Monday, March 12, 2007

Hiking-Geology Report, cont'd

Field observations about active fissures:
A. The depicted fracture and fault locations were studied both at the Virgin River and near the up-dipping and wrenching of the Kaibab, both near the Town of Virgin and west of Three Falls near Toquerville. The unusual fracturing and breakage of the Kaibab formation indicates that there is a local anomaly in the crust due to on-going strain. The active fractures allowed the Virgin River to create a channel through the rim of the Hurricane scarp, even though the scarp is higher structurally and topographically relative to the drainage of the Virgin to the east.
B. Other evidence for an anomaly in the crust is the presence of the Pah Tempe hot springs, several clustered dormant volcanoes, spreading fractures, fissures, and faulting, as well as the accentuated up-dipping of the stratigraphy in the region.
C. The fissures and fractures are linear, indicating that they result from locally accented stresses, not caverns made by the solution of limestone in the Redwall or a similar layer some 300 meters beneath the surface.
D. The Virgin captured local drainage, superseding the Ash and Laverkin Creeks. This indicates that a major change in local stresses in Pleistocene time offset the previous NW-SE strains, to allow N-S fractures to become dominant.
E. The Virgin River outlet through the Hurricane scarp represents a regional boundary to the Hurricane linear scarp, where the Colorado plateau is uplifted more toward the south than to the north. This could happen because there is more unloading to the south (erosion), but also because the direction of stress has been redirected near the River.
The overall conclusions about this supposed fracture and lateral fault feature are as follows;
A. The only evidence for lateral movement is at the graben face- which indicates small-scale right lateral faulting;
B. The trace of the NW-SE fracture system is mainly along the PVM Creeks and springs, and in the large fissures above the Virgin River;
C. There is definitely an anomalous weak zone from Hiway 9 to the quarry north of Laverkin;
D. Fissures aligned NW-SE and their orthogonals are opening presently above the Virgin River (near the town of Virgin); and
E. Otherwise most of the faulting and fracturing is on an N-S alignment, as is the thrusting above and east of the town of Toquerville.
F. All conclusions about the stresses, faulting, and anticlines- monoclines are aggravated by the following sequence of events:
I. The Laramide was a compressional event (about 100 million years ago), caused by a NE-ward subduction of Pacific plate under the whole CP- all the way to the Rocky Mountains (this compression is exemplified by trends of N-S features, such as hogbacks, at the AZ border and NE-SW trends near the PVM and Virgin anticline;
II. At 41 ybp (years before present), as shown by the Hawaiian-Emperor Seamount chain, a change in direction of the Pacific basin movement from N-S to NW-SE occurred. This was probably caused by the crossing of a large transform fault by the Hawaiian volcanic location and outpouring, across which the parallel segments on either side of the transform moved at differing velocities. This change of direction of Pacific plate movement would affect the Basin and Range (B&R, caused by extension) all the way to the CP edge, gradually dying out as it approached Utah;
III. Near 21 million ybp, during extensional time (the West was being pulled apart- opposite to compression, which creates shortening of the earth’s Crust), the PVM rose as an igneous intrusion, creating local compression in an otherwise extensional time. This is similar to a salt dome, which rises in extensional cracks, but which locally creates folding or doming of sedimentary beds, which are due to compression;
IV.In Miocene time, erosion uncovered the PVM, allowing border rock to slide (detach) from the main granite-like mass, carrying overlying sedimentary beds downward to the SE. This is complicated by the non-appearance of this action on the SW side of PVM. Why would beds slide downward on only one corner of the protruding mass? There are isolated masses of intrusives near the Wet Sandy Creek and on the east side of PVM, which may be dikes, and this would increase the likelihood that they rose along the supposed weakness shown by the Wet Sandy Creek linear.
V.In Pliocene time, 5 mybp continuing until now, the Hurricane N-S faulting rejuvenated, and this is conjectured to have occurred due to a slowing of the earth’s spin (length of day), allowing the equatorial bulge to decrease (a lessening of the centrifugal force creating the bulge would cause the earth to shrink in the equator to mid-latitude zone, including Hurricane area). This bulge is only on the order of a Earth diameter increase of 20 miles, but could create fractures in a N-S orientation, as it shrunk;
VI.In Pleistocene time (2 mybp and later), the N-S fracturing in an extensional regime allowed the Hurricane fault to wax and wane, depending on the Milankovich cycle (due to the wobble, precession and oval pattern of the earth’s movement around the sun). This cycle is at most 84,000 years, and would create loosening and tightening of the rocks near the H fault, causing cycles of movement- these are shown by terraces above the present H scarp;
VII.Vulcanism within the Hurricane City limits has locally created compression, in an otherwise extensional regime, similar to the Salt Domes described above. This has caused local thrusting and folding, which must be eliminated in a region-wide study. Extension and erosion, causing unloading of sediments, is causing the local area to rise as it loses weight to the Virgin River- which carries away the mass.