Wednesday, October 17, 2007
Detail of Structural Rearrangements to N of Crater
A Young Hogback has been Created by Surface Sediments rotating into the void left by Magma evacuation (greatest along the Hurricane Fault)
Ancient and Current Earth Movements near Hurricane and its namesake Fault
Refer to accompanying photos, to see the movements revealed in the rocks outcropping near the lands bordering the Hurricane Fault, Hf: S 26, 27, 34 & 35, T41S, R13W, Utah.
Hikes and Field Trips during the years 2004-07, by our local Earth Science group ( www.PorOgle.blogspot.com ), have shown that the Earth’s Crust is moving continuously in the region near Hf, in several ways:
1. The Colorado Plateau, CP, is uplifting- relative to the Basin and Range, B&R, to the west- along Hf;
2. Additional subsidence along the same fault occurs on the west side, due to past evacuation of magma through several vents. Although the vulcanism is not active now, the shrinkage of the cooling subsurface rock causes the overlying surface rock to sink into the contracting space and form Hogbacks running north to south, N-S (more so near the fault);
Look Carefully, and you will see changes in dip on the west side of the Hogback, as distance from nearby vents increases
3. CP, in addition to uplifting, is rotating along Hf in a right lateral manner, as shown by slickensides on the east side of Hf;
4. There is undulation of the rock segments along Hf, where some crust moves more than its neighboring outcrops. This creates lenses of contrasting-movement rock, as well as grabens along Hf;
5. There are large fracture systems which intersect Hf, creating multi-faulting segments in the outcrops. These appear to trace NW-SE, contrasting with the N-S trending Hf. At the intersection there is instability, such as in the Laverkin quarry. One such trace moves SE from the Pine Valley Mountains, PVM, along the Wet Sandy Creek towards Toquerville (possibly feeding the Ash Creek Spring there), and then traces along Hf until reaching the quarry- where the trace moves SE toward the large fissures above the Virgin River, near the diversion dam west of the town of Virgin;
6. The Crust occasionally shakes from unknown causes, with earthquakes, such as happened in 1992; and,
7. There is abnormal expansion due to geothermal activity, such as that at Pah Tempe and near the Veyo volcanic cone.
8. Large scale fracture systems, which by definition have little movement along them, exhibit shear in NW-SE linears and probably maintain openings in the sheared rock- so that fluids may move slowly along the linears.
All of the above movements or tendencies manifest themselves in the rocks as shown in accompanying photos. The trick is to verify the movements by looking at what appears to be static earth, but is in actuality motion at a rate which the eye cannot directly determine. Rather, the motion must be found from clues such as open fissures, inability of erosion to keep up with the uplift or downdrop, unsteady abnormal temperatures, spring water anomalies, strange rock movements, and landslides.
Dip Irregularities are noticeable along the West edge of the Hogback (looking north)
Basalt Flows entered the ancient Virgin River, as well as through Faults and Fractures