Long‐Term Lithospheric Strength and Upper‐Plate Seismicity in the Southern Central Andes, 29°–39°S

Abstract

We examined the relationship between the mechanical strength of the lithosphere and the distribution of seismicity within the overriding continental plate of the southern Central Andes (SCA, 29°–39°S), where the oceanic Nazca Plate changes its subduction angle between 33°S and 35°S, from subhorizontal in the north (<5°) to steep in the south (∼30°). We computed the long‐term lithospheric strength based on an existing 3D model describing variations in thickness, density, and temperature of the main geological units forming the lithosphere of the SCA and adjacent forearc and foreland regions. The comparison between our results and seismicity within the overriding plate (upper‐plate seismicity) shows that most of the events occur within the modeled brittle domain of the lithosphere. The depth where the deformation mode switches from brittle frictional to thermally activated ductile creep provides a conservative lower bound to the seismogenic zone in the overriding plate of the study area. We also found that the majority of upper‐plate earthquakes occurs within the realm of first‐order contrasts in integrated strength (12.7–13.3 log Pam in the Andean orogen vs. 13.5–13.9 log Pam in the forearc and the foreland). Specific conditions characterize the mechanically strong northern foreland of the Andes, where seismicity is likely explained by the effects of slab steepening.

Lower and Upper Plate Controls on Crustal Seismicity in the Southern Central and Patagonian Andes

“…Our results confirm previous studies, indicating a southward decrease in upper‐plate seismicity (Gutscher et al., 2000; Nacif et al., 2017; Rodriguez Piceda et al., 2022). However, our updated and comprehensive analysis, including extensive datasets, reveals a major drop in the upper‐plate seismicity at a specific latitudinal area, which is not straightforward to explain.…”

“…(2000) suggested that areas of flat subduction in the Andean margin release more seismic energy, thus suggesting a first‐order control of lower plate geometry on upper‐plate seismicity. In the study area, recent studies have confirmed this hypothesis, showing a decrease in seismic activity south of the Pampean flat‐slab (Lupari et al., 2015; Nacif et al., 2017; Olivar et al., 2018; Rivas et al., 2019; Rodríguez Piceda et al., 2022; Venerdini et al., 2020). This pattern of seismicity has been recently interpreted as related to changes in lithospheric strength to the south of the flat subduction segment (Rodríguez Piceda et al., 2022).…”

“…Another possibility is that the massive difference in seismicity concerning the Andean margin at lower latitudes responds to a change in the upper‐plate lithospheric strength south and north of 38.5°S, which could produce a difference in the seismotectonic behavior along the active margin (Ibarra et al., 2021; Rodriguez Piceda et al., 2022; Tassara & Yáñez, 2003). An analysis of effective elastic thickness (Te), a proxy of the long‐term lithosphere strength (Watts, 1992), shows a gradual increase in Te along strike the forearc sector toward the 33°–39°S with Te values mostly between ∼30 and 15 km and a sudden decrease south of 39°S to values around 15–10 km (Tassara et al., 2007) (Figure 6b).…”

“…The seismic activity along the active margin, from Colombia to Patagonia, is mainly driven by ongoing plate convergence (6.1 cm/yr, Norabuena et al., 1999) and associated orogenic processes (e.g., Alvarado et al., 2009; Dewey, 1972; Farías et al., 2010; Jordan et al., 1983; Malavé & Suárez, 1995; Ojeda & Havskov, 2001; Pardo et al., 2002; Wigger, 1988). However, the reasons why some segments of the Andean margin are characterized by an intense seismic activity on the upper plate while others have more discrete seismicity is still debated (Gutscher et al., 2000; Nacif et al., 2017; Rodríguez Piceda et al., 2022).…”

Late Miocene Exhumation of the Western Cordillera, Ecuador, Driven by Increased Coupling Between the Subducting Carnegie Ridge and the South American Continent

“…Important changes in the dip of the subducting plate may also have an impact on mantle dynamics and influence dynamic topography of the upper plate (e.g., Dávila et al., 2010; Eakin et al., 2014; Gérault et al., 2015; Liu, 2015). In addition to the subduction of bathymetric anomalies and its impact on upper‐plate processes, inherited lithospheric‐scale heterogeneities and strength contrasts in the upper plate exert first‐order controls on deformation, uplift, and magmatic processes, thus influencing the build‐up of topography in subduction orogens (e.g., Horton & Fuentes, 2016; Isacks, 1988; Rodriguez Piceda et al., 2022; Waldien et al., 2022).…”