The Colorado Lineament: a Middle Precambrian Wrench Fault System

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

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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.

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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. adjacent granitic plutons. The youngest of of the faults. Some 30 miles southeast of Blackstone's these to be cut by the shear zone is dated at The amount and sense of displacement Wyoming Lineament, there is a broad, 1760 m.y. B.P. and the oldest to be on the shear zones is poorly documented. northeast-trending zone of shearing that emplaced in the zone is dated at 1720 m.y. The trend of the fault belt is roughly paral- contains mylonite, cataclasite, and phyllo- B.P. (Hills and others, 1975). Blocks of lel to the regional grain of the basement nite. It was first described by Houston and metadolomite occur in the fault zone 10 km rock (Lovering and Goddard, 1950, Pis. 1, McCallum (1961) in the central part of the or more northeast of their outcrop position 2; Cohee and others, 1961), and displace- Medicine Bow Range; they called it the on the northwest side of the fault, suggest- ments of rock units across the faults are not "Mullen Creek—Nash Fork shear zone." A ing a left-lateral slip. The steeply northeast obvious. Folds associated with the cata- Precambrian origin for the faulting was in- plunging syncline in the younger metasedi- clastic deformation afford possible clues. ferred by Houston and Parker (1963), and ments, sheared along its southeast limb, is The Homestake shear zone (Fig. 1, no. 2) in this was later substantiated by radiogenic interpreted as due to lateral movement on the northern Sawatch Range is bordered by dating (Hills and others, 1968). Extensions the fault (Houston and Parker, 1963). The tight folds with near vertical axes. Tweto of the zone northeast across the Laramie occurrence of remnants of miogeosynclinal and Sims (1963, p. 1009) related the folds Range and southwest across the Sierra rocks along the northwest side of the fault to action of a horizontal shearing couple at Madre to the Colorado-Wyoming border and the absence of these to the south the time of folding, but the sense of rotation have been identified (Houston, 1971; Hills suggest either strong upward movement of is obscure. Moench and others (1962) and others, 1975; Houston and others, the southeast block or profound lateral noted minor folds related to a wide 1975; Divis, 1977). The zone appears to movement. northeast-trending zone of cataclasis in the turn westerly where it crosses the Sierra The Mullen Creek—Nash Fork zone (Fig. central Front Range. They recognized a B Madre. 1, no. 6) may be taken as an approximate lineation subparallel to the fold axes that Basement rocks on opposite sides of the northern boundary for a broad belt of Pre- probably relates to an axis of internal rota- Mullen Creek—Nash Fork fault are litholog- cambrian faults that extends to the south- tion, and an A lineation, subnormal to B, ically and structurally dissimilar (Houston ern margin of the Colorado Mineral Belt, characterized by rodding and slickensides. and others, 1968). Metamorphic rocks on marked by the Idaho Springs—Ralston shear Although attitudes for the lineations range the northwest side are mainly quartzo- zone (Tweto and Sims, 1963, p. 998). In all, through a wide angle, the average orienta- feldspathic gneisses marked by a north-to- the belt of Precambrian faulting is at least tions lead to the conclusion that movement northwest structural trend. Those on the 160 km wide. Although the nature and ex- on the shear zone was mainly up dip. Ab- southeast contain up to 40% amphibolite, tent of Precambrian movement is poorly bott (1970, p. 100-110) drew the same presumably derived from basaltic sills or known, the character and spacing of shear conclusion from similar folds and lineations flows, and structural axes trend northeast. zones within the belt suggest that aggregate associated with the Skin Gulch shear zone. The fault zone appears to mark the displacement across it may have been very He noted, however, that in outcrop pattern boundary between ancient rocks that large. It is unlikely that a fault system of this the shear zone in the area of his investiga- characterize the basement province of cen- magnitude was confined to the region of its tion resembles a cymoid loop with a sense tral Wyoming and a much younger base- exposure in north-central Colorado and of left-lateral wrench. According to W. A. ment in Colorado. Ages for the former are southeastern Wyoming. Its extension to the Braddock (1976, personal commun.), many mainly 2,400 m.y. or more; those for the southwest across the Colorado Plateau and of the shear zones show in places a steep latter are 1,750 m.y. or less. to the northeast under the High Plains is a lineation that trends down the dip. Whether Overlying the basement rocks on the distinct possibility. the lineation is kinematically related to A north side of the fault zone in the Medicine (direction of transport) or B (axis of inter- Bow mountains, there is 35,000 ft of mildly PRECAMBRIAN FAULTING ON nal rotation) has for the most part not been deformed and somewhat metamorphosed THE COLORADO PLATEAU determined. rocks, mainly of sedimentary origin. The sequence is divisible into two units, the Flagstaff-Grand Canyon Region, Arizona Mullen Creek—Nash Fork Shear Zone, older consisting mainly of quartzite and Southeastern Wyoming metavolcanics, the younger containing Distribution of major northeast-trending quartzite, metadolomite, and slate. The fault zones along the Grand Canyon and in Soon after Lovering and Goddard (1950) older unit is broadly folded along a north- the vicinity of Flagstaff, Arizona, is shown defined the Front Range portion of the Col- east trend; the younger unit is exposed in a on Figure 1 (nos. 7 through 10). The faults orado Mineral Belt, Blackstone (1951, large syncline that plunges steeply north- are grouped mainly into three discrete 1953) called attention to a northeast- east, the southeast limb of which has been lanes, forming the Sinalaya, Bright Angel, trending zone of structural discontinuity cut out against the shear zone (Hills and and Mesa Butte systems. A fourth occur-

