Layman's Introduction

The Colorado Plateau is a 1000 kilometer diameter uplift, similar to a layer cake, when viewed from the side. It is a flat-and-level cake which has a larger than normal thickness of crust underneath it, although when viewed in cross-section it is much eroded at the surface. It is very colorful, since when driving along its road cuts and river channels, it displays the erosion of many ages and colors of sedimentary formations, or beds.

This is not to say that there are no distortions in the layer cake; infrequently there are intrusions of igneous rocks, various crumpling of the beds by compression (sideways), and large slices or faults noticed- which cause various colors and type of rocks to be found facing each other laterally. Its' elevation is at least 6000 (~2 km) feet, on average, and it displays sedimentary beds mostly in the Mesozoic (65- 225 million years age), although significant Paleozoic (previous) as well as Cenozoic (subsequent) rocks are found.

The CP is very noticeable, since it is generally higher than its neighboring rocks, by several thousand feet, except for its NE portion. There is a transition on the west and SW sides, where the surface tapers off to the west gradually- called the Transition Zone- and it is in this zone where its edge can be investigated without concern that one is in the further west zone called the Basin and Range (B&R). It is bounded on the SW and south by the Mogollon Rim (through most of AZ), on the west by the B&R (Utah), on the east by the Rio Grande rift, and on the north by the Uinta Mountains. The NE side is indeterminate, since this is the termination of the underlying plate subduction, which created it.

The CP was initially created after the Mesozoic, by the thrusting of the Pacific Ocean floor and its underlying rocks under the Plateau during the Nevadan, Sevier, and finally Laramide orogenies, via a plastic layer- called the asthenosphere- which allows sliding of brittle rocks over the upper part of the mantle. These, although named separately, were gradual movements eastwardly (NE-wardly mostly) which shoved up old mountain roots in its path- creating the named Nevadan and Sevier orogenies. The dominant movement, which shoved up the CP a km or so, was the last of the thrustings (subductions), terminating against the Rocky Mountains. This subduction is now dead, since the Pacific movement has now turned to a northward direction, but the results of the previous action are apparent in the CP in many places. We will study some of them, where N-S orientation of linear features, called hogbacks, anticlines, swells, and great faults remain in the exposed ancient beds (these are generally oriented perpendicularly to the direction of thrust).

The movement of the CP which is seen best is the later stretching and fracturing of the whole West USA (particularly west of the Hurricane fault), where compression of the rocks was replaced by an extension, when the Pacific movement turned northward (about 41 mybp- million years before present), and this turn can be readily seen in the linear presentation of the Hawaiian Islands- Emperor seamount chain, as a 60 degree turn to the SE (present islands), from the previous N-S orientation (undersea mountains). Since that time, there has been no significant compression of the CP rocks, except for local volcanoes or other igneous uplifts- rather there has been a NW-SE oriented fracturing and stretching of the formations seen at the surface. This fracturing of the rocks on a macro (small) or kilo (large) scale can be seen as one walks over the exposed surface. It can be measured readily with a compass, and tells whether there is the normal regional SE-NW fracturing or whether there is a local anomaly yielding a departure from the normal (such as would occur with a volcano, slump, geothermal event, or other distortion).

Besides fracturing and faulting (shearing of the rocks), there are other events occurring in the CP and its transition zone. There is a general uplift by thermal expansion (such as would happen by heating an iron bar), there is rotation of the Plateau by shearing forces, there is differential uplift causing local anomalies such as the Hurricane fault, tilting caused by differential stresses across a given large section of rock, and uplift caused by unloading (large-scale erosion). Unloading, or rebound, is similar to that occurring in Scandinavia, where glacial ice of many thousands of feet thickness added weight to the ground surface (thereby depressing it), and then when the ice melted the surface rebounded upward.

The transition zone, where LaVerkin and Hurricane are located, is a zone where there are many expressions of faulting, geothermal release, and uplift, so that the mechanism of the uplift may be studied- all of these are clues to the action generated deep in the crust of the earth and the underlying mantle. There are speculations that the uplift is simply one due to radioactive decay of elements thrust under the CP during the Laramide, and that now the decay is sufficient to cause a heat buildup. This would express itself as vulcanism, if the shearing created faults and fractures along which the heat could readily escape. If the CP proper contained this heat with its insulating blanket of thick shaly rocks, the Plateau would simply expand upwardly.

The first guess in the days before understanding earth subductive tectonics was that the mantle contained circulating thermal currents (rotation of fluid or plastic mass, as in a teakettle), causing the CP to be moved as a unit by advection- separating it from the extending part of the B&R by this same laterally-moving advection.

Another speculation for the heating of the CP is that the subduction during the Laramide left low-density rocks under-compacted according to their depth. Gradually the rocks would compact with the overlying weight of the depth at which they finally found themselves, and in the case of Basalt being compacted to a new crystalline form with higher density, called Eclogite, heat of crystallization would be released.

A combination of the above factors probably occurred, but whatever mechanism is found to be dominant, it elevated the CP uniformly internally, but suddenly quite unevenly at the beginning of the transition zone- the Hurricane cliffs. This is a sharp transition- called a scarp- and occurs for over a hundred miles with about the same elevation difference looking E-W along the N-S expression (the Hurricane fault).

We can easily measure or observe the shear, fracture and uplift with an altimeter, tape, clinometer, and compass. The interpretation of these data is not so simple, but we can model the results and note how other geological circumstances agree (or disagree) with the physical model.

Several features are apparent along the Hurricane fault:

The latest cliffs are fairly uniform in elevation change along the scarp;
Volcanoes stretch along the N-S path, being most noticeable in the Hurricane-LaVerkin area;
Many distortions occur on the west of the fault, most noticeably in the Virgin River crossing (but the beds are mostly flat and regular to the east);
The sedimentary beds tilt upwardly to the west east of the fault, and continue this up-to-the-west feature in some younger beds on the west side;
Fractures and river orientations change when crossing the fault;
Laramide compressions are numerous on the west side, but rare on the east side of the fault;
Basalt flows occur on both sides of the fault, and sometimes cover earlier gravel flows in stream channels (conglomerates)- these may be used for dating, by radioactive decay of K40 and other elements; and
There is a progression of vulcanism toward the west with time.

My intention is to hike along many of these features, and make measurements of fractures orientations, faulting strike (orientation on the surface) and dip, vulcanism ages and location relative to other features, springs and seeps location and temperatures, orientation of fissures and surface anomalies, and distortions noticed. From all of these observations, one can integrate them in a model of the dynamics of the uplift (at least on this side). The Hurricane fault is a linear feature, which should give clues as to the underlying mechanism, when looked at near the scarp.