Tuesday, November 20, 2007

Stress analysis of the regime surrounding Pine Valley Mts.PVM

PVM igneous Intrusion (Monzonite) is shown in Red, as contrasted to surrounding sedimentary Rocks

Google Earth photo shows the SE edge of PVM and its draining Wet Sandy Creek. Quail Lake is at the SW corner of the map, and Anderson Junction is near the Center (Orient onto the Occidental I-15)

There are flat and level beds almost alongside the Monzonite melt, indicating that they have not been distorted by sliding of large masses of monzonite Pine Valley Mountain Foothills, PVM, sedimentary Carmel outcrop, or by the intrusion of the hot mass

Pine Valley Mountain Foothills, PVM, sedimentary Claron outcrop

The PVM rose as a melt in Miocene times (21 mybp, million years before present, laccolith-shaped, and oriented NE-SW), and developed a mushroom-shaped overhang at about the kilometer depth in its late stages. This overhang is due to the mechanics of stress changes, as the melt rose. At approximately 1 km, the resisting stress of overburden is less than that of splitting the rock vertically, and the melt works laterally, lifting the over-lying rock (this same result has been noticed for rising Salt Domes in the Gulf Coast, USA). This is noticed in the photo shown for the Oak Grove Campground area, where the flat and level Tertiary rock is un-deformed by the rising melt, while downhill there are remnants of melt which have fallen past the weak sedimentary beds. This means that the dropped Monzonite mass has not slid over the sediments; rather it was above and to the SE of them in days when there was a lot more Mesozoic and Tertiary rock (before erosion). As the mountain and its surrounding sedimentary rock eroded in the 21 million years just past, the detached Monzonite sunk further in elevation, now displaying portions SE of the older and once deeper sedimentary rock.

Tertiary Claron (pink to white youngest limey siltstone) within 200 meters of Tm Intrusion is almost flat and level- indicating that it has not incurred monzonite sliding past it .
Study the accompanying Google Earth geographic map, to notice several things:
1. Wet Sandy Creek traces SE, to point toward Toquerville and its large Ash Creek spring;

Though SE would be the normal eroding direction for streams falling off the NE-SW oriented Pine Valley Mts, the wash east of Oak Grove trends toward the south

2. While most creeks and washes descending from PVM are somewhat irregular, Wet Sandy is fairly consistently tracing NW-SE, making a linear trend with the headwaters of Santa Clara River on the NW side of PVM;
3. Large isolated portions of Monzonite outcrop near Anderson Junction, which are seemingly anomalous; and
4. The NW-SE fracture trend is not obvious, once the Hurricane fault is intersected.
Although the trend is strongly indicated, we have no proof that this is the same NW-SE trend noticed in the Fissures near the Virgin River, or of that noticed at the Laverkin Quarry.

Re-tracing the path to the Monzonite (PVM) outcrop east of the Oak Grove campground:
Hikers in November 07 returned to the arroyo just east of the campground, to view the contact of Tertiary Claron, Tc, with the 21 mybp laccolith- seen as pink sedimentary beds within a few hundred meters of the Monzonite, Tm, sheer cliffs.
Although rubble obscures the actual contact, the continuity of Tc can be seen over one hundred meters NE-SW, to determine that Tc has not been shoved or rotated significantly (the dip of Tc into the igneous uplift is not over 5 degrees, and the formation on both sides of the arroyo is concordant). This is shown in the accompanying photos.
Also shown is a photo with an outcrop of Tm which indicates that the arroyo yielding the bare Tc has been caused by a large fracture (insufficient displacement to classify as faulting) tracing N-S into and through Tm. This was surprising, since most of the fractures in the surrounding sedimentary rock show NW-SE orientation. The N-S fractures, and faulting, are considered to be less than 2 million years of age (similar to the latest Hurricane fault expression), as contrasted to the NW-SE older set in this area. Hence the stresses causing Hf are present in this area, some 10 km distant from the fault.

Notice the Vertical Fracture which extends upward as far as can be seen- this has allowed increased erosion (forming an arroyo or wash in a N-S orientation)

Those taking this hike should remain in the arroyo until getting close enough to touch Tc, because of the thickness of manzanita and scrub oak- which is essentially impenetrable. This is a boulder-hopping hike, which requires climbing and jumping over the outhouse-sized boulders.
Re-stating the conclusions noted from a previous hike:
1. Tc was relatively unaffected by the younger intrusion of Tm, remaining fairly flat and level. This eliminates the conclusion that Tm fragments slid over Tc, moving SE downhill with time, in this area. There are segments of Tm further to the SE near Anderson Junction of I-15 which evidently did slide over the soft sediments, using them as a lubricant and carrying some Tc with the younger Tm.
2. There is possibility that Tc slid somewhat downhill, moving along the curved face (starting almost vertically, but gradually moving horizontally with time), which results in Tc dipping-down to the cliff face as the whole sedimentary column dropped (rotating due to the smaller cliff slope angle). This yields a small dip into the monzonite- less than 5 degrees for this location. Other locations to the east have a more marked dip, strongly indicating that Tc and older beds slid- taking their overlying Tm with them.

Notice the Large Dip Angle of Mesozoic beds tilting into the PVM Monzonite
Mesozoic and later Tertiary Rocks border Tm, and the elevations of each must be measured to determine which have fallen from original high positions after being lifted and penetrated by Tm
3. Underneath the pink Tc lake limestones and siltstones, there are thick and hard conglomerates, which were searched for imbrication. These showed no flow direction indication, and appear to be storm-generated- leaving pebbles and cobbles dumped into sand or silt.

Southeast of the Tc outcrop, there are blocks of Monzonite which have obviously slid down to their present position.

Linear Slickensides were found SE of the Tc outcrop in Mesozoic rock, indicating that some sliding occurred downhill of Tc

Oncoliths were found in the Mesozoic downhill of the Claron Tc