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Active Faulting and Northward Growing of the Arauco Peninsula, South-Central Chile (37°30’S)

Active Faulting and Northward Growing of the Arauco Peninsula, South-Central Chile (37°30’S)

U N I V E R S I D A D D E C O N C E P C I Ó N DEPARTAMENTO DE CIENCIAS DE LA TIERRA 10° CONGRESO GEOLÓGICO CHILENO 2003

ACTIVE FAULTING AND NORTHWARD GROWING OF THE ARAUCO PENINSULA, SOUTH-CENTRAL (37°30’S)

MELNICK, D.1, ECHTLER, H.1, PINEDA, V.2, BOHM, M.1, MANZANARES, A.1, VIETOR, T.1

1GeoForschungsZentrum (GFZ) Potsdam, Germany [email protected], [email protected], [email protected], [email protected], [email protected] Telegrafenberg, D-14473 Potsdam 2Universidad de Concepción, Departamento de Ciencias de la Tierra [email protected], Barrio Universitario s/n, Casilla 160-C, Concepción

INTRODUCTION The Arauco Peninsula is located in the Pacific coast of the Southern Central Andes (Fig. 1). This uplifted block of continental shelf is the biggest, in extension (~2,250 km2) and westward coastline displacement (~30 km), of the entire Pacific margin of the southern hemisphere. The shelf in the Arauco area records temporal and spatial discontinuous marine and continental fore- arc basin formation since Late (Arcos and Elgueta, 1993; Pineda, 1986), developed on top of a crystalline Permo- basement (Glodny et al., 2002; Hervé, 1977). Alternating episodes of uplift/ and subsidence/sedimentation are evident from the geological record and paleogeographic reconstructions (Pineda, 1986), possibly controlled by the alternating cycles of accretion/erosion as described for the frontal wedge (Bangs and Cande, 1997). Seismicity data, field observations, a merged topography/bathymetry digital elevation model, and detail bathymetric profiles led us to interpret that the Arauco - Concepción area is actively uplifting and that the Peninsula is probably growing towards the north. Coseismic uplift related to flexural bending during megathrust nucleated on a supposed ‘locked’ convergence zone has been proposed as mechanism for surface uplift (Barrientos and Ward, 1990). Our data shows that interseismic deformation may also significantly contribute to uplift processes along the Nazca- Southamerican active margin. inherited crustal-scale structures, like the Bío-Bío and Gastre zones (Fig. 1), seem to control the block-behaviour of the Arauco - Concepción basins since the Late Cretaceous (Echtler et al., 2003).

GEOLOGICAL AND TECTONIC SETTING The Arauco and other nearby Senonian-Tertiary fore-arc basins have been well studied during extensive coal mining (Wenzel, 1985) and gas/oil exploration (Arcos and Elgueta, 1993; Fuenzalida, 2002). Their basement is a Permo-Triassic accretionary complex and - related granitoides forming a paired metamorphic belt (Glodny et al., 2002; Hervé, 1977). Late Cretaceous to Quaternary marine and continental sedimentary sequences were deposited in independent basins, limited by major NW trending crustal-scale basement discontinuities as the Gastre and Bío-Bío fault zones (Echtler et al., 2003). These structures, together with basement

Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad high development (Arcos and Elgueta, 1993) and regional uplift/subsidence episodes (Pineda, 1986) controlled the geometry, infill and temporal evolution of these basins. Thickness of the sedimentary sequences varies dramatically along and across strike (Mordojovich, 1981), showing temporal discontinuous transgressive-regressive cycles of marine to continental facies (Pineda, 1986).

Figure 1. Regional tectonic setting of the Arauco Peninsula and regional seismicity from NEIC. : after (Tebbens and Cande, 1997) indicating age in Ma, dark gray: crust produced by the Chile ridge. (SA plate): Hatched: Arauco Peninsula, BBF: Bío-Bío fault, GFZ: zone, LOFZ: Liquiñe-Ofqui fault zone.

After a period of fast subsidence during the (Reuther et al., 1998), represented by the fossil-bearing marine of the Tubul Fm. (Pineda, 1986), most of the actual exposed Peninsula was uplifted during the Quaternary, excepting the Lebu and Lavapie highs (Pineda, 1986). Regional uplift rates, although with large error bars, show accelerating rates for the Late -Holocene to recent (Nelson and Manley, 1992).

The -Quaternary wedge at the Arauco latitude records interrupted phases of subduction erosion and accretion (Bangs and Cande, 1997). Currently the wedge is in a non-to-little frontal accretion mode (SPOC, 1960) with ~2-3 km of glacial in the trench (Bangs and Cande, 1997). Basal accretion has been proposed as a possible uplift mechanism (Echtler et al., 2003).

