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

Richardson (1909) described the structure of the Franklin Mountains as follows (Fig. 1, Table 1):

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.