Shear wave anisotropy beneath Nicaragua and Costa Rica: Implications for flow in the mantle wedge
Bibliographic record
Abstract
We present new shear wave splitting data from local events in Costa Rica and Nicaragua recorded by the temporary (July 2004 to March 2006) 48‐station TUCAN broadband seismic array. Observed fast polarization directions in the fore arc, arc, and back arc range from arc‐parallel to arc‐normal over very short distances (<5 km when plotted at raypath midpoints) making the direct interpretation of individual splitting measurements in terms of flow tenuous, even when considering variations in the relationship between lattice‐preferred orientation and deformation (e.g., B‐type dislocation creep in olivine). Therefore, we tomographically invert the splitting measurements to find a three‐dimensional model of crystallographic orientation in the wedge. We assume the elastic constants of olivine and orthopyroxene with hexagonal symmetry and use a damped, iterative least squares approach to account for the nonlinear behavior of splitting when considering three‐dimensional ray propagation and distributions of anisotropy. The best fitting model contains roughly horizontal, arc‐parallel olivine [100] axes in the mantle wedge down to at least 125 km beneath the back arc and arc, which we interpret to indicate along‐arc flow in the mantle wedge. Pb and Nd isotopic ratios in arc lavas provide additional evidence for arc‐parallel flow and also constrain the direction (northwest, from Costa Rica to Nicaragua) and minimum flow rate (63–190 mm/a). With only slightly oblique subduction at 85 mm/a of the relatively planar Cocos Plate, the most likely mechanism for driving along‐arc transport is toroidal flow around the edge of the slab in southern Costa Rica, generated by greater slab rollback in Nicaragua. Two important implications of this arc‐parallel flow are the progressive depletion of the mantle source for arc lavas from Costa Rica to Nicaragua and the possible need for significant decoupling between the wedge and downgoing plate.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".