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?