Coastal Deformation and Great Subduction Earthquakes, Isla Santa María, Chile (37°S)
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Coastal deformation and great subduction earthquakes, Isla Santa María, Chile (37°S) Daniel Melnick† GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany, and Institut für Geowissenschaften, Universität Potsdam, Postfach 601553, 14415 Potsdam, Germany Bodo Bookhagen‡ Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA, and Institut für Geowissenschaften, Universität Potsdam, Postfach 601553, 14415 Potsdam, Germany Helmut P. Echtler GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Manfred R. Strecker Institut für Geowissenschaften, Universität Potsdam, Postfach 601553, 14415 Potsdam, Germany ABSTRACT and crustal seismicity reveal active reverse- INTRODUCTION fault cored anticlines surrounding Isla Santa Isla Santa María at the active margin of María; the principal fault apparently roots Uplift and subsidence are fi rst-order phenom- south-central Chile is the result of earth- in the plate-interface thrust. These reverse ena of tectonically active coasts along subduc- quake-related uplift and deformation in the faults in the upper plate result from inversion tion margins (e.g., Plafker, 1972; Ando, 1975). forearc since at least late Pleistocene time. of late Cretaceous to early Pliocene normal Emergent coasts are usually characterized by Field mapping, dating of key depositional faults and rift structure of the Arauco forearc differential uplift within distinct segments that horizons, and analysis of seismic-refl ection basin. Positive inversion of these inherited may be sustained as morphotectonic units on profi les reveal ongoing deformation in this structures started between 3.6 and 2.5 Ma time scales of 105 to 106 yr. During earthquakes, sector of the Chilean forearc. The 30 km2 and resulted in continuous shortening rates these areas appear to act as semi-independent island is located ~12 km above the interplate of ~0.8 mm/yr. The seismic-refl ection profi les rupture zones that guide deformation (e.g., seismogenic zone and 75 km landward of show that the asymmetric tilt domains and Kaizuka et al., 1973; Ando, 1975; Matsuda et the trench. It is situated near the southern progressive syntectonic sedimentation are al., 1978; Taylor et al., 1987; Thatcher, 1990; termination of the Concepción earthquake linked to the position of the island in the fore- Berryman, 1993b; Pandolfi et al., 1994; Ota rupture segment, where Charles Darwin limbs of two converging anticlines, whereas and Yamaguchi, 2004). Coseismic land-level measured 3 m of coseismic uplift during a M their backlimbs have been removed by cliff changes caused by subduction earthquakes fol- > 8 megathrust earthquake in 1835. Perma- retreat. The 2 m uplift contour of the 1835 low a sinusoidal deformation pattern across the nent postearthquake deformation from this earthquake is parallel to the strike of active margin, with uplift along the shelf and coast, and earthquake and an earlier event in 1751 is faults and antiforms in the Arauco-Concep- subsidence farther inland (e.g., Plafker and Sav- registered by emerged, landward-tilted abra- ción region. The close relation among the age, 1970; Savage, 1983; Hyndman and Wang, sion surfaces. Uplift at ~2 m/k.y. and tilting asymmetric uplift and tilt of the island, mod- 1995). This idealized distribution of coseismic at ~0.025°/k.y. of the island have been fairly ern deformation patterns, and reverse faults deformation is dramatically altered, however, constant throughout the late Quaternary and rooted in the plate interface suggests that when pre-existing faults in the upper plate are have resulted in emergence of the island above slip on the plate interface thrust infl uences, triggered by a megathrust event, such as during sea level ~31 k.y. ago. The island is composed localizes, and segments surface deformation the M 9.2 Alaskan earthquake in 1964 (Plafker, of a late Pleistocene upper, tilted surface with during large interplate earthquakes. Fur- 1972). In some subduction zones, crustal faults two asymmetric tilt domains, and Holocene thermore, the link between positive inversion have controlled coastal deformation patterns lowlands characterized by uplifted and tilted of pre-existing structures, uplift, and tilt pat- and forearc basin formation over million-year strandlines. Industry offshore seismic-refl ec- terns in the forearc emphasizes the impor- time scales, and their subdivisions seem to gov- tion profi les covering an area of ~1800 km2 tance of inherited structural fabrics in guid- ern seismotectonic segmentation, rupture prop- ing plate-boundary deformation. agation, and susceptibility to local earthquake hazards (e.g., Berryman et al., 1989; Goldfi nger †Corresponding author e-mail: melnick@gfz- Keywords: Chile margin, forearc deformation, et al., 1992; Barnes et al., 2002; Johnson et al., potsdam.de. ‡Present address: Department of Geological and subduction earthquakes, tectonic inversion, re- 2004; Bruhn and Haeussler, 2006; Briggs et al., Environmental Sciences, Stanford University, Stan- verse faults, crustal seismicity. 