Circum-North Pacific Map: Explanatory Notes
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EXPLANATORY NOTES

SUMMARY TERRANE MAP

This summary tectonostratigraphic map depicts major on-land and offshore geologic/tectonic units in the Circum-North Pacific region, that comprises the Russian Far East, Alaska, the Canadian Cordillera, and adjacent offshore regions. The summary terrane map is published separately in paper format at a scale of 1:10,000,000 in U.S. Geological Survey Open-File Report 96-727 and Geological Survey Open-File 3428 (Nokleberg and others, 1997c). The summary terrane map is also used as a geologic base for a suite of two mineral deposit maps published at a scale of 1:1,5,000,000 and five metallogenic belt maps published at a scale of 1: 10,000,000 in in U.S. Geological Survey Open-File Report 97-161 and Geological Survey Open-File 3446 (Nokleberg and others, 1997b). The summary terrane map is a summary of a more detailed presentation of the onshore and offshore geology of the region at a scale of 1:5,000,000 (Nokleberg and others, 1994b). This more detailed map also contains descriptions of terranes, overlap assemblages, and other major geologic units, an extensive bibliography of cited references, and 188 stratigraphic columns of terranes, cratons, and craton margins. This summary map is one of a series of reports and maps on the mineral deposits, metallogenesis, and geology of the Circum-North Pacific. Additional detailed information on mineral deposits and interpretation of metallogenic belts for the region are available in: (1) U.S.G.S. Open-File Report 93-339 on the metallogenesis of mainland Alaska and the Russian Northeast (Nokleberg and others, 1993); and (2) U.S.G.S. Open-File Reports 96-513-A and B (paper and CD formats, respectively) (Nokleberg and others, 1996a, 1997a), the signficant deposits and placer districts of the region.

This summary map and the more detailed map are the result of extensive geologic mapping and associated tectonic studies in the Russian Far East, Hokkaido Island of Japan, Alaska, the Canadian Cordillera, and the U.S.A. Pacific Northwest in the last few decades. Geologic mapping suggests that most of this region can be interpreted as a collage of fault- bounded tectonostratigraphic terranes that were accreted onto continental margins around the Circum-North Pacific mainly during the Mesozoic and Cenozoic (Parfenov, 1984, 1991; Howell and others, 1985; Watson and Fujita, 1985; Parfenov and Natal'in,1985, 1986; Jones and others, 1987; Monger and Berg, 1987; Fujita and Cook, 1990; Zonenshain and others, 1990; Natal'in, 1991, 1993; Moore and others, 1994; Silberling and others, 1992; Nokleberg and others,1994a-c; Plafker and Berg, 1994; Tabor, 1994; Monger and Nokleberg, 1996).

A key definition for the map is tectonostratigraphic terrane which is defined below as a fault-bounded geologic entity or fragment that is characterized by a distinctive geologic history that differs markedly from that of adjacent terranes (Jones and others, 1983; Howell and others, 1985). Most tectonostratigraphic terranes (hereafter referred to as terrane) are fault- bounded, stratigraphically coherent assemblages that formed before accretion, i.e. tectonic juxtaposition, to adjacent units. A few terranes are fault-bounded structural complexes, mainly subduction zone or accretionary-wedge complexes. The terranes are bounded by various types of major faults or fault zones named sutures. Paleontologic, stratigraphic, and paleomagnetic evidence suggests that some terranes were originally widely separated from one another, or from the cratons of either North America or North Asia by distances of as much as thousands of kilometers (Plafker and Berg,1994; Monger and Nokleberg, 1996). Other terranes are interpreted to be displaced from one another or from another position on the same continent by distances of only hundreds of kilometres or less.

On this map the terranes are interpreted according to their inferred tectonic environments. These environments are (1) cratonal; (2) passive continental margin; (3) metamorphosed continental margin; (4) continental-margin arc; (5) island arc; (6) oceanic crust, seamount, and ophiolite; (7) accretionary wedge and subduction zone; (8) turbidite basin; and (9) metamorphic terrane for terranes that are too highly deformed and metamorphosed to determine the original tectonic environment. For terranes with complex geologic histories, the chosen color indicates the tectonic environment most prevalent during this history of the terrane. The tectonic environments inferred for igneous rocks are temporal (pre- and post-accretion) and genetic (subduction-related, rift-related, and collisional related (anatectic)).

