Monday, March 12, 2007
Colorado Plateau and the Hurricane Fault
Hurricane Fault Geology and Hiking (2007)
The little town of Hurricane, Utah is picturesquely situated to allow easy hiking and exploring of the western border of the Colorado Plateau and its adjacent transition zone (west of CP) to the Basin and Range country. The fault is in the city limits of its namesake and borders the next town to the north- Laverkin. The Virgin River forms the boundary between these two towns, and makes a deep canyon through the young volcanic deposits extruded in the last few hundred thousand years (even though there is no significant canyon west of Hurricane where hiway 9 crosses the Virgin or at the town of Virgin to the east). There are at least thirty hikes which can be made within a ten mile radius, which yield vital clues about the incipience and dynamics of the Hurricane fault.
The Colorado Plateau, CP, is an oval geographic province (originally defined as the drainage area for the Colorado River), whose boundary is well-defined in Hurricane- as a steep scarp rising at least 200 meters for its’ last cycle. The diameter of CP may be remembered as something less than 1000 kilometers, and it may be visually noted as a province which is higher in elevation than its neighbors and subject to considerably more erosion. It stands out to the visitor as an erosional province, colorful due mainly to its almost level red and yellow sandstones and contrasting igneous rocks- particularly near Hurricane.
Hikes, East CP, in Moreno Valley, NM
The geology was fairly obvious about my cabin in the Mesozoic- there was steeply inclined Jurassic Entrada sandstone forming a lake on my north neighbor’s ranch, dipping down to the east. Then, to the east there was an outcrop of Cretaceous Dakota sandstone forming a backdrop behind a smallholder’s house (forming a picturesque view of three evergreens, frequently photographed). Then, on the east side of the valley, there were older- but higher in elevation- Triassic and Paleozoic sandstones. We lived at the leading edge of a Laramide thrust, where Paleozoic outcropped above Mesozoic. Some Precambrian isolated outcrops remained (generally surrounded by Mesozoic to Paleozoic sedimentary rocks) - shists to granite like rock. To the south (Angle Fire) there were mostly Pennsylvanian and older sandstones terminating against Tertiary volcanics. This area was the termination of the great Laramide thrusts, where lighter-weighing sediments had over-ridden younger rocks of the Mesozoic. The thrust had a strike of NW-SE, as compared to N-S in northeast AZ and Utah. Just to the east of this thrust, the Rockies terminated, and the N-S oriented Oligocene melts (Cimarron Mts.) resulted from the later extensional dynamics of the mid-tertiary. Gold ore was found in the Tertiary, whereas other metals were mined in the PC to the west.
The highest peak in New Mexico lay 5 miles to the NW of our cabin- this 13,000+ foot mountain was situated above the villages of Taos and Questa in the Colorado Plateau transition zone, containing the Rio Grande rift zone- a young extensional feature. For those interested in studying the front of the Laramide thrusting and subsequent tectonics, this area had all the clues: NE-thrusts of Paleozoic over Mesozoic, PC intrusions of granite and shists, down dipping to the east Mesozoic sediments, mid-Tertiary intrusions of gold-bearing ore from extensional dynamics, and finally extrusions of basalt from Miocene volcanics. The advance of the Laramide front could be seen to:
a. Exhibit compressional tectonics in the post-Dakota to pre-Poison Canyon (Eocene) sedimentary rocks;
b. Cover the Mesozoic sandstones with the older Sangre do Cristo and Magdalena formations;
c. Create down dip to the east of pre-Dakota beds;
d. Be followed by, first, intrusion of the Cimarron uplift (Oligocene Dacite?) at the eastern edge, and second, by Miocene basalt extrusions about Angel Fire and finally, extrusions about the Rio Grande River (Pliocene) last;
e. The Poison Canyon, Epc, was deposited down to the east, as a result of the post-Laramide uplifts- creating a bloody red sandstone to the east of the Rockies (Sangre de Cristo Mt.);
f. There was some violent extrusion associated with the first volcanics, exemplified by the Crystal Tuff (at Angel Fire ski area), but in the rift there were mostly basaltic strato-volcano classic cones;
g. The Cimarron N-S chain was the oldest uplift, but the Rio Grande valley is still rising- as shown by the down cutting of the river through the previously high-elevation uplifted area;
h. The area of the rift all the way to Santa Fe is still geothermally hot, as shown by the springs at Ojo Caliente and the geothermal area of Jemez and Fenton Lake, where the Hot Dry Rocks program was initiated. The progression of the heat and rifting is evidently east to west (with time), opposite of that expected from a movement of the NA plate westward over the western edge of a spreading zone (subducted during the Laramide);
i. Vulcanism is still present near the Rio Grande rift, e.g. Capulin (Chokecherry) Crater- east of Raton, and Carrizozo flow near Alamogordo, both only of the order of 1000 years age.
