ABSTRACT Forearc topography and inferred paleotopography are key constraints on the processes acting at plate interfaces along subduction margins. We used along-strike variations in modern topography, trench sediment thickness, and instrumental seismic data sets over >2000 km of the Chilean margin to test previously proposed feedbacks among subducted sediments, plate interface rheology, megathrust seismicity, and forearc elevation. Observed correlations are consistent with subducted sediments playing a prominent role in controlling plate interface rheology, which, in turn, controls the downdip distribution of megath-rust seismicity and long-term forearc elevation. High (low) rates of trench sedimentation promote long-term interseismic coupled offshore forearc uplift (subsidence) and onshore forearc platform subsidence (uplift). Low trench sedimentation rates also promote deeper megathrust seismic slip, enhancing short-wavelength coastal zone uplift. Shallowing of subducting slabs contributes to a reduction in coastal zone–onshore forearc relief, in turn preventing formation of onshore forearc basins. The extremely low denudation rates of hyperarid northern Chile have allowed better reconstructions of the histories of paleoel-evations and paleoclimate compared to other sections of the forearc. Even if these histories are not sufficiently resolved to unequivocally assign causality among climate variability, changes in plate interface frictional properties, and forearc elevation, they are consistent with the onset of hyperaridity in the coastal zone at 25–20 Ma (1) triggering long-term, long-wavelength offshore forearc subsidence and onshore forearc uplift, and (2) accelerating short-wavelength coastal zone uplift.

The dynamics of the erosive central Andean forearc vary significantly along strike. In northern Chile at 20°S–27°S, and particularly at 22°S–25°S, the forearc in the Coastal Cordillera has been undergoing extension since at least the Pliocene, reactivating steep E-dipping faults of the Mesozoic Atacama fault system. This has been explained by forearc uplift driven by underplating, shallow slab dip, subduction of bathymetric features, and elastic rebound during the earthquake cycle. These processes, however, are active over a much wider area of the central Andean forearc than Coastal Cordillera extension and therefore cannot explain why extension is localized to the northern Chilean onshore outer forearc. We compiled crustal seismicity and the depth of lithospheric boundaries from existing studies to investigate other possible explanations for onshore forearc extension. At 22°S–25°S, seismicity increases above the background subduction-related level present to the north and south. Extensional focal mechanisms, consistent with steep E-dipping faults active at depths up to ~40 km, are also present onshore within this latitude range, but absent to the north and south; this is consistent with the distribution of mapped active fault scarps. The Salar de Atacama crust is seismically active at depths up to ~40 km. Thick lithosphere is present beneath the forearc, and the longitudinal axis of thickest lithosphere is deflected to the east at the latitude of the Salar de Atacama. To the east, the Puna Plateau lithosphere has been thinned by lithospheric removal events. The most robust correlation with onshore forearc extension is the thick, cold, strong crust and mantle lithosphere beneath the anomalous Salar de Atacama in the inner forearc of northern Chile. The combination of underplating-driven outer forearc uplift, the presence of the preexisting structure of the Atacama fault system favorable for reactivation, and the negative buoyancy beneath the Salar de Atacama is inferred to drive Coastal Cordillera normal faulting at this latitude. Recent (<10 Ma) lithospheric removal beneath the Puna Plateau to the east may have enhanced the effect of the negatively buoyant Salar de Atacama lithosphere on the forearc. This implies that both preexisting lithospheric structure and lithospheric processes in the hinterland may influence forearc dynamics.

The forearc of Central Chile (33°–34°S) is formed by three N-S–trending morphostructural units, including, from west to east, the Coastal Cordillera, the Central Depression, and the Principal Cordillera. The Cenozoic sedimentary rocks that could represent the erosional material generated throughout the morphotectonic evolution of these units accumulated in the marine Navidad Basin. The age of the marine deposits is controversial, as foraminifer biostratigraphy indicates that marine deposition started during the late Miocene, whereas 87 Sr/ 86 Sr data indicate that deposition started during the early Miocene. We carried out single heavy mineral microprobe analysis and standard heavy mineral analysis of these deposits in order to qualitatively identify the geological units subjected to erosion in the central Chilean forearc during Cenozoic times. Our analysis focused mainly on unweathered and unaltered detrital garnet, pyroxene, and amphibole. The textural characteristics and geochemical signature of these minerals were used to determine their original rock type; their magmatic affinity, in the case of pyroxenes of volcanic origin; and their metamorphic grade, in the case of amphiboles of metamorphic origin. We have also compared the composition of detrital garnet, pyroxene, and amphibole with preexisting chemical data of these minerals in the possible source rocks, which, along with the analysis of the detrital heavy mineral suite in each sample, allows us to determine the specific geological unit from which they were generated. Three erosional-depositional stages are recorded by our analysis. Whereas the chemistry of pyroxene and amphibole characterized volcanic-subvolcanic sources within the present-day Central Depression for the first stage, the Central Depression and the Principal Cordillera for the second stage, and the Principal Cordillera for the third stage; the composition of garnet is indicative of metamorphic and plutonic sources within the Coastal Cordillera during all three stages. If marine deposition inside the Navidad Basin started during the early Miocene, the provenance results would record erosion and deposition contemporary with volcanic activity. On the other hand, if marine deposition started during the late Miocene, the provenance results show a retrograde erosive response to landscape for a regional uplift event proposed for that period in the study area. Also, assuming that provenance results are directly related to the action of faults, our data indicate that the main relief-generating fault during the early stages of Andean uplift corresponds to the Los Ángeles–Infiernillo Fault, rather than the San Ramón Fault, as stated by the proposed morphotectonic models for the study area. In addition, the ubiquitous provenance from the Coastal Cordillera is more likely to represent the erosion of nearshore basement rocks affected by faulting along the eastern border of the Navidad Basin, rather than uplift and erosion of the Coastal Cordillera, as previously considered. Single-mineral geochemical analysis of detrital pyroxene and amphibole can be used in other sedimentary basins related to arc-magmatic systems with short transport distances, like the ones in the western Andean border, where these minerals tend to be largely unweathered. In particular, our work represents an advance in this field, as the chemistry of detrital amphibole has not been used before to discriminate source rocks presenting different geochemical signatures.

