THE 'COAST RANGE MEGALINEAMENT' IN SOUTHEASTERN ALASKA--HOW DOES IT DIFFER FROM THE 'COAST SHEAR ZONE'?
BREW, David A., <email> and FORD, A.B., U.S. Geological Survey, MS 904, 345 Middlefield Road, Menlo Park, CA 94025-3591
'Coast Shear Zone' (CSZ) is being used for a crustal-scale geomorphic and geostructural feature described over twenty years ago (Brew and Ford, CJES, v. 15, p. 1763-1772.) as the 'Coast Range Megalineament' (CRML). This zone is one of five shear zones (Brew and Ford, 1996, GSA Abs., v.28, no. 7, p. A-444; 1997: in press, USGS. Prof. Paper) associated with the boundary between the Insular and Intermontane superterranes. The CRML, the youngest of the 'Coast shear zones'; is a narrow, NNW-SSE-trending, essentially straight fault zone interpreted to post-date the 50-Ma Coast Mountains batholith. It is recognizable because of its Pleistocene/Holocene erosional expression for over 800 km at the the western edge of the Coast Mountains Complex. It terminates at the Denali fault system to the NNW and dies in the Coast Mountains SSE of Prince Rupert, British Columbia. The zone is a feature in the 'southeastern Alaska coincident zone' (Brew and Ford, 1985, USGS. Circular 967). The zone varies from < 1 km to a few km in width and is parallel to the strike of the adjacent rocks. No piercing points have been identified. Country rocks to the W belong to the Gravina overlap assemblage, Alexander and Wrangellia terranes, and (in two places) tthe Nisling terrane. Rocks to the E belong to the Wrangellia and Nisling terranes. This establishes that the CRML/CSZ is not the actual Insular-Intermontane superterrane boundary. The zone is the E limit of 95-Ma Wrangell-Revilla belt plutons and the W limit of the 55-70 Ma Great Tonalite Sill and 50 Ma Coast Mountains batholith plutons. Metamorphic rocks to the W record lower T/P conditions than those to the E and the zone is the western margin of uplift of the Coast Mountains in SE Alaska. Shear-sense indicators have not been reported; there is no evidence for lateral movement. The CRML/CSZ follows a prominent aeromagnetic high and a gravity 'step'; the ACCRETE project reports it is associated with a gradual 2 km deepening of the Moho to the E.
COMPARISON OF THE SALMON RIVER SUTURE ZONE AND COAST SHEAR ZONE
MANDUCA, C. A., Geology Dept., Carleton College, Northfield, MN 55057; TIKOFF, B., Geology and Geophysics, Rice Univ., Houston, TX 77005; and SILVER, L. T., Geological & Planetary Sciences, Caltech, Pasadena, CA 91125
The Salmon River Suture Zone (SRSZ) is a major zone of deformation localized along the boundary between accreted material and the Precambrian/Paleozoic continental margin in west-central Idaho. Similar to Coast Shear Zone in western British Columbia/southeast Alaska, the SRSZ is a composite structure representing at least three deformation events. The oldest structures in the SRSZ are west-directed thrusts within the accreted terranes, and amphibolite-facies gneisses associated with epidote-bearing plutonism along the crustal boundary. Epidote-bearing tonalite emplaced relatively late in this deformation sequence is 118 Ma (U/Pb on zircon). Magmatism migrated progressively eastward with emplacement of the Little Goose Creek complex (111 Ma; U/Pb on zircon) and the Payette River tonalite (90 Ma; U/Pb on zircon). The Payette River tonalite is part of a narrow (~10 km) body extending along the western side of the Idaho Batholith (~280 km). The youngest penetrative deformation, forming the Western Idaho shear zone, is sub-vertical and concentrated along the accreted terranes/continental margin boundary. Shear fabrics postdating emplacement of the Payette River tonalite display dextral, transpressional kinematics. All fabrics were subsequently tilted 20-30ø W on E-dipping, Miocene domino-style normal faults, in a broad area surrounding the suture zone. The structural and intrusive history of the SRSZ is very similar to that of the Coast Shear Zone, both in style and relative timing. Structures related to initial accretion and burial are not well preserved. Early west-directed thrusting occurs west of the crustal boundary and is coeval with epidote-tonalite plutonism. Emplacement of an elongate, late-stage, vertical tonalite sill occurs east of the crustal boundary and post-dates thrusting. The youngest penetrative fabrics form a narrow, late-stage vertical shear zone localized along the crustal boundary. A major difference between the areas is that events in Idaho apparently preceded those in Alaska by 20-30 m.y. The similar histories may represent a particular time-transgressive accretion event or a general response of steeply-dipping crustal boundaries to long-lived terrane tectonics.
