Monday, February 4, 2008

Hurricane Fault Transition Zone has many Clues to Uplift of Colorado Plateau

Part of Hurricane Fault, Hf, can be Measured for Orientation near Hiway 9 east of Laverkin (notice the alignment of a saddle with the fracture direction)

A Monocline or Hogback has risen across the Virgin due to cooling of Subsurface Magma at the face of the Hurriane fault (as evidenced by open cracks on its east side and dropping house lots on the Laverkin cliffs to the East)

Google Photo above shows the Virgin River and its border between Hurricane and Laverkin

Hurricane (Stout) Park Volcano, SPC- S27 41S 13W, named Radio Tower Volcano, by UGS
Access to this hike is made in the Town of Hurricane, going north on 200 West street from Hiway 9, until the city park is reached on the west side (750 West). Turn left, or west, to the gravel road which runs along the boundary of the park, and drive to the end, about 200 yards. Park on the shoulder at the park corner (bounded by a fence corner). Cross a ditch and at the fence corner, enter the volcanic boulder-strewn pasture land. Use either the existing faint trail past the waste storage pond to the west, or hike through open sloping land on the volcanic skirt to the NW. Proceed to the top and walk along an old cattle road to the north rim of the crater. This is just above the Virgin River, where the countryside with all of its’ impressive views are presented. We will look down on the river, the Mesozoic rocks, and the young volcanic detritus toward the north and west.

Section 27 has a 100k+ year extrusion (old lava flow) with a slight crater remaining. Hike with an objective of overlooking the Virgin River at the crater rim, to see the shear cliffs of basalt from several flows and sedimentary rock. A great sweep of the Laramide Virgin anticline, lava flows, and young faulting can be evaluated from a single stance.

Stratigraphy west of Hurricane fault (Hf) is highly contorted, while remaining almost flat and level to the East

Try to focus on the following Features shown, looking to the north and NE:
1. The Hurricane Mesa (high plateau with towers on top) to the east- which has flat and almost level strata (beds of sediments, which are laid parallel to each other);

Hurricane Mesa is to the East (Right of Photo), and is almost flat and level, compared to formations west of it in the Transition Zone
2. Tilted pink-colored monocline or Hogback, just across the Virgin to the north;
3. Basalt flows into the Previous Virgin canyon, as shown by black rock higher in elevation than the present river bottom;

Notice that the Black Basalt has moved into all available crevices

4. Basalt rock which is irregular on the base and sides (indicating filling of old canyons and faults and fissures);
5. Pine Valley Mountains, PVM, to the north and NW- which are an intrusion of granite-like rock rising in the Miocene, about 21 million years ago; and
6. The Virgin Anticline between PVM and the Virgin, which were compressed upward in the Laramide Compression from the west about 100 m.y. ago (notice that it is almost parallel to PVM).

It can be seen that there are at least two flows (there are at least 3 craters in the area) of Quaternary basalts, Qb, which flowed out onto a sedimentary surface, secondly flowed into river canyons, and finally, which were cut by the latest Virgin River erosion (forming the latest canyon- bypassing some of the older ones). In an outcrop on the north side of the Virgin can be seen the Ps or Pleistocene sediments (rubble), underlying a volcanic tuff- both conformable with each other, but not with the underlying Mesozoic and overlying rock. These are now at a 20 degree unconformity with the underlying sedimentary Triassic rock, Trs. These were laid down by the volcano on the south side of the river, occurring also on the north side by that same volcano (dipping in the opposite direction, as seen below Sky Mountain Golf on the south side of the Virgin), and are tentatively dated as Ps, and show that there was no Virgin River then where they now occur, or to the north of the Radio Tower Volcano. There is also an uplift of the Mesozoic to the east, which indicates a Hogback or tilted monocline pointing northward towards the Pine Valley Mountains; this is not a Laramide structure, although the incipience of the anticline several miles to the north might have been.

From below, the basalt which flowed into the Canyon appears more massive

It appears that the 100k years old vulcanism has created compressive stresses (shoving upward), which accentuated the 100m year Laramide structures in the 200m+ years old Mesozoic rocks. There are several faults and fracture indications in the cliff walls and on the surface Jurassic Navajo Jn on the north side of the River. These generally have a NW-SE trend, which is the normal strain pattern for this part of the Western US over the preceding Epoch (Pliocene) and earlier. The N-S patterns, associated with the recent strains and the Hurricane Fault are not as apparent in this location. One mile to the west and further, river bed directions do have this orientation. In this location the river orients either NW-SE or NE-SW, indicating a transition zone for the stress pattern just west of the Hurricane Fault. This is probably due to the rising vulcanism; some young stream patterns on the east side of the crater orient N-S, indicating the age of the new stress system. The author thinks that this stress change has occurred in the last 2-3 m.y, and that this might be offset by local anomalies, such as vulcanism.
It is believed that the Main Hurricane Fault is one mile east of the volcano, as evidenced by vertical beds at the abandoned Laverkin Power plant on the Virgin River, and the fault extends eastward in splays near the Pah Tempe Hot Springs (a 4000 feet separation). The volcano likely is associated with the uplift of the Colorado Plateau, at the Hurricane fault Hf, but the statistics of volcano location depend upon fracture systems more than upon the plane of the fault- and at the ground surface appear random in areal location.

Several Flows found flat ground, a shallow Virgin canyon, and finally a Gorge