2 May 2012
To recap the week so far here on Mountain Beltway: On Monday we looked at some sweet vertical boudinage along the plane of tectonic cleavage (not to mention those folds in a (formerly) horizontal granite dike, now bearing vertical axial planes), and then on Tuesday we looked at a horizontal cut through that same outcrop, and it showed moderately distorted lava pillows, And now today, let’s look at one more neat feature from this same outcrop: horizontal joints, formed almost perfectly perpendicular to the vertical foliation:
You’re looking at a vertical outcrop plane there, and so the white-ish bands (quartz veins) are oriented more or less horizontally. Here, we zoom in to the propagating tip of one joint, with fibrous minerals perpendicular to the joint’s surface (parallel, presumably, to the extension direction):
More massive milky quartz can be seen to the left, in the most widely-dilated portion of the vein.
Though of a different style (brittle) than the boudinage (brittle/ductile), the orientation of these structures is consistent with the same kinematics (vertical extension).