CONTRACTIONAL STRUCTURES IN THE WESTERN SKEENA FOLD BELT: AN INDICATION OF CRETACEOUS SINISTRAL CONVERGENCE WITHIN THE COAST BELT
EVENCHICK, Carol A., Geological Survey of Canada, 101-605 Robson Street, Vancouver, BC, V6B5J3, email.
The Skeena Fold Belt is a thin-skinned fold and thrust belt which occupies much of the width of the northern Intermontane Belt of the Canadian Cordillera. It is composed largely of northwest trending contractional structures, but includes domains of northeast to north-northeast trending contractional structures along its western margin. The fold belt roots to the west in the Coast Belt and is Cretaceous and possibly earliest Tertiary in age. The north-northeast and northeast trending structures are enigmatic because they are orthogonal to the vast majority of contractional structures in the Cordillera. Similar orientations of structures occur sporadically for at least 900 km along the boundary of the Coast and Intermontane belts, from 350 km north-northwest of the Skeena Fold Belt to100 km south-southeast of it. A speculative explanation for the origin of the north-northeast trending structures is that they are a result of southeast shortening along the Cordilleran margin during a period of sinistral convergence. Sparse cross-cutting relationships suggest that they are overprinted by north-trending contractional structures, which may record a transition to the dominant northwest trend of the Skeena Fold Belt. Because the rocks are as young as latest Jurassic or earliest Cretaceous in age, the sinistral convergence and transition to dextral convergence must also be Cretaceous. In the region northwest of the Skeena Fold Belt the age of north-northeast trending structures is limited to between 135 Ma and 106 Ma. Plate motion studies by other workers, based on the hot spot reference frame, suggest a transition from sinistral to dextral oblique convergence along the northern Cordilleran margin in mid-Cretaceous time. The interpretation of structures in the western Skeena Fold Belt as resulting from sinistral convergence provides structural evidence on a large scale for the proposed Cretaceous relative plate motions. Possible candidates for the Early Cretaceous sinistral shear zone would be earlier structures along or near the present Denali Fault and Coast Shear Zone.
PALEOMAGNETIC INDICATIONS OF CRUSTAL TILT AFFECTING THE QUOTTOON AND DUNDAS ISLAND PLUTONS IN THE PORTLAND INLET AREA
Butler, Robert F., and Gehrels, George E., Dept. Geosciences, Univ. of Arizona, Tucson, AZ 85721, email
Paleomagnetic directions have been determined from 12 sites (all normal polarity) of the mid-Cretaceous Dundas Island pluton and 27 sites (dominantly reversed polarity) from the Paleocene Quottoon pluton east of the Coast Shear Zone. The observed mean directions are: inclination (I) = 51.9°; Declination (D) = 67.7°; a95 = 6.9° for Dundas Island pluton; I = 80.3°; D = 41.7°; a95 = 3.1° for Quottoon sites on the south side of Portland Inlet; and I = 43.9°; D = 64.4°; a95 = 8.6° for Quottoon sites on the south side of Pearse Canal (~10 km north of Portland Inlet). When compared to expected Paleocene directions at the sampling locations, the Quottoon paleomagnetic directions observed from the south side of Portland Inlet are marginally discordant. However, the Quottoon directions observed from north of Portland Inlet are strongly discordant with F±DF = 30.7°±7.1° and R±DR = 75.0°±11.0°. Given concordant paleomagnetic directions recorded by Eocene plutons east of Prince Rupert, B.C., an explanation of this discordant direction by latitudinal transport would require >4000 km northwards transport in ~10 m.y. (= 40 cm/yr velocity). Thus it is much more reasonable to interpret this discordant paleomagnetic direction as the result of ~45° east-side-up tilt of the panel of crust containing the Quottoon pluton north of Portland Inlet. One important conclusion of the present study is that the Coast Shear Zone does not separate plutons with discordant paleomagnetic directions from those with concordant directions. The large difference between paleomagnetic directions observed within the Quottoon pluton on opposite sides of Portland Inlet indicates the presence of a major deformation zone within the inlet. This fault or shear zone must have accommodated major north-side-up displacement. If the ~45° east-side-up