Conclusions relative to Extensional tectonics, from Moreno Valley:
1. Tertiary to Recent stress manifestations indicate that N-S is the orientation of rivers (Rio Grande, Cienaguilla north to Eagle Nest, and Black Lake drainage south), uplifts (Cimarron’s, Sangre de Cristo, and Sandias), and dikes (Mt. Baldy);
2. Although the Laramide produced NE movement (strikes of exposures are NW-SE) until Oligocene times, where the later geographical orientations were N-S;
3. Early extrusions were more violent in the region, but later ones were more basaltic flows (CO-NM border all the way to Llano Quemado);
4. Geothermal gradients are still abnormally high, and ground water chemistry still produces abnormal compounds.
These conclusions are the same for the areas studied in AZ and Utah, that is, extensional dynamics followed the Laramide compressional tectonics; recent stretching of the western USA has produced igneous activity which progressed from east to west, and which became mainly basaltic with time; the Colorado Plateau seems to have been affected mainly by being elevated by unusual heat- it may have been intruded and suffered extrusions, but the stratigraphy remained relatively flat and level.
Whatever happened after the Laramide and subsequent uplift of the Colorado Plateau, happened on both sides of the plateau- extension of the sedimentary rocks to allow intrusions of granite-like rock to form in N-S linears (Cimarron and Henry Mts) and then later to allow both basalt flows and rivers to orient in N-S patterns (Rio Grande, Colorado Rivers and small creeks such as Coal pits and Huber washes in Zion Park). The trend is westward with time, for the progression of the intrusions and later extrusions, hence the extensional tectonics cannot be attributed to subduction of a spreading zone underneath the westward-moving north American plate- which would have caused an opposite-moving surface effect.
When a plot of basaltic progression is made with time passage, it is seen that extrusions proceeded from the Rio Grande rift clockwise since Oligocene times around the southern and southwestern borders of CP, remaining near St. George, Utah at present. Some young basaltic cones remain fairly uneroded near the town of St. George and Hurricane, and hot springs occur.
The dominant question for the Colorado Plateau remains: what is shoving the CP upward, creating unusual heat and allowing the Plateau to remain relatively flat (stratigraphicly viewed), undistorted, and high in elevation, while all around the edges there is geological turmoil and extension?
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.
Slickensides and Grabens
Although the Hurricane scarp is well-developed near the town of Hurricane and further south, exhibiting sheer walls which cannot be climbed by automobiles, this is not the case east of the town of Laverkin. In this area, Hiway 9 easily climbs the faulted area, on the way to Zion Park, and allows hikers to examine the differing geology which exhibits this gradual ascent. And although the rim generally has sedimentary beds which dip up to the west, here the opposite occurs. These beds are ascended via switchbacks, which occur near or cross a graben and monoclines. A Graben, which is the German word for grave or ditch, is a normal-faulted drop of a block of rock which occurs whenever the earth is pulled apart (extension), allowing the graben block to fall into the earth along steeply-dipping slices in the earth. In this case the dip is about 70-80 degrees from the horizontal, making a mirror image on either side of the dropping block. For this case, the angle of drop perpendicular to the block can be measured by noting the striations made by the falling block (parallel scrape marks), as it drops into a pre-existing fracture in the earth. Further, the dropping is sufficiently slow, so that water enters the sliding plane- leaving a trail of hard shining surfaces of silicate coating in an otherwise carbonate rock (CaCO3 limestone). The silicate is brought in by warm water and precipitated in the cooler rock.
See these features by studying the photos shown further down in the manuscript, or by clicking on the links shown on the right side of this Blog (as titles or as Items in the Archives).