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rence of interest is the Chapparral-Shylock basement foliation, and later strata are es- in the underlying basement, probably a system, near Prescott, southwest of Flag- sentially flat, field evidence for wrench fault. Within the anomaly zone, northeast- staff. The faults are expressed fqr the most movement is not easily established. trending faults break the Mesozoic cover part in Phanerozoic rocks. Limited base- Wilson (1939) concluded that deforma- and could be due to Laramide movement on ment exposures show zones of shearing up tion of the older (pre-Apache) rocks in cen- an ancient basement structure, as demon- to 3 km wide, containing mylonite in tral Arizona took place during a Mazatzal strated for the Bright Angel and Mesa Butte places. Together the fault systems define a revolution that involved folding, faulting, fault systems in Arizona. belt of faulting comparable in extent, and and emplacement of large granitic plutons. Hite (1975) defined a northeast-trending on the trend of, the belt in Colorado. Silver (1965) regarded this as the most fracture zone near Moab, Utah, as the Although many of the faults are known widespread and intense deformational Roberts Rift. The zone parallels the Col- to have had complex histories, and some event in the Precambrian of Arizona. He orado River for about 30 km (Fig. 1, no. were active recently, a Precambrian ances- obtained dates of 1715 m.y. B.P. for a pre- 13) and appears to have experienced sub- try for the fault belt appears to be well es- deformational rhyolite and 1660 m.y. B.P. stantial left-lateral strike slip, presumably in tablished. Ransome (1908) recognized a for a postdeformational granite, which Laramide time. Deep-seated regional struc- Precambrian origin for the Bright Angel bracket the Mazatzal orogeny. Similar dates tures parallel to the Colorado River had fault zone, later confirmed by Maxson and were obtained for rocks in the Grand Can- been noted previously (Joesting and others, Campbell (1934) and Maxson (1961). Pre- yon (Pasteels and Silver, 1966). In the ab- 1966; Mutschler and Hite, 1969). From cambrian movement on a part of the Mesa sence of other data, it is tempting to sup- data provided by surface mapping, Butte system was noted by Krieger (1965). pose that Precambrian faulting in the region geophysics, and deep drilling, Hite specu- Anderson (1967) documented a minimum was initiated during the Mazatzal episode. lated that the Roberts Rift and other north- Precambrian displacement of 8 km on the east structures in the Paradox region are re- Chaparral fault, which in places is overlain Southeastern Utah lated to a system of basement wrench faults by unbroken Cambrian strata. (Hite, 1975, Fig. 7) with a history similar to Regional mapping by Shoemaker and From the Grand Canyon region to the that of the Mineral Belt faults in Colorado. others (1974), using ERTS-1 photography, mountains of central Colorado, the base- revealed normal faulting and alignment of ment surface is buried beneath relatively THE COLORADO LINEAMENT Cenozoic eruptive centers in Phanerozoic thick, flat-lying Phanerozoic rocks. The cover along extensions of the Bright Angel dominant structural features in the cover Assuming a subsurface connection be- and Mesa Butte fault systems for a distance are northwest-trending faults and folds of neath the central part of the Colorado of about 300 km. Reference to aeromagne- the Paradox-Uncompahgre complex. How- Plateau, the combined Arizona and Col- tic