tilt of the Quottoon pluton north of Portland Inlet is accommodated by vertical displacement across the Coast Shear Zone, the sense of that displacement would be west-side-up across the shear zone. The Dundas Island pluton has a discordant paleomagnetic direction compared to the mid-Cretaceous expected direction at a paleo-position 1000 km south: F±DF = 21.8°±5.8° and R±DR = 95.7°±10.7°. Although commonly interpreted as evidence for ~3000 km of post-mid-Cretaceous northwards transport, this discordant direction can also be accounted for by ~45° east-side-up tilt. Given the agreement with the tilt required to account for the discordant direction from the Quottoon pluton north of Portland Inlet, we favor east-side-up tilt as the explanation for the discordant paleomagnetic direction from the Dundas Island pluton.
SEISMIC REFLECTION PROFILES REVEAL EXTENSIONAL FABRIC OF DIXON ENTRANCE, SE ALASKA
DIEBOLD, John B., L-DEO, Palisades NY 10964-8000, email; DAS, Triparna, and HOLLISTER, Lincoln S., Princeton University, Princeton, NJ 08544.
Multichannel seismic [MCS] reflection profiles, acquired as part of the ACCRETE project, imaged crustal reflectors and Moho on both sides of the fundamental pair of structural boundaries separating the Alexander, Wrangellia, and North America terranes. The eastern margin of the Alexander terrane features strong crustal reflectors, mostly dipping gently (ca. 12 degrees) westward. Moho reflections within this zone are also stronger and more continuous than those seen in the terranes to the east. A broad Moho arch, about 3 km high, 100 km wide, and striking NNE, is defined by two E-W profiles, 45 km apart, and a third, arch-parallel profile. It is likely that this crustal arching is the result of the Neogene extension forming the Queen Charlotte Basin, but two oddities remain to be explained. First, mid-crustal faulting, as expressed in the pattern of dipping reflectors, does not match that predicted by most simple shear models. Second, the overall strike of the arch (unless it has been offset by some unknown, very recent strike-slip fault) is oblique both to terrane boundaries, and to graben and half-graben structures previously mapped in Dixon Entrance and Hecate Strait. Several possible explanations can be explored. Terrane boundaries splay out just north of Dixon entrance; south of Ketchikan, the Taku terrane and the Gravina belt disappear. Also, the strike of the Wrangellia/North American boundary, marked by the coast shear zone, rotates 35 degrees northward, beginning a dogleg towards the east, partially accommodating the splaying of the terrane boundaries. The strike of the Dixon Entrance Moho arch is subparallel to the coast shear and terrane boundaries to the north, suggesting that the crust there may be in a different tectonic regime than that forming Hecate Strait, to the south.
CRUSTAL STRUCTURE AND THE MOHO ACROSS THE COAST SHEAR ZONE
SMITHSON, Scott B., Morozov, Igor B., Dept. of Geology and Geophysics, Univ. of Wyoming, Laramie, WY 82071-3006, email, and Hollister, Lincoln S., Dept. of Geology, Princeton Univ., Princeton, NJ 08544
We discuss the relief of the Moho and the pattern of crustal reflections observed during the '94 wide-angle ACCRETE experiment. Several prominent crustal reflectors steeply dipping to the northeast were imaged west of the Coast Shear. These reflectors, associated with the mid-Cretaceous thrust system, terminate under the surface expression of the Coast Shear. Near the western side of the Coast Shear, several strong short southwest-dipping interfaces were identified. On the northeast side of the Coast Shear, two prominent reflectors were imaged within the middle- to lower crust. Within the top of Paleogene arc, we identified a NE dipping fabric associated with a ductile top to the east or NE deformation. A localized thin body above the Moho probably corresponds to the S-wave 'bright spot' observed before. Although the Moho discontinuity exhibits no sharp step beneath the Coast Shear, a significant Moho relief is present, with the Moho deepening from about 25-26 km west of the Coast Shear to 31 km east of it. Taken together, these features suggest that the Coast Shear extends subvertically into a great depth, apparently close to the Moho boundary. At the same time, the Coast Shear does not seem to offset the Moho directly.
