Neotectonics of the Pedro de Valdivia Area, northern Chile

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1 Neotectonics of the Pedro de Valdivia Area, northern Chile Angelo Villalobos 1*, Joaquín Cortés-Aranda 1, Luis Astudillo 1, Rodrigo Riquelme 1 and Arturo Jensen 1 (1) Universidad Católica del Norte, 0610 Avenida Angamos, Antofagasta, Chile. * Presenting Author s 1. Introduction Upper plate faulting is one of the main processes leading Cenozoic landscape evolution in the hyperarid Coastal Forearc of northern Chile (~18 S-23 S). There, predominant kinematic styles are represented by NS striking normal faults, NNW to NW trending reverse faults and EW striking reverse faults [e.g. Allmendinger and González, 2010]. Long term activity along these faults has conducted the construction of conspicuous scarps that form mountain range fronts of even 500 m height [González et al., 2003; Allmendinger et al., 2005]. The most recent expression of activity along these faults is given by scarps developed on young alluvial gravels [e.g. González et al., 2006; Cortés et al., 2012]. Despite this evidence, no important (Mw>7) earthquakes have been registered along them during historical times [e.g. Comte et al., 1994]. However, a paleoseismological survey performed during the FONDECYT project (GG) concluded that faults like Mejillones, Salar del Carmen and Chomache (Figure 1a) have triggered Mw~7 events during late Pleistocene-Holocene times. a Figure 1. a) ASTER DEM map showing the study area with the main faults and key localities. CCF= Cerro Cordón Fault; SCF= Salar del Carmen Fault; SVF= Sierra Valenzuela Fault; CF= Colupo Fault; WAF= Western Antucoya Fault; EAF= Eastern Antucoya Fault; b) WAF and EAF controlling a graben basin; and c) Normal faults leading the staircase-type topography in the area. F= Fault; HG= Halfgraben basin. In this contribution we provide neotectonic evidence of Quaternary activity along normal and reverse faults in the Carta Pedro de Valdivia area (Figure 1a). These faults are located just above the northern termination of the interplate seismogenic gap where the 1877 Iquique Mw 8.7 earthquake occurred [Comte and Pardo, 1991]. Null

2 information about the recent activity of upper plate faults in this area is available, thus we intend to give a first approach to precise the seismic hazard that they represent. 2. Structural context The Pedro de Valdivia area is located in the eastern part of the Coastal Cordillera at (Figure 1a). There, the most important faults are part of the Atacama Fault System [Arabasz, 1971], which formed in Late Jurassic-Early Cretaceous as a sinistral strike slip fault [e.g. Scheuber and Andriessen, 1990]. This system extends for more than 1000 km between 21 S and 26 S and its main branch in the Pedro de Valdivia area is given by the Salar del Carmen Fault (SCF; Figure 1a). In general, structures in the study area are represented by sets of NNE and NW faults that have produced a graben and halfgraben basins that flank mountain ranges of less than 2000 ma.s.l (Figure 1b). This configuration constitutes a staircase-type topography (Figure 1c). The limits between ranges and halfgraben are dominated by piedmonts composed by at least two generations of alluvial surfaces partially buried by colluvial deposits. In most of the cases alluvial surfaces and also colluvial deposits are clearly affected by fault displacement. At the northern part of the area, near the Antucoya Mine (Figure 1a), the most important structures are given by NNE normal faults that define a graben filled with ~600 m of alluvial gravels. We named these faults as Western (WAF) and Eastern (EAF) Antucoya faults (Figure 1a-b). Further north, the Colupo Fault (CF) has promoted the uplifting of a NS mountain range where stratified crystalline rocks outcrop (Figure 1a). Also, NW reverse faults occur at this part of the area, spatially related to the intrusion of hypabyssal bodies. One of these faults is the Grava Fault (GF; Figure 1a). At the southern part, west of Algorta (Figure 1a), NNE normal faults have conducted the uplifting of NNE oriented mountain ranges where sedimentary, volcanic and intrusive rocks outcrop. The most important structures flanking mountain ranges are the Cerro Cordón, Salar del Carmen and Sierra Valenzuela faults (Figure 1a). Immediately east of these ranges, halfgraben basins filled with alluvial deposits exist (Figure 1c). 3. Neotectonic evidence Neotectonics of the Pedro de Valdivia area is mainly expressed in piedmont surfaces preserved at this part of the Coastal Cordillera. Also, in some sites we found that recent fault activity has affected crystalline rocks in the core of mountain ranges. We report two types of neotectonic features: i) Fault scarps and ii) Tensional cracks. i) Fault scarps. These morphological features have been abundantly identified in alluvial gravels and rarely affecting crystalline rocks. Scarps constructed in alluvial gravels have been produced by normal and reverse faulting. Normal fault scarps are typically 0.5 to 4 m height and are dominated by a debris slope facies [sensu Wallace, 1977]. A well example of this kind of scarps is along the Colupo Fault (CF), where several subsidiary normal faults dislocate a piedmont surface (Figure 2a). At this site, we observed knick points along gullies that dissect some of the scarps (Figure 2b). Although the topography of the area allows inferring that the dominant process conducted by faults is extension, the deformation of the youngest deposits has occurred

