Timing Determination of Phase B of the Yanshanian Movement in the Eastern North China Craton: Evidence from Dating of A Ductile Shear Zone in Sihetang, Northeastern Beijing
Bibliographic record
Abstract
The orogenic belt is an important Mesozoic tectonic feature in north of the North China Craton (NCC). It was named after its position by Wong (1927, 1929, Bulletin of Geological Society of China). The Yanshanian orogeny is believed to consists of two phases (A and B), represented by two regional angular unconformities, but their timing has not been defined. Both phases are characterized by crustal shortening. Phase A is marked by the unconformity beneath the andesite volcanic rocks of the Tiaojishan Formation and Phase B by the unconformity beneath volcanic rocks of the Zhangjiakou Formation. The famous Sihetang ductile shear zone developed within the Yunmeng area in the central segment of the Yanshan tectonic belt, and was the product of Phase B of Movement. The Sihetang shear zone is located in the north margin of the Early Cretaceous Yunmengshan pluton, with hanging wall thrusting to near SSW. Adjacent metamorphic basement, Meso-and-Neoproterozoic cover layers, and Late Jurassic to Early Cretaceous plutons were involved in the ductile shear zone deformation. Many syntectonic dykes present in the Shihetang shear zone were found to emplace in three stages, which correspond to three different stages of the shear belt (early, middle and late) based on their geometries within the shear zone. This study collected the zircons from dykes of each stage for dating. LA-ICP-MS zircon U-Pb ages for the three stages are 143.0±2.1 Ma, 140.8±1.4 Ma, and 137.5±2.4 Ma respectively, consistent with dyke emplacement order constrained from structural analysis. This also suggests that 143~138 Ma should be the time period for the B phases of the Sihetang ductile shear zone deformation. This result couples with that of the zircon of volcanic rocks sampled from neighboring strata around the shear zone.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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 teacher head, 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".