H30: Supplement to H28 in the Hurricane Fault Zone—
Fracture-Stress Change
Access
Access to this hike is the same as that for H28.
On State Highway 9, just east of Laverkin, Utah, and just above the Hurricane Fault (HF) scarp, an old gravel road leads toward the town of Virgin (Virgin Quadrangle, S13, T41S, R13W). This old road has gravel switchbacks just south of the paved highway. This area contains an anomaly for the HF scarp, because the road climbs the scarp gradually, allowing motor access to the plain below the Hurricane Mesa. Ordinarily, within the 40-km (64-mi) investigative area, the scarp is too steep for roadways except at canyons, which are generally dead ends.
Observations
In previous investigations we found that a gravel quarry just below this feature displays many slumps and faults, indicating that the scarp is weak rather than resistant. In addition, this study area is anomalous geologically. The entire area north of this weakened feature has several splays of the HF as well as saddles in the volcanic deposits, indicating unusual tectonic activity. Consequently, we have hiked this area to determine whether a crossing of two systems exists: (1) the most recent (Pleistocene) uplift of the N–S HF, and (2) a pre-existent NW–SE major fracture resulting from the Miocene intrusion of the Pine Valley Mountains (PVM).
With a Brunton Compass, we measured several portrayals which indicate that the rock columns have been rotated, fractured, faulted, and slumped. These measurements were taken in Permian Kaibab (Pk) Limestone (which, generally, is highly resistant and competent), and helped determine the various movements of the Pk as it reacted both within the fault splays and on both sides of the HF. Attached photos display some of the outcrops that demonstrate features listed below:
1. The northward plunge of Pk as it approaches the
paved highway (toward the quarry);
2. The dip of a graben wall created by faulting east
of the HF;
3. The track of fault movement as the graben block
slides down the fault plane, creating slickensides;
4. The orientation of various fracture systems and their accompanying orthogonal fractures on a map view; and
5. The variation in plunge of Pk as it crops out to
the west of the faulting, indicating small (100-m, 300-ft) fault blocks and
their saddles just above the wall of the graben.
We determined that the N-S valley below the switchback (pointing toward hiway 9 and becoming deeper in that northward direction) is a local graben that trends 150°–330° from north, and points toward a saddle in the basalt–Paleozoic contact near the town of Toquerville. That it is a graben is indicated by the slope of the walls (cuts in the not-quite-level sediments) on both sides of the N–S valley; both upthrown walls on either side of the valley point down toward the center of the valley (typical of normal faulting) and surround a large water tank storage facility (the tank is surrounded by soil-like oxidized rubble that has iron compounds converted to limonite-like remains). In the following discussion, we present our reasons for determining the valley to be a graben, however small in size.
Discussion
Since the Oligocene, the entire region has been subjected to extension, except for local compression caused by volcanism and intrusion (Hurricane volcanoes and PVM laccoliths). These features are distant enough to eliminate local compression near the graben. The fault planes dip toward the graben (where the water tank is located), which would be consistent with normal faulting. However, the dips of bedding and fractures above the graben are not necessarily consistent with the local graben. The dominant fractures align with the graben axis, but other parallel systems are oriented at 165°–345° from north (with omnipresent orthogonal fractures).
The west side of the graben is bounded by an upthrown block that is lower than the upthrown block on the east side. This fact indicates that the Pk dropped differentially as it approached the main HF to the west. This differential drop could result from local gravity effects, but would not explain the presence of the graben proper. Incidentally, this phenomenon is opposite the general stratigraphy near the HF elsewhere, where the strata on the east side generally dip up to the west. Furthermore, the graben tilts to the north with the canyon becoming more pronounced (similar to the Pk outcrops) in the down-dropped fault block. The average dip of the Pk is about 10° down to the north, with three segments or individual 100-m (300-ft) blocks separated by small saddles, each dipping north 5°–10°. These orientations do not conform with the sliding of the graben downward, where slickensides suggest the movement to be 25° from vertical toward the south.
Slickensides show that the down dropping graben block moved southward twice as much as the tilt of the Pk outcrops (25° vs. 10°), so that the west wall was subjected to more movement than that caused by dip down to the north. This motion indicates that the graben was being rotated southward (clockwise, looking at the wall of the upthrown block to the west), simultaneously with the tilt of the strata down to the north. This action would be caused by a right-lateral fault.[1] Because this right movement is larger in magnitude than the dip of the strata, a dominant crustal movement other than the HF is indicated. In my opinion, such movement is the shear action of small lateral displacement tracing 165°–345° from north, occurring simultaneously with the downdrop of the graben block.
This orientation does not align along the NW–SE path of the Santa Clara River tributary, Wet Sandy Creek, Toquerville Spring, and anomalies to the north. Furthermore, I propose that the HF has allowed movement along a counter-clockwise rotation of the Colorado Plateau (CP) to offset the suspected original NW–SE path toward the fissures near the Virgin River to the SE. The configuration would be shaped like a backward S (a gentle almost-straightened Z, _∕‾), where the HF has caused a turn of the fracture path toward the south until reaching the Laverkin quarry where the fracture system resumes the original NW–SE path.