In addition to terranes, the map also depicts postaccretion units that include: (1) Cenozoic and Mesozoic overlap assemblages of sedimentary and volcanic rocks that were deposited across two or more terranes; (2) Cenozoic and Mesozoic basinal deposits that occur within a terrane or on the craton; (3) plutonic rocks. For simplicity, post- accretionary sedimentary, volcanic, and plutonic rocks are combined into individual postaccretion units. These overlap assemblages and basinal deposits formed mainly during sedimentation and magmatism that occurred after accretion of terranes to each other or to a continental margin. Overlap assemblages provide minimum ages on the time of accretion of terranes. Some Cenozoic and Mesozoic overlap assemblages and basinal deposits, as well as fragments of terranes, are extensively offset by movement along postaccretion faults.

Significant differences exist between the representation of onshore and offshore geology. These are: (1) the offshore part of the map is depicted in a more schematic fashion than the onshore part because of more limited data and because the offshore terranes and early Cenozoic and older overlap assemblages generally are obscured by extensive late Cenozoic sedimentary cover that is not shown unless thicker than two kilometres; (2) marginal contacts of offshore Cenozoic and Cretaceous sedimentary basins do not match contacts of onshore Cenozoic and Cretaceous sedimentary units because offshore basins are limited to those regions with sediment thicknesses greater than two kilometres; and (3) for simplicity, the major onshore Cenozoic sedimentary basins are generally not defined and described separately because the onshore part of the map is designed to emphasize terranes and overlap volcanic assemblages that are crucial for both for tectonic and metallogenic analyses published elsewhere (Nokleberg and others,1994a-c).

Several key geologic sources were used in the compilation of the map. For Alaska, the basic outcrop pattern for the map is from Beikman (1980), Gehrels and Berg (1992, 1994), Barker and others (1994), Brew (1994), and Moll-Stalcup and others (1994). The distribution of terranes is from Jones and others (1987), Monger and Berg (1987), and Brew (1994), with modifications by Grantz and others (1991), Worall (1991), Nokleberg and others (1994a-c), the cited references, and the Alaskan co-authors of this report. For the Canadian Cordillera, the basic outcrop pattern is from Monger and Berg (1987), Wheeler and others (1988), and Wheeler and McFeeley (1991) with modifications by the Canadian authors. For the northern part of the Russian Far East,the basic outcrop pattern is from Sosunov (1985) with modifications by the Russian authors. For the southern part of the Russian Far East,the basic outcrop pattern is from Krasny (1991) and Bazhanov and Oleinik (1986) with modifications by the Russian authors. For the offshore areas of the central and eastern north Pacific Ocean, the basic pattern is from Atwater (1989) and Atwater and Severinghaus (1989).

TECTONIC DEFINITIONS

The following definitions are modified from Coney and others (1980), Jones and others (1983), Howell and others (1985), Monger and Berg (1987), Wheeler and others (1988), and Wheeler and McFeeley (1991), with modifications by the authors.

Accretion
Tectonic juxtaposition of two or more terranes, or tectonic juxtaposition of terranes to a continental margin.

Accretionary wedge and subduction zone terrane
Fragment of a mildly to intensely deformed complex consisting of varying amounts of turbidite deposits, continental margin rocks, oceanic crust and overlying units, and oceanic mantle. Divided into units composed predominantly of turbidite deposits or predominantly of oceanic rocks. Units are interpreted to have formed during tectonic juxtaposition in a zone of major thrusting of one lithospheric plate beneath another, generally in zones of thrusting along the margin of a continental or an island arc. May include large fault-bounded units with a coherent stratigraphy. Many subduction zone terranes contain fragments of oceanic crust and associated rocks that exhibit a complex structural history, occur in a major thrust zone, and possess blueschist facies metamorphism.

Basinal deposits
An assemblage of sedimentary and lesser amounts of volcaniclastic and volcanic rocks that were deposited on a single terrane after accretion, or onto a craton margin or craton after a major orogenic event. Includes some foreland and successor basin deposits, and forearc, intra-arc, and backarc deposits.

Collage of terranes
Groups of tectonostratigraphic terranes, generally in oceanic areas, for which insufficient data exist to separate units. Craton. Chiefly regionally metamorphosed and deformed shield assemblages of Archean and Early Proterozoic sedimentary, volcanic, and plutonic rocks, and overlying platform successions of Late Proterozoic, Paleozoic, and local Mesozoic and Cenozoic sedimentary and lesser volcanic rocks.

