Early Tertiary Anaconda Metamorphic Core Complex, southwestern Montana

2004 ◽  
Vol 41 (1) ◽  
pp. 63-72 ◽  
Author(s):  
J Michael O'Neill ◽  
Jeff D Lonn ◽  
David R Lageson ◽  
Michael J Kunk
Keyword(s):  
Sedimentary Rocks ◽  
Shear Zones ◽  
Detachment Fault ◽  
Integrated System ◽  
Metamorphic Core Complex ◽  
Granitic Rocks ◽  
Core Complex ◽  
Early Tertiary ◽  
Late Tertiary ◽  
Metamorphic Core

A sinuous zone of gently southeast-dipping low-angle Tertiary normal faults is exposed for 100 km along the eastern margins of the Anaconda and Flint Creek ranges in southwest Montana. Faults in the zone variously place Mesoproterozoic through Paleozoic sedimentary rocks on younger Tertiary granitic rocks or on sedimentary rocks older than the overlying detached rocks. Lower plate rocks are lineated and mylonitic at the main fault and, below the mylonitic front, are cut by mylonitic mesoscopic to microscopic shear zones. The upper plate consists of an imbricate stack of younger-on-older sedimentary rocks that are locally mylonitic at the main, lowermost detachment fault but are characteristically strongly brecciated or broken. Kinematic indicators in the lineated mylonite indicate tectonic transport to the east-southeast. Syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging-wall rocks in low-lying areas between fault blocks and breccia zones. Muscovite occurs locally as mica fish in mylonitic quartzites at or near the main detachment. The 40Ar/39Ar age spectrum obtained from muscovite in one mylonitic quartzite yielded an age of 47.2 + 0.14 Ma, interpreted to be the age of mylonitization. The fault zone is interpreted as a detachment fault that bounds a metamorphic core complex, here termed the Anaconda metamorphic core complex, similar in age and character to the Bitterroot mylonite that bounds the Bitterroot metamorphic core complex along the Idaho-Montana state line 100 km to the west. The Bitterroot and Anaconda core complexes are likely components of a continuous, tectonically integrated system. Recognition of this core complex expands the region of known early Tertiary brittle-ductile crustal extension eastward into areas of profound Late Cretaceous contractile deformation characterized by complex structural interactions between the overthrust belt and Laramide basement uplifts, overprinted by late Tertiary Basin and Range faulting.

Crustal structure and early Tertiary extensional tectonics of the Omineca belt at 51°N latitude, southern Canadian Cordillera

1996 ◽  
Vol 33 (12) ◽  
pp. 1596-1611 ◽  
Author(s):  
Bradford J. Johnson ◽  
Richard L. Brown
Keyword(s):  
Seismic Data ◽  
Shear Zones ◽  
Core Complex ◽  
Crustal Rocks ◽  
Early Tertiary ◽  
Geological Information ◽  
Extensional Strain ◽  
Palinspastic Restoration ◽  
Metamorphic Core ◽  
Normal Sense

A crustal cross section through the Omineca belt at the latitude of the Trans-Canada Highway has been drawn to satisfy available surface geological information and Lithoprobe seismic data from this part of the Cordilleran hinterland. Palinspastic restoration of Tertiary normal-sense shear zones leads to the conclusion that the Omineca belt at latitude 51°N was extended in the Eocene by approximately 45 km, 20–25% of the width of the belt. It is shown that the Okanagan–Eagle River fault, which defines the western margin of the Shuswap metamorphic core complex, is likely to have accommodated approximately 30 km of displacement. Restoration of this fault and of 15 km displacement on the Columbia River fault (eastern margin of the Shuswap complex) juxtaposes upper-crustal rocks with similar stratigraphic, structural, and metamorphic characteristics and indicates that the crust was over 50 km thick prior to Eocene extension. Comparison of the crustal geometry in the present and restored sections suggests that extensional strain was partitioned such that the upper crust was most highly attenuated above the central Shuswap complex, whereas the lower crust was most greatly stretched beneath the Intermontane and western Omineca belts.

Permian rifting and detachment faults and their role in Alpine collisional tectonics

2021 ◽  
Author(s):  
Nikolaus Froitzheim ◽  
Linus Klug
Keyword(s):  
Normal Fault ◽  
Southern Alps ◽  
Detachment Fault ◽  
Metamorphic Core Complex ◽  
Rift System ◽  
Early Permian ◽  
Core Complex ◽  
Detachment Faults ◽  
The North ◽  
Metamorphic Core

<p>The Permian was a time of strong crustal extension in the area of the later-formed Alpine orogen. This involved extensional detachment faulting and the formation of metamorphic core complexes. We describe (1) an area in the Southern Alps (Valsassina, Orobic chain) where a metamorphic core complex and detachment fault have been preserved and only moderately overprinted by Alpine collisional shortening, and (2) an area in the Austroalpine (Schneeberg) where Alpine deformation and metamorphism are intense but a Permian low-angle normal fault is reconstructed from the present-day tectonometamorphic setting. In the Southern Alps case, the Grassi Detachment Fault represents a low-angle detachment capping a metamorphic core complex in the footwall which was affected by upward‐increasing, top‐to‐the‐southeast mylonitization. Two granitoid intrusions occur in the core complex, c. 289 Ma and c. 287 Ma, the older of which was syn-tectonic with respect to the extensional mylonites (Pohl, Froitzheim, et al., 2018, Tectonics). Consequently, detachment‐related mylonitic shearing took place during the Early Permian and ended at ~288 Ma, but kinematically coherent brittle faulting continued. Considering 30° anticlockwise rotation of the Southern Alps since Early Permian, the extension direction of the Grassi Detachment Fault was originally ~N‐S and the sense of transport top-South. In this area, there is no evidence of Permian strike-slip faulting but only of extension. In the Schneeberg area of the Austroalpine, a unit of Early Paleozoic metasediments with only Eoalpine (Cretaceous) garnet, the Schneeberg Complex, overlies units with two-phased (Variscan plus Eoalpine) garnet both to the North (Ötztal Complex) and to the South (Texel Complex). The basal contact of the Schneeberg Complex was active as a north-directed thrust during the Eoalpine orogeny. It reactivated a pre-existing, post-Variscan but pre-Mesozoic, i.e. Permian low-angle normal fault. This