I can't help but share a virtual outcrop I just made from one of my favorite places for structural geology, Mosaic Canyon! This canyon lies at the northern end of the Panamint Range in Death Valley National Park in Eastern California. Here, the Noonday Dolomite has undergone general shear within the footwall of the Tucki Detachment/Low Angle Normal Fault; note extreme flattening of the layers (overall thickness of these units has decreased from >1 km to ~100m), and formation of asymmetric vein boudins in more competent layers indicative of general shear.
Note the overprinting of this more ductile fabric by increasingly brittle deformation; first, by semi-brittle shear zones with significant adjacent formation of small veins, then by fully brittle, small offset faults. This transition from ductile to brittle deformation occurred as these footwall rocks were exhumed and subject to lower pressure and temperature conditions shallower in the crust due to slip on the fault.
This fault is one of the early manifestations of the transition from subduction to a transform plate boundary along the west coast of the US due to collision of the mid-ocean ridge separating the Pacific and Farallon plates (around 25-30 MYA). This transition caused areas of crust that had been greatly thickened during subduction to stretch, and the Tucki Detachment is one of the structures accommodating that process (see Lutz et al., 2021 for an excellent description). Authigenic clay age dating suggests a timing of last motion on the detachment of ~16.9 MA (Haines et al., 2016). These fabrics record a North-South extension direction (Hodges, 1987), which is inconsistent with the NW/SE stretching direction characteristic of deformation today in Death Valley.
Also visible in the upper left corner of the outcrop is the "mosaic" for which Mosaic Canyon is named--perhaps a topic better left to a later post!
Earthmover and Industrial Tyre Specialist at T&C Site Services Ltd
2wDale - where are the Christmas decorations ? Or is it too early