Conclusions
The investigation discussed above has led to the following conclusions.
1. A fracture system, oriented NW–SE has developed in response to the NE–SW uplift of the PVM since the Miocene. This fracture system is expressed by the Pine Valley Town and campground tributary of the Santa Clara River, which is oriented similarly to the Wet Sandy Creek; the entire system proceeds toward the Toquerville Spring. Analyses of the spring water could confirm this origin by finding the ratio of Potassium/Chlorine (K/Cl), which would be anomalously large (>0.2) for intrusive rock of high orthoclase feldspar content and its derivatives of illite and dissolved K. If the Fluorine/Cl (F/Cl) were high instead, the ratio would indicate that the spring has flowed under the basalt of volcanic emissions near the HF. Should both K and F values be large in the spring water, both circumstances would be inferred.
2. Upon interference from the most recent uplift of the HF scarp, the NW–SE trend was disrupted. Since then (Pleistocene), the trend has not only been intersected, but the HF has allowed right lateral movement of the fracture system where intersection occurs.
3. As displayed to the south on a graben wall above Highway 9 on a gravel switchback road, the orientation of slickensides shows that the area has been subjected to extension (producing the graben and fractures) on a more southward path along the HF until reaching the graben. Thereafter the orientation of the fractures returns to NW–SE toward the large fissures above the Virgin River west of the town of Virgin. The slickensides show that not only did extension and downdrop occur in the Mesozoic rocks to the west, but that the west wall of the graben moved along a right-lateral fault system.
4. In the Pk outcrop, orthogonal fracturing occurs persistently at nearly right angles to the dominant fractures. The dominant fractures align with the large saddle to the north where the HF juxtaposes basalt against Pk or older Paleozoic sediments.
5. For all this conjecture to be consistent, the NW–SE fracture system would have to be dying out as opposed to the emergence of N–S faulting and fracturing- which are now increasing. Evidence of this scenario is emergence of N–S drainage in the last 2 m.y. (exemplified in the lower Colorado River, portions of the Rio Grande River near the CP, local drainage of Coal Pits Creek, and finally the trend of the Pacific plate moving toward Alaska as shown by new volcanism on the Island of Hawaii along the Mauna Kea–Mauna Loa–Loihi trend, contrasting to the older NW-SE trend). These occurrences suggest that the shear producing the NW fractures of the Western United States and the orientation of the California San Andreas system is being superseded by a system of N–S faulting and it’s accompanying orthogonal fractures.
One way to monitor this fracture-stress change in the study area is to look at the outcrops of flat strata, and measure the dominant and intersecting parallel fracture systems. The rocks of the older system will be cemented more than those of the younger system, which will be opening with the ongoing stress. I have studied such intersecting fracture systems in Arizona on flat sandstone mesas where calcite or silica cement displays one fracture system sealed and the other system open (with displacement of one of the intersecting fractures having occurred along the younger).
Features to investigate further:
a. Since anomalies have occurred at intervals along the suspected NW-SE trend, we need to find evidence of a continuity of shear or lateral displacement, rather than local occurrences widely separated from each other. Although the linear trend of the Santa Clara-Wet Sandy creeks on either side of PVM is strongly indicative, there is little evidence of displaced rocks. The small graben near hiway 9, and its depth, indicate a lateral displacement of about 50-100 meters in less than 1 m.y. (using the sine of 30 degrees against the depth of the visually-presented canyon). This would calculate a minimum lateral displacement of .05mm/year or more. This would not necessarily indicate a general rotation of the CP, but rather a local lateral shear crossing the H fault (release of strain as the graben dropped, relative to the CP).
b. There should be located a flat mesa of Pk along the anomaly, showing the crossing NW-SE and N-S fractures, which intersect and show displacement of one fracture against the other. This would then be labeled faulting, rather than fracturing- which has more uncertainty. Again, the lack of sealing cementation agents would indicate the younger (as well as the fracture which is continuous rather than being displaced along the other).
c. The intrusive dikes near hiway I-15 should be studied further, to determine whether there is evidence of gravity sliding, rather than the asserted movement along a fracture system (evidenced only at present by the direction of the grain trend in the outcrop and by the nearness to the projected NW-SE creek orientation).
d. In AZ, there has been published a map of linears (fracture systems at the surface) for the entire state. There might have been something similar to this done in Utah, which has remained obscure. Conventional geologists are averse to anything not in their training, and would mot be willing to accept work of this sort, as was the case in AZ. I got a copy of the linear map anyway, though it was suppressed. I had already been convinced of the value of this type of ignored information, from my fieldwork in Turkey, which showed parallel linears (and even lines of anomalous springs, where other measurements were unavailable). Anomalous springs show up in AZ also, one of which is a line proceeding from Soda to Winter Cabin Spring, brushing Montezuma Well. This linear continued monotonously until reaching the WC, where it made an abrupt 45 degree turn. At the lip of Montezuma, one could stand on one of the N-S fractures, while peering into the well.