H23: Toquerville west 03/01/06

For those interested in hiking further along the N-S linear displayed west of Laverkin and at the same structure west of Ash Creek (a hike west of Toquerville), take hiway 9 in Hurricane and hiway 17 north at the Laverkin intersection, proceeding to Westfield paved road in Toquerville. Follow this road west until Sunset Drive, where you will turn west again until the road reaches open fields, near the power line easement. Park there and start hiking north along the Power line gravel road. We will follow the N-S linear (hogback and eroded almost vertical beds) wherever it leads. We have found that the monocline, which at first appeared to be part of a Laramide structure, is a young fault block, similar to one found with the same orientation west of the Hurricane fault, at the AZ border (near Honeymoon canyon). The orientation, N-S, is similar, as is the up dip to the west shown near Honeymoon canyon, in Mesozoic rocks. I see now that this feature occurs, whenever there is a major canyon which depicts a large block of faulted rock rotating into the hurricane fault zone on the west side of the main fault (CW, looking northward at a cross-section). Evidently, the transition zone is pulling away from the CP, allowing the strata on the west side to rotate into the gap caused by removal of magma deep below. The magma pushes up on the east side of the fault, causing the layer cake sedimentary rocks there to dip up to the west also.

This is an easy hike, mostly along level ground. At the west end of our trek, we will descend into a canyon carved into basalt rock, to find evidence of the hogback and faulting in the bottom of the arroyo. Although colorful geological displays are seen at the high starting hilltop, the descent will allow only views of the canyon walls. We will possibly encounter Mesozoic rocks which are the remains of the hogback in the lower canyon walls. Wear rugged boots, as basalt boulders will be the dominant rocks in the arroyo. We hope to see fissures wherever the N-S fractures are active.

Assessment of the 2 km wide sedimentary column west of the Hurricane fault:

1. The Hurricane fault has been investigated from the AZ border to the village of Toquerville, and found to have highly disturbed sediments on the west side of the fault (as shown by Mesozoic and younger rocks), whenever there are volcanic vents and outlets nearby. This creates a change of fracture pattern, and allows major drainage channels to develop. This happens at the Virgin, Honeymoon Canyon, and at Gould’s wash- which evidently is the youngest of the three. There are some fossil canyons leading west across the main Hurricane scarp which evidence a similar geological disturbance, but the canyons do not flow water now, as they have been marginalized by subsequent local uplifts. Consequently they are not eroded cleanly, so that the dynamics are not as obvious. This includes the zone at the Laverkin quarry, and at the cliffs created by a secondary splay of the H fault, near Toquerville;

2. A zone of highly re-arranged fault blocks occur on the west side of the H fault (whenever there is a river canyon to view them), which is about 2 km in extent E to W, and these blocks dip down to the east into the disturbed zone. It appears that the zone west of the H fault is being extended, allowing the blocks to rotate into the zone, filing the void created by evacuated magma (erupted in nearby volcanoes). The east sides of the blocks are not dragged up against the west side of the fault, as would be the case for normal faults (to produce strata with dip up to the east), rather they are rotated into the vacated space. This can only happen whenever the zone is being extended, not compressed onto the fault. Consequently with extension, there are fissures developing on either side of the main fault, causing N-S openings to continue to occur (being extensional, on both sides). These are noticeable east of the Laverkin hogback, north of the Virgin River, and above the Virgin near the diversion dam. I have seen them in Gould’s canyon also, but you can’t sort these there from simple gravity spalling of cliffs. I believe that they occur north of Toquerville, as seen from a distance. They appear to cut through the old Virgin anticline north of Toquerville. You might ask the question “How can a block of crust fall into a part of the transition zone near the Hurricane fault, when the magma pushing up into the volcanoes must be a compressive event?” This is a valid response, and the answer can be: The block doesn’t rotate when there is compression of up-coming magma, but only rotates into space created by later cooling of the magma. During a cooling period, the remaining magma underground shrinks, and pulls in the overlying crust. Some space has to be filled as the rock shrinks with cooling.

3. The key to the occurrence of disturbed sedimentary rocks on the west side of the H fault seems to lie with the rise of magma through volcanic outpourings. These vents disturb the regional stress-fracture pattern, and create a local fracture pattern which allows drainage of streams to more easily follow the fractures. The original vents must have followed some earth weakness which was pre-existent, so that there is some factor which is dominant beside the location of fractures. Factors which would cause the deep crustal fracture pattern to change locally would include:

a. sharp changes of the crustal thickness, E-W, which could only be measured by Geophysical means;

b. anomalous heat sources, such as unusual concentrations of radioactive materials in the deep crust (this might be indicated by hot springs or radioactivity);

c. localization of unusually dense or hard rocks and minerals in the crust, indicated by gravity surveys;

d. up-warping of the asthenosphere, causing sharp changes of the crustal base- noticed with Seismics or earthquake patterns; and

e. transitions from plasticity to rigidity, or the reverse, caused by mineralization changes (this might be noticed in volcanic rock compositions).

4. The N-S shear zone seems to diverge to the east, near the Toquerville Pk cliffs, since the lineations reach the Virgin anticline there. Evidently there is an anomaly created by the uplift of the PV laccolith, and we will not try to evaluate further north. There is a major Pre-Cambrian weakness displayed beneath the PV mountains, and this ancient event has allowed the PV to rise discordantly from general B&R trends (usually N-S);

5. We will have to investigate the anomalous faulting and discordant blocks existing at the Laverkin quarry and east of Toquerville (south of the dominating Pk cliffs), to resolve this remaining mystery.

Observations made on 1 March 06, S10 T41S R13E:

A. The lineament which ran N-S west of Laverkin town and creek appears only as ravines west of the town of Toquerville. Although the compass indicates that it trends NE-SW, the compass is erroneous due to significant iron in the underlying basalt. The topographic map indicates that there is a slight re-orientation of the trend to the NNE-SSW, but this is offset just to the north of Toquerville, and the overall average trend is N-S. It remains a hogback and monocline wherever it can be seen south of Toquerville, but plunges down to the north just south of Toquerville town and disappears below a thick basalt outcrop there. There are N-S ravines and gullies on either side of this basalt, but no sedimentary rocks can be found. There is a flume for irrigation running on the east and south sides of the basalt, but the ditch contains no pre-basalt rocks- not even conglomerates. There is considerable level white scale, such as would emerge from hot water flows, but no sedimentary rocks. Evidently, the volcanic flow obscures the evidence of previous outcrops, as it is quite thick.

B. The Hurricane fault makes at least two splays just east of this basalt flow and the town of Toquerville, and it seems that the change of orientation to NE-SW of the lineation is related to this fault complication. The sedimentary beds observed east of this basalt flow are distorted in several directions, and the changes must be due to interference caused by the combination of the PV uplift and the intersection of the Hurricane fault with the Virgin anticline. We will investigate this further.

C. On a map view, the N-S lineament continues north of the vague Virgin anticline, as indicated by saddles and contours. There is a mine in S28 T40S on a hill oriented N-S 2 miles west of the H fault, and this is the last N-S oriented structure south of the PV mountains. Beatty Hill in S9 is another, but it is some 5 km west of the Hurricane fault. We will investigate these structures, which are north of the I-15 Freeway.