Monday, March 12, 2007

Huricane Fault and Geology

The Hurricane fault is essentially a north to south earth slice on the western edge of CP, originating near the Tushar Mountains north of Cedar City and extending (altogether over 100 miles) past the Grand Canyon to the south. It is currently rising on the east side by at least two-tenths millimeter, 2/10 mm, per year, and seems to cycle at least several times in a million years (as evidenced by dating of basalt flows from nearby volcanoes). Three of these cycles can be seen ascending the scarp east of town on the way to Zion National Park (Hiway 9) as erosional terraces, but other cycles are more subtle. There have been times in the last million years, when the country drained by the present Virgin River has been almost leveled by erosion, allowing boulders from the Pine Valley Mountains to the north to roll SE-ward all the way to Virgin town (impossible now, due to the scarp in its path). The erosion about Virgin town is rapid near the Virgin River, and the town now sits in a bowl whose only drainage is to the west through an increasingly deep canyon (even deeper than the base of the scarp at the CP boundary). This canyon was formed in response to a damming of the Virgin by young lava flows from craters situated in the town of Hurricane. Lakes formed behind the dams, allowing rapid down-cutting when the highly-fractured basalts were breached by cyclical floods. The various basalt flows originating in the city limits (at least 3 from 3 separate craters), may be distinguished by their positions inside the Virgin canyon- the youngest basalts now residing at the lowest levels, having filled older canyons which were carved in previously higher elevation flows. This feature is generally true for the whole region- the oldest basalts now form high mesas, with the younger flows occurring where erosion has stripped off old rocks to allow the almost-liquid lavas to flow in canyons below the edges of the high basalt-capped mesas (which are more resistant to erosion). Stating again, the highest mesas have the oldest basalt flows at their tops, with the youngest basalts occurring at lower elevations, (erosion has removed rock to allow the younger basalts to flow in valleys closer to the present land surface elevation). This is reverse to the normal case, where older rocks occur lower in elevation and position compared to the younger.
In addition to the uplift of the CP at the obvious scarp, there is an additional movement at the scarp, which is due to vulcanism. Volcanoes generally occur near the scarp, since this large slice in the earth allows rock which is normally hot at the depth where it occurs to exit in the weakness at the scarp boundary. Rock becomes molten at about 2000 degrees C, and this normally occurs at depths of about 100 km, when the gradient in the earth’s crust is 2 degrees C per 100 meters (20 degrees/km). In the Hurricane transition zone, the thickness of the earth’s crust varies from about 40 km on the CP to 25 km at the transition zone edge, so that the basaltic lavas have originated from below the crust (in the mantle, where rocks are more fluid- plastic).
When an eruption occurs, heat and rock mass are lost to the surface of the earth, with the lava pushing upward to form a lip of the CP beds up to the west (layers of sediments can be readily seen as one climbs up the scarp of the CP, dipping upwardly to the west). After the hot mass has been expelled (extrusion) via craters, cooling occurs particularly at the scarp where the open fractures are located. The resulting cooling at the scarp face allows the rock on its west side to sink into the resulting shrinkage zone. This creates a monocline and a hogback to the west of the scarp, and is particularly noticeable near the young volcanic craters. The ones studied were the Laverkin hogback just west of Ash Creek, the Honeymoon Canyon near the AZ border, and Gould’s wash just east of Sullivan’s knoll. This sinking and formation of a monocline (single slope, with a hogback on the west edge) causes an additional settling along the Hurricane fault, on the order of a half centimeter yearly- much in excess of that movement along the fault proper.