The Colorado Lineament: A middle Precambrian wrench fault system LAWRENCE A. WARNER Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309 ABSTRACT at widely scattered localities early in the the northeast was apparent by the turn of century. During the past two decades, the the century, following vigorous develop- The "Colorado Lineament" is the name number of such discoveries has increased ment of mining that began in about 1860. herein assigned to a northeast-trending belt steadily, and detailed studies concerning the The alignment of mineral districts is strik- of Precambrian faults that traverses the influence of Precambrian faulting on later ingly depicted on a map compiled by Fisch- Rocky Mountains of Colorado and the events have been made in several areas. The er (1946). A belt of silicic plutons, emplaced Colorado Plateau and is followed along faults of partipular interest to this report are in Laramide and later time, had been shown much of its trend by the Colorado River. It those of northeasterly trend, which appear to coincide approximately with the belt of has been established that northeast faults in to constitute the master set. Although mineralization (Burbank and others, 1935; the Colorado Mineral Belt have a Pre- hinted at by previous workers, the concept Lovering and Goddard, 1938). cambrian ancestry; other faults in the sys- has been largely overlooked that these Although Precambrian faults had been tem have been identified in north-central faults comprise a broad belt of Precambrian recognized within the Mineral Belt (Lover- Colorado. Similar faults in the Grand Can- shearing, extending across most of the ing and Goddard, 1950, p. 54 and PI. 2), yon region have been traced across the Col- Cordilleran region of the United States and the nature and extent of the Precambrian orado Plateau into Utah, defining an exten- probably continuing into the northern mid- faulting and its influence in localizing sion of the zone in Colorado. The entire belt continent. Laramide intrusives and ore deposits was constitutes a fault lineament more than The nature and extent of the Precambrian not fully appreciated until documented by 1,100 km long and 160 km wide. fault belt thus defined make it comparable Tweto and Sims (1963). Subsequent work The northern margin of the belt is the to the San Andreas system of California and by Warner and Robinson (1967, p. 108- Mullen Creek—Nash Fork shear zone, to similar Phanerozoic fault belts elsewhere. 110), Abbott (1970), Punongbayan (1972), which traverses southeastern Wyoming, Such wrench fault systems appear to and work now in progress by doctoral stu- separating an ancient basement terrain characterize many continental plate mar- dents under the direction of W. A. Brad- (3=2,400 m.y. old) in central Wyoming from gins. dock at the University of Colorado, have younger rocks (^1,750 m.y. old) in Col- Acceptance of this analogy raises two is- served to broaden and confirm the thesis of orado. Gravity and aeromagnetic data, to- sues for consideration. First, proximity of Tweto and Sims. gether with radiometric ages obtained for the Precambrian fault system, at the time of A broad belt of Precambrian shear zones deep-well samples, suggest an extension of its origin, to the margin of the ancestral traverses the basement cores of the Rocky this boundary beneath the high plains and North American continent is implied. Sec- Mountains in Colorado (Fig. 1). Igneous in- into the northern midcontinent region. ondly, the mechanism for forming trusions and metallization occur mainly The pattern that emerges resembles a longitudinal wrench fault systems, what- along the southern margin of this belt, fault system of the San Andreas type. ever it may be, apparently has been opera- where the ancient faults apparently were Although the record is fragmentary, the tive through a large portion of the history of most vigorously reactivated in Laramide sum of the evidence suggests that the system Earth. time. Individual shear zones within the belt formed adjacent to the southeastern margin Arguments for the probable existence, in range from narrow bands of mylonite and of the ancestral North American continent what is now middle North America, of a pseudotachylite to broad bands of milder in connection with Penokean orogeny 2000 Precambrian fault system of the San An- cataclasis, the former commonly contained to 1700 m.y. B.P. It appears to represent a dreas type, and consideration of its tectonic within the latter. Some of the cataclastic Precambrian counterpart of Phanerozoic implications, form the bases for this report. zones are more than 1 km wide and can be wrench fault systems that have formed traced for tens of kilometres. The zones of commonly along continental plate margins PRECAMBRIAN SHEAR ZONES shearing are separated by