STRAIN PARTITIONING AND MULTIPLE FABRIC DEVELOPMENT DURING SYNKINEMATIC PLUTON EMPLACEMENT, NORTHWESTERN BRITISH COLUMBIA
KLEPEIS, K.A., Dept. of Geology and Geophysics, University of Sydney, NSW 2006, Australia, email; CRAWFORD, M.L., Dept. of Geology, Bryn Mawr College, Bryn Mawr, PA 19010; GEHRELS, G.E., Dept. of Geosciences, Univ. of Arizona, Tucson, AZ 85721.
Metasedimentary and metaigneous rocks of the late Cretaceous to Paleogene Coast Plutonic Complex south of the Alaska-British Columbia border preserve a history of changing styles of deformation during emplacement of synkinematic tonalite plutons. This history allows us to examine the effects of pluton emplacement and host rock melting on fabric geometry, shear zone evolution and kinematics, and the partitioning of contractional and extensional strains during orogen-parallel displacements. Across the eastern margin of the ~58 Ma Quottoon pluton near Quottoon Inlet, pre-intrusion fabrics record regional south-directed thrusting along moderately N to NE-dipping planes. This fabric is best preserved in zones of minimal intrusive activity but can also be recognized in metasediment screens within and between deformed tonalite sills. In contrast with this regional trend, ~58-55 Ma deformation during pluton emplacement and melting produced large spatial variations in the orientation and kinematics of fabrics and shear zones. This variability can be correlated with differences in (1) bulk rock composition, (2) amount of melt present during deformation and (3) proximity of rocks to pluton margins. Areas dominated by metaigneous rocks and melt-present deformation preserve steep, NNW-striking oblique-dextral shear zones and fabrics. Pre-intrusion strains are mostly obliterated by melting. Finite strain studies using stretched veins show that some areas dominated by melt experienced ENE-WSW contraction, whereas others experienced ENE-WSW extension. In contrast, country rocks adjacent to sill margins where deformation is mostly solid-state are affected by moderately N-dipping, ~55 Ma extensional shear zones and fabrics that reorient and reactivate older fabrics. Reactivation of the older N-dipping fabric suggests that a preexisting E-W structural anisotropy partially controlled sill emplacement. These results show that the partitioning of contractional and extensional strains during pluton emplacement was highly variable, especially in the ENE-WSW direction but that all areas experienced NNW-SSE stretching and displacement on surfaces of different orientations.
LATEST CRETACEOUS AND PALEOGENE HISTORY OF THE CENTRAL REGION OF THE COAST OROGEN AT THE BRITISH COLUMBIA-ALASKA BORDER
CRAWFORD, M.L., Dept. Geology, Bryn Mawr College, Bryn Mawr, PA 19010, email; GEHRELS, G.E., Dept. Geosciences, Univ. Arizona, Tucson, AZ 85721; KLEPEIS, K.E., Dept. Geology and Geophysics, Univ. of Sydney, Sydney, NSW 2006, Australia; CRAWFORD, W.A., Dept. Geology, Bryn Mawr College, Bryn Mawr, PA 19010.