The Western Escarpment of the Andes at 18.30°S (Arica area, northern Chile) is a classical example for a transient state in landscape evolution. This part of the Andes is characterized by the presence of >10,000 km 2 plains that formed between the Miocene and the present, and >1500 m deeply incised valleys. Although processes in these valleys scale the rates of landscape evolution, determinations of ages of incision, and more importantly, interpretations of possible controls on valley formation have been controversial. This paper uses morphometric data and observations, stratigraphic information, and estimates of sediment yields for the time interval between ca. 7.5 Ma and present to illustrate that the formation of these valleys was driven by two probably unrelated components. The first component is a phase of base-level lowering with magnitudes of∼300–500 m in the Coastal Cordillera. This period of base-level change in the Arica area, that started at ca. 7.5 Ma according to stratigraphic data, caused the trunk streams to dissect headward into the plains. The headward erosion interpretation is based on the presence of well-defined knickzones in stream profiles and the decrease in valley widths from the coast toward these knickzones. The second component is a change in paleoclimate. This interpretation is based on (1) the increase in the size of the largest alluvial boulders (from dm to m scale) with distal sources during the last 7.5 m.y., and (2) the calculated increase in minimum fluvial incision rates of ∼0.2 mm/yr between ca. 7.5 Ma and 3 Ma to ∼0.3 mm/yr subsequently. These trends suggest an increase in effective water discharge for systems sourced in the Western Cordillera (distal source). During the same time, however, valleys with headwaters in the coastal region (local source) lack any evidence of fluvial incision. This implies that the Coastal Cordillera became hyperarid sometime after 7.5 Ma. Furthermore, between 7.5 Ma and present, the sediment yields have been consistently higher in the catchments with distal sources (∼15 m/m.y.) than in the headwaters of rivers with local sources (<7 m/m.y.). The positive correlation between sediment yields and the altitude of the headwaters (distal versus local sources) seems to reflect the effect of orographic precipitation on surface erosion. It appears that base-level change in the coastal region, in combination with an increase in the orographic effect of precipitation, has controlled the topographic evolution of the northern Chilean Andes.

The Coastal Cordillera of northern Chile is the only subaerial part of the South American continental crust in direct contact with the subducted Nazca plate. Deformation of the cordillera can be directly related to plate coupling at the subduction zone. Knowledge of uplift rate variation along the coast is however limited, mainly due to difficulties in dating and correlating the discontinuous remnants of marine terraces. This paper uses geomorphology and the stream gradient index (SGI) to examine 80 drainages along 200 km of coastline between 21°S and 23°S. Selected catchments are located on similar geology (basaltic andesite and granodiorite) and within the same coastal climate zone. The southernmost part of the transect overlaps with an area of known uplift rates based on dated marine terraces. Characteristics of the SGI index, combined with the geomorphology, enable the coast to be divided into two sectors. Extrapolation of the SGI values from an area of known (sector 1) to unknown uplift rates (sector 2) enables an estimate of relative uplift rates for sector 2. Sector 1 has known uplift rates of 240–384 mm k.y. −1 . The SGI results suggest sector 2 has overall uplift rates which may be 2–3 times greater than sector 1. The large-scale (70–100 km) differences in apparent differential uplift reflected by the SGI along the Coastal Cordillera are tentatively attributed to aseismic ridge subduction. Smaller-scale (30 km) variations in the SGI associated with the major regional drainages in sector 2 suggest that the latter drainages may be focused in structural lows (with low SGI values) controlled by NE-SW– to E-W–trending reverse faults, although further work is required to clarify this.