The graben can be seen to originate just uphill of hiway 9 on the south side, and it becomes deeper in vertical extent as it proceeds to the north (it tilts downward, underneath the hiway). This shows that the graben is a local feature, and not one connected with the fault mechanism in a more general sense (that is, it is created locally; it is only noted to occur within a kilometer distance). Further to the north, the graben is truncated by the appearance of a monocline. At this point, the graben which occurred in Pk at the surface, Permian Kaibab limestone, gives way to a monocline which outcrops entirely in the Mesozoic. Obviously, there is a crossing of the otherwise N-S Hurricane fault by an additional stress feature to create the abutment. It is conjectured that there was a previous NW-SE fracture system, which has been crossed by the N-S Hurricane fault. The NW-SE feature is believed to have originated in Miocene times (Pine Valley Mountains- which is incised by the fracture- originated at 21 mybp, as an intrusion dated by radioactivity); the Hurricane fault was rejuvenated in Pliocene to present time. This NW-SE feature shows up well in the nearby PVM, as a fracture system which has allowed both the headwaters of the Santa Clara and the Wet Sandy Creeks to align along a common NW-SE axis (even though the two creeks are on opposite sides of the mountain chain). The anomaly also expresses itself in a large spring at Toquerville, in a line of saddles heading towards Laverkin, and with a series of en echelon faults near the Laverkin rock quarry.
Insert a photo of saddles and slumping earth at the quarry:
See these features by studying the photos shown further down in the manuscript, or by clicking on the links shown on the right side of this Blog (as titles or as Items in the Archives).
The graben can be seen to originate just uphill of hiway 9 on the south side, and it becomes deeper in vertical extent as it proceeds to the north (it tilts downward, underneath the hiway). This shows that the graben is a local feature, and not one connected with the fault mechanism in a more general sense (that is, it is created locally; it is only noted to occur within a kilometer distance). Further to the north, the graben is truncated by the appearance of a monocline. At this point, the graben which occurred in Pk at the surface, Permian Kaibab limestone, gives way to a monocline which outcrops entirely in the Mesozoic. Obviously, there is a crossing of the otherwise N-S Hurricane fault by an additional stress feature to create the abutment. It is conjectured that there was a previous NW-SE fracture system, which has been crossed by the N-S Hurricane fault. The NW-SE feature is believed to have originated in Miocene times (Pine Valley Mountains- which is incised by the fracture- originated at 21 mybp, as an intrusion dated by radioactivity); the Hurricane fault was rejuvenated in Pliocene to present time. This NW-SE feature shows up well in the nearby PVM, as a fracture system which has allowed both the headwaters of the Santa Clara and the Wet Sandy Creeks to align along a common NW-SE axis (even though the two creeks are on opposite sides of the mountain chain). The anomaly also expresses itself in a large spring at Toquerville, in a line of saddles heading towards Laverkin, and with a series of en echelon faults near the Laverkin rock quarry.
Insert a photo of saddles and slumping earth at the quarry:
Geology Report with Fissures, cont'd
9. Fissures in Kaibab Limestone (Adjacent to Virgin town and Virgin River), 1/11/06
The Kaibab limestone at S29 & 30 T41S R12W has fissures some 200 meters from the Virgin Canyon, which at this location is over 100 meters deep. The fissures run parallel to the canyon in places, but some run at varying angles. At first glance they might look like sinkholes. The interesting features are:
a. The fractures trace N-S parallel to the Hurricane fault, but do not when there are perpendicular turns of the river.
b. The large fissures have N-S fractures in the blocks bordering them at slopes of up to 20 degrees from vertical, and they also have NW-SE fractures as well- some of these are 5 meters deep.
c. Gravity appears to have forced large blocks of limestone to rotate slowly toward the canyon and also has opened up the fissures wider at the surface of the ground near the Virgin. These blocks are not parallel to the fractures that cross them.
d. Sinkholes are usually rounded, as seen at the ground surface, but some of the fissures and fractures near the Virgin are at the top of mounds.
e. Sinkholes created by the dissolution of carbonate rocks (dolomite and limestone) under the surface are not linear in shape.
f. The Virgin River makes perpendicular turns, tracing either N-S or E-W in this Section 29, but further east it makes NW-SE or NE-SW channels.
g. The Kaibab limestone away from the river looks distorted and wrenched. As far as a half kilometers away from the canyon, the surface yields rectangular blocks of limestone at the surface which are separate from the others and which have soil between them.
Hiking-Geology Report, cont'd
Field observations about active fissures:
A. The depicted fracture and fault locations were studied both at the Virgin River and near the up-dipping and wrenching of the Kaibab, both near the Town of Virgin and west of Three Falls near Toquerville. The unusual fracturing and breakage of the Kaibab formation indicates that there is a local anomaly in the crust due to on-going strain. The active fractures allowed the Virgin River to create a channel through the rim of the Hurricane scarp, even though the scarp is higher structurally and topographically relative to the drainage of the Virgin to the east.
B. Other evidence for an anomaly in the crust is the presence of the Pah Tempe hot springs, several clustered dormant volcanoes, spreading fractures, fissures, and faulting, as well as the accentuated up-dipping of the stratigraphy in the region.