2006). Some of these faults are rooted in the ford, California 94305, USA. plate-interface thrust, and mechanical coupling GSA Bulletin; November/December 2006; v. 118; no. 11/12; p. 1463–1480; doi: 10.1130/B25865.1; 14 fi gures; 1 table; Data Repository item 2006176. For permission to copy, contact [email protected] 1463 © 2006 Geological Society of America Melnick et al. with the megathrust during the earthquake cycle sampled material is uncontaminated because of: Platform, formed by uplifted Cenozoic marine is thus expected (e.g., Park et al., 2000; Barnes (1) stratigraphic concordance—ages of lower and continental sediments, which are the focus et al., 2002; Bruhn and Haeussler, 2006). stratigraphic levels are older; (2) thickness of of this paper; (2) the Coastal Cordillera, a This study focuses on the Chile active margin, the sedimentary sequence—the samples were Paleozoic accretionary complex and magmatic which is composed of distinct coastal segments taken from fresh sea-cliff exposures where no arc constituting the crystalline continental base- undergoing long-term differential uplift and roots or other exotic material was present, sev- ment; (3) the Central Depression, a low-lying subsidence manifested in a rich array of coastal eral meters below the top of the sequence; and basin fi lled by Pliocene-Quaternary conglomer- landforms. Historic accounts of great subduc- (3) the type of material—dating was performed ates; and (4) the Main Cordillera, characterized tion earthquakes in Chile indicate the protracted on centimeter-size charcoal and large pieces of by high topography and an active volcanic arc existence of discrete rupture segments (Lom- well-preserved wood. (e.g., Mpodozis and Ramos, 1989). nitz, 1970, 2004; Kelleher, 1972; Nishenko, Tectonic uplift rates were calculated using the Isla Santa María is in the Coastal Platform, 1985; Comte et al., 1986; Thatcher, 1990; Beck radiocarbon ages and the sea-level curve from part of the Arauco Basin, a late Cretaceous to et al., 1998). Figure 1A shows the southern seg- Siddall et al. (2003). Uplift rates derived from Quaternary forearc depocenter fi lled with at least ments. However, it is not known over what time optically stimulated luminescence ages of late 3.1 km of marine and continental sediments, as scales these rupture segments prevail and what Holocene strandlines and a detailed topographic revealed by ENAP exploration wells (Mordojo- their infl uence is with respect to overall land- survey based on measurements with a laser the- vich, 1981; González, 1989; Elgueta and Arcos, scape development. We present the deformation odolite are presented in Bookhagen et al. (2005) 1994). The Arauco Basin contains the Campan- history for Isla Santa María in the south-central and Bookhagen et al. (2006). Active structures ian to Maastrichtian Quiriquina Formation, the Chile margin (Fig. 1A), where continuous late offshore were interpreted from migrated seis- Paleocene to Eocene Lebu Group, the late Mio- Quaternary uplift and tilting are recorded in mic-refl ection profi les provided by Empresa cene to early Pliocene Ranquil Formation, and the geomorphology and coastal deposits of this Nacional del Petróleo (ENAP, the Chilean state the late Pliocene to Pleistocene Tubul Forma- island, which experienced meter-scale coseis- oil company) (Fig. 2C), and crustal seismicity tion (e.g., Biró, 1979; Pineda, 1986; Elgueta and mic uplift during the last great interplate earth- data recorded by the ISSA (Integrated Seismo- Arcos, 1994; Le Roux and Elgueta, 1997; Finger quake (Fig. 2C). The island is located near the logical experiment in the Southern Andes) local et al., 2006) (Fig. 2A). Hydrocarbon exploration transition between two distinct seismotectonic network (Fig. 2D). The seismic stratigraphy was models of forearc basins in Chile between 34 sectors, the Concepción and Valdivia segments correlated with ENAP exploration boreholes and 45°S, supported by structural data from coal (Fig. 1A). We use detailed geologic mapping, and outcrops in the Arauco Bay area (Figs. 2C mines in the Arauco Basin, suggest that various stratigraphy, geomorphology, and radiocarbon and 2A) (Mordojovich, 1981; González, 1989; phases of subsidence and extension occurred dating to demonstrate that the island is being dif- Vietyes et al., 1993; Elgueta and Arcos, 1994). between the Late Cretaceous and early Pliocene ferentially uplifted and tilted. Furthermore, we The ISSA data were collected