| Autor: |
David C. Engebretson, M.Clark Blake, Russell F. Burmester |
| Rok vydání: |
2000 |
| Předmět: |
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| Zdroj: |
Tectonophysics. 326:73-92 |
| ISSN: |
0040-1951 |
| DOI: |
10.1016/s0040-1951(00)00147-5 |
| Popis: |
The Eastern San Juan Islands expose an island arc ophiolite sequence and one or more ocean floor terranes. Cover rocks of the former, including red radiolarian tuff and arc-derived sandstone, have no penetrative fabric and lack high-pressure minerals. In contrast, pillow basalts and ribbon cherts of the latter are strongly deformed and the overlying metagraywacke is folded with a penetrative axial plane cleavage. All ocean floor rocks have aragonite in veins, indicative of higher-pressure conditions than experienced by the island arc rocks. A downward magnetization is common to the ocean floor rocks, consistent with widespread remagnetization during the Cretaceous normal superchron. This magnetization is carried by magnetite and, in red rocks, by hematite; fluid flow during or after deformation and high P metamorphism likely facilitated remagnetization at low temperature. The magnetic directions are somewhat scattered but most are moderately to steeply down to the southeast. They are far from directions expected with or without northward transport of Baja BC. Rotation of observed directions into agreement with those expected for either present or more southerly locations also rotate structural elements in the host rocks. Three alternative rotation histories explored here all produce structural reconstructions that differ considerably from the present structure. A tectonic history of the eastern San Juan Islands based on any of these reconstructions would be superior to one based on present attitudes of structures. This is because at least a constraint that the restored paleomagnetic directions match a paleomagnetic field direction is satisfied. Choices among alternative reconstructions would become clearer with better understanding of relative ages of specific structures and their magnetizations. Improved geologic understanding of the regional setting where the rocks could have been deformed and remagnetized would also help. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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