and gravity maps of Arizona (Sauck and ever, gravity and aeromagnetic surveys in orado fault belts comprise a system of Pre- Sumner, 1971; West and Sumner, 1973) re- the Paradox Basin (Joesting and Case, cambrian faults about 160 km wide and veals northeast-trending anomaly belts 1960) revealed subtle expressions of trans- 1,100 km long (Fig. 1). Zeitz and others parallel to the major fault systems. Positive verse northeast-trending structural bound- (1969, p. 1710) noted that such a fault sys- magnetic anomalies ranging up to 700 aries in the basement. Subsequent investi- tem is consistent with regional aeromagne- gammas, probably related to metavolcanic gations at two localities, one covering the tic data. Traversing the Colorado Plateau amphibolite in the Vishnu schist, permit ex- La Sal uplift and the other the Blanding ba- and the Rocky Mountains of Colorado, it is tensions of ancestral Sinalaya, Bright Angel, sin, confirm the presence of northeast- followed along much of its length by the and Mesa Butte faults for more than 400 trending basement structures in the region Colorado River. It seems appropriate to km (Shoemaker and others, 1974, p. 376- (Fig. 1, nos. 11, 12). label this feature the "Colorado Linea- 379, Fig. 7). The Blanding structure (Case and Joest- ment," using the term "lineament" in the The age and nature of the Precambrian ing, 1961), buried beneath some 8,500 ft of sense in which it has been applied to other faulting in Arizona is little better known Paleozoic-Mesozoic strata, is defined by a large-scale linear structural discontinuities. than in Colorado. As many as nine episodes zone of low density and moderately mag- The ages, nature, and extent of Precam- of faulting have been postulated for the netized basement material about 30 km brian movements along the system are Bright Angel fault, mainly Precambrian wide and more than 80 km long, centered poorly known. In places, large lateral dis- (Shoemaker and others, 1974, p. 273-276), on the town of Blanding and trending about placement is indicated; in others, the evi- including normal, reverse, and transcurrent N50°E. The zone is interpreted to be a dence appears to support substantial verti- displacements. Anderson (1967) interpreted block of granite bounded on either side by cal movement. Fragmentary data suggest displacements on the Shylock and Chapar- faults. The northwest boundary can be that parts of the system were active inter- ral faults as due to right-lateral slip, and traced for 160 km to near Uravan, Col- mittently over a long interval of Precam- Shoemaker and others (1974, p. 382) orado. brian time, beginning perhaps 1500 to suggested that Precambrian displacements West and northwest of the La Sal Moun- 2000 m.y. B.P. Once formed, the fractures on the northeast-trending faults in northern tains, a series of linear magnetic trends is were utilized to relieve regional stresses re- Arizona were controlled by right-lateral aligned northeasterly, parallel to a promi- lated to subsequent orogenic events. In wrench movement. Huntoon and Sears nent gravity trend and transverse to the re- gross aspect, the Colorado Lineament re- (1975) found no evidence for lateral slip on gional Phanerozoic structures (Case and sembles the linear fault belts that developed the Bright Angel and related faults. How- others, 1963). The anomalies are inter- along many Phanerozoic mountain systems. ever, since the faults are subparallel to preted to relate to a structural discontinuity The fact that its trend is roughly parallel

