SW U. S. diabase province: A 1. 1-Ga intrusion event of middle Grenville and middle Keweenawan age
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Diabase in the southwestern US intrudes Middle Proterozoic stratified rocks as sills and Early and Middle Proterozoic crystalline rocks as subhorizontal sheets and subvertical dikes. It is discontinuous in a broad belt extending from western Texas to southeastern California. The best known intrusions are sills in Middle Proterozoic strata in Death Valley, Grand Canyon, and central Arizona. Sparse to rare dikes in some of these strata trend mostly north but range from north-northeast to west-northwest. Diabase dikes widespread in crystalline rocks in western Arizona and adjacent parts of southeastern California strike from north to west-northwest, but are predominantly northwesterly. Dikes and sheets are also present in crystalline rocks in the southern Pinaleno Mountains, southeastern Arizona, where dikes strike west-northwest. The northwest trend of the diabase province and prevalent northwesterly trend of dikes in crystalline rocks suggest that intrusion was controlled by an approximately horizontal least compressive stress field roughly parallel to the Grenville Front. Radiometric ages of Arizona and California diabase indicate emplacement at [approximately]1,100 Ma. Paleomagnetic poles from diabase sills and enclosing stratified rocks in Arizona correlate with poles reported from middle and early-late Keweenawan rocks of Lake Superior. Emplacement of the diabase coincides with: (1) the middle Keweenawanmore » eruptive and intrusive episode of the Midcontinent Rift System; (2) a major episode of (middle) Grenville thrusting and deformation documented in the Van Horn area; and (3) a time of abrupt reversal in North American apparent polar wander. These interrelated manifestations presumably arose in response to a major episode of plate interaction and collision between North American and a plate that encroached from the southeast.« lessKeywords:
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A structurally simple, 35‐km‐thick, north facing stratigraphic succession of Late Archean to Middle Proterozoic rocks is exposed near the Montreal River, which forms the border between northern Wisconsin and Michigan. This structure, the Montreal River monocline, is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the Keweenawan Supergroup (Middle Proterozoic) along the south limb of the Midcontinent rift, and disconformably underlying sedimentary rocks of the Marquette Range Supergroup (Early Proterozoic). These rocks lie on an Archean granite‐greenstone complex, about 10 km of which is included in the monocline. This remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition. Tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension. The faults responsible for the monocline are newly recognized components of a well‐known regional fault system that partly closed and inverted the Midcontinent rift system. Resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 +/−20 Ma and followed very shortly after extension that formed the Midcontinent rift system.
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Only fragmentary direct information is available on the basement complex of the Southern Peninsula of Michigan because of limited and poorly distributed basement drill holes. This has encouraged the use of geophysical methods, primarily gravity and magnetic, to study the Precambrian formations. A basement configuration map prepared from magnetic depth estimates and basement drill tests confirms that the basement surface under the Southern Peninsula has the form of an oval depression reaching a maximum depth of more than 15,000 ft (4.5 km) below sea level on the western shore of Saginaw Bay. A basement topographic high is associated with the Howell anticline and a roughly north-south-striking basement trough plunges into the basin from the common boundary point of Indiana, Ohio, and Michigan. Aeromagnetic and Bouguer gravity anomaly maps, together with isotopic dates of samples obtained from basement drill holes and extrapolation of known Precambrian geologic trends, indicate that four basement provinces underlie the Southern Peninsula. The Penokean province can be traced geophysically from Lake Michigan into the Southern Peninsula, where it is characterized by east-southeast-striking geophysical anomalies. Central and southwestern Michigan is underlain primarily by felsic rocks correlating with the Central province. Basement rocks in southeastern Michigan strike north-northeast and are interpreted to be metamorphosed intrusive and extrusive rocks and mafic and felsic gneisses of the Grenville province. The Grenville front strikes south-southwest from Sagi aw Bay to west of the Howell anticline and from there due south to the Michigan Ohio boundary. A Keweenawan rift zone characterized by mafic intrusive and extrusive rocks and by uplifted gneisses transects all of the provinces and extends from Grand Traverse Bay to southeastern Michigan. Another subparallel ancient rift zone may be present in southwestern Michigan. These zones were formed during an episode of crustal extension in Keweenawan time. Subsequent deformation of the sedimentary rocks within the basin generally has been associated with movement along lines of basement weakness apparently related to the rift zone and Penokean structural trends.
