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Tectonic Events at the Intersection Between the Hope Fault and the Purcell Trench, Northern Idaho

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Tectonic Events at the Intersection Between the Hope Fault and the Purcell Trench, Northern Idaho Tectonic Events at the Intersection Between the Hope Fault and the Purcell Trench, Northern Idaho GEOLOGICAL SURVEY PROFESSIONAL PAPER 719 Tectonic Events at the Intersection Between the Hope Fault and the Purcell Trench, Northern Idaho By JACK E. HARRISON, M. DEAN KLEINKOPF, and JOHN D. OBRADOVICH GEOLOGICAL SURVEY PROFESSIONAL PAPER 719 The right-lateral Hope Fault was active intermittently from Precambrian to Eocene whereas the Purcell Trench is a Late Cretaceous graben UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 70-189044 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - (paper cover) Stock Number 2401-2085 CONTENTS Page Page Abstract ________________________ 1 General geology Continued Introduction _____________________. 1 Structure Continued General geology _________________ 3 Folds _________________. 8 Faults _________________- 10 Rock types ___________________ 3 Isotopic age data _____________ 11 Belt Supergroup __________. 3 Rocks in and near the Purcell Trench 11 Purcell sills ______________. 4 Regional correlations ____________. 12 Lakeview Limestone ____________ 5 Geophysical data ________________. 13 Migmatite ________________ 5 Physical properties _ _ . 14 Quartz monzonite _________ 5 Gravity interpretations ______ ____ 15 Granodiorite _______________ 5 Gravity anomalies ________. 18 Diabase-diorite _____ ____________ 6 Magnetic interpretations __________ 18 Sandpoint Conglomerate ___________ 6 Magnetic anomalies _________ 19 Granodiorite and dacite porphyries 6 Sequence of tectonic events _______ _. 20 Surficial deposits __________ Regional implications _____________- 21 Structure ________________. References cited _________________. 23 ILLUSTRATIONS Page PLATE 1. Maps showing the surface and bedrock geology, Bouguer gravity, and aeromagnetics of the Purcell Trench-Hope fault intersection, northern Idaho _____________________________In pocket FIGURE 1. Index map showing location of study area with reference to the Pur­ cell Trench and the Hope fault _____________________ 2 2. Photographs of the Sandpoint Conglomerate _________ 7 3. Diagrams illustrating possible stresses to form tensional openings north of the Hope fault ______________________ 8 4. Photograph of clay model illustrating top view of folds formed in upper plate of thrust ___________________________ 9 5. Diagram showing density of rock types in and near the Purcell Trench ________________________________..___ 16 6. Diagram showing magnetization of rocks in and near the Purcell Trench _____________________________________ 17 7. Map showing principal geologic and tectonic provinces, eastern Wash­ ington, northern Idaho, and northwestern Montana __.. 21 8. Map showing major faults in northern Idaho and northwestern Montana ____________________________________ 22 TABLES Page TABLE 1. Chemical analyses of granodiorite and granodiorite porphyry._. 8 2. Isotopic data on some rocks in and near the Purcell Trench _ 11 3. Physical properties of rocks in and near the Purcell Trench __ 14 in TECTONIC EVENTS AT THE INTERSECTION BETWEEN THE HOPE FAULT AND THE PURCELL TRENCH, NORTHERN IDAHO By JACK E. HARRISON, M. DEAN KLEINKOPF, and JOHN D. OBRADOVICH ABSTRACT ing which the north block moved east. This movement re­ The intersection between the Purcell Trench, a topographic sulted in dilation in and near the trench where granodiorite depression extending from Coeur d'Alene Lake northward and dacite porphyries later filled faults. Both the Purcell for about 280 miles, and the Hope fault, a major west-north­ graben and the Selkirk Mountains were offset 3-4 miles at west-trending right-lateral fault at least 100 miles long, is that time. in the Idaho panhandle. The zone of intercept in the Purcell Trench is largely hidden by glacial debris. INTRODUCTION Recent geologic mapping of all bedrock exposures in and near the trench has been used with gravity and aeromagnetic Two major structural features, the Hope fault data and potassium-argon age determinations to interpret and the Purcell Trench, intersect in northern Idaho the tectonic events in the area. The Purcell Trench appears (fig. 1). Although the features have been known for to be a structural feature in the United States that extends about 60 years, the exact nature of each and the for a few miles into Canada where it joins a series of valleys without obvious structural control. The structural part of relation between them have not been fully under­ the trench includes a graben that contains the Upper(?) stood, largely because the trench is filled by glacial Cretaceous Sandpoint Conglomerate. The part of the trench debris that covers all but a few outcrops. Additional in the United States is near the east edge of the Kootenay complications have been the uncertainty in the age arc mobile belt, which is a zone of intense deformation and of higher grade