Quartz Porphyroclast Mylonites, Boc Granite, Czech Republic
|
| In the Saxo-Thuringian there are intensely sheared mylonites after a coarse granite. These mylonites have two interesting features: one is that they commonly have quartz acting as the strongest mineral forming porphyroclasts surrounded by weak, stretched feldspars (this page, see paper by Zavada et al., 2007 JGR, see also Menegon et al., 2011, JGR), the other is that they were later folded and their lineation is curved in spectactular ways (associated page, click here). This page also depicts the overprinting of thrusting to the E by normal movement to the west in the Sphynx outcrop in the Krusny Hory Mountains. |
Quartz Porphyroclast Mylonites: Boc-Type (pronounced Botch)
Quartz porphyroclast mylonite in Boc. Notice intensely stretched feldspars (not retrogressed). Asymmetry indicative of top to left shearing. |
 Quartz porphyroclast mylonite. Notice intensely stretched feldspars (not retrogressed). Asymmetry indicative of top to left shearing. Joehstadt outcrop upper nappe of the Saxo-Thuringian. Photo close to parallel to lineation and perpendicular to foliation. |
 Same rock as previous but shear intensity has increased approaching ultramylonites and quartz porphyroclasts now form long lenses. Photo parallel to lineation and perpendicular to foliation. |
Overprinting of thrusting by normal movement, Sphynx outcrop, Menec
Sheared porphyritic granite at Sphynx with top-to-right (E) shear sense. |
Sheared Kfs phenocryst top-to-right (E) shear sense. Top clast is fractured by a synthetic shear zone allowing clockwise rotation of the sub-clasts. |
Sheared Kfs phenocryst top-to-right (E) shear sense. |
Interacting Kfs porphyroclasts, top-to-right (E) shear sense. |
Same outcrop as photographs above, but a narrow band (diagonally from finger to the upper right corner) shows normal movement (left side down, to the E) parallel to S-planes. |
|
Shear planes spaced 5 cm apart overprinting pre-existing C-plane of thrusting and indicating left-side-down (to the east) movement. |
Same as previous. |