Thermobarometric constraints on the structural evolution of the Coast Mountains batholith, central southeastern Alaska

Thermobarometric data from amphibolite-facies metamorphic rocks west of the Coast Mountains batholith provide important constraints on the structural evolution of the mid-Cretaceous Sumdum–Fanshaw fault system and Late Cretaceous – Paleocene Le Conte Bay shear zone in central southeastern Alaska. Ductile structures that make up the Sumdum–Fanshaw fault system record the east-directed underthrusting of the Alexander terrane and Gravina belt beneath the Ruth assemblage (Yukon–Tanana terrane) and Taku terrane. These structures are truncated to the east by the Le Conte Bay shear zone. Temperature and pressure estimates calculated from the garnet–biotite geothermometer and garnet–rutile–ilmenite–plagioclase–quartz geobarometer suggest juxtaposition of the Gravina belt and Yukon–Tanana terrane at relatively deep levels (>7 kbar) during mid-Cretaceous time. Rocks west of the Le Conte Bay shear zone yield thermobarometric estimates of 465–890 ± 50 °C and 7.1–11.8 ± 1 kbar (1 kbar = 100 MPa). Late Cretaceous and Paleocene metamorphism associated with the Le Conte Bay shear zone reflects synkinematic emplacement of tonalitic intrusions along the western margin of the Coast Mountains batholith. Thermobarometric results from samples adjacent to the tonalite bodies record uplift and retrogression and suggest tonalite emplacement at 7.5–7.7 ± 1 kbar. An eastward increase in thermobarometric estimates observed in Thomas and Le Conte bays is inferred to record uplift and east-side-up tilting of rocks west of and within the Le Conte Bay shear zone during Late Cretaceous and Paleocene time. Rocks within the Le Conte Bay shear zone were apparently rapidly (1.5–2 mm/a) uplifted to shallow crustal levels prior to mid-Eocene time. Thermobarometric results for the Petersburg region are similar to those previously reported along the western flank of the northern Coast Mountains batholith.

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U–Pb geochronology of Late Cretaceous and early Tertiary plutons in the northern Coast Mountains batholith

Canadian Journal of Earth Sciences ◽

10.1139/e91-082 ◽

1991 ◽

Vol 28 (6) ◽

pp. 899-911 ◽

Cited By ~ 32

Author(s):

George E. Gehrels ◽

William C. McClelland ◽

Scott D. Samson ◽

P. Jonathan Patchett ◽

David A. Brew

Keyword(s):

Shear Zone ◽

Late Cretaceous ◽

Detrital Zircons ◽

Northern Coast ◽

Coast Mountains ◽

Western Margin ◽

Early Proterozoic ◽

Isotopic Signatures ◽

Metasedimentary Rocks ◽

Early Tertiary

U–Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic–granodioritic bodies in central portions of the batholith yield ages of 59–58 Ma, coeval with younger phases of the tonalitic sheets. Large granite–granodiorite bodies in central and eastern portions of the batholith were emplaced at 51–48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Variations in the age, shape, location, and degree of fabric development among the various plutons indicate that Late Cretaceous – Paleocene tonalitic bodies were emplaced into a steeply dipping, dip-slip shear zone that was active along the western margin of the batholith. Postkinematic Eocene plutons were emplaced at shallow crustal levels. Inherited zircon components in these plutons range in age from mid-Paleozoic to Early Proterozoic and are coeval with detrital zircons in adjacent metasedimentary rocks. These old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons.

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Contrasting tectono-stratigraphy and structure in the Coast Belt near Chilko Lake, British Columbia: unrelated terranes or an arc – back-arc transect?

Canadian Journal of Earth Sciences ◽

10.1139/e94-152 ◽

1994 ◽

Vol 31 (11) ◽

pp. 1700-1713 ◽

Cited By ~ 4

Author(s):

Paul J. Umhoefer ◽

Margaret E. Rusmore ◽

G. J. Woodsworth

Keyword(s):

