Lower and Upper Plate Controls on Crustal Seismicity in the Southern Central and Patagonian Andes. Issue 1 (25th January 2023)

• Record Type: Journal Article
• Authors: Correa‐Otto, Sebastian; Gianni, Guido M.
• Is Part Of: Tectonics. Volume 42:Issue 1(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-01-25
• Subjects: slab tear -- slab break‐off -- crustal seismicity -- Andes; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2022TC007335
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: We analyze crustal seismological records to assess the origin of the variable seismic activity along the southern Central and Patagonian Andes (30°–55°S), an area associated with latitudinal changes in the lower‐plate geometry, upper‐plate deformation, and topography. We carried out a statistical analysis of the seismicity records at crustal depths of 0–50 km from an updated earthquake catalog. Studying the seismicity by its area covered, depth, latitude, and time, this analysis reveals a consistent and drastic decrease in the number of earthquakes south of 38.5°S. We note that the low seismicity area is spatially correlated with a thin, hot, and weak (at least in the forearc region) upper plate and the subduction of a young/warm lower plate with short slab depths (∼150–400 km) beneath Patagonia. Notably, the reduced upper plate seismicity is mimicked at depth by a similar decrease in lower plate seismicity associated with the presence of relatively shallow slabs south of 38.5°S. This observation allows us to suspect a genetic connection between lower plate properties and the striking reduction in the upper‐plate seismicity. We tentatively suggest that the young/warm and short Nazca and Antarctic subducting plates, left after Neogene slab detachments, modified the thermomechanical upper plate structure, triggering the drastic decrease in crustal earthquakes. This process was likely aided by an inherited thin continental margin. This study reveals that slab detachment in non‐collisional settings can impact upper‐plate seismicity. Plain Language Summary: We studied shallow earthquakes that occur in the crust, Earth's outermost layer, to better understand the variability in the seismic activity of the southern Andes. This area presents changes from north to south in the geometry and deformation of the tectonic plates that give origin to this massive mountain range. We studied a large earthquake database for the first 50 km below the Earth's surface, over the last 50 years. This analysis reveals a drastic decrease in the number of earthquakes that occur south of the latitude 38.5°. We discuss possible factors for this significant reduction of seismicity, and by pairing our results with seismic tomographies, studies that use seismic waves to create modeled images of the Earth's interior, we propose that the abrupt decrease in the seismic activity is related to the presence of small fragments of detached tectonic plates, that we refer to as short oceanic slabs. We discuss how the latter and an inherited thin continental margin would have jointly affected crustal properties and the earthquakes in the area. This study reveals that besides the commonly acknowledged effects of slab detachment for this kind of setting, this process may also impact the seismicity on the surface. Key Points: We analyze crustal seismicity in the southern Central and Patagonian Andes between 30° and 55°S Our analysis reveals a drop on upper‐plate seismicity south of 38.5°S coinciding with short slabs left after Neogene slab detachments Slab detachments of the Nazca and Antarctic plates modified the upper plate structure causing the drastic reduction of crustal seismicity