Figure 2. Generalized map showing Bouguer gravity along northeast extension of Colorado Lineament. Gravity contours adapted from Woolard and Joesting (1964).

to the regional grain of the basement rocks Ralston fault. Droullard (1963) described a the Great Plains region is suggested. No in- along its course is compatible with this Laramide fault zone that can be traced for dication has been found of the wide fault analogy. 100 km from near Wheatland, Wyoming, belt south of this margin as exposed in the northeastward along the south flank of the mountains of Colorado. If it lies buried be- NORTHEAST EXTENSION Hartville uplift. The zone lies along the ex- neath the Phanerozoic cover of the north- tension of the Mullen Creek—Nash Fork central United States, there can have been Geological and Geophysical Evidence fault and could have a Precambrian antece- little or no movement on the faults since the Approximate boundaries for the Col- dent in the underlying basement. cover was deposited. orado Lineament where it crosses the Eastward of the western high plains, Bouguer gravity anomalies (generalized Rocky Mountain front are the Idaho geologic evidence for an extension of the from Woolard and Joesting, 1964) between Springs-Ralston shear zone on the south lineament is meager. Subsurface data for the Rocky Mountain front and the Cana- and the Mullen Creek—Nash Fork shear northwestern Nebraska (Lidiak, 1972) are dian Shield and along the potential exten- zone on the north. Both of these zones ap- too sparse to show whether the trend noted sion of the Colorado Lineament are shown pear to have possible extensions onto the by Edwards in Colorado continues. A on Figure 2. Where it crosses the mountain high plains (Fig. 1, nos. 14, 15). From a northeast-trending fault has been noted in front, the Mullen Creek — Nash Fork zone study of basement samples obtained from basement rocks beneath central South follows a trough in the gravity contours deep wells in northeastern Colorado, Ed- Dakota (Bayley and Muehlberger, 1968; which loses its identity beyond the wards (1963) noted a northeast-trending Lidiak, 1971). This feature is shown on Wyoming-Nebraska border. Beginning in boundary between granitic (on the south- Figure 2 in relation to the Mullen Creek- northeastern South Dakota and continuing east) and metamorphic (on the northwest) Nash Fork fault. across Minnesota, a well-defined gravity terranes that lies approximately on a north- A possible extension of the northern trough trends northeast, bounded by the eastward extension of the Idaho Springs- margin of the Colorado Lineament across —50-mgal contour. This value approxi-

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UPPER PRECAMBRIAN (Y) TERRANES ATORNEAR DEEPLY OMITTED SURFACE BURIED Wj — Wg BOUNDARY MIDDLE PRECAMBRIAN (X) TERRANES I I k \| (LOCALLY INCLUDE Y OR W) 1 ''' 1 1 (MOREY AND SIMS, 1976)

LOWER PRECAMBRIAN (W) TERRANES FAULT ZONE (LOCALLY INCLUDE Y OR X)

W| -- GREENSTONE TERRANE SCALE AND EQUIVALENT ROCKS

W,-- GNEISS TERRANE es:

Figure 3. Generalized map of Precambrian terranes in north-central United States.

mates the calculated Bouguer gravity for the northeast extension. A detailed discussion Superior province have for some time been region; however, values within the trough of the Precambrian geology is beyond the known to exist in central Wyoming (Can- are significantly lower than those for the scope of this report. Pertinent information non and others, 1966; Reed and Zartman, surrounding area. The gravity trough is fol- is summarized in Figures 3 and 4. It is con- 1973), whereas rocks in the Colorado Front lowed by a narrow, linear magnetic ridge, venient to treat the terranes as structural Range have dates of approximate Chur- bordered on the south by a similar magnetic provinces in terms of their times of consoli- chillian age and younger (Hedge and others, valley; these anomalies are among the more dation to form essentially stable segments 1967; Peterman and others, 1968; Peter- striking features on the magnetic map of of the continental craton, a practice that has man and Hedge, 1968; Vera and Van Minnesota (Zietz and Kirby, 1970). Sims been followed for the Canadian Shield. Schmus, 1974). The boundary between the (1972) pointed out that gravity and magnet- A time classification for rock provinces of Front Range and Wyoming provinces is ic anomalies parallel the rock fabric in cen- the Canadian Shield has been given by marked by the Mullen Creek-Nash Fork tral and northern Minnesota and that grav- Stockwell (1964, 1965, 1973) and a similar shear zone in southeastern Wyoming ity lows associated with magnetic highs are classification for the western Great Lakes (Houston and others, 1968; Hills and commonly related to northeast-trending region was given by Sims and Morey others, 1968; Houston, 1971). bands of granitic rock. Comparison with (1972). Of major interest is the early Pre- A thick sequence of mildly deformed the geologic map of Minnesota (Sims, cambrian (W) Superior province, added to metasedimentary rocks overlying the base- 1970) indicates that the anomaly belt in the basement during Kenoran-Algoman ment in the Medicine Bow Range along the question follows in part a band of granite. orogeny, and the middle Precambrian (X) north side of the shear zone appears to be However, the regularity and continuity of Churchill province resulting from Hud- related to deposits similarly situated with the feature, in comparison to other such sonian-Penokean orogeny. These provinces respect to presumed extensions of the fault combinations noted for the geologic, grav- of the central Canadian Shield are repre- in the Sierra Madre uplift to the southwest ity and magnetic maps of Minnesota, sented in the United States west and south and the Hartville uplift to the northeast suggest that this anomaly belt may be struc- of Lake Superior, the former mainly by the (Houston and others, 1968, p. 144; Divis, turally as well as lithologically controlled. greenstone-granite complex of northern 1977). The latter contains extensive de- Minnesota and the latter mainly by rocks of posits of sedimentary iron information. The Relation To Basement Terranes the Animikie Group (Morey, 1973) and the Medicine Bow sequence is thought by Marquette Range Supergroup (Cannon, Houston to correlate with the Animikie The nature and arrangement of basement 1973) in eastern Minnesota and northern Group in the Lake Superior region. Both are terrane along the path of the Colorado Wisconsin and Michigan. depicted as miogeosynclinal deposits with Lineament are of interest in regard to its Rocks presumed to correlate with the eugeosynclinal counterparts to the south-

