Tectonosedimentary evolution of the Coastal Cordillera and Central Depression of south-Central Chile (36°30′-42°S)
Graphical abstract
Introduction
Convergent margins are classified as erosive or accretionary depending on their material-transfer modes (e.g., Von Huene and Scholl, 1991; Clift and Vannucchi, 2004; Kukowski and Oncken, 2006). Erosive margins form in areas where trench sediment thickness is less than 1 km and convergence rates exceed 6.0 cm/a. They are characterized by trench retreat and forearc subsidence triggered by subduction erosion (Von Huene and Scholl, 1991; Clift and Vannucchi, 2004). Accretionary margins develop in regions where trench sediment thickness exceeds 1 km, and the rate of ortogonal convergence is <7.6 cm/a (Clift and Vannucchi, 2004). In accretionary margins, trench sediment is frontally obducted to the upper plate forming an accretionary prism, which can be described as a particular case of a submarine fold-and-thrust-belt (Clift and Vannucchi, 2004; Maksymowicz, 2015). The accreted material tends to accumulate in front of and beneath the accretionary prism by frontal accretion and underplating, forming a wedge-shaped body that grows with time (Cloos and Shreve, 1988; Von Huene and Scholl, 1991; Brandon et al., 1998; Clift and Vannucchi, 2004). Forearc basins typically form between the accretionary wedge (also known as subduction complex), which acts as a dam to pond sediment, and the arc (Dickinson and Seely, 1979; Dickinson, 1995). Forearc basins are subjected to little contractional deformation since this is mostly concentrated in the accretionary wedge (Dickinson, 1995). Sediment in these basins is typically derived from the arc area, reaches thicknesses between 1000 to >10.000 m, and accumulate at rates that range from 25 m/my to more than 250 m/my. Subsidence in forearc basins is thought to be caused principally by the growing tectonic load of the accretionary wedge (Dickinson, 1995). As the subduction complex grows, forearc basins typically shoal with time, evolving from turbiditic to shallow marine and even continental deposition (Dickinson and Seely, 1979; Dickinson, 1995). The morphology of accretionary wedges and forearc basins can adopt different configurations determined by the elevation of the trench-slope break and the sedimentation rate. This configuration typically varies with time (see Fig. 6 in Dickinson and Seely, 1979).
Coastal Cordilleras or Coast Ranges are typically considered as uplifted, subaerially exposed, forearc highs (e.g., Brandon et al., 1998). They are characteristic of mature continental convergent margins classified by Dickinson and Seely (1979) as ridged forearcs. The best-studied Coastal Cordilleras occur along the Cascadia margin (Brandon et al., 1998, and references therein). Other well-known examples are Kodiak Island in the eastern Aleutian margin, Shikoku of the southeast Japan margin, the Island of Crete of the Hellenic margin, southern Iraq and western Pakistan of the Makran margin, and northeast New Zealand of the Hikurangi margin (Brandon et al., 1998 and references therein). Coastal Cordilleras run parallel to the trench, and they are typically flanked by a forearc depression located between them and the volcanic arc (Brandon et al., 1998). Most authors ascribe uplift and emersion of Coastal Cordilleras to the thickening of an accretionary wedge (e.g., Brandon et al., 1998; Clift and Vannucchi, 2004).
The Chilean margin presents a Coastal Cordillera that runs parallel to the trench along most of its length. Between this range and the Andean Cordillera there is a low lying area, typically interpreted as a forearc basin, known as the Central Depression (e.g., Jordan et al., 1983; Horton, 2018). Several studies have been carried out in the tectono-stratigraphic evolution of the Andean Cordillera (e.g., Mpodozis and Ramos, 1989; Charrier et al., 2007; Armijo et al., 2010; Farías et al., 2010; Giambiagi et al., 2014). Fewer works have been performed in the genesis of the Coastal Cordillera and the Central Depression, most of them being carried out in central and south-central Chile (~33°-42°S) (e.g., Brüggen, 1950; Farías et al., 2008b; Rehak et al., 2008; Melnick et al., 2009). As a consequence, the causes and timing for the development of these morphotectonic units are debated. The genesis of the Coastal Cordillera and the Central Depression at the cited latitudes has been assigned to the Cretaceous (Gana and Wall, 1997), Late Oligocene-Early Miocene (Muñoz et al., 2000), Late Miocene (Farías et al., 2008b), or Pliocene (Brüggen, 1950; Melnick et al., 2009). The geologic evolution of these physiographic units has been related to very different causes including the uplift of the entire margin and the creation of the Central Depression by downwarping triggered by extensional tectonics (Brüggen, 1950), or by differential fluvial erosion (Farías et al., 2006; Farías et al., 2008b); to basal accretion driving the uplift of the Coastal Cordillera (Rehak et al., 2008); to the trenchward growth of the Andes that generated a foreland basin by orogenic loading (the Central Depression) and a fore-bulge (the Coastal Cordillera) in the forearc area (Armijo et al., 2010); or to the widening of the crustal root caused by Andean shortening that triggered passive surface uplift in the Coastal Cordillera by isostatic rebound (Giambiagi et al., 2014) (see chapter 4.3 for more details). Also not well understood is the origin of the latitudinal differences in elevation of these morphostructural units; the Coastal Cordillera is considerably low or even absent in some areas of the Chilean margin, whereas junction ridges connecting the Coastal and Andean Cordilleras characterize some parts of the Central Depression (e.g., Rehak et al., 2008; Farías et al., 2008b).
