A Recent Tear in Subducting Plate Explains Seismicity and Upper Mantle Structure of Southern Costa Rica. (10th December 2020)

• Record Type: Journal Article
• Authors: Bourke, J.; Levin, V.; Linkimer, L.; Arroyo, I.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 21:Number 12(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-12-10
• Subjects: receiver functions -- seismology -- subduction -- tectonics; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2020GC009300
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Past studies of southern Costa Rica have generated a multitude of tectonic scenarios to account for different data sets. Flat slabs, detached slabs, and slab windows have been proposed to address the uplift of the Cordillera de Talamanca (CT), cessation of volcanism, and absence of deep seismicity beneath southern Costa Rica. In this study, we investigate the crust and the upper mantle along the southwest flank of the CT using the receiver function methodology. We observe two regional positive P‐to‐S converted pulses at delay times of ∼2–4 s and ∼5–8 s. The first likely represents a gradational crust‐mantle boundary of the upper plate. The second represents a similar impedance increase ∼50–60 km deep that extends from central Costa Rica to Panama. Compared to well‐located seismicity, this boundary is offset to the NE from the Cocos plate Benioff zone beneath northern CT, and remains observable through a gap in seismicity farther to the southeast. This offset makes it difficult to interpret this feature as related to the presently subducting lithosphere. Instead, we propose that the 50–60 km deep boundary marks the Moho of a lithospheric fragment left behind under the CT in the course of Panama Triple Junction migration through Costa Rica over the last 10 Ma. Our interpretation accounts for the geophysical, geochemical, structural, and geomorphic observations in the literature explaining the complex geodynamic scenario observed in southern Costa Rica. Plain Language Summary: Subduction of oceanic plates is typically associated with both intense seismic activity and volcanism. A segment of the subduction zone in southern Costa Rica and Panama lacks both of these attributes at present. The area has an unusual tectonic environment that is changing relatively rapidly (on the time scale of 1–3 million years) due to geometry and motion directions of three plates, Cocos, Nazca, and Caribbean, that are joined here forming a triple junction. Alternate scenarios proposed to account for the recent rise of very high mountains, the cessation in volcanism, and the absence of seismicity in the area include subduction gaps, flat subduction, and subducted lithosphere lost just recently (geologically speaking). We develop new seismological constraints on Earth internal layering in the upper 100 km beneath southern Costa Rica. We find an unexpected near‐horizontal boundary 50–60 km beneath this region. It is well below the crust mantle boundary of the Caribbean plate, and it is not aligned with well‐located seismicity of the currently subducting Cocos plate. We interpret this feature as a fragment of previously subducted oceanic lithosphere stranded due to the rapid rearrangement of plate configuration in Costa Rica‐Panama border region. Key Points: This study constrains the seismic properties beneath the Cordillera de Talamanca of southern Costa Rica We observe an anomalous impedance increase with depth in the upper mantle beneath the upper plate Moho that is also separated from the subducting Cocos plate We propose that this feature is a piece of detached lithosphere caused by the migration of the Panama Triple Junction through Costa Rica