metamorphic rocks thought to be magmatism that began at least as early as the late Pre- cambrian. The present trench is a young feature formed confined to the west wall of the trench and the con­ at the time of intrusion of the Kaniksu batholith, about 100 sequent uncertainty in the kind and amount of m.y. (million years) ago. trench faulting necessary to bring these rocks adja­ The first break along the Hope fault probobly formed in cent to low-grade metamorphic rocks on the east Belt time. The Hope fault now has an apparent right-lateral wall. offset of 16 miles, of which 3-4 miles are early Tertiary. Subsidiary faults fan out to the northwest from the main In 1903 Daly (1912) originally identified the topo­ Hope fault at its junction with the Purcell Trench. This graphic trench during his survey along the 49th fault system offsets the Purcell Trench and graben, as well parallel. He (1906, p. 597) named it the Purcell as the Selkirk Mountains that form the west wall of the Trench. By "trench" Daly (1906, p. 596) meant "a trench. Some faults in this system contain granodiorite and long, narrow, intermontane depression occupied by dacite porphyries that are about 50 m.y. old. Major intrusive rocks in the area are quartz monzonite and two or more streams (whether expanded into lakes granodiorite, both of the Kaniksu batholith. These rocks are or not) alternately draining the depression in oppo­ about 100 m.y. old. site directions." He described the Purcell Trench as The sequence of tectonic and intrusive events in the area extending from Bonners Ferry northward about 200 in the Precambrian was open folding on north-trending axes, miles into Canada, through the valleys of Kootenay faulting on the Hope fault, intrusion of Purcell sills at least 870 m.y. ago, and block faulting. In the Late Cretaceous Lake and the Duncan River to the mouth of the the events were intrusion of quartz monzonite of the Kaniksu Beaver. He (1912, p. 600) also noted that "the con­ batholith about 100 m.y. ago accompanied by thrusting from structional profiles [of the Rocky Mountain and west to east; migmatization and metamorphism of Prichard Purcell Trenches] may have been those of grabens Formation and Purcell sills to form schists, gneisses, and * * *." Daly's geologic map (1912, sheet 6) shows amphibolite; and cross folding on tight upright to over­ turned major east-trending and minor north-trending axes. pre-Belt rocks faulted against Belt rocks at the Still later, perhaps 95 m.y. ago, granodiorite of the Kaniksu Purcell Trench. batholith was emplaced, the trench faults were formed, and Calkins (1909) justified extension of the Purcell the Sandpoint Conglomerate was dumped into the Purcell Trench south from Bonners Ferry about 80 miles "at graben that was forming east of the rising Selkirk Moun­ tains. Tectonism accompanying the final pulse of intrusion least to the southern end of Lake Coeur d'Alene." He culminated in right-lateral movement on the Hope fault, dur­ also identified and named the Hope fault, which he HOPE FAULT-PURCELL TRENCH INTERSECTION, IDAHO 116° 115° CANADA__ UNITED I STATES --Area of this figure 50 100 MILES I FIGURE 1. Location of study area with reference to the Purcell Trench and the Hope fault. recognized as a major fault occupying the Clark boundary. Fyles (1964, 1967) studied part of the Fork valley for about 40 miles of its length southeast Kootenay Lake and Duncan Lake areas in detail. from Hope, Idaho, and reported that some evidence He mapped several faults parallel to and near the of faulting was found in the Purcell Trench. His valleys that contain the lakes, but he showed no geologic map shows metamorphic rocks of ques­ master structure in the valleys. He (1964, p. 9, 10) tioned older Precambrian age which are separated used the term "Purcell Trench" in only a topo­ from Belt rocks by Coeur d'Alene Lake at the south graphic sense. end of the Purcell Trench. Kirkham and Ellis (1926, pi. 3, p. 32, 33) identi­ Schofield (1915, p. 168) argued that the trench fied what they called the Kootenai fault in the owes its present form to modification of a preexist­ Purcell Trench. On their geologic cross section the ing valley; Rice (1941, p. 4, map 603A) concurred fault is shown as high angle; the text suggests and showed no fault in the north-northwest-trend­ downthrow to the east but also points out that the ing valley occupied by Kootenay Lake, although he fault may be an east-dipping thrust. They believed did show several north- to northeast-trending faults that rocks on both sides of the trench belong to the in and near the Purcell Trench at the international Prichard Formation, in contrast to the hypothesis INTRODUCTION of Daly who believed that the schistose rocks on the The narrowest part of the topographic trench is at west were pre-Belt. Elmira, Idaho, where the walls are only 11/2 miles Anderson (1930, pi. 14) made the first geologic apart; this affords the nearest approach to continu­ map that showed the intersection between the Hope ous outcrop anywhere in the United States part of fault and the Purcell Trench.
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