3 mainly by reverse faulting. Scarps produced by reverse faulting are 1 to 5 m height and dominated by a debris slope facies, although free face is still preserved in some cases. The best example of this type of scarps occurs along the Grava Fault (GF), west of the Antucoya Mine (Figure 1a, 2c). There, a reverse fault promotes the uplifting of an alluvial surface in a mountain front where crystalline rocks outcrop (Figure 2c). Moreover, at this site the fault dislocates even colluvial deposits (Figure 2c). Reverse fault scarps have been also recognized affecting crystalline rocks. Along the northern termination of the Grava Fault (GV), a remarkable fault scarp puts into contact intrusive and volcanic rocks (Figure 2d). There, the fault scarp is dominated by a free face that dips 80 SW with almost no accumulation of debris facies (Figure 2d). Figure 2. a) Normal faults affecting a piedmont surface in the periphery if the Colupo Fault; b) Knick points in gullies displaced by faults showed in a); c) Grava Fault displacing alluvial gravels (AG) and colluvial deposits (CD) at the mountain front of volcanic rocks occur (J3i). The scarp is dominated by the debris slope (DS) facies; d) Grava Fault putting into contact volcanic (J3i) and hypabissal (Hyp) rocks. The free face (FF) dominates the scarp profile. ii) Tensional cracks. Tectonic cracks were identified in two kinds of sites. The first one is the periphery of the most conspicuous identified normal fault scarps (e.g. SCF). There, tensional cracks affect alluvial gravels composing the piedmont surfaces and even the sedimentary deposits of active channels (Figure 3a). These cracks are hundreds of meters long and oriented N5-20E. They present apertures of 1-15 cm and are partially filled with fine sediments arranged in subvertical layers parallel to the crack walls. Another site where these cracks exist corresponds to the hanging wall of reverse faults (Figure 3b). There, these cracks are parallel to the fault strike, providing an irregular appearance to the scarp profile. According to McCalpin [1996], these cracks are the result of the collapse of the deformed material. 4. Discussion Normal faulting has dominated the long term deformation in the study area. This is inferred from the graben-halfgraben topography that characterizes the zone. On the

4 other hand, field observation suggests that both normal and reverse faults have experienced activity during the Quaternary. We base this statement on the fact that the youngest sedimentary deposits composing piedmont surfaces and colluvium are clearly deformed. Locally, normal faulting has been observed affecting Quaternary deposits as for instance along the Colupo Fault (Figure 2a-b). Nevertheless, it seems that normal faulting is subsidiary in comparison with reverse faulting during the Quaternary. This latter deformation style has been observed, for example, along the Grava Fault (Figure 2c). Based on this evidence, we postulate that compression is the main process controlling upper plate fault activity during the Quaternary at this part of the Coastal Forearc. Figure 3. a) Tensional cracks on alluvial deposits at the periphery of normal faults in the area. b) Tensional cracks in the hangingwall block of reverse faults in the area. Scarp geometry suggests discrete slip events along normal and reverse faults in the area. Based on their heights, we interpret that the causative slip events are coherent with Mw~7 earthquakes [sensu Wells and Coppersmith, 1994]. This appears to be clearer in the case of reverse faults affecting crystalline rocks, as the Grava Fault does at its northern termination. Indeed, this fault has generated a prominent free face dominated scarp of around 1.7 m height, which was very likely constructed by a single earthquake. With the available evidence, we think that faults in the Pedro de Valdivia area must be considered active. However, we plan to develop detailed paleoseismological and neotectonic studies in the near future to constrain their seismic potential. Also, further investigations for evaluating the stress build up along them as consequence of the subduction earthquake cycle are needed. With this latter approach, we could explore the dynamical relationship between upper plate fault activity and the stages of the subduction cycle. In this way, we point to better precise the seismic risk in the Coastal Forearc due to both, the 1877 gap and upper plate fault activity. References Allmendinger, R. W., G. González, J. Yu, G. Hoke, and B. Isacks Trench-parallel shortening in the northern Chilean forearc: Tectonic and climatic implications, Geol. Soc. Am. Bull., 117(1), , doi: /b Allmendinger, R. W., and G. González Neogene to Quaternary tectonics of the coastal Cordillera, northern Chile, Tectonophysics, 495(1 2), , oi: /j.tecto Arabasz, W. J Geological and geophysical studies of the Atacama Fault System in northern Chile, PhD thesis, 264 pp., Calif. Inst. of Technol., Pasadena. Allmendinger, R. W., G. González, J. Yu, G. Hoke, and B. Isacks Trench-parallel shortening in the northern Chilean forearc: Tectonic and climatic implications, Geol. Soc. Am. Bull., 117(1), , doi: / B Comte, D., and M. Pardo Reappraisal of great historical earthquakes in the northern Chile and southern Peru seismic gaps, Nat. Hazards, 4, Cortés A., J., G. González L., S. A. Binnie, R. Robinson, S. P. H. T. Freeman, and G. Vargas E Paleoseismology of the Mejillones Fault, northern Chile: Insights from cosmogenic 10Be and optically stimulated luminescence determinations, Tectonics, 31, TC2017, doi: /2011tc González, G., Dunai, T., Carrizo, D., y Allmendinger, R Young displacements on the Atacama Fault System, northern Chile from field observations and cosmogenic 21Ne concentrations: Tectonics, v. 25, p. TC3006. González, G., Cembrano, J., Carrizo, D., Macci, A., and Schneider, H. 2003, The link between forearc tectonics and Pliocene-Quaternary deformation of the Coastal Cordillera, northern Chile. Journal of South American Earth Sciences, v. 16, p McCalpin, J Paleoseismology, Academic Press, California, USA. Scheuber, E., and P. M. Andriessen The kinematics significance of the Atacama Fault zone, northern, Chile, J. Struct. Geol., 21, Wells, D.L. y Coppersmith, K.J New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bulletin of the Seismological Society of America, vo. 84, pp Wallace, R. E Profiles and ages of young fault scarps, northcentral Nevada, Geol. Soc. Am. Bull., 88, , doi: / (1977)88<1267:PAAOYF>2.0.CO;2.

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