C. The fissures and fractures are linear, indicating that they result from locally accented stresses, not caverns made by the solution of limestone in the Redwall or a similar layer some 300 meters beneath the surface.
D. The Virgin captured local drainage, superseding the Ash and Laverkin Creeks. This indicates that a major change in local stresses in Pleistocene time offset the previous NW-SE strains, to allow N-S fractures to become dominant.
E. The Virgin River outlet through the Hurricane scarp represents a regional boundary to the Hurricane linear scarp, where the Colorado plateau is uplifted more toward the south than to the north. This could happen because there is more unloading to the south (erosion), but also because the direction of stress has been redirected near the River.
The overall conclusions about this supposed fracture and lateral fault feature are as follows;
A. The only evidence for lateral movement is at the graben face- which indicates small-scale right lateral faulting;
B. The trace of the NW-SE fracture system is mainly along the PVM Creeks and springs, and in the large fissures above the Virgin River;
C. There is definitely an anomalous weak zone from Hiway 9 to the quarry north of Laverkin;
D. Fissures aligned NW-SE and their orthogonals are opening presently above the Virgin River (near the town of Virgin); and
E. Otherwise most of the faulting and fracturing is on an N-S alignment, as is the thrusting above and east of the town of Toquerville.
F. All conclusions about the stresses, faulting, and anticlines- monoclines are aggravated by the following sequence of events:
I. The Laramide was a compressional event (about 100 million years ago), caused by a NE-ward subduction of Pacific plate under the whole CP- all the way to the Rocky Mountains (this compression is exemplified by trends of N-S features, such as hogbacks, at the AZ border and NE-SW trends near the PVM and Virgin anticline;
II. At 41 ybp (years before present), as shown by the Hawaiian-Emperor Seamount chain, a change in direction of the Pacific basin movement from N-S to NW-SE occurred. This was probably caused by the crossing of a large transform fault by the Hawaiian volcanic location and outpouring, across which the parallel segments on either side of the transform moved at differing velocities. This change of direction of Pacific plate movement would affect the Basin and Range (B&R, caused by extension) all the way to the CP edge, gradually dying out as it approached Utah;
III. Near 21 million ybp, during extensional time (the West was being pulled apart- opposite to compression, which creates shortening of the earth’s Crust), the PVM rose as an igneous intrusion, creating local compression in an otherwise extensional time. This is similar to a salt dome, which rises in extensional cracks, but which locally creates folding or doming of sedimentary beds, which are due to compression;
IV.In Miocene time, erosion uncovered the PVM, allowing border rock to slide (detach) from the main granite-like mass, carrying overlying sedimentary beds downward to the SE. This is complicated by the non-appearance of this action on the SW side of PVM. Why would beds slide downward on only one corner of the protruding mass? There are isolated masses of intrusives near the Wet Sandy Creek and on the east side of PVM, which may be dikes, and this would increase the likelihood that they rose along the supposed weakness shown by the Wet Sandy Creek linear.
V.In Pliocene time, 5 mybp continuing until now, the Hurricane N-S faulting rejuvenated, and this is conjectured to have occurred due to a slowing of the earth’s spin (length of day), allowing the equatorial bulge to decrease (a lessening of the centrifugal force creating the bulge would cause the earth to shrink in the equator to mid-latitude zone, including Hurricane area). This bulge is only on the order of a Earth diameter increase of 20 miles, but could create fractures in a N-S orientation, as it shrunk;
VI.In Pleistocene time (2 mybp and later), the N-S fracturing in an extensional regime allowed the Hurricane fault to wax and wane, depending on the Milankovich cycle (due to the wobble, precession and oval pattern of the earth’s movement around the sun). This cycle is at most 84,000 years, and would create loosening and tightening of the rocks near the H fault, causing cycles of movement- these are shown by terraces above the present H scarp;
VII.Vulcanism within the Hurricane City limits has locally created compression, in an otherwise extensional regime, similar to the Salt Domes described above. This has caused local thrusting and folding, which must be eliminated in a region-wide study. Extension and erosion, causing unloading of sediments, is causing the local area to rise as it loses weight to the Virgin River- which carries away the mass.
A. The depicted fracture and fault locations were studied both at the Virgin River and near the up-dipping and wrenching of the Kaibab, both near the Town of Virgin and west of Three Falls near Toquerville. The unusual fracturing and breakage of the Kaibab formation indicates that there is a local anomaly in the crust due to on-going strain. The active fractures allowed the Virgin River to create a channel through the rim of the Hurricane scarp, even though the scarp is higher structurally and topographically relative to the drainage of the Virgin to the east.
