Table of Contents
Three tectonic provinces within the Cordilleran System, are analyzed in terms of tectonic framework. (1) Precambrian terrain of North-Central Colorado; (2) The Pacific Northwest; and (3) The San Andreas-Garlock fault system and associated features. In each case a close relationship is seen to exist between: (1) Arcuate mountain development; (2) Precambrian basement fabric; (3) Major fault patterns; and Major mineral occurrences. The arcuate trends developed when the basement was still mobile. Later competent deformation rejuvenated pre-existing lines of weakness.
Tectonic Analysis of the Precambrian Terrain of North-Central Colorado
The Precambrian terrain of North-Central Colorado is dominated by several north-eastward directed convex mountain arcs. The present discussion will concentrate on the arc whose axial trace trends northeastward from the northern Sawatch range through the Gore and Williams ranges. This axial trend continues on through the batholith south of Granby reservoir, and meets the Great Plains near Fort Collins, Colorado. The arcuate nature of this trend is outlined by the courses of the Eagle, Arkansas, Blue, Fraser, Arapahoe, Buckhorn and Cache la Poudre. (South Fork) rivers.
The following discussion will indicate that there is an intimate relationship between the major fault patterns of North Central Colorado and the overall arcuate framework.
The smooth arcuate pattern of foliation trends is disturbed mainly in two types of areas:
- In the strongly cross folded apex portions of the individual mountain arcs.
- At Junctions with the adjoining Medicine Bow and Laramie range arcs (along the South Fork of Cache La Poudre River); and at the junction with the northern arcuate portion of the “Pike’s Peak Range” (along Duck Creek, Chicago Creek and Ralston Creek).
The strongest fault trends of Worth-Central Colorado are closely associated with either the arcuate foliation pattern or with the disturbances in this pattern. These fault trends fall into three main groups:
Group (1) Northwest Trending Faults, Including the “Breccia Reefs”: These, faults are essentially parallel to the northwest-trending portions of the individual mountain arcs. Many of these faults tend to swing around to a west-northwest-trend in sympathy with either major or subsidiary bends In the foliation pattern.
Group (2) North-Northeast-Trending Faults such as the Mosquito, Berthoud Pass, Apex, and Dory Hill Faults: These faults are also parallel to the Precambrian foliation trends and are believed to have developed initially as thrust or high angle reverse faults.
Group (3) East-Northeast-Fault Trends: This group includes faults trending from northeast to east-northeast. The east-northeast direction is more pronounced.
Tectonic Analysis of the Pacific Northwest
The Pacific northwest, like the Colorado Rockies, is dominated by an arcuate system of mountain chains and associated lineaments. The major mineral deposits of the region are closely associated with this arc-lineament system. Thus an understanding of the evolution of this system is fundamental for exploration of the region.
The Columbia river arcuate embayment is the most striking feature of the entire, area. This arc is strongly convex to the east, and is bounded on the north by the Kootenay arc and bounded on the south by the southwest Wyoming arcuate thrust zone. The north limb of the Columbia river arc is delineated by the Lewis and Clark line (also known as the Montana lineament), a great west-northwest-trending lineament system which reaches from Spokane on the northwest to central Montana on the southeast.
The south limb of the Columbia river arc is less distinct, being marked by a northeast-trending chain of granodiorite and quartz monzonite intrusions. These intrusions, like the Boulder batholith, are younger than the Idaho batholith and appear to have been injected into a relatively competent framework during the Laramide orogeny (Anderson; Larsen and Schmidt).
The position of Butte is unique in the overall structural pattern of the region. It is located on the south flank of the Montana apex and on that portion of the east-southeast-trending Lewis and Clark line where it bends southeastward to join the east-southeast-trending Absaroka-Nye- Bowler lineament systment. The change in strike of the Lewis and Clark line near Butte may have an important bearing on the localization of this mining camp. The “apex concept” is of more proven merit in regional exploration, however.
To summarize, the Archean trends south of the Lewis and Clark line appear to be dominated by north-northeast foliation trends, while the Archean trends north of the Lewis and Clark line (Little Belt Mountains) appear to be dominated by east-southeast (ESE – WNW) trends. The Columbia river arcuate embayment was superimposed upon this pre-existing Archean framework and the development of this arc is simply a history of “Consequent” tectonics.