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The name Falls is given to a northeast-trending zone of diverse geologic features that can be traced northeastward from the Idaho batholith in the Cordilleran miogeocline of the United States, across thrust belt structures and basement rocks of west-central and southwestern Montana, through the cratonic rocks of central Montana, and into southwesternmost Saskatchewan, Canada. The zone is well represented in east-central Idaho and west-central Montana where geologic mapping has outlined northeast-trending, high-angle faults and shear zones that: (1) extend more than 150 km (93 mi) from near Salmon, Idaho, northeastward toward Anaconda, Montana; (2) define a nearly continuous zone of faulting that shows recurrent movement from middle Proterozoic to Holocen time; (3) controlled the intrusion and orientation of some Late Cretaceous to early Tertiary batholithic rocks and early Tertiary dike swarms; and (4) controlled the uplift and orientation of the Anaconda-Pintlar Range. Recurrent movement along these faults and their strong structural control over igneous intrusions in this region suggest that northeast-trending faults represent a fundamental tectonic feature of the region. Figure Geologic features that are similar to those mapped in the Salmon-Anaconda region are present to the southwest and the northeast. In central Idaho, these structures include numerous northeast-trending faults and pronounced topographic lineaments that cut across the southern part of the Idaho batholith, and a northeast alignment of Tertiary igneous rocks that cut the Idaho batholith and adjacent rocks. East and southeast of the Anaconda-Pintlar Range, subparallel, high-angle faults and topographic lineaments are present in the Highland, Pioneer, Ruby, and Tobacco Root Mountains. High-angle faults may have in part controlled the orientation of the northeast-elongate Boulder batholith. Northeast-trending structures are not easily traced across the thrust belt of western Montana or across he Lewis and Clark line. In the central Montana plains, northeast of the disturbed belt, however, a broad zone of colinear, northeast-trending structures is present, and includes: parallel, buried basement highs that in part controlled depositional patterns of some Paleozoic and Mesozoic sedimentary rocks; major physiographic features, such as the remarkably straight, 175-km (109-mi) long segment of the Missouri River, and equally long, buried river channels in southwestern Saskatchewan; a northeasterly alignment of highly differentiated igneous rocks and a belt of ultrabasic intrusions and related diatremes; End_Page 1350------------------------------ and a well-defined pattern of northeast-trending gravity and aeromagnetic anomalies underlying this part of central Montana and southwesternmost Saskatchewan. Taken together, all these geologic features define a broad, northeast-trending zone at least 150 to 200 km (93 to 125 mi) wide and more than 1,000 km (620 mi) long. The zone is approximately colinear but not demonstrably continuous with the well-exposed boundary in eastern Saskatchewan and Manitoba between the Archean Superior and the Proterozoic Churchill provinces of the Canadian Shield. This boundary is also characterized by: high-angle faults, shear zones, and topographic lineaments; pronounced linear gravity and magnetic anomalies; igneous intrusions; and fault controlled depositional patterns and mineralization. That the Great Falls lineament is controlled by a similar Precambrian boundary between the Archean Wyoming province of southwestern Montana and early Proterozoic terrane to the north is speculative; however, the geologic features found along the Great Falls lineament share many common characteristics with features present along the Archean-Proterozoic boundary in Canada. End_of_Article - Last_Page 1351------------
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Abstract A chronology of Late Cenozoic deformation and related igneous events in the Gold Hill area of the northern Deep Creek Mountains, western Utah was determined by field relationships. The emplacement of a Late Eocene/Early Oligocene quartz monzonite intrusion was followed by as many as three periods of normal faulting which produced faults with east-northeast, northeast, and northwest trends. Leuco-granite dikes commonly occur along the traces of these faults. A previously unrecognized low-angle normal fault was identified in this study. The subhorizontal feature truncates the above mentioned faults and probably moved during the Late Oligocene. The low-angle fault is displaced by normal faults with northeast and north trends. Leuco-granite, andesite, and diabase dikes are often associated with these faults. Olivine basalts occur along faults with east and north-northeast trends. These basalts are interpreted to be approximately 14–12 m.y. old as are the faults along which they were extruded. Late Miocene extension was probably in an east-west direction as indicated by the emplacement of north-trending quartz-adularia dikes at 8 m.y. A high-angle normal fault trends northwest and cuts features of all previously discussed ages and orientations.