cating that the major rock suite, consisting GREAT LAKES GREAT PLAINS ROCKY MOUNTAINS M.Y.B.P. CANADIAN SHIELD of metavolcanic and plutonic rocks, com- 500 PALEOZOIC - pares in age with the Marquette Range N >1- Supergroup in Michigan. However, Rb-Sr data for 2,800-m.y.-old gneisses and mig- t1 1,000 - V GRENVILLIAN KEWEENAWAN LLANO (TEXAS) bJ matites in central Wisconsin indicate high PIKES PEAK (COLO.) a. a. initial 87Sr/86Sr ratios, suggesting that they I a: ELSONIAN NEMAHA (KAN., NEBR.) Ul were derived from a more ancient granitic SILVER PLUME (COLO.) CL 1,500 a. terrane, possibly an extension of that in ^ ','///',•///'' • ID '/////////// ' •' '/ ///// / / / southern Minnesota (Van Schmus and An- 'MAZATZAL (ARIZ.) , y'/////////-, BLACK HILLS ' '/, HU0S0NIAN '//// / BOULDER CREEK (COLO.) X derson, 1977). / PE NOKEAN / ^ -V '//////////S* ///////, 2,000 - ' '''/,'////'' y / / /",> ''///;// / ///

PR0TER0Z0I C U1 The northern boundary of the Mortonian cr a. 1 terrane was established by Morey and Sims a along a line extending southwest from the 2,500 - 2 KENORAN CENTRAL WYOMING ALG0MAN western tip of Lake Superior (Fig. 3). It is | parallel to, and lies some 50 mi south of, the (W ) gravity-magnetic anomaly belt discussed in 3,000 1 the previous section and noted on Figure 2.

BE ART00TH-STILL WATER The alignment of these features with a (WY0M.- MONT.) northeast extension of the Colorado Line- 3,500 - MORTONIAN ament strengthens the argument for such an PRECAMBRIA N extension and suggests that the northern margin of the Mortonian terrane may be a