On both sides of Portland Inlet at the British Columbia-Alaska border the segment of the Coast Orogen east of the Coast shear zone records the construction and subsequent uplift of a batholith complex containing plutons ranging in age from 72 to 51 Ma. The batholith can be subdivided into a pre-60 Ma group of tonalite-leucotonalite-granodiorite bodies affected by penetrative W- to SW-directed compressional ductile deformation shared with the country rocks and post-60 Ma diorite to granite plutons that postdate this deformation. Metasedimentary host rocks to the batholith contain relic metamorphic assemblages that resemble those of the mid-Cretaceous thrust belt west of the Coast shear zone. Additional links with rocks of the western side of the orogen are formed by the occurrence of 88-89 Ma tonalite bodies both east and west of the Coast shear zone. Monazite grains in garnet from one site in migmatitic country rocks yield Th-Pb (UCLA ion probe) ages from 71.5 ± 2.3 Ma to 65.0 ± 2.6 Ma, documenting metamorphic mineral growth in the country rocks during pre-60 Ma pluton emplacement. The batholith is segmented into kilometer to tens of kilometer size blocks bounded by late, steep, dip-slip shear zones that cut all units with the possible exception of 53-51 Ma granite plutons on the east. These narrow shear zones, that we interpret as related to uplift of the batholith complex, have two dominant trends: NW strike with east side down displacement and E-W strike with north side down displacement. The westernmost of the NW striking shear zones lies within the 5-7 km wide ~55 Ma Coast shear zone that bounds the western side of the batholith. The age of a felsic dike emplaced in one of the E-W shear zones is 51 Ma. A moderately to steeply east-dipping fabric in the 53-51 Ma granite bodies records east side down displacement in a narrow zone along the contact between these bodies and the low grade supracrustal rocks of the Stikine terrane on the east.
DEXTRAL TRANSPRESSIVE SHEARING AT THE EASTERN MARGIN OF THE QUOTTOON PLUTON: IMPLICATIONS FOR THE EVOLUTION OF THE COAST SHEAR ZONE
ANDRONICOS, Christopher, L., Dept. of Geosciences, Princeton University, Princeton, New Jersey, 08544, email; HOLLISTER, Lincoln, S., Dept. of Geosciences, Princeton University, Princeton, New Jersey, 08544; DAVIDSON, Cameron, Department of Geology, Beloit College, Beloit, WI 53511
A multi-stage deformation history is preserved at the eastern margin of the Quottoon pluton (QP), near the Skeena River, British Columbia. Moderately north-northeast dipping fabrics record melt present southwest-directed thrust shearing. Thrusting resulted in the formation of east-plunging kilometer scale recumbent folds. These folds fold a mineral stretching lineation and gneissic fabrics, indicating they are second generation structures. Recumbent folds are deflected and refolded into northwest plunging kilometer scale folds with steeply east dipping axial surfaces going west towards a subvertical northwest striking dextral shear zone. Dextral shearing produced moderately northwest-plunging (330, 30) mineral lineations, parallel to fold hinges. From east to west, across strike of the shear zone, lineations are steepened, with the steepest lineations (on average) occurring within the QP. This geometric feature, coupled with the crosscutting of the shear zone by the QP, indicates lineations produced during dextral shearing were steepened during pluton emplacement. We suggest that these pre-QP structures were produced during partitioned transpression, with synchronous movement on shallow and steep fabrics. Before intrusion of the QP these structures would have coincided with the eastern margin of the Coast shear zone. Therefore, we conclude that the transpressive structures described here record part of the deformation history of the Coast shear zone prior to the intrusion of the QP. These structures were subsequently overprinted by intrusion of the QP and deformation events that followed pluton emplacement.
THE COAST PLUTONIC COMPLEX: A CONSEQUENCE OF A LEAKY BAJA BC TRANSPRESSIVE FAULT SYSTEM?
HOLLISTER, Lincoln S., email and ANDRONICOS, Christopher L., email Dept. of Geosciences, Princeton Univ., Princeton NJ 08544.