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Volcanic and sedimentary rocks and structures record the Tertiary structural evolution of the lower Colorado River region in southwestern Arizona and southeastern California. A late Eocene or early Oligocene (prior to ∼33 Ma) episode of faulting is indicated by medium‐ to coarse‐grained arkosic rocks in the Chocolate and southern Trigo Mountains. Much of the area was relatively quiescent tectonically during extrusion of volcanic rocks from ∼33 to 22 Ma, but the southernmost part was periodically uplifted and eroded into its underlying crystalline rocks. A major episode of extensional deformation and tilting occurred after deposition of welded tuff about 22 Ma and affected the entire area from the Palo Verde Mountains on the west to the Kofa Mountains on the east. Extension‐related faulting quickly waned and had largely ceased by about 20 Ma in the Kofa Mountains; elsewhere the timing is poorly constrained. By ∼13 Ma, thick alluvial fans filled many grabens and half grabens among tilted fault blocks throughout the area. Volcanism in the lower Colorado River region may have been coincident with a broad structural depression now oriented east‐west. The northern limit of the volcanic terrane defines a tilt‐domain boundary. The northern boundary, reaching from the New Water Mountains in Arizona to the little Chuckwalla Mountains in California, ultimately evolved to separate a terrane of relatively untilted crystalline horsts on the north from a series of east or northeast dipping fault blocks on the south. The southern margin is less well defined but is subparallel to the northern boundary and to the Chocolate Mountains anticlinorium.
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Middle-late Proterozoic tectonics of west Texas and eastern New Mexico: A geophysical interpretation
The Midcontinent rift system forms one of the most prominent gravity features in North America. The geophysical anomaly extends in an arc from Oklahoma to Lake Superior and then into Kentucky. The Midcontinent Rift System was active between 1.19--1.01 Ga as indicated in the Lake Superior region by age dating of intrusive igneous rocks. The authors suggest that the period of formation of the Midcontinent rift was a time of extensive igneous activity in Texas and New Mexico which is represented by intrusions beneath the Central Basin Platform (TX-NM), Pajarito Mountain in the Sacramento Mountains (NM), the Mundy Breccia in the Franklin Mountains (TX), lava flows in the Allamore Formation near Van Horn (TX), and the Crosbyton geophysical anomaly (east of Lubbock, TX). These bodies were intruded between 1.07--1.22 Ga. These bodies and other bodies located by geophysical anomalies and wells drilled into mafic Precambrian rocks may be related to the Midcontinent rift system by a rift jump or splay. Alteratively this magmatism could be related to Grenville age tectonics in Texas. The Central Basin Platform, is divided in two on gravity maps by the Abilene minimum, an ENE trending 900 km long 100 km wide gravity low. This gravitymore » low is not associated with basement topography and must be an intra-basement feature. One possible source of this body is a continental arc batholith. This batholith must have been intruded near a suture zone prior to 1166 Ma the age of the Central Basin Platform intrusion. A possible age for this intrusive would be 1,350--1,400 Ma. This age would coincide with formation of the Chaves Granite Gneiss terrain and the Red River-Tillman metamorphic belt, a possible foreland basin. This date would also coincide with regional metamorphism in north-central New Mexico.« less
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The western metamorphic belt of the Sierra Nevada consists of two eugeosynclinal terranes separated by the Melones and Sonora faults. Subvertical, bedded Mesozoic volcanic rocks metamorphosed to low greenschist facies predominate to the west, whereas Paleozoic metamorphic rocks of higher grade and greater structural complexity predominate to the east. In order to study the structural development of the faults, 121 samples of basalt and diabase were collected for paleomagnetic analysis from three Jurassic formations, the Logtown Ridge and Penon Blanco formations west of the Melones fault and the Sonora dike swarm to the east of the Sonora fault. A northwesterly, downward directed magnetization occurs in each unit. Three fold tests and a conglomerate test on the two formations west of the faults show that the magnetization is secondary, postdating Nevadan (Late Jurassic) folding and is probably coeval with peak metamorphism. An average of five paleomagnetic poles from the Sierra Nevada, three derived from the secondary magnetizations given herein and two previously published, all of probable Kimmeridgian age, yields λ′ = 67.2°N, ϕ′ = 161.2°E, and α 95 = 6.5°. Southeasterly magnetizations also occur in the Logtown Ridge Formation and Sonora dike swarm. Directions from the Sonora dikes are approximately antipodal to the secondary directions and are reversed; magnetizations from the Logtown Ridge Formation yield similar results only if corrected for the tilt of bedding. The Logtown Ridge magnetizations (tilt‐corrected) yield a pole position near to that expected for North America. The data from the Sonora dikes require a tilt correction of 25°–30° toward the south‐southwest about a horizontal axis parallel to the regional structure in order to yield a North American pole position. We conclude that the eastern wall rocks of the Melones and Sonora faults have been rotated 25°–30° in response to Nevadan deformation in contrast to the western wall rocks, which have been rotated about 90°.
