Hurricane Fault (within 40 km of Virgin R.) 2006 Model

A layman's view of the Zion Park to St. George, Utah area:

Southern Utah's beautiful scenery is mainly the result of erosion of the colorful Mesozoic rocks of the Colorado Plateau. The Plateau has been elevated to an average 2 kilometer elevation, by two ancient events, which made the high plateau subject to rapid erosion:

1. Part of the Pacific basin has been shoved underneath the Western USA all the way to the Rocky Mountains, by crustal movements which are known to have been active for several billion years. When these rocks were thrust under the continent, they began to heat by decay of radioactive elements and by crystalline re-arrangements- consequently, the Plateau expanded upwardly, and
2. The Basin and Range (B&R) Province, or the mountainous region from western Utah to eastern California, has dropped relative to the Colorado Plateau, because of shrinkage of the earth's crust westward of the Hurricane cliffs, as the earth slows over a long period of time; a decrease in velocity would cause a decrease in the ellipsoidal configuration at the equatorial bulge. This results in a decrease of the average elevation of the Great Basin, and a relative drop from CP to B&R.

Consequently, the flat and almost level sandstones of the Plateau have remained undistorted, relative to the highly faulted and distorted B&R, but are subject to larger wind velocities, precipitation, and runoff (causing erosion) at the higher elevations.

In the Hurricane, Utah area, there have been young volcanoes added to the scenery; three of these are in the city limits. They are dormant, and are not expected to erupt again, but new ones will appear at almost random times and add new cones to the region. Except for weather, these are the only cataclysms expected. This province between Hurricane and St. George is called a transition zone, and it allows a gradual change from the high plateau to the east to the lowlands of the B&R. Under this zone is a gradually thinning crust (westwardly), which exhibits many abrupt changes in the exposed rocks and landforms.

There are many gradual, subtle changes in the landforms- such as cliffs forming along the rivers, deposition of gravel beds in the creeks, and sand dunes forming as sand is loosened from the many sandstones exposed. These events create the colorful and varied landforms which all of us enjoy.

Technical Modeling:

Another year's hiking and analyzing the cliffs and outcrops within 25 miles of the Pah Tempe Hot Springs on the Virgin River has yielded sufficient information to form a model for the dynamics of the present Hurricane fault and scarp, from Pliocene to present time:

1. The scarp is a regional phenomenon, not connected with the larger N-S Wasatch fault, and much younger than it in incipience. It has formed since mid-Pliocene time, due to the shrinkage of the hemisphere by slowing of the globe, producing a large N-S fracture as centrifugal force decreases on the non-spherical or ellipsoidal portion of the earth. This causes cracks to develop along the western edge of the portion of the crust which is thicker- compared to the further west portion under the Basin and Range, B&R). That this is a regional phenomenon is partially indicated by the monotonous 200 meter scarp, which extends southwardly to the Grand Canyon, as well as northwardly from Hurricane town;
2. The H fault is complicated by a geothermal association, such that after the N-S weakness was exploited, magma intruded this feature and produced vulcanism. In the zone of interference caused by the older Pine Valley laccolith, the intersection of two movements has produced hot springs. The magma expressions producing the volcanoes and hot springs is somewhat statistical- exploiting fractures on both regional and local scales- but is most disruptive near the present Virgin crossing, producing concentrated vulcanism (3 cones in Hurricane town) and hot waters with exotic components. The magma is narrow and elongated, not rounded as with a batholith or laccolith, and pushes up the rocks bordering the east side of the fault (causing the edges to become up-dip to the west);
3. The Hurricane scarp is crossed by weaknesses produced by the Pine Valley laccolith, where NE-SW interference has caused disruptions to occur at the Virgin River and further north. Consequently, Pk near Toquerville is shoved up more to the NW on the east side of the fault than was exhibited by the Laramide compressive forces. It appears that the Virgin anticline has been further compressed by the PV intrusion, as it mushroomed to the SE, similar to salt domes which take right turns as they encounter the 1 km depth, where it is easier to lift the overburden than to split the overlying rock vertically. This compressive and resulting shear stress has rotated Pk (and the Virgin anticline) in the vicinity of the Virgin River east of Pah Tempe, allowing fissures to develop parallel to the two dominant stresses- NE-SW and NW-SE. The hot magmatic acidic waters have exploited these weaknesses to allow subsurface flow channels to develop and assist the Virgin in eroding the present canyon crossing the Hurricane scarp;
4. Although the dominant expansion of the CP is caused by excessive heat (which cannot escape due to the thick blanket of sediments), there is also lesser expansion upwardly of sediments in the transition zone, as load is removed from the zone by erosion. This can be seen in the Virgin River bed, with elevation measurements on conglomerates laid down in Pleistocene time. Simultaneously, the down drop of the B&R produces an apparent uplift (a relative contrast) of the CP.
5. The Hurricane fault is young- near 1 m.y. age- which is shown by occurrence of rounded Pine Valley granite-like boulders east of the present fault, beneath 1 m.y. basalts in North Creek near the town of Virgin. This could only occur if there were no scarp in their SE-wardly path at the time they moved downhill. That the fault is young, as is the Zion highlands, is shown by Laverkin River drainage proceeding SW-wardly or southerly before the oldest basalt flows occurred. The dominant highlands were the PV Mountains, before the transition zone began to sink in Pliocene time, which is shown by conglomerates near the present H fault with imbrication going SE-wardly or southerly. Before the Pliocene age, Zion was elevated, but so must have been the transition zone. Creeks were flowing mainly from the PV highlands, and lesser so from Zion highlands compared to now.
6. It is expected that drainage pre-basalt age proceeded southward or SW-ward towards the large Fort Pierce creek valley; this evidence is covered or eroded away, and must be checked by finding conglomerates further south.
7. The Hurricane fault will continue to cause relative elevation differences across it, but in a gradually decreasing manner. The vulcanism seems to be in a downward mode, as must be the associated elevation change rate. This conclusion can be checked, relatively, with the east side of the CP, where there are active thermal events- such as hotspots under the Jemez Mountains, and along the Rio Grande rift. There the earth has a N-S fracture manifestation also, probably due to the same earth-contracting feature, and geothermal manifestations are more pronounced (the dynamics as seen by geothermal activity is more extreme).

All of the conclusions for the Hurricane fault modeling must be checked for verification with the outcrops in the 80 kilometer circle of investigation.