The local structures of the Arauco and Concepción basins have been described and mapped in detail due to coal exploitation (Wenzel, 1985), hydrocarbon exploration (Arcos and Elgueta, 1993; Fuenzalida, 2002) and field mapping by the Earth Science Department, U. de Concepción (2000-2002). Geologic maps display mostly ~NE trending normal faults, affecting Senonian to Recent deposits (Arcos and Elgueta, 1993; Boettcher, 1999; Pineda, 1986. Microtectonic analyses show a roughly radial extensional pattern. The larger and older normal faults, that affect the Senonian to strata, with up to hundreds of meters throw, have been related to Tertiary continental -like basin formation (Arcos and Elgueta, 1993; Fuenzalida, 2002). Small normal faults with meter-scale throw, that affect the Quaternary infill are interpreted as extensional structures related with uplift, or general fore-arc extension in relation with differential back-arc shortening in the Central Andes (Vietor et al., 2003). Few reverse faults and folds where recognized in the Arauco area (Fig. 2 and 5). They can be explained as compressive zones related to clockwise block rotation as proposed by Echtler et al., (2003) and Vietor et al., (2003) witch might also be related to surface expressions of active reverse faulting (further explained).

The subducting Nazca plate (Fig. 1) under the Arauco Peninsula is formed by ~31 Ma oceanic crust produced by the East Pacific Rise (Tebbens and Cande, 1997). Currently underthrusted at 66 mm/a as determined by GPS measurements (Angermann et al., 1999) or ~80 mm/a, averaged from the past 3 Ma based on magnetic anomalies (Gripp and Gordon, 1990; Somoza, 1998). The Mocha Fracture Zone is currently subducting under the Peninsula. Previous studies have attributed the uplift of the Peninsula to this slightly southward migrating oceanic discontinuity (Boettcher, 1999). Mayor changes of the Nazca plate occur at the Valdivia Fracture Zone system ~40°S (Fig. 1), that separates oceanic crust produced by the Chile to the south and East Pacific ridges to the north (Tebbens and Cande, 1997).

CRUSTAL SEISMICITY AND SURFACE DEFORMATION The seismicity of the study area was registered during the ISSA2000 campaign, with 65 stations located between 36°-40°S, that operated during January to April 2000 for around 100 days (6). The spatial analysis of fore-arc crustal micro-seismicity shows two major clusters of events around the Arauco Peninsula (Fig. 2). The first cluster is located immediately north of the Peninsula (~37°S, 73°30´W), around the Santa María Island. The second cluster is more scattered and located south of the Peninsula (~38°S, 73°30´W). Cross sections along these clusters (Fig. 2) show that the earthquakes define linear trends, representing activity along two 60-70° seaward dipping faults. The two faults extend continuously from the coupling zone at ~20 km depth to ~3- 5 km. Interplate and crustal upper plate events can be well differentiated along the cross sections, since they show almost perpendicular trends (Fig. 2). These two clusters limit the actual extent of the Arauco Peninsula, indicating an individual and uplifting block behaviour.

Based on the geometry, spatial distribution of the seismicity and correlation with surface features, we name the northern cluster as the Santa María fault zone (SMFZ). The SMFZ is interpreted as a NE-SW striking, high-angle reverse blind fault zone (Fig. 2). The map-extent of this fault is from the Lavapie point, along the eastern side of the Santa María Island platform, to the limit of the Hualpén and Tumbes Peninsulas with the Concepción-Talcahuano peneplain (Fig. 2). The Bío-Bío canyon is strongly controlled by faults as noticed by Pineda (1999), and the eastern part is controlled by the SMFZ. One crustal event (Mw=6,2; depth 5 km) from the NEIC dataset can be correlated with activity along the SMFZ, although with lower localization precision that the ISSA2000 dataset. The focal mechanism solution shows a NNE trending high-angle reverse fault (Fig. 2). Further current analysis of localizations and focal mechanism solutions will provide more information about the kinematics and geometry of the SMFZ.

Activity along the SMFZ might be correlated with surface uplift during historical megaearthquakes. The 1835 uplift was measured and described by Fitz Roy and Darwin: 3 m at the Santa María Island, 2,4 m in the southern Arauco Peninsula and 1,8 m in Tubul, in Talcahuano the uplift was 1,2 to 1,5 m but decreased to 0,6 m three months after the earthquake (in Brüggen, 1950). The 1939 Chillán-Concepción event produced 0,6 m of coseismic uplift of the Tumbes-Hualpén block. This was measured by a topographic leveling done by the I.G.M. and described by Brüggen (1950). Uplift in the area is evident from geomorphological observations such as uplifted marine terraces and abrasion platforms, river drainage and coastline retreat as shown by the position of paleostrandlines (Kaizuka et al., 1973; Mardones, 1999; Nelson and Manley, 1992). Although no geodetic data is available, based on our observations and the ISSA2000 seismicity dataset (Bohm et al., 2002), we infer also an important interseismic uplift for the Santa María Island.

