Characterization of the depocenters and the basement structure, below the central Chile Andean Forearc: A 3D geophysical modelling in Santiago Basin area

Abstract

Since the last century, several geological and geophysical studies have been developed in the Santiago Basin to understand its morphology and tectonic evolution. However, some uncertainties regarding sedimentary fill properties and possible density anomalies below the sediments/basement boundary remain. Considering that this is an area densely populated with more than 6 million inhabitants in a highly active seismotectonic environment, the physical properties of the Santiago Basin are important to study the geological and structural evolution of the Andean forearc and to characterize its seismic response and related seismic hazard. Two and three‐dimensional gravimetric models were developed, based on a database of 797 compiled and 883 newly acquired gravity stations. To produce a well‐constrained basement elevation model, a review of 499 wells and 30 transient electromagnetic soundings were used, which contribute with basement depth or minimum sedimentary thickness information. For the 2‐D modelling, a total of 49 gravimetric profiles were processed considering a homogeneous density contrast and independent regional trends. A strong positive gravity anomaly was observed in the centre of the basin, which complicated the modelling process but was carefully addressed with the available constrains. The resulting basement elevation models show complex basement geometry with, at least, eight recognizable depocenters with maximum sedimentary infill of ~ 500 m. The 3‐D density models show alignments in the basement that correlates well with important intrusive units of the Cenozoic and Mesozoic. Along with interpreted fault zones westwards and eastwards of the basin, the observations suggest a structural control of Santiago basin geometry, where recent deformation associated with the Andean contractional deformation front and old structures developed during the Cenozoic extension are superimposed to the variability of river erosion/deposition processes.

Tectonosedimentary evolution of the Coastal Cordillera and Central Depression of south-Central Chile (36°30′-42°S)

“…Most of the forearc became a positive, mountainous area as indicated by seismic, borehole, and gravimetric data that show an irregular paleorelief carved on Oligo-Miocene and older rocks (e.g., García, 1968;Rubio, 1990;Elgueta et al, 2000;Yáñez et al, 2015;González et al, 2018). The difference in elevation between the lowest and the highest parts of this paleorelief is approximately 1000 m (see González et al, 2018). Sedimentation resumed in the forearc during the lower Pliocene (Fig.…”

“…Plio-Pleistocene deposits in the forearc have highly variable thicknesses that reach more than 2000 m. Thus, the accommodation space cannot have been created by a global sea-level rise and the consequent fluvial sedimentation to maintain river equilibrium profile since maximum eustatic sea-level changes during the last 5 Ma are just on the order of 100 m (Haq et al, 1987). In addition, González et al (2018) noted that the lowest part of the paleorelief carved in Oligo-Miocene or older rocks in central Chile is located at elevations of 200 m below the present sea level, which indicates that the Central Depression has been subjected to subsidence. Information from ENAP wells in the Central Depression between ~41 • and 42 • S show that the thickness of Plio-Pleistocene deposits increases considerably to the east and reaches ~2250 m near the limit with the Andean Cordillera (Fig.…”

The geophysical signature of Oyut deposits, Oyu Tolgoi, Mongolia

“…9. Results of inversion products were effectively located with porphyry copper sulphide mineralization data, and density and magnetics susceptibility distribution have good spatial correlations (Gonzalez, et al, 2018) with copper mineralization. In the next section, the results of the study are considered in terms of sections AA and BB shown in Fig.…”

Shallow Anatomy of the San Ramón Fault (Chile) Constrained by Geophysical Methods: Implications for its Role in the Andean Deformation

“…The Central Depression basin is filled with Pleistocene to Holocene sedimentary and volcaniclastic deposits (e.g., Fock et al, 2005). Geophysical studies have inferred an average depth of 250 m for the Central Depression in the vicinity of Santiago, with a maximum depth of 500 m (González et al, 2018;Yáñez et al, 2015). The geology of the western flank of the Main Cordillera is dominated by volcanic and sedimentary rocks of the Abanico Formation (Late Eocene to Early Miocene), which is overlain by the volcanic Farellones Formation (Miocene).…”

“…Although some seismicity clusters suggest the likely presence of east dipping faults closer to the western flank of the Main Cordillera (see, e.g., Armijo et al, 2010;Farías et al, 2010), there are no records of clear seismic clustering along the trace of the SRF at depths <5 km (see the discussion in Estay et al, 2016). Gravimetric studies have indicated the presence of a sediment depocenter in the northeastern part of the Santiago basin, whose eastern margin partially coincides with the northern segment of the SRF (González et al, 2018;Yáñez et al, 2015). However, these studies do not show any distinctive feature in the structure of the basin along the central and southern segments of the SRF, suggesting that the SRF did not play a key role in the development of the Santiago basin (Yáñez et al, 2015).…”