Tectonic Analysis of the San Andreas-Garlock Fault System
The San Andreas-Garlock fault system is still the outstanding structural puzzle of California as witnessed by the great diversity of opinion which continues to exist concerning the stress of orientation responsible for the faulting. Possibly the greatest difficulty arises from the fact that most authors have attempted to explain the fault pattern in terms of a simple conjugate shear system. It is more probable that the San Andreas and Garlock faults did not originate as conjugate shears (Benioff, Hewett). The writer feels that the puzzle becomes simplified if the problem is examined in terms of arc-lineament tectonics and basement fabric.
Many of the previous interpretations were limited-to southern California, and have been handicapped by their geographical limitation. Menard and St-Amand have discussed the relationship of the San Andreas fault system to other major structural features of the North Pacific ocean but did not bring many of the continental structures into their interpretations.
The following regional structural features should be accounted for in any interpretation of the San-Andreas-Garlock fault systems
- The Arcuate Nature of the Mountain Systems along the Western Edge of North America: These features can be explained only in terms of a dominant ENE-WSW stress system.
- The East Northeast-Trending Major Fractures of the Northern Pacific Ocean Floor, and the Relative Elevation and Geomorphology of the Ocean Floor between these Fractures: The Murray fracture system appears to be the oceanic continuation of the Garlock-Big Pine-Santa Ynez-Malibu-Pinto Mountain fault system.
- Dominant Structural Trends in the Precambrian Basement East of the Sierra Nevada and Southern California Batholiths: The strongest Precambrian trends are east-northeast as evidenced by the predominant east-northeast Precambrian foliation, folding and thrust directions (Anderson, 1951). Foliation trends in the Precambrian Pelona schist, which is confined to the vicinity of the Garlock fault, are generally parallel to that fault (Wiese). Subsidiary weakness directions are associated with north-northwest Precambrian faulting and north-northeast normal faulting. The north-northwest Precambrian faults are of a tear origin according to E.D. Wilson.
- Structural Features Associated with the Emplacement of the Sierra Nevada and Southern California Bathollths; The work of Mayo (1937) indicates that the regional north-northwest trend of folding in the pre-Nevadan geosyncline influenced the emplacement of the Sierra Nevada batholith, while east-northeast lines of weakness exerted a subsidiary control. Mayo (1937) suggests further that a north-south fault pattern evolved within the batholith as a result of right lateral shifting along the north-northwest lines of weakness.
- The Pattern of Cretaceous and Tertiary Sedimentation which Developed with the Evolving Tectonic Framework: Maximum isopach trends are associated with east-northeast faulting in the Transverse ranges and with north-northwest faulting in the Los Angeles, Cuyama and San Joaquin basins (Reed and Holllster; Corey, Barbat, Schwade et al, Slmonson,).
- The Relative Amount of Offsetting along the Various Elements of the Strike-slip Fault System of California: Hill and Dibblee suggest that the east wall of the San Andreas moved south while the west wall remained stable.
This past year makes us realize the extent to which price and economic cut-off may stimulate or curtail exploration. But exploration does not consist solely in the discovery of sources of material.
If you would build you must have bricks or stones, mortar, wood. If you would explore the universe, as man is doing, you must have a working knowledge of physics, mathematics, some chemistry, geology and astronomy. And you must understand them and be able to work out with them they are your tools. And here sadly an impressionistic attempt frequently (lacking form, color and design does not pass muster. Nature does not generally unlock her secrets to approximations, and guesses. Effort, care and persistence are necessary, discouragement and failure are frequent.
The first question today might be, do future demands warrant exploration? Over the long term there can be no doubt, based on the increase of world, population and the increase in consumption per person. This requires, however, active effort to broaden base metal usages as compensation to the inroads of other materials.
As to future developments. All sciences, even exploration, move from general to detailed knowledge from empiricism to formulation, from few to manifold concepts. Thus geologically we shall increasingly acquire detailed knowledge of the physical and chemical forces present and reacting in this exquisitely balanced, cyclic earth of ours.
We shall develop additional rapid and automatic airborne means for sampling the existing physical forces and chemical traces near the earth’s surface, and also largely automatic, i.e, computer means, for systematically evaluating and correlating these measurements to conditions within the crust. Improved measurements and corrective processes will carry our mineral search deeper into the earth’s crust where today broad seismic and electrical measurements reach to the Moho phase discontinuity and beyond.