Earthquakes of the southern cluster are more dispersed, probably due to the higher structural complexity of the area, controlled by the Arauco Peninsula-Nahuelbuta Range-Gastre fault zone conjunction. Several faults have been mapped along this contact (Arcos and Elgueta, 1993; Boettcher, 1999; Pineda, 1986) although no kinematic information has been reported, because vegetation and dunes cover the area. The geometry of this cluster as observed from the E-W cross section, indicates also an east-vergent high-angle reverse fault. This fault is Probably responsible of the active uplift of this part of the Peninsula, as evidenced by geomorphological observations such as deep incised gorges on the flat surface of the Pliocene Tubul Fm. (Black triangle; Fig. 2) and catchments-damming that produced the Lanalhue and Lleu-Lleu lakes (Mardones, 1999). Therefore, we name this fault the Lanalhue - Lleu-Lleu fault zone (LLFZ). A prominent NW trending scarp covered by a dune field, southeast of Punta Morguilla (Fig. 2), might be correlated with recent activity of the Gastre fault zone, witch crosses the margin along the Lanalhue lake (Fig. 2). The asymmetric shape of the Peninsula, with a close bay to the north and a long NW trending shore in the south, may reflect a certain degree of strike-slip deformation, possibly accommodated by clockwise block-rotations in the Arauco area (Fig. 2) as proposed by Echtler et al., (2003).

Based on: (i) the aligned geometry of the hypocenters, (ii) position in the active convergent margin, (iii) vertical extension of the active zone, from the coupling zone at ~20 km to depths of ~3 km and (iv) geomorphological and geological correlations with surface deformation observed during historical earthquakes, we interpret these faults as part of an active seaward-dipping reverse block-system, of the Nazca-Southamerican fore-arc complex.

Figure 2 (Next page). Merged topography/bathymetry digital elevation model and regional structures of the Arauco Peninsula area. Cross sections along the SMFZ and LLFZ, seismic data from ISSA2000 (Bohm et al., 2002), stippled rectangles indicate seismic data used for the two E-W profiles. Focal mechanism from NEIC. All reverse faults are blind.

BATHYMETRY The merged topography/bathymetry digital elevation model of the Arauco area, compiled from I.G.M. topographic and Chilean Navy bathymetric maps (Fig. 2), shows two major discontinuities in the Arauco Bay: a NNW trending scarp that extends from the eastern part of the Santa María Island platform to the north, through the structurally controlled Bío-Bío canyon (Pineda, 1999) and the SMFZ, and a set of NE trending faults, extending from the Lavapie high to the Hualpén-Tumbes Peninsulas eastern scarp. The first scarp might control the block- behaviour of the Arauco Peninsula, decoupling block-rotation and differential uplift.

The depth-break immediately east of the Santa María Island platform, limits the shallow (<100 m) Arauco Bay and an outer high (Figs. 2 and 3). The northern prolongation of this scarp is a branch of the Bío-Bío canyon, which deeply incises the shelf and frontal slope. This outer high is located on the limit of the continental shelf and frontal slope, and extends to the north of the canyon. Shallow basins form east of this high. Detailed bathymetric profiles were done on board the R/V Agor Vidal Gormaz, from the Chilean Navy (Fig. 3). The profiles show rugose textures typical of rocky ocean-floor, indicating no recent sedimentation, probably due to the active uplift of this area. The textures of these profiles contrast with the flat-bottom of the inner-shelf bays.

Figure 3. Shaded elevation model, main structural segmentation and map of areas with rocky ocean floor. Contrasting texture in detailed bathymetric profile ST. REFLECTION SEISMIC PROFILES Offshore Reflection profiles were shot by ENAP (Empresa Nacional del Petroleo), the Chilean national oil company, during hydrocarbon exploration in the last decades (Arcos and Elgueta, 1993; Fuenzalida, 2002). Line 19, an E-W striking profile, located between the Arauco Peninsula and Santa María Island shows the structure of the basin, correlated with the drill-logs (Arcos and Elgueta, 1993; Fuenzalida, 2002). Two main unconformities can be clearly seen, Eocene / and Miocene/Pliocene. The second developed during the Miocene-Pliocene as showed by the syn-rift and sag deposition of the Ranquil and Tubul Fms (Fuenzalida, 2002). The normal faults that can be recognized in the seismic lines controlled basin formation during Eocene to Miocene times. The same structures are mapped in the Lota area coal mines as NE trending faults (Wenzel, 1985) (Fig. 2). From these observations, we deduce that the NE trend was the main direction of the Tertiary basins of the Arauco-Concepción area.