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Apparent polar wander
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The direction of remanent magnetization for 102 sites in Eocene volcanic and volcaniclastic rocks of the O'Brien Creek Formation, Sanpoil Volcanics, and Klondike Mountain Formation suggests approximately 25° of clockwise rotation of a 100 by 200 km area in northeastern Washington. The volcanic rocks consist chiefly of rhyodacite and quartz latite flows, with intercalated ash flow tuff and volcaniclastic layers. These rocks have been sampled at 102 sites distributed among five volcanotectonic depressions: the Toroda Creek, Republic, Keller, and First Thought grabens and the Spokane‐Enterprise lineament. The volcanic rocks probably range in age from 55 m.y. to about 48 m.y., and the 50‐ to 48‐m.y.‐old volcanic rocks within this suite appear to be rotated as much as the older rocks. Previous investigators have shown that 40‐m.y.‐old and younger plutonic rocks of northwestern Washington are not rotated; hence we infer that the north‐central Washington rocks were rotated to their present declination between 48 and 40 m.y. B.P. (during the middle and/or late Eocene). During early Eocene time this region was extended in a westward direction through crustal necking, gneiss‐doming, diking, and graben formation. Internal deformation of the region related to this crustal extension was extreme, but most bedrock units that were formed concurrent with the crustal extension were probably in place prior to the rotation; hence we infer that the rotation was chiefly accommodated by movement on faults peripheral to the sampled area. Faults active during Paleogene time appear to define boundaries of a triangular crustal block (the Sanpoil block), encompassing much of northeastern Washington, northern Idaho, northwestern Montana, and adjacent parts of British Columbia. The faults include the Laramide thrusts of the Rocky Mountain thrust belt, the strike‐slip faults of the Lewis and Clark line, and strike‐slip faults of the Straight Creek‐Fraser zone. We suggest that during early Eocene time the Sanpoil block was extended westward through crustal necking and dilation and then during the middle Eocene was rotated clockwise and thrust over the craton in a final stage of Laramide thrusting. The “motor” driving these deformations presumably was interaction of North America with oceanic lithosphere off its western margin; such interaction probably involved right‐oblique underthrusting and dextral shear.
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Tectonic interpretations of new heat flow data from Louisiana, Mississippi, Alabama, and panhandle Florida, and radiometric dates for subsurface igneous rocks from Mississippi suggest a complex basement configuration of both continental and oceanic crust for the Gulf Coast region. Original (early Paleozoic) North American continental terrain, characterized by average heat flow (~ 1.0 heat flow units), is postulated to extend no further south than a northwest-trending boundary linking the truncation of the southern Appalachian Mountains with the Ouachita Mountains. A northwest-trending zone of thinned continental basement and thick sedimentary deposits with anomalously low heat flow and Mesozoic volcanic rocks exists in southern Alabama and pinches out in northern Mississi pi (Black Warrior basin and central Mississippi deformed belt). South of that belt, a zone of high heat flow (1.5 to 2.1 hfu), subsurface Mesozoic basalts, and feldspathoidal rocks characterizes northern Louisiana and west-central Mississippi. This zone is distinguished by mobilized, thermally conductive salt diapirs and alkalic igneous rocks. The Wiggins uplift with Paleozoic granitic rocks in southernmost Mississippi and its westerly continuation in southern Louisiana yield average heat flow values and are interpreted to represent residual fragments of a South American continental mass that was convergent in the late Paleozoic Era and divergent during the early to middle Mesozoic Era. End_of_Article - Last_Page 1680------------
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The Gogebic iron range extends from northwestern Wisconsin into northern Michigan. The eastern end of the Gogebic range lies roughly forty miles east of Ironwood, Michigan. Recent geologic mapping in this area indicates that Archean and Early Proterozoic rocks underwent multiple periods of deformation. This study focuses primarily on the initial, D1, period of deformation as observed in the western part of the map area. Here, structural data indicate that bedding was folded about fold axes that plunge gently to the northeast. Well developed axial planer foliation strikes east-northeast and dips southeast. None of these features occur in underlying Archean greenstones and gneissic rocks. Rather, gneissic foliation strikes consistently northwest and dips southwest, thus indicating that a decollement fault must separate structures in Early Proterozoic rocks from underlying Archean basement. This deformation occurred during the Penokean orogeny. It was caused by collision of the Wisconsin Magmatic Terranes with a continental foreland to the north along the Niagara suture.
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