Along the ACCRETE transect, 54-55oN, evidence of a late-Cretaceous to middle-Eocene deformation history is preserved within the Coast Plutonic Complex. The earliest (75-65 Ma) fabrics were formed during partitioning of oblique convergence onto contemporaneous northwest striking dextral transcurrent shear zones and shallowly north dipping thrust fabrics. We propose a cause and effect relationship between this transpressive shearing and the origin of plutons of the Coast Plutonic Complex. Basalt magma intruded into the crust from conduits produced where the transpressive shear zones passed into the mantle. Basalts provided the heat for melting of the crust and formation of the tonalite plutons. Deformation was localized to places where rocks were softened by heating and melting. Continuing episodes of basalt underplating and the consequent melting and recrystallization at high temperature, especially during the terminal phase of uplift and exhumation between 60 and 50 Ma, further obscured some of the evidence of the prior deformation. This correlation of pluton generation to transpressive shear zones may account for the restriction of the late-Cretaceous to middle-Eocene thermal and deformation events to the east side of the Coast shear zone. The Coast shear zone thus may define the western boundary of the hypothetical Baja British Columbia fault zone of Cowan et al (1997) across which 1500-2000 km of cumulative displacement may have occurred between late-Cretaceous and early-Tertiary.
THE CENTRAL GNEISS COMPLEX: AN ASYMMETRICAL EXTENSIONAL GNEISS DOME BOUNDED BY THE COAST SHEAR ZONE
CHARDON, Dominique, and ANDRONICOS, Christopher, L., email Dept. of Geosciences, Princeton Univ., Princeton NJ 08544
Foliation trajectories (based on published data from several sources) within the Coast Plutonic Complex (CPC) between 54 and 55 degrees latitude north, define an asymmetrical dome cored by supracrustal rocks of the Central Gneiss Complex (CGC). The dome contains synkinematic sills that are concordant with the domal foliation. Elongation lineations on the dome envelop are N plunging along its western limb and shallowly ENE plunging along the eastern limb that is defined by the Shames river mylonite zone. This shear zone deforms rocks of the CGC, the synkinematic 53 Ma Kasiks pluton (which displays pervasive C/S structures along its roof), older plutonic suites and overlying low-grade volcanic rocks. Kinematic indicators are consistent with top-to-the-ENE extensional shearing. Shearing resulted in a steep metamorphic field gradient on the eastern limb of the dome from granulite to greenschist facies conditions, implying thinning across the shear zone. Greenschist facies fabrics are superimposed on amphibolite facies fabrics within the shear zone, indicating cooling during deformation. The dome is bounded to the west by the steeply dipping 58 Ma Quottoon pluton and steep foliations with stretching lineations plunging gently to the north. We propose that the Eocene structural and magmatic pattern of the CGC resulted from the exhumation of the ductile lower crust of the CPC along the Shames river detachment into an asymmetrical extensional gneiss dome between 53 and 48 Ma. The structural history of the steeply dipping western limb of the dome has been controlled by the earlier (58 Ma and/or older) anisotropy defined by the great tonalite sill (Quottoon pluton) and Coast shear zone.
LATE CRETACEOUS TO EOCENE PLUTONISM AND DEFORMATION NEAR TERRACE, BRITISH COLUMBIA
GAREAU, S.A., and WOODSWORTH, G.J., Geological Survey of Canada, 101-605 Robson St., Vancouver, BC V6B 5J3, email; FRIEDMAN, R.M., Dept. Earth & Ocean Sciences, University of British Columbia, 6339 Stores Rd., Vancouver, BC V6T 2B4.