The cited studies show the incertitude in the genesis, timing, and latitudinal variability of the Coastal Cordillera and Central Depression. It is noteworthy that some authors relate the tectono-stratigraphic evolution of these morphostructural units to the development of the Andean Cordillera (e.g., Armijo et al., 2010; Giambiagi et al., 2014) whereas others ascribe their origin to accretionary processes occurring exclusively in the forearc (e.g., Lohrmann et al., 2006; Rehak et al., 2008; Melnick et al., 2009). Also debated is the relationship of the Coastal Cordillera and Central Depression genesis with extensional (Brüggen, 1950; Muñoz et al., 2000), or contractional deformation (Rehak et al., 2008; Melnick et al., 2009), passive uplift (e.g., Giambiagi et al., 2014), or even differential erosion (Farías et al., 2006; Farías et al., 2008b).
The forearc of south-central Chile (36°30′-42°S. Fig. 1) is a key area to understand the genesis of these morphostructural units because there are widespread exposures of marine and continental late Cenozoic deposits that can help to constraint their evolution (Fig. 2). To investigate the tectono-sedimentary development of the.
Coastal Cordillera and Central Depression in this area, we based our study on stratigraphy, sedimentology, geochronology (UPb, LA-ICP-MS), structural geology, and geomorphology. Based on our findings and previous studies in this region, we explore the causes of the geologic evolution and latitudinal variability of these morphostructural units. Our studies reveal that their development is complex and related to different factors that varied with time. In particular, we show that tectonic processes in the Andean Cordillera had an important influence on the geological evolution of the forearc.
Section snippets
Geologic setting
The Chilean margin has been an ongoing subduction area, probably since Paleozoic times (Oliveros et al., 2019). In south-central Chile, the oceanic Nazca plate subduces under the South American continent at a convergence rate of 66 mm/a, with an azimuth of about N78°W (Kendrick et al., 2003). There are two major bathymetric features of the oceanic plate along the Chilean margin: 1) the Chile Rise, an active spreading center that forms a triple junction between the Nazca, South American, and
Sedimentology of the Mininco, Rodados Multicolores, Nochaco, and Cañete formations
We analyzed different stratigraphic columns of the Mininco, Rodados Multicolores, Nochaco, and Cañete (Fig. 8, Fig. 9). Outcrops of these units typically occur along.
roadcuts and riverbanks. Sections are typically small and badly preserved, which prevents a refined sedimentologic interpretation.
Cenozoic tectono-sedimentary evolution of the forearc
Zircon and apatite fission-track data obtained by Glodny et al. (2008) in the present Coastal Cordillera of south-central Chile (36°-42°S) indicate very slow exhumation in this area since ~200 Ma (see chapter 2.1.1). The sedimentary record indicates subsequent intervals of uplift and subsidence, but these events did not lead to exhumation detectable by the apatite fission-track method (Glodny et al., 2008). Sedimentary and igneous activity was scarce in the forearc during the Mesozoic and the
Conclusions
Our studies indicate the following stages on the tectono-sedimentary evolution of the forearc of south-central Chile (36°30′-42°S) during the late Cenozoic.
1) A regional event of extensional tectonics during the Oligocene-early Miocene resulted in the genesis of a series of basins that extended from the present Chilean coast to the retroarc in Argentina. After a period characterized by widespread volcanism, progressive extension and crustal thinning led to a widespread marine transgression that
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
AE was founded by Conicyt, Fondecyt Regular projects 3060051, 11080115, and 1151146, and by the Universidad de Concepción, Chile. MPR was founded by Conicyt, Fondecyt project 3180710, Chile. LS was founded by ANPCyT (PICT 2016-2252), University of Buenos Aires (grant UBACYT20020150100166BA, 20020190100234BA) and CONICET (Grant 11220150100426CO), Argentina. DO work was partially funded by the project UNRNPI 40-A-631, Argentina. We thank ENAP for access to seismic and well data and Dr. María
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