FIELD EVIDENCES FOR BLOCK-ROTATION AND COMPRESSION The shoreline of the Arauco Peninsula shows a series of SW trending points and bays on the western Pacific side (Fig. 2). The presence of this complex shoreline is due to stratigraphic and structural control. Transtensional faults affecting limestone and siltstone layers where recognized. These faults produce tectonic breccias up to 100 m wide. Although slickenslides where not easy to find in Arauco, the few collected data were of good quality. Sinistral movements along NNE to NE trending faults were observed. This data are compatible with clockwise block rotation in the forearc (Fig. 2), as has been proposed by Echtler et al., (2003) based on reactivation patterns and the saw-tooth morphology of the Peru-Chile trench and coastline at these latitudes.

During the current survey, immediately east of the Llico town, compressive structures where identified affecting the Miocene Ranquil Fm. Along the road from Arauco to Llico, an anticline is exposed as already noticed by Fuenzalida (2002). The limbs dip 25° and 30° and the axis trends ~NS and plunges slightly to the south (Fig. 5). Along the coast, the same anticline about ~2 km wide, is cut by a 30° west-vergent ramp and is affected by a tight asymmetric anticline with the same vergence (Fig. 5). The axial zone of this is affected by several small normal faults, possibly moment-bending or postseismic extension related.

Figure 4. South view of the Tubul-Ranquil Fms. fault contact, and lower hemisphere stereoplot of the measured striations.

East of the El Fraile beach, the contact between the Miocene Ranquil Fm. and Pliocene Tubul Fm. was observed as a N20°W striking transtensional normal/sinistral fault with very well preserved wedge shaped striations plunging 40° to 50° to the north (Fig. 2 and 4). This zone can be related to the eastern limit of the Lavapie clockwise rotating block (Fig. 2). The northern prolongation of this fault passes trough the western side of the Santa Maria Island. Further east, only small normal faults affecting the Pliocene where recognized, as previously reported (Reuther et al., 1998).

Figure 5. View to the south of the anticline affecting the Miocene Ranquil Fm. Above, along the road from Arauco to Llico. Below, along the beach.

DISCUSSION The ISSA2000 dataset emphasize that high-angle blind reverse faults, as the SMFZ and LLFZ must have a major role during the observed active uplift and coseismic deformation. Probably these faults zones are responsible for the slip transfer from the coupling zone, where megathrust Mw> 8 earthquakes are nucleated (Barrientos and Ward, 1990), to the surface. This was previously suggested for the 1960 Mw=9,5 event by Plafker and Savage (1970). Historical descriptions from the Arauco-Concepción area and Mocha Island (38°30’S) (Brüggen, 1950; Nelson and Manley, 1992) show that during megaearthquakes, ~50% of the coseismic uplift is compensated during postseismic relaxation processes, accommodated by pre-existing faults of the Tertiary basins. Reactivation of these NE to NNE trending Tertiary basin-bounding normal faults and block rotation, as postulated by Vietor et al., (2003) and Echtler et al., (2003), might explain the current geography of the forearc in the area, characterized by an uplifting outer-ridge peninsula system and subsiding inner-shelf basins as the Arauco and Concepción bays. This geography is actively transforming and may, considering the uplift of the Santa Maria Island, produce a land bridge with the Arauco Peninsula in the future.

The Andean fore-arc at the Arauco latitude shows uplift since and during the Quaternary. This behaviour might be attributed to: changes along strike in physical parameters of the lower plate (Tebbens and Cande, 1997), influence and reactivation of major pre-Andean and/or inherited shear zones (Echtler et al., 2003), although the detailed mechanisms are still poorly understood. These changes are observed since the Pliocene/Pleistocene boundary and are associated with: (i) the westward migration and narrowing of the volcanic arc south of the Arauco latitude, by a possible increase of the subduction angle (Stern, 1989), (ii) eastward migration and also narrowing of the arc at around 33°-34°S, by decrease of the subduction angle, or subduction erosion (Stern, 1989), (iii) decreasing subduction velocities (Somoza, 1998; Gripp and Gordon, 1990), (iv) fore-arc wedge changes from accretionary to little or non-accretionary modes (Bangs and Cande, 1997; SPOC, 1960) and (v) ~2-3 km of trench infill by glacial sediments during the Pleistocene (Bangs and Cande, 1997). These factors open discussion to explain the Quaternary uplift of the Arauco area as related with changes in the mass transfer mode due to changed plate- boundary conditions or due to regional discontinuities of the Arauco block.

ACKNOWLEDGMENTS We would like to thank ENAP for giving us access to seismic lines and especially to Ricardo Fuenzalida and Constantino Mpodozis (ENAP-Sipetrol) for fruitful discussions, René Fuenzalida for his help in the field, Klaus Bataille and Gonzalo Hermosilla (U. Concepción) for logistic help. This work was supported by the GeoForschungZentrum Potsdam Southern Andes Project, SFB 267 “Deformation Processes in the Andes” and IQN “International Quality Network” Universität Potsdam.

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