Paleomagnetic Determination of Vertical Axis Block Rotation near the Doty Fault in southwestern Washington
Bibliographic record
Abstract
In this paper I present the results of paleomagnetically derived vertical axis rotations (VARs) of sites in two different flows of the Columbia River Basalt (CRB) – the 16 Ma Sentinel Bluffs member of the Grande Ronde flow and the 12 Ma Pomona Member of the Packsack Lookout – near the Doty fault in southwestern Washington. In two field seasons, I collected 99 cores from 14 sites, 11 in the Grande Ronde flow and three in the Pomona member flow. Of the 227 specimens that I demagnetized, 212 had well-defined magnetic directions. Positive fold and reversal tests results confirm the primary magnetization in both flows of the CRB, which is carried by magnetite and titanomagnetite. Using virtual geomagnetic poles (VGPs), I compared the tectonically corrected site mean directions from this study against reference directions measured in the same lithologies in the stable eastern Washington which has had little rotation since the Miocene. The resulting VARs in the Grande Ronde flow show a large range of rotations, from 68° counterclockwise (CCW) to 9.6° clockwise (CW) (Table 4), with an average CCW rotation of 22.3°. The Pomona Member sites all show CCW rotation between 0.5° and 2.5°, with an average CCW rotation of 2.1°. Previous paleomagnetic regional studies have consistently found an approximately 14-16° CW rotation in CRBs located on the forearc block. The CCW rotations of the CRBs adjacent the Doty fault suggest that fault activity, specifically sinistral strike-slip motion, occurred between 16 and 12 Ma, and possibly in the time since then. I hypothesize that an escape structure or wrench zone may be present in the region due to the concentrated, homogeneous stress present as the rotating Oregon-Washington forearc block squeezes the Puget lowlands block against the Canadian coast mountains. I propose that significant additional paleomagnetic research is needed in the area to determine the extent of the area with CCW rotation. This will also help create a more resolved map of where the boundaries of the rotating blocks are, how they may be rotating in relation to one another, and to help map the network of smaller faults facilitating the rotations.
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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 | 0.000 |
| 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 | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| 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.
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