The Terrace area straddles the boundary between Intermontane (IB) and Coast (CB) belts. The IB encompasses the southern margin of Bowser Basin, late Paleozoic to Middle Jurassic Stikine strata (both included in ST), and the fault-bounded Kitselas-Kitsumkalum block (KKB) distinguished from conventional Stikine terrane based on ductile deformation and metamorphism. In ST, Late Cretaceous Bulkley intrusions are represented by undeformed stocks (86 Ma, U-Pb) that cut little-metamorphosed Late Jurassic strata. An 80.9 Ma (U-Pb) dike that cuts metamorphosed and deformed Early Jurassic Kitselas volcanics is ductilely foliated and lineated. A Late Paleocene (59-60 Ma, U-Pb) intrusive body in KKB is heterogeneous, but dominantly titanite-bearing granodiorite; it locally has strong ductile fabrics. Eocene (50-53 Ma, U-Pb and K-Ar) titanite-bearing plutons (Carpenter Lake suite) and 47-48 (U-Pb) biotite granite are discordant, show no ductile deformation, and intrude most other units. Ductile deformation in the IB is restricted to KKB; it post-dates, at least in part, 60 Ma and ceased by 52 Ma. It may be related to uplift along faults of KKB. In the CB west of the Terrace area, ductilely deformed biotiteñmuscoviteñgarnet-bearing plutons of about 83.5 Ma (U-Pb) age are roughly coeval, but differ petrographically from the Late Cretaceous hornblende>biotite Bulkley intrusions near Terrace. 67-78 Ma plutons present in the eastern CB (and including parts of the Ponder pluton) have no known counterparts in the IB near Terrace. The 60 Ma old pluton in KKB is coeval with parts of Quottoon pluton. Eocene plutons in CB, including Kasiks and parts of Ponder plutons, are roughly correlative with the Carpenter Lake suite in the IB. Young deformational and plutonic history in KKB resembles more that of the Coast Plutonic Complex than that of ST.
THE QUOTTOON IGNEOUS COMPLEX: A RECORD OF TECTONIC EVOLUTION ALONG THE COAST SHEAR ZONE
THOMAS, Jay B., email, and SINHA, A.K., Dept. of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
The quartz dioritic Quottoon Igneous Complex (QIC) is a major Paleogene (65-56 Ma) magmatic body in NW British Columbia and SE Alaska that was emplaced along the Coast shear zone (CSZ). The QIC contains three different igneous suites that provide information about source regions, magmatic processes and evolving tectonic regimes that changed from a dominantly convergent to dominantly strike-slip regime from 65 to 55 Ma (Klepeis and Crawford, 1996). Field observations suggest that the QIC can be subdivided into three suites. First, the easternmost suite (e.g., along Steamer Passage) has a pervasive solid-state fabric (239, 68 SE), common mafic enclaves and dikes, and high color indices (~36). Second, the western suite (e.g., along Quottoon Inlet) has only a weak fabric (327, 78 NE) developed in the magmatic state (aligned feldspars, melt filed shears), and intermediate color indices (~28). The third mappable suite occurs along the Skeena River where solid-state fabrics are well developed (342, 63 NE), the rocks have intermediate color indices (~25), and contain abundant metsedimentary screens. Geochemical (Rb, Sr, Ba, La/Yb) and isotopic (87Sr/86Sr)i data preclude linkages amongst the three suites by fractional crystallization or AFC processes. Rocks of the Skeena River transect, although geochemically similar to other portions of the QIC, have the lowest 87Sr/86Sri (0.70442 to 0.70528). Derivation of the other suites from this isotopically juvenile magma by fractionation and/or mixing processes in unlikely. In contrast, the western suite (87Sr/86Sri = 0.70611 to 0.70753) can be interpreted as parental magma for the eastern suite (87Sr/86Sri = 0.70565 to 0.70654) that was modified by addition of a mantle component (87Sr/86Sri = 0.70350). Therefore, we interpret the evolution of the QIC to have proceeded according to the following petrologic sequence of events: 1. A quartz dioritic magma, mixed with a mantle derived mafic magma, was emplaced along the CSZ during a compressional tectonic regime. This event resulted in the pervasively foliated eastern suite of the QIC. 2. A quartz dioritic magma (Skeena River suite) was emplaced into a compressional tectonic environment that was derived from an isotopically distinct reservoir. 3. Finally, emplacement of the homogeneous western suite quartz dioritic suite occurred into a tectonically static environment. We suggest that the construction of the QIC represents magmatic evolution and emplacement that coincided with tectonic evolution of the CSZ.
