUniversitaet Potsdam, DEU, Federal Republic of Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200624
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