4,000 - LOWE R fault zone, a view shared by Sims (1976a, p. ARCHEOZOI C 1 1105). In northern Michigan, Keweenawan Figure 4. Major Precambrian tectonic and (or) thermal events for parts of central and western North America. Time boundaries adapted from James (1972). rifting may have displaced the Mortonian boundary southward from its original trend (Sims, 1976a, his Figs. 5, 6). east. A probable extension of the Penokean Kornik (1969) southwest of Hudson Bay. orogenic belt from Minnesota to southeast- Basement rocks west of this boundary then Discussion ern Wyoming and northern Colorado has become part of the Churchill province, been suggested (Hills and others, 1968, p. which is interpreted as having developed on Earlier interpretations for the evolution 1776; Houston, 1971, p. 21). the margin of the Superior terrane (Wilson of North America (Gastil, 1960; Tilton and A more complex relationship is indicated and Brisbin, 1962), the two having been Hart, 1963; Engel, 1963) may require mod- by the geochronology of basement samples joined as a result of Hudsonian orogeny. ification to include two or more nuclear obtained from wells in the northern mid- A further complication was introduced elements in the protocontinent. It seems continent region. Goldich and others by the discovery of early Archean ages for likely that a substantial fragment of the era- (1966) noted that basement ages are mainly the Morton and Montevideo gneisses along ton in the Lake Superior region predated Churchillian in a broad belt extending the Minnesota River Valley in southwestern the Superior province of the southern northwesterly from Nebraska through Minnesota (Goldich and others, 1970; Canadian Shield (Sims, 1976a, p. 1102). It South Dakota to Montana (Fig. 3). The Goldich and Hedge, 1974). These rocks, is also likely that remnants of a similar an- Black Hills appear to lie near the western dated at 3550 to 3800 m.y. B.P., are the cient terrane may exist in the northern margin of the belt; hence, the term "Black oldest known on the North American con- Rocky Mountain region. Detrital zircon Hills orogeny" has been applied by Goldich tinent. On the basis of structural, lithologic, from a gneiss in the Beartooth Range, to deformation that affected the system. geochronological, and geophysical data, northern Wyoming, yielded an age of more The appearance of older rocks in the Black Morey and Sims (1976) proposed that than 3,100 m.y. (Goldich and others, 1966, Hills (Goldich and others, 1966; Zartman much of southern Minnesota and parts of p. 5395). Rocks of similar age have been and Stern, 1967) and beneath northeastern northern Wisconsin and Michigan are un- identified at several localities in Wyoming South Dakota and much of North Dakota derlain by similar rocks or their reconsti- and Montana (Catanzaro and Kulp, 1964; were interpreted by Goldich to indicate that tuted counterparts, which together com- Giletti, 1966; Heimlich and Banks, 1968; rocks of the Superior province underlie the prise the remnant of a sialic protocontinent Nunes and Tilton, 1971). The dates fall northern midcontinent region, the Black that was more extensive at one time. Rocks considerably short of those in southern Hills system amounting to a tectonic over- of this anicent terrane, mainly gneisses of Minnesota, but indicate that one or more print. amphibolite and granulite facies, were pre-Algoman events occurred in the Rocky This interpretation was modified by formed during or prior to a Mortonian Mountain region. Lidiak's (1971) study of the buried Pre- orogenic event that culminated about 3,500 Rocks of the Superior terrane are com- cambrian basement in South Dakota. He m.y. ago. monly interpreted to have formed largely concluded that the boundary between lower The southern boundary of this terrane is on an oceanic crust and to have been and middle Precambrian rocks beneath indefinite. Studies by Van Schmus and supplied in part from island-arc assem- eastern South Dakota extends northwes- others (1975) failed to indentify any Pre- blages (Morey and Sims, 1976, p. 150- terly across North Dakota and thence cambrian W rocks in the basement of east- 151). Whether the protocontinental ele- northerly through Manitoba to connect ern and central Wisconsin. Age determina- ments were united as a result of Algoman- with the boundary between the Superior tions for samples from this region fall Kenoran orogeny to form a single craton, or and Churchill provinces established by mainly between 1,860 and 1,900 m.y., indi- oceanic segments remained between them,