Late Cretaceous ◽

Structural Evolution ◽

Fault System ◽

The West ◽

Volcanic Arc ◽

Coast Mountains ◽

Magmatic Arc ◽

Clastic Rocks ◽

Oblique Convergent ◽

Back Arc

Stratigraphy and structural styles vary greatly in two areas of the Coast Belt near Chilko Lake (Chilcotin Ranges in the east and Coast Mountains in the west). No definite continuity between the two belts has been established in the pre-mid-Cretaceous geology, but this area may be a long-lived, episodic magmatic arc and nearby arc-related basin. The stratigraphic contrasts may reflect inherent differences between an arc and related basinal sequence. Triassic volcanic-arc sequences are part of the Stikine (western belt) and Cadwallader (eastern belt) terranes, which may be part of the same arc. The Jurassic is represented by one dated pluton in the west compared with almost continuous deposition of volcanogenic clastic rocks in the east. Lower Cretaceous sequences in the west and east may represent a volcanic arc and back-arc basin. The Taylor Creek Group (Albian) is the first definitive link between the two belts and represents an arc and intra-arc or back-arc basin. The structural evolution of the two belts also differs significantly. The early Late Cretaceous Eastern Waddington thrust belt comprises all major structures in the west, but only has minor expression in the east. Most of the structures in the east are part of the latest Cretaceous(?) to early Tertiary dextral-strike-slip, Yalakom fault system. These differences were most likely caused by the Late Cretaceous change from nearly orthogonal subduction to a dextral-oblique convergent margin.

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LATE CRETACEOUS UNDERPLATING OF THE KLUANE-COTTONWOOD-MACLAREN SCHIST BENEATH THE NORTHERN COAST MOUNTAINS ARC, YUKON AND ALASKA

10.1130/abs/2019cd-329846 ◽

2019 ◽

Author(s):

Trevor S. Waldien ◽

◽

Sarah M. Roeske ◽

Jeffrey A. Benowitz

Keyword(s):

Late Cretaceous ◽

Northern Coast ◽

Coast Mountains

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Nd and Sr isotopic constraints on the petrogenesis of the west side of the northern Coast Mountains batholith, Alaskan and Canadian Cordillera

Canadian Journal of Earth Sciences ◽

10.1139/e91-085 ◽

1991 ◽

Vol 28 (6) ◽

pp. 939-946 ◽

Cited By ~ 30

Author(s):

Scott D. Samson ◽

P. Jonathan Patchett ◽

William C. McClelland ◽

George E. Gehrels

Keyword(s):

Late Cretaceous ◽

Crustal Thickening ◽

Northern Coast ◽

Crustal Material ◽

The West ◽

Crustal Component ◽

Coast Mountains ◽

Early Proterozoic ◽

Depleted Mantle ◽

Wrangellia Terrane

Nd and Sr isotopic ratios are reported from 15 samples of plutons of the northern Coast Mountains batholith (CMB), between. the Alexander–Wrangellia terrane and the Stikine terrane of southeastern Alaska. Samples of plutons that are part of the Late Cretaceous – Eocene CMB suite have a range in initial εNd of −3.0 to −0.2 and 87Sr/86Sr of 0.70494–0.70607. There is no correlation of isotopic ratio with age, lithology, or geographic location of these plutons. Two plutons that are probably older than the bulk of the CMB plutons have present-day εNd values of −6.8 and −2.6.The Late Cretaceous – Eocene plutons have Nd depleted-mantle model ages (tDM) of 620–1070 Ma. These data indicate that the northern CMB must contain a significant component of old, evolved continental crust. The presence of an old crustal component is further demonstrated by inherited zircons of average Early Proterozoic age contained in some plutons. The mid to Late Proterozoic tDM ages of the CMB plutons are therefore a result of a mixture of Early Proterozoic crustal material with. younger, juvenile crust. The most likely source of this old crustal component is the Yukon–Tanana terrane, a fragment composed of ancient crustal material that occurs within and directly to the west of the northern CMB. The juvenile component is probably a combination of material derived from the mantle and from anatexis of the surrounding juvenile terranes. Crustal anatexis may have occurred as a result of the intrusion of mafic melts related to subduction along the outboard margin of the Alexander–Wrangellia terrane, by crustal thickening due to the underthrusting of the Alexander–Wrangellia terrane beneath the Yukon–Tanana and Stikine terranes, or by a combination of both processes.

