An interpretation of the structural geology of the Franklin Mountains, Texas
Earl M. P. Lovejoy

Abstract:

Richardson (1909) described the structure of the Franklin Mountains as follows (Fig. 1, Table 1):
  1.  "The long, narrow Franklin Range, rising 3,000 ft above broad lowlands, resembles a 'basin range' fault block of westward-dipping rocks, but it differs from the type by being complexly faulted internally. [p. 9]
  2.  "The structure of the Franklin Mountains viewed from a distance appears simple. The strata strike parallel to the trend of the range and dip westward at steep angles. But the simplicity is only apparent, for the distribution of the rocks show that the range is traversed by many faults. As a whole the long, narrow mountain belt bordered by broad waste-covered deserts, the western slopes coinciding with the dip of the rocks and the steeper eastern face exposing eroded edges of the strata, presents the general appearance of an eroded fault block of the basin-range type. [p. 9-10]
  3.  "On the east [of the range] the position of the hypothetical fault along the base of the range is completely concealed by wash. [p. 10]
  4.  "On the west the dislocation consists of two parallel faults at the base of the range between the foothills and the main mountain mass. These faults can be followed for several miles and probably border the entire range. The greatest displacement appears in the central part of the range ... indicating a throw of more than 2,500 ft.... The easternmost of these parallel faults along the western base of the mountains has a relatively small throw, indicated by steeply tilted lower Paleozoic strata abutting against the rhyolite porphyry, but farther north the throw is reversed and increased in amount by the cross fault which separates the North Franklin and Central Franklin blocks.... [p. 10]
  5.  "One of the main faults of the range is the longitudinal one which separates the North Franklin and Cassiterite blocks.
  6.  "An important transverse fault separates the North Franklin and Cassiterite blocks on the north from the Central Franklin block on the south.
  7.  "At the southwest end of the range a small wedge-shaped block in which the Hueco limestone outcrops enters the South Franklin block, the Fusselman and Montoya limestone outcropping west of it. [This "wedge-shaped block" of Hueco Ls. is part of the Western Boundary fault zone; the Fusselman and Montoya Ls. form the Crazy Cat landslide—E.M.P.L.J.
  8.  "... major deformation of the El Paso district probably developed during the close of the Cretaceous period or early in Tertiary time ... [when] the mountain blocks and intervening basins were outlined."
Harbour (1972) mapped (1:24,000) the northern Franklin Mountains. He showed that the "longitudinal fault," "one of the main faults of the range" (Richardson, quote 5 above), was actually a series of low-angle normal faults, one of which Harbour mapped as the base of a landslide south of the Tin Mine. Harbour noted that the "stratigraphic displacement" on his "west limb" (which I refer to as the Western Boundary fault zone) of his "Avispa thrust fault" is more than 7,800 ft at 31°52'30" N. latitude (Figs. 2 and 3), decreasing north to about 2,500 ft in Avispa Canyon where he considered the "Avispa thrust fault" trace to leave the western front of the range and to change trend to become the "important transverse fault," earlier noted by Richardson (quotes 4 and 6). Harbour did not extend the boundary fault north of Avispa Canyon (cf. quote 4 above).
Harbour (1972, p. 67-69) wrote (cf. Richardson, quote 8):

"Cenozoic structural features in the Franklin Mountains trend north to northwest and are a result of both compression and tension, which culminated in uplift of the mountain block above the surrounding basins. Structural features caused by compression include thrust faults and, on the west slope of the mountains, anticlines and synclines. Structural features caused by tension include high- and low-angle normal faults and, probably, the Avispa fault. The pattern of the structural features suggests that compression preceded tension, for thrust faults are most logically attributed to a single period of compression followed by relaxing tension at which time the mountains were uplifted along a normal fault near their eastern base.
"Regional studies indicate that the pattern of early Cenozoic compressional structural features and later normal faults persists throughout the Basin and Range province in western Texas and southern New Mexico. ... More recent descriptions of New Mexico's Organ Mountains (Dunham, 1935, p. 142-147) ... tend to confirm the conclusions of King (1935, p. 244-251) that in western Texas and northern Mexico uplift of the present mountain ranges took place along normal faults after a period of compression." [see quote from Dunham below]
Harbour noted that compression is believed to have occur-red locally in early Tertiary time and that "the age of the ... [boundary] faults ... is uncertain," although noting that "the initial uplift ... is generally placed in Miocene time."
Harbour noted that Dunham (1935, p. 176) "believed that much if not most of the mountain building occurred at the end of Pliocene time."
Harbour continued:
"However, proved displacements on faults involving the basin is generally less than 400 ft in this region. Mountain building may still be continuing as shown by recent fault scarps along the fronts of the Franklin ... Mountains."
Harbour also noted (cf. quote 3 above):
"Perhaps the most striking feature of the Franklin Mountains' structures is the curvature of the fault planes. ... Recent fault scarps in the Cenozoic deposits are markedly sinuous and the Fusselman Canyon fault is a high-angle normal fault with a U-shaped trace. Other curving fault planes are the Avispa fault and the low-angle normal faults."
It is of interest to note that Dunham (1935, p. 149-150) actually wrote:
".. there are many gaps in the evidence bearing in Miocene, Pliocene, and Quaternary structural changes. The events ... are not entirely proved by local evidence, but in the absence of such data it seemed advisable to make the story consistent with the general history of the period in the Rio Grande region."
I suggest that an analysis of the structural geology of the Franklin Mountains not only fills some of these gaps, but gives rise to a considerably different tectonic interpretation not consistent with the published general history of the period in the Rio Grande region.

Citation:

  1. Lovejoy, Earl M. P., 1975, An interpretation of the structural geology of the Franklin Mountains, Texas, in: Las Cruces Country, Seager, William R.; Clemons, Russell E.; Callender, Jonathan F., New Mexico Geological Society, Guidebook, 26th Field Conference, pp. 261-268.

More information...