B. Other evidence for an anomaly in the crust is the presence of the Pah Tempe hot springs, several clustered dormant volcanoes, spreading fractures, fissures, and faulting, as well as the accentuated up-dipping of the stratigraphy in the region.
C. The fissures and fractures are linear, indicating that they result from locally accented stresses, not caverns made by the solution of limestone in the Redwall or a similar layer some 300 meters beneath the surface.
D. The Virgin captured local drainage, superseding the Ash and Laverkin Creeks. This indicates that a major change in local stresses in Pleistocene time offset the previous NW-SE strains, to allow N-S fractures to become dominant.
E. The Virgin River outlet through the Hurricane scarp represents a regional boundary to the Hurricane linear scarp, where the Colorado plateau is uplifted more toward the south than to the north. This could happen because there is more unloading to the south (erosion), but also because the direction of stress has been redirected near the River.
The overall conclusions about this supposed fracture and lateral fault feature are as follows;
A. The only evidence for lateral movement is at the graben face- which indicates small-scale right lateral faulting;
B. The trace of the NW-SE fracture system is mainly along the PVM Creeks and springs, and in the large fissures above the Virgin River;
C. There is definitely an anomalous weak zone from Hiway 9 to the quarry north of Laverkin;
D. Fissures aligned NW-SE and their orthogonals are opening presently above the Virgin River (near the town of Virgin); and
E. Otherwise most of the faulting and fracturing is on an N-S alignment, as is the thrusting above and east of the town of Toquerville.
F. All conclusions about the stresses, faulting, and anticlines- monoclines are aggravated by the following sequence of events:
I. The Laramide was a compressional event (about 100 million years ago), caused by a NE-ward subduction of Pacific plate under the whole CP- all the way to the Rocky Mountains (this compression is exemplified by trends of N-S features, such as hogbacks, at the AZ border and NE-SW trends near the PVM and Virgin anticline;
II. At 41 ybp (years before present), as shown by the Hawaiian-Emperor Seamount chain, a change in direction of the Pacific basin movement from N-S to NW-SE occurred. This was probably caused by the crossing of a large transform fault by the Hawaiian volcanic location and outpouring, across which the parallel segments on either side of the transform moved at differing velocities. This change of direction of Pacific plate movement would affect the Basin and Range (B&R, caused by extension) all the way to the CP edge, gradually dying out as it approached Utah;
III. Near 21 million ybp, during extensional time (the West was being pulled apart- opposite to compression, which creates shortening of the earth’s Crust), the PVM rose as an igneous intrusion, creating local compression in an otherwise extensional time. This is similar to a salt dome, which rises in extensional cracks, but which locally creates folding or doming of sedimentary beds, which are due to compression;
IV.In Miocene time, erosion uncovered the PVM, allowing border rock to slide (detach) from the main granite-like mass, carrying overlying sedimentary beds downward to the SE. This is complicated by the non-appearance of this action on the SW side of PVM. Why would beds slide downward on only one corner of the protruding mass? There are isolated masses of intrusives near the Wet Sandy Creek and on the east side of PVM, which may be dikes, and this would increase the likelihood that they rose along the supposed weakness shown by the Wet Sandy Creek linear.
V.In Pliocene time, 5 mybp continuing until now, the Hurricane N-S faulting rejuvenated, and this is conjectured to have occurred due to a slowing of the earth’s spin (length of day), allowing the equatorial bulge to decrease (a lessening of the centrifugal force creating the bulge would cause the earth to shrink in the equator to mid-latitude zone, including Hurricane area). This bulge is only on the order of a Earth diameter increase of 20 miles, but could create fractures in a N-S orientation, as it shrunk;
VI.In Pleistocene time (2 mybp and later), the N-S fracturing in an extensional regime allowed the Hurricane fault to wax and wane, depending on the Milankovich cycle (due to the wobble, precession and oval pattern of the earth’s movement around the sun). This cycle is at most 84,000 years, and would create loosening and tightening of the rocks near the H fault, causing cycles of movement- these are shown by terraces above the present H scarp;
VII.Vulcanism within the Hurricane City limits has locally created compression, in an otherwise extensional regime, similar to the Salt Domes described above. This has caused local thrusting and folding, which must be eliminated in a region-wide study. Extension and erosion, causing unloading of sediments, is causing the local area to rise as it loses weight to the Virgin River- which carries away the mass.
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