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Late Cretaceous to Paleocene Tectonometamorphic Evolution of the Blanchard River Assemblage, Southwest Yukon: New Insight into the Terminal Accretion of Insular Terranes in the Northern Cordillera

Lithosphere ◽

10.2113/2020/2298288 ◽

2020 ◽

Vol 2020 (1) ◽

Author(s):

Lianna Vice ◽

H. Daniel Gibson ◽

Steve Israel

Keyword(s):

British Columbia ◽

Late Cretaceous ◽

Amphibolite Facies ◽

Contact Metamorphism ◽

Northern Coast ◽

Coast Mountains ◽

Tectonic Events ◽

Western Cordillera ◽

History Of ◽

Northwestern North America

Abstract The Intermontane-Insular terrane boundary stretches over 2000 kilometers from British Columbia to Alaska in the western Cordillera. Juxtaposed between these terranes is a series of Jura-Cretaceous basinal and arc assemblages that record a complicated and contested tectonic evolution related to the Mesozoic-Paleocene accretionary history of northwestern North America. In southwest Yukon, west-verging thrust faults facilitated structural stacking of the Yukon-Tanana terrane over these basinal assemblages, including the Early Cretaceous Blanchard River assemblage. These previously undated compressional structures are thought to be related to the final collapse of the Jura-Cretaceous basins and the tectonic burial of the Blanchard River assemblage resulting in amphibolite facies metamorphism. New in situ U-Th-Pb monazite ages record at least three tectonic events: (1) the tectonic burial of the Blanchard River assemblage to amphibolite facies conditions between 83 and 76 Ma; (2) peak burial was followed by regional exhumation at ca. 70-68 Ma; and (3) intense heating and ca. 63-61 Ma low-pressure contact metamorphism attributed to the intrusion of the voluminous Ruby Range suite, which is part of the northern Coast Mountains batholith. The tectonometamorphic evolution recorded in the Blanchard River assemblage can be correlated to tectonism within southwest Yukon and along the length of the Insular-Intermontane boundary from western British Columbia through southwestern Yukon and Alaska. In southwest Yukon, these results suggest an asymmetric final collapse of Jura-Cretaceous basins during the Late Cretaceous, which relates to the terminal accretion of the Insular terranes as they moved northward.

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Implication of the structural evolution of the Adola Belt on shear zone-hosted gold mineralization

Geoscientific Research in Northeast Africa ◽

10.1201/9780203753392-98 ◽

2017 ◽

pp. 541-547

Author(s):

H. Worku

Keyword(s):

Shear Zone ◽

Gold Mineralization ◽

Structural Evolution

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Influence of inherited structural domains and their particular strain distributions on the Roer Valley Graben evolution from inversion to extension

10.5194/se-2020-23 ◽

2020 ◽

Author(s):

Jef Deckers ◽

Bernd Rombaut ◽

Koen Van Noten ◽

Kris Vanneste

Keyword(s):

Late Cretaceous ◽

Fault System ◽

Normal Faults ◽

Geological Model ◽

Structural Domains ◽

Stress Directions ◽

Roer Valley Graben ◽

Western Border ◽

Border Fault ◽

As Stress

Abstract. After their first development in the middle Mesozoic, the overall NW-SE striking border fault systems of the Roer Valley Graben were reactivated as reverse faults under Late Cretaceous compression (inversion) and reactivated again as normal faults under Cenozoic extension. In Flanders (northern Belgium), a new geological model was created for the western border fault system of the Roer Valley Graben. After carefully evaluating the new geological model, this study shows the presence of two structural domains in this fault system with distinctly different strain distributions during both Late Cretaceous compression and Cenozoic extension. A southern domain is characterized by narrow ( 10 km) distributed faulting. The total normal and reverse throw in the two domains was estimated to be similar during both tectonic phases. The repeated similarities in strain distribution during both compression and extension stresses the importance of inherited structural domains on the inversion/rifting kinematics besides more obvious factors such as stress directions. The faults in both domains strike NW-SE, but the change in geometry between them takes place across the oblique WNW-ESE striking Grote Brogel fault. Also in other parts of the Roer Valley Graben, WNW-ESE striking faults are associated with major geometrical changes (left-stepping patterns) in its border fault system. This study thereby demonstrates the presence of different long-lived structural domains in the Roer Valley Graben, each having their particular strain distributions that are related to the presence of non-colinear faults.