PALEOGENE MAGMATISM IN NW BRITISH COLUMBIA-SE ALASKA: EVIDENCE FOR CHANGE FROM TRANSPRESSION TO EXTENSION
SINHA, A.K. and THOMAS, Jay B., email, Dept. of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
Paleogene magmatism in NW British Columbia is dominated quartz
dioritic and granodioritic compositions that are interpreted to be melts
derived from an amphibolitic lower crust (Sinha and Thomas, 1996). Of the
numerous Paleogene plutons, the Ponder pluton has long been considered
to be one of the largest igneous bodies in the Canadian Cordillera. As
such its thermal impact on the crust and associated deformation would be
significant. Field relationships and geochemical data (Hutchison, 1982;
Peters, 1984; Sisson, 1985) have been used to suggest that the Ponder pluton
is a normally zoned pluton, emplaced at mid-crustal depths, with a rim
of dioritic composition grading into a core region of granodiorite.
However, through the utilization of new geochemical data coupled with
detailed structure-composition sections across the Ponder pluton in NW
British Columbia, we suggest that the Ponder pluton is more likely to be
an igneous complex (Ponder igneous complex (PIC)) composed of nine distinct
igneous sheets. The PIC has a cumulative thickness of ~10 km and an along
strike length of ~200 km. Although details of emplacement mechanisms for
individual sheets are lacking, the magmatic construction of a ~10 km thick
igneous stratigraphy required rapid uplift, perhaps associated with extensional
processes. Published geobarometry of measurably deeper levels of emplacement
along the western margin are adequately explained by the multiple sheet
model where the western contact is associated with the lowest structural
sheet. The igneous stratigraphy proposed by us crosses the Portland Canal
into SE Alaska and is exposed along the canal as four separate igneous
bodies (sheets). Although direct stratigraphic and geochemical correlations
are not currently possible along the canal, it is likely that the PIC reflects
a change in tectonic regimes. The Coast shear zone and the early phases
of the Quottoon Igneous Complex (QIC) record evidence of compressional
tectonics at 65 Ma, while the PIC provides an unique record of large-scale
magmatism associated with extensional tectonics at ~54 Ma. The approximately
10 Ma time interval recorded in the contrasting magmatic complexes (QIC,
PIC) identifies the rapid transition from compressional to extensional
tectonics.
METCALF, James, and DAVIDSON, Cameron, Department of Geology, Beloit College, Beloit, WI 53511, email.
The NNW trending Coast shear zone in northern British Columbia separates high-temperature metamorphic and plutonic rocks of the exhumed Paleogene arc to the East from the high-pressure, lower temperature rocks of the Cretaceous thrust belt to the West. In the Skeena river area, the ca. 58.6 Ma Quottoon pluton (tonalite) defines the eastern boundary of the Paleogene arc and cross cuts fabrics formed during southwest directed thrusting and dextral shearing within the Coast shear zone (Andronicos et al., this meeting). Magmatic foliations within the Quottoon pluton defined by aligned biotite and hornblende generally strike parallel to the Coast shear zone and dip steeply northeast or are vertical; lineations defined by the preferred alignment of hornblende typically plunge 20-70 NNW. We quantitatively mapped the variability in the intensity of the fabric elements present in the Quottoon pluton using the autocorrelation function (ACF; Panozzo-Heilbronner, 1992). The 3ellipticityý of the 96th contour is used as a measure of fabric intensity and varies from 0.36 to 0.84, where low numbers indicate an intense (well-developed) fabric and 1 indicates no fabric (i.e. no correlation in any direction). ACF grain sizes (see Davidson et al., 1996) vary from 0.21 to 1.87 mm. In general, ACF ellipticities increase and ACF grain sizes decrease within the Quottoon pluton toward the western boundary of the Paleogene arc, suggesting that more strain was recorded by the rocks in the west while they were partially molten. This apparent strain gradient could be due to strain concentration within the Coast shear zone along the western margin of the pluton as the pluton crystallized, or from the pluton crystallizing at different rates while it was being deformed. The latter explanation implies that a horizontal temperature gradient was present between the Paleogene arc and western thrust belt at the time of intrusion of the Quottoon pluton.
Davidson, C., Rosenberg, C., and Schmid, S.M. (1996) Synmagmatic folding
of the base of the Bergell pluton, Central Alps.
Tectonophysics, 265, 213-238.
Panozzo Heilbronner, R. (1992) The autocorrelation function: an image
processing tool for fabric analysis.
Tectonophysics, 212, 351-370.
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address: Department of Geology
Princeton University
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