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is a matter of conjecture. It is assumed for purposes of this report that a craton was achieved essentially for central North America prior to Churchill time. The southern portion of the post- Algoman craton was affected by major orogeny, extending from the Great Lakes region to western Arizona, about 2,000 to •SAN ANDREAS SYSTEM 1,700 m.y. ago (Fig. 4). Proximity of the COLORADO' orogenic system to the then continental X\V>>». v 1 - margin is a matter of some importance to '"""! LINEAMENT an interpretation of the Colorado Linea- ment. In the Colorado-Wyoming Rocky Mountains, this orogenic activity appears to have marked the continental border; no rocks predating the orogeny have been identified south of the Mullen Creek—Nash Fork shear zone. The situation in Arizona is less certain, due mainly to widespread Phanerozoic cover. A major problem in interpretation exists in the Great Lakes region, for which data are most abundant. The Animikie Group and related metasedimentary and metavolcanic rocks in the Figure 5. Colorado Lineament in relation to major wrench fault systems in southwestern United States. western Great Lakes region and similar rocks in southeastern Wyoming are regarded as largely miogeosynclinal deposits sumed that the belt was submarginal to the tina, and related faults (Grantz, 1966; that grade southward into eugeosynclinal then southeastern border of the North Roddeck, 1967; Ovenshine and Brew, counterparts (Hills and others, 1968; Van American continent. This is not to suggest a 1972) along the eastern Pacific margin, and Schmus and others, 1975; Van Schmus, narrow belt that rigorously followed the similar faults in Japan (Kaneko, 1966), the 1973, Sims, 1976a). In general, deforma- continental boundary. Indeed, the defor- Philippines (Allen, 1962), New Guinea tion, plutonism, and metamorphism related mation zone may have in places been very (Krause, 1965), and New Zealand to Penokean orogeny increase southward, wide and parts of it may have extended (Wellman, 1955), bordering the western suggesting an approach to the cratonic substantially inland. Pacific. Faults of comparable age and type boundary. However, the relationship of the are found in the Tethyan belt, extending Penokean orogenic belt to the continental TECTONIC IMPLICATIONS from the Mediterranean to the East Indies margin remains in question. Sims (1976a, (Wellman, 1965; Ambrayses, 1970; Katali, 1976b) interpreted the Penokean system as Assuming an extent from Lake Superior 1970; Abel-Gawad, 1971; Tjia, 1972). an intra-cratonic mobile belt, a view re- to western Arizona and a width of 160 km Wrench fault systems related to Paleozoic cently adopted by Van Schmus and Ander- or more, the Colorado Lineament compares mountain chains have been identified in the son (1977). A similar interpretation has favorably with Phanerozoic longitudinal Appalachians of the United States and been made for Proterozoic-Paleozoic wrench fault systems, such as the San An- Canada (Wilson, 1962; Reed and Bryant, orogenic systems in central Europe (Krebs dreas and the Walker Lane—Texas Linea- 1964; Webb, 1967), the Caledonides of and Wachendorf, 1973). Lacking details for ment (Fig. 5). Although there are as yet few Great Britain (Kennedy, 1946; Storetvedt, the southern boundary of the Penokean data to demonstrate substantial transcur- 1974) and Svalbard (Harland, 1972), and belt, one may also speculate that the system rent movement along the Colorado fault the Ural Mountains of Russia (Safanov, was marginal to the then continental plate belt, the resemblance to these later systems 1937). The Colorado Lineament appears to but in places extended inland and over- is otherwise striking. be the first such Precambrian fault system to printed much older basement rocks. This The Colorado Lineament then joins a be recognized. A careful search of Pre- pattern is noted in later mobile belts, includ- growing list of longitudinal wrench fault cambrian terranes probably will reveal ing parts of the Appalachian and systems that characterize the marginal others. Cordilleran systems. zones of continental plates. Benioff (1962) The mechanics of longitudinal wrench Although the paleotectonic model that noted that about 75 percent of all tectonic faulting in relation to modern plate- emerges from these considerations is earthquakes are generated on such faults. tectonics models is in need of clarification. clouded with uncertainty, the sum of the Best known are the Mesozoic-Cenozoic Early models (Hess, 1962; Dietz, 1966; evidence is taken to indicate that the Col- fault systems that form a girdle around the Vine, 1966) emphasized mobility of the orado Lineament developed as a major fault Pacific Ocean (Allen, 1965; Burk and ocean basins and pictured the continents as system along the trend of a cordilleran Moores, 1968). These include the Atacama playing a purely passive role in crustal de- mobile belt formed as a result of Penokean (St. Amend and Allen, 1960), the San An- formation. Although the concept of conti- and related orogeny in the interval 2,000 to dreas (Dickinson and Grantz, 1968), nental drift requires that the continental 1,700 m.y. ago. For this report, it is as- Chatham Strait, Denali, Fairweather, Tin- plates move, the notion has persisted that

such movements are secondary and related Zell Peterman. I am grateful for their help- p. 251-271. to spreading processes in the oceanic plates. ful suggestions for its improvement and as- Case, J. E., and Joesting, H. R., 1961, Precam- brian structures in the Blanding basin and Morgan (1968) called attention to the im- sume full responsibility for any remaining Monument upwarp, southeastern Utah: portance of strike-slip faulting in permitting errors of omission or inclusion and for the U.S. Geol. Survey Prof. Paper 424D, movement between rigid plates on a sphere. conclusions reached. p. 287-291. However, the tendency has been to classify Case, J. E., Joesting, H. R., and Byerly, P. E., 1963, Regional geophysical investigations fault systems at plate boundaries as trans- REFERENCES CITED in the La Sal Mountains area, Utah and form faults (Wilson, 1965), which are ge- Colorado: U.S. Geol. Survey Prof. Paper, netically related to oceanic plate dynamics. Abbott, J. 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