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Palaeozoic tectonic and sedimentary evolution and hyrdrocarbon prospectivity in the Bornholm area

Danmarks Geologiske Undersøgelse Serie A ◽

10.34194/seriea.v34.7054 ◽

1994 ◽

Vol 34 ◽

pp. 1-23

Author(s):

Ole Valdemar Vejbæk ◽

Svend Stouge ◽

Kurt Damtoft Poulsen

Keyword(s):

Late Cretaceous ◽

Foreland Basin ◽

Fault System ◽

Structural Development ◽

The South ◽

Alum Shale ◽

Present Distribution ◽

Lower Palaeozoic ◽

Wrench Fault ◽

Uplift And Erosion

The present distribution of Palaeozoic sediments in the Bornholm area is a consequence of several different tectonic regimes during the Phanerozoic eon. This development may be divided into three main evolutionary phases: A Caledonian to Variscian phase encompassing the Lower Palaeozoic sediments. The sediments are assumed originally to have showed a gradual thickness increase towards the Caledonian Deformation Front located to the south. This pre-rift development may be further subdivided into three sub-phases: A period of slow sedimentation on a relatively stable platform as recorded by the uniformly low thicknesses of the Cambrian to Lower Silurian sediments. A period of foreland-type rapid sedimentation commencing in the Llandoverian to Wenlockian, continuing in the Ludlovian and possibly into the Devonian. The period is characterized by /olding and uplift of the Caledonides to the south causing tectonic loading of the foreland and resultant rapid sedimentation in the foreland basin. A period of gravitational collapse causing minor erosion during the Devonian. The transition to the second major phase in the Phanerozaic structural development, during which the Sorgenfrei-Tornquist zone came into existence, is recorded by regional deposition of Carboniferous sediments. These sediments are, however, mostly removed by tater erosion. A syn-rift phase characterized by sedimentation in graben areas and expanding basins commencing in the Rotliegendes and continuing through the Triassic, Jurassic and Lower Cretaceous. This phase was probably initiated by a Late Carboniferous- Early Permian tensional dominated right-lateral wrench fault system within the Sorgenfrei-Tornquist zone. A Post-rift development phase dominated by Late Cretaceous carbonate sedimentation. During Late Cretaceous and Early Tertiary times the Bornholm area was strongly affected by inversion tectonism caused by compressional strike-slip movements. This resulted in reverse faulting and uplift and erosion of former basinal areas. Understanding the two latter phases is important for understanding the present distribution of the Palaeozoic. A key to understanding the hydrocarbon potential of the area is the maturation of the organic matter in the main potential source, the Ordovician Upper Alum Shale. Maturity was mainly achieved during the Silurian to Late Palaeozoic time, and little further maturation took place later. The Upper Alum Shale is accordingly expected to be overmature in the main part of the study area and mature in the Hano Bay Basin. This reflects the assumed primary uniform thickness of the Lower Palaeozoic, with a general thinning towards the northeast. A Caledonian to Variscian phase encompassing the Lower Palaeozoic sediments. The sediments are assumed originally to have showed a gradual thickness increase towards the Caledonian Deformation Front located to the south. This pre-rift development may be further subdivided into three sub-phases: A period of slow sedimentation on a relatively stable platform as recorded by the uniformly low thicknesses of the Cambrian to Lower Silurian sediments. A period of foreland-type rapid sedimentation commencing in the Llandoverian to Wenlockian, continuing in the Ludlovian and possibly into the Devonian. The period is characterized by /olding and uplift of the Caledonides to the south causing tectonic loading of the foreland and resultant rapid sedimentation in the foreland basin. A period of gravitational collapse causing minor erosion during the Devonian. The transition to the second major phase in the Phanerozaic structural development, during which the Sorgenfrei - Tornquist zane came into existence, is recorded by regional deposition of Carboniferous sediments. These sediments are, however, mostly removed by tater erosion. A syn-rift phase characterized by sedimentation in graben areas and expanding basins commencing in the Rotliegendes and continuing through the Triassic, Jurassic and Lower Cretaceous. This phase was probably initiated by a Late Carboniferous- Early Permian tensional dominated right-lateral wrench fault system within the Sorgenfrei-Tornquist zone. A Post-rift development phase dominated by Late Cretaceous carbonate sedimentation. During Late Cretaceous and Early Tertiary times the Bornholm area was strongly affected by inversion tectonism caused by compressional strike-slip movements. This resulted in reverse faulting and uplift and erosion of former basinal areas. Understanding the two latter phases is important for understanding the present distribution of the Palaeozoic. A key to understanding the hydrocarbon potential of thearea is the maturation of the organic matter in the main potential source, the Ordovician Upper Alum Shale. Maturity was mainly achieved during the Silurian to Late Palaeozoic time, and little further maturation took place later. The Upper Alum Shale is accordingly expected to be overmature in the main part of the study area and mature in the Hano Bay Basin. This reflects the assumed primary uniform thickness of the Lower Palaeozoic, with a general thinning towards the northeast.

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