septa of un- during episodes of mountain building. IN COLORADO AND WYOMING sheared rock several kilometres wide. Relationship of the shearing deformation INTRODUCTION The Colorado Mineral Belt to other major Precambrian events in the region appears to have been established, Field relationships indicating the pres- The localization of mineralized districts although details are uncertain. Braddock ence of Precambrian faults in basement ex- in Colorado along a northeast-trending belt (1970) recognized three episodes of folding, posures on the Colorado Plateau and in the extending diagonally across the state from each with a distinct axial cleavage, in the Rocky Mountains of Colorado were noted La Plata on the southwest to Jamestown on northern Front Range. The earliest folding Geological Society of America Bulletin, v. 89, p. 161-171, 5 figs., February 1978, Doc. no. 80201. 161 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/2/161/3444075/i0016-7606-89-2-161.pdf by guest on 25 September 2021 162 L. A. WARNER is accompanied or followed by low-grade northern Front Range between 1390 and began with the early folding and met- regional metamorphism; the latest is fol- 1450 m.y. B.P. was not accompanied by amorphism and occurred partly between lowed by metamorphism of almandine- major tectonism, although local deforma- Silver Plume and Pikes Peak emplacement amphibolite grade and coincides approxi- tion has been recognized. Emplacement of times (Tweto and Sims, 1963, p. 999- mately with emplacement of Boulder Creek the Pikes Peak batholith in the southern 1000). Hedge (1967) suggested that cata- granodiorite. Dates for the later metamor- Front Range about 1000 m.y. B.P. (Hutch- clasis in the central Front Range may have phic event fall between 1700 and.1800 m.y. inson, 1960; Hedge, 1970) has not been begun about 1450 m.y. B.P. and continued B.P. (Peterman and others, 1968). Em- tied to a major tectonic event. Cataclastic until about 1200 m.y. B.P. The Homestake placement of the Silver Plume, Sherman, deformation related to some of the shear shear zone (Fig. 1, no. 2) is in part later than and related plutons in the central and zones in central and northern Colorado granite dated as 1,390 m.y. old (Pearson EXPLANATION L AR AM 10 E AND LATER Jß * PLUTONS EXPOSE 0 PRECAMBRIAN ANTICLINAL FOLD FAULT OR SHEAR ZONE: 1. CENTRAL FRONT RANGE SYSTEM (LOVERING AND GOODARD, 1950-, TWETO AND SIMS, 1963) 2. HOMESTAKE SYSTEM, SAWATCH RANGE (TWETO AND SIMS, 1963) 3. GUNNISON SHEAR 20NE (TWETO ANO SIMS, 1963) 4. MOOSE MT. SHEAR ZONE (PUNONGBAYAN, 1972) 5. SKIN GULCH SHEAR ZONE (ABBOTT, 1970) 6. MULLEN CREEK-NASH FORK SHEAR ZONE (HOUSTON AND McCALLUM, 1961) 7. SINALAYA SYSTEM (SHOEMAKER ET. AL., 1974) 8. BRIGHT ANGEL SYSTEM (SHOEMAKER ET. AL., 1974) 9. MESA BUTTE SYSTEM (SHOEMAKER ET. AL.. 1974) 10. SHYLOCK-CHAPARRAL SYSTEM (ANDERSON, 1967) 11. BLANDING STRUCTURE (CASE AND JOESTING, 1961 ) 12. LA SAL STRUCTURE (CASE ET. AL-, 1963) 13. ROBERTS RIFT (H ITE , 1975) 14. SUBSURFACE BOUNDARY BETWEEN GRANITIC (S.E.) AND METAMORPHIC (N.W.) TERRANES (EDWARDS, 1963) 15. HARTVILLE FAULT (DROULLARD, 1963) COLORADO SLIlü I V I tvyc ! i [ J /ri Y>/®/ À I» COMPI LEO FROM VARIOUS SOURCES, INCLUDING COHEE ET. AL. (1963), BAYLEY AND MUEHLBERGER (1968), KING (1973), KING ET. AL. (1974) Figure 1. Tectonic map of the Rocky Mountain region, showing Colorado Lineament. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/2/161/3444075/i0016-7606-89-2-161.pdf by guest on 25 September 2021 THE COLORADO LINEAMENT: PRECAMBRIAN WRENCH FAULT SYSTEM 163 and others, 1966). Abbott (1970) obtained passing through the northern Laramie others, 1968, Fig. 10). Ages for the whole-rock Rb-Sr dates on mylonites in the Range and across the northern ends of the metasediments are uncertain. Poorly de- Skin Gulch shear zone (Fig. 1, no. 5) that Medicine Bow and Sierra Madre uplifts in fined Rd-Sr isochrons date the older unit at cluster around 1200 m.y. B.P. He recog- southeastern Wyoming. He called this zone 1840 m.y. B.P. and the younger unit at nized two subsequent episodes of brittle the "Wyoming Lineament." The features 1650 m.y. B.P. (Hills and others, 1968, p. fracture prior to Laramide faulting. Clearly, that he noted are mainly Laramide; he did 1770, 1773). The dates are regarded as ap- more age data are needed. In the absence of not attempt to relate the zone to older proximate minima for the time of deposi- details, it may be supposed that the cata- basement structures. Precambrian faults tion. clastic deformation extended over a sig- along its trend have not been recognized, An age for the Mullen Creek—Nash Fork nificant time span. There is, as yet, no evi- but subsequent discoveries suggest that they shear zone is suggested by its relationship to dence to preclude an earlier origin for many may be present.