Wednesday, April 25, 2007

Sample Earth Science Hike, PVM Laccolith, Utah

Oak Grove Earth Science Hike along the Wilderness Area Boundary (S8, 9 T40S R14W): Apr 10/07, PVM
To access this border of the Pine Valley Mountains, SE side, exit I-15 on the north side of the freeway at the town of Leeds, Utah, then proceed northward on the Silver Reef road for two miles to a fork leading northward toward the Oak Grove Campground (graveled after crossing the creek). Just short of 8 miles from I-15, take a right fork to the trailhead to Columbine Spring, skirting the Wilderness area (this is a 100 meter dead end road, ending at the trailhead). Hiking north, there is a Tee at which good views to the east, of sedimentary outcrops, may be accessed. Follow this trail to the lowest point where you can then access the contact of igneous and sedimentary rocks by walking uphill cross-country in a dry wash.
This area has intrusive rocks, Tip, in contact with older Tertiary and Mesozoic outcrops; expect the following scenario:
1. The Miocene 21mybp intrusion (Pine Valley Mountains) rose to a kilometer or less depth (below ground surface) in Tertiary times, and then it moved outwardly and formed the shape of a mushroom on the SE side of the melt. I expect this since this is the situation with many salt domes (which are also hot plastic-flow rocks). At about this depth, it is easier to lift the overburden than to vertically fracture it, and the melt moves laterally- forming a sill or dikes. This problem has been solved by engineering, and the depth of the change in direction depends upon Poisson’s ratio and for the sediments to have essentially no strength for shear (the Tertiary Claron Tc or underlying Cretaceous most likely fits this requirement). The lateral hoop stress is about twice the normal stress (S: lateral=μ x Vertical, overburden stress, ~ .5 x depth x 1.2 psi/foot x 2 for a hoop, whereas Sv is ~ 1 psi/ft) for an intrusion pushing either outward or upward), and the rising melt cannot easily vertically fracture the overlying rocks- rather the movement proceeds laterally.
Note: Tc is divided into at least two vertical sections by geologic mappers, and the lower is the pink or red found on this hike; it has a total thickness no greater than 2300 feet (about 700 meters) near Cedar City, Utah. If the melt reached the upper Tc, it would have been no more than 1000 feet below ground surface in Miocene time.
2. The melt (quartz monzonite) was an intrusion- never reaching the surface of the ground. Nevertheless, there it is exposed in the Pine Valley Mountains (PVM). Since large crystals and grains are portrayed in the igneous rocks (requiring a long slow cooling period), they definitely were covered while cooling- otherwise the rock would be glassy or aphanitic, without large crystals. The fact that the ground mass in the rock has fine crystals surrounding larger grains indicates that the large grains formed at depth with the slow cooling; the large grains would have formed before the hot mass reached a shallow depth where more rapid cooling of finer crystals occurred. To become exposed, the overlying Tertiary and Cretaceous rocks must have been eroded later, exposing some kilometer thickness of igneous rock (as seen now- from about 7,000 at the sedimentary contact to 10,000 feet at the highest mountain). This has happened in the 21 my time since emplacement. Since the soft sediments eroded rapidly, compared to the igneous rock, there was exposed to the air the PVM and its surrounding sills and dikes.
3. The surface geological map shows some granite-like igneous rocks (monzonite) with Tc on three sides in the lands to the SE of PVM, near Anderson Junction at I-15. These rocks are all too low in the section (compared to their original appearance higher up the PVM); consequently they must have moved since their emplacement. This is shown by the appearance of older rocks, such as Jn, Navajo sandstone, in a fairly normal circumstance nearby. For the case where Tc borders the intrusive rocks at an abnormally low elevation, one of the following must have occurred:
a. A dike or sill occurred below the top of Tc (at less than one km depth, in Miocene times), and after erosion of Tcu (upper) some 3000 or more feet, the whole mass moved down the mountain by creep. Keep in mind that under the overhang of igneous sills there would have occurred a normal section of sedimentary rock which was by-passed by the upwardly-buoyed laccolith nearby (to the NW). When Engineer M. King Hubbard proposed this mechanism 50 years ago for large-scale thrusting, it was derided by geologists as “solving geological problems, such as thrusting and generation of overpressure, by lubrication”. Nevertheless, after the dynamics of earth movements were investigated using physics, it was found that this is the likely mechanism for the movement of large blocks of rock associated with Geopressure- particularly over rock with a large component of fine grains (clay or shale). The overlying block of earth slides over a temporary cushion of over-pressured shale, allowing easy movement; or
b. The block of igneous rock on the SE side of PVM, surrounded by Tc was higher in elevation originally, but has since dropped with normal faulting. I think this likelihood is low, since the nearby columns of igneous rock as far away as five miles have performed similarly. Most igneous masses on the SE side of PVM do not have bordering Tc, so they could either have been original dikes off the main laccolith or could have detached separately. This detachment mechanism has been investigated extensively in AZ, and the underlying rock is usually highly metamorphosed. I find none of this in this location, and the likelihood is again low; or
c. The dikes are still in their original position, at least for those not having bordering Tc or Cretaceous rocks; they would not have moved (for these outcrops), but could have occurred because of the previously investigated Weak Zone on the SE side of PVM. It is interesting that there is no obvious Tip on the west side of PVM, and that extrusion occurs on the north side. Further, the cursory look at the west side seems to have sediments down-dipping against PVM (the opposite of what you would expect for a compressive intrusion); hence the melt incorporated the silicates to yield the characteristic quartz monzonite and did not shove north or westward. Extrusion (basalts and other flows) on the north side indicates that extension occurred there.