An Interpretation of the Structural Geology of the Franklin Mountains, Texas Earl M
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New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/26 An interpretation of the structural geology of the Franklin Mountains, Texas Earl M. P. Lovejoy, 1975, pp. 261-268 in: Las Cruces Country, Seager, W. R.; Clemons, R. E.; Callender, J. F.; [eds.], New Mexico Geological Society 26th Annual Fall Field Conference Guidebook, 376 p. This is one of many related papers that were included in the 1975 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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LOVEJOY Department of Geological Sciences The University of Texas at El Paso El Paso, Texas 79968 INTRODUCTION actually a series of low-angle normal faults, one of which Harbour mapped as the base of a landslide south of the Tin Richardson (1909) described the structure of the Franklin Mine. Harbour noted that the "stratigraphic displacement" on Mountains as follows (Fig. 1, Table 1): his "west limb" (which I refer to as the Western Boundary 1. "The long, narrow Franklin Range, rising 3,000 ft above fault zone) of his "Avispa thrust fault" is more than 7,800 ft broad lowlands, resembles a 'basin range' fault block of at 31°52'30" N. latitude (Figs. 2 and 3), decreasing north to westward-dipping rocks, but it differs from the type by being complexly faulted internally. [p. 9] about 2,500 ft in Avispa Canyon where he considered the "Avispa thrust fault" trace to leave the western front of the 2. "The structure of the Franklin Mountains viewed from a distance appears simple. The strata strike parallel to the range and to change trend to become the "important trans- trend of the range and dip westward at steep angles. But verse fault," earlier noted by Richardson (quotes 4 and 6). the simplicity is only apparent, for the distribution of the Harbour did not extend the boundary fault north of Avispa rocks show that the range is traversed by many faults. As Canyon (cf. quote 4 above). a whole the long, narrow mountain belt bordered by broad waste-covered deserts, the western slopes coinciding Harbour (1972, p. 67-69) wrote (cf. Richardson, quote 8): with the dip of the rocks and the steeper eastern face "Cenozoic structural features in the Franklin Mountains exposing eroded edges of the strata, presents the general trend north to northwest and are a result of both com- appearance of an eroded fault block of the basin-range pression and tension, which culminated in uplift of the type. [p. 9-10] mountain block above the surrounding basins. Structural features caused by compression include thrust faults and, on 3. "On the east [of the range] the position of the hypo- the west slope of the mountains, anticlines and synclines. thetical fault along the base of the range is completely Structural features caused by tension include high- and low- concealed by wash. [p. 10] angle normal faults and, probably, the Avispa fault. The pat- 4. "On the west the dislocation consists of two parallel tern of the structural features suggests that compression pre- 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, indi- cated 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 Frank- lin 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 lime- stone 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 land- slide—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)1 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 1. The report was prepared after Harbour's death (Harbour, 1972, p. 2) by his friends who noted that they "hope that they have not seriously modified his intended meaning in any part of the report." ceded tension, for thrust faults are most logically attributed It is of interest to note that Dunham (1935, p. 149-150) to a single period of compression followed by relaxing ten- actually wrote: sion at which time the mountains were uplifted along a there are many gaps in the evidence bearing in Miocene, normal fault near their eastern base. .. Pliocene, and Quaternary structural changes. The events "Regional studies indicate that the pattern of early Cenozoic ... are not entirely proved by local evidence, but in the compressional structural features and later normal faults per- absence of such data it seemed advisable to make the story sists throughout the Basin and Range province in western consistent with the general history of the period in the Rio Texas and southern New Mexico. ... More recent de- Grande region." scriptions 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 I suggest that an analysis of the structural geology of the Mexico uplift of the present mountain ranges took place Franklin Mountains not only fills some of these gaps, but gives along normal faults after a period of compression." [see rise to a considerably different tectonic interpretation not con- quote from Dunham below] sistent with the published general history of the period in the Rio Grande region. 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 A WORKING HYPOTHESIS initial uplift ... is generally placed in Miocene time." Two concepts underlie my working hypothesis: 1) range Harbour noted that Dunham (1935, p. 176) "believed that tilting occurred mainly late in range uplift; 2) surficial land- much if not most of the mountain building occurred at the sliding and subsurficial gravity gliding accompanied range up- end of Pliocene time." Harbour continued: lift and tilting. Richardson (1909) did not consider these "However, proved displacements on faults involving the basin processes. Harbour (1972) mapped landslides to the north and is generally less than 400 ft in this region. Mountain building may still be continuing as shown by recent fault scarps along south of the Tin Mine (Fig. 2) and recognized low-angle the fronts of the Franklin ... Mountains." normal faults on the northeast end of the range. I believe landsliding and gravity gliding explain most of the internal Harbour also noted (cf. quote 3 above): structures of the range; if so, their local and regional tectonic "Perhaps the most striking feature of the Franklin Mountains' significance is considerably changed. structures is the curvature of the fault planes. ... Recent fault scarps in the Cenozoic deposits are markedly sinuous Range Uplifting and Tilting and the Fusselman Canyon fault is a high-angle normal fault with a U-shaped trace.