Craton margin
Chiefly Late Proterozoic through Jurassic sedimentary rocks deposited on a continental shelf or slope. Consists mainly of platform successions. Locally has, or may have had an Archean and Early Proterozoic cratonal basement.

Cratonal terrane
Fragment of a craton.

Continental margin arc terrane
Fragment of an igneous belt of coeval plutonic and volcanic rocks and associated sedimentary rocks that formed above a subduction zone dipping beneath a continent.

Foreland basin
Trough or depression filled with clastic deposits that was deposited adjacent to an orogenic belt.

Island arc terrane
Fragment of an igneous belt of plutonic rocks, coeval volcanic rocks, and associated sedimentary rocks that formed above an oceanic subduction zone.

Metamorphic terrane
Fragment of a highly metamorphosed or deformed assemblage of sedimentary, volcanic, or plutonic rocks that cannot be assigned to a single tectonic environment because the original stratigraphy and structure are obscured. Includes intensely-deformed structural melanges that contain intensely-deformed fragments of two or more terranes.

Metamorphosed continental margin terrane
Fragment of a passive continental margin, in places moderately to highly metamorphosed and deformed, that cannot be linked with certainty to the nearby craton margin. May be derived either from a nearby craton margin or from a distant site.

Oceanic crust, seamount, and ophiolite terrane
Fragment of part or all of a suite of deep-marine sedimentary rocks, pillow basalt, gabbro, and ultramafic rocks that are interpreted as oceanic sedimentary and volcanic rocks and the upper mantle. Includes both inferred offshore oceanic and marginal ocean basin rocks, minor volcaniclastic rocks of magmatic arc derivation, and major marine volcanic accumulations formed at a hotspot, fracture zone, or spreading axis.

Overlap assemblage
A post-accretion unit of sedimentary or igneous rocks deposited on, or intruded into, two or more adjacent terranes. The sedimentary and volcanic parts either depositionally overlie, or are interpreted to have originally depositionally overlain, two or more adjacent terranes, or terranes and the craton margin. Overlapping plutonic rocks, which may be coeval and genetically related to overlap volcanic rocks, link or stitch together adjacent terranes, or a terrane and a craton margin.

Passive continental margin terrane
Fragment of a craton margin. Post-accretion rock unit. Suite of sedimentary, volcanic, or plutonic rocks that formed in the late history of a terrane, after accretion. May occur also on adjacent terranes or on the craton margin either as an overlap assemblage or as a basinal deposit. A relative-time term denoting rocks formed after tectonic juxtaposition of one terrane to an adjacent terrane.

Pre-accretion rock unit
Suite of sedimentary, volcanic, or plutonic rocks that formed in the early history of a terrane, before accretion. Constitutes the stratigraphy and igneous geology inherent to a terrane. A relative-time term denoting rocks formed before tectonic juxtaposition of one terrane to an adjacent terrane.

Seamount and oceanic plateau
Major marine volcanic accumulations generally formed at a hotspot, fracture zone, or spreading axis.

Subterrane
Fault-bounded unit within a terrane that exhibits a similar but not identical geologic history relative to another fault bounded unit in the same terrane.

Superterrane
An aggregate of terranes that is interpreted to share either a similar stratigraphic kindred or affinity, or a common geologic history after accretion. A synonym is composite terrane.

Terrane
A fault-bounded geologic entity or fragment that is characterized by a distinctive geologic history that differs markedly from that of adjacent terranes. Constitutes a physical entity, i.e. a stratigraphic succession bounded by faults, inferred faults, or an intensely-deformed structural complex bounded by faults. Some terranes may be displaced facies of other terranes.

Turbidite basin terrane
Fragment of a basin filled with deep-marine clastic deposits in either an orogenic forearc or backarc setting. May include continental-slope and continental-rise turbidite deposits, and submarine-fan turbidite deposits deposited on oceanic crust. May include minor epiclastic and volcaniclastic deposits.

MINERAL DEPOSIT AND METALLOGENIC BELT MAPS

These mineral deposit and metallogenic belt maps depict the locations of major mineral lode deposits, placer districts, and metallogenic for the onshore parts of the Russian Far East, Alaska, and the Canadian Cordillera. Sheets 1 and 2 depict at a scale of 1:5,000,0000 the significant lode mineral deposits and placer districts for region. Sheets 3-7 depict at a scale of 1:10,000,000 the metallogenic belts for the regions. Sheets 3-5 depict pre-accretionary metallogenic belts, Sheet 6 depicts accretionary metallogenic belts, and Sheet 7 depicts post-accretionary metallogenic belts. All seven sheets utilize the summary terrane map of the Circum-North Pacific (Nokleberg and others, 1997c). This display of mineral deposit and metallogenic belt patterns on the summary terrane map allows the interpretation of the origin of individual and belts of mineral deposits with respect to geologic host rocks.

Sheets 1 and 2 also contain a summary table for lode mineral deposits that states the mineral deposit number for each quadrant, deposit name, deposit model, and major metals. A similar summary table for placer districts states mineral deposit number for each quadrant, deposit name, deposit model, major metals. Lode deposits and placer districts are numbered separately within individual quadrants bounded by integer values of 4 of latitude and 6 of longitude. The quadrants are numbered from west to east,and are lettered from south to north. In the summary mineral deposit tables, the major metals are the known potentially valuable metals reported for each deposit, and are listed in order of decreasing abundance and (or) value, and are shown by standard chemical symbols. The type of lode deposit is an interpretation that was made by examining the summary of the deposit and then classifying the deposit using the deposit models previously described.

A metallogenic belt is defined as geologic unit (area) that either contains or is favorable for a group of coeval and genetically-related, significant lode and placer deposit models. Metallogenic belts are subdivided into (1) pre-accretionary deposits that formed early in the geologic history of each tectonostratigraphic terrane and are thereby unique to each terrane; (2) (syn)accretionary deposits that formed during periods of major structural juxtaposition, regional deformation, and penetrative deformation that generally occurred during collision of now adjacent terranes; and (3) post-accretionary deposits that consist mainly of igneous-arc-related deposits that formed late in the geologic history of groups of terranes, and generally occur in two or more adjacent terranes. The metallogenic belts defined in this report are based on the significant deposits of the region which were selected to be representative of the metallogeny of the region.

Additional detailed information on lode mineral deposits and placer districts for the region are available in U.S.G.S. Open-File Reports 96-513-A and B on lode mineral deposits and placer districts of the Russian Far East, Alaska and the Canadian Cordillera (Nokleberg and others, 1996a, 1997a).

Metallogenic belts are subdivided into (1) pre-accretionary deposits that formed early in the geologic history of each tectonostratigraphic terrane and are thereby unique to each terrane; (2) (syn)accretionary deposits that formed during periods of major structural juxtaposition, regional deformation, and penetrative deformation that generally occurred during collision of now adjacent terranes; and (3) post-accretionary deposits that consist mainly of igneous-arc-related deposits that formed late in the geologic history of groups of terranes, and generally occur in two or more adjacent terranes. The metallogenic belts defined in this report are based on the significant deposits of the region which were selected to be representative of the metallogeny of the region.

Sheets 3-7 also contain a summary table for lode mineral deposits for each metallogenic belt. The tables state the mineral deposit number for each quadrant, deposit name, deposit model, major metals, and metallogenic belt. A similar summary table for placer districts states mineral deposit number for each quadrant, deposit name, deposit model, major metals. Lode deposits and placer districts are numbered separately within individual quadrants bounded by integer values of 4 of latitude and 6 of longitude. The quadrants are numbered from west to east, and are lettered from south to north. In the summary mineral deposit tables, the major metals are the known potentially valuable metals reported for each deposit, and are listed in order of decreasing abundance and (or) value, and are shown by standard chemical symbols. The type of lode deposit is an interpretation that was made by examining the summary of the deposit and then classifying the deposit using the deposit models previously described. A metallogenic belt is defined as geologic unit (area) that either contains or is favorable for a group of coeval and genetically-related, significant lode and placer deposit models.

Additional detailed information on detailed descriptions of lode deposits and placer districts are available in U.S.G.S. Open-File Reports 96-513-A and B which are tabular data bases (paper and CD formats, respectively)of significant metalliferous lode mineral deposits and placer districts of the Russian Far East, Alaska and the Canadian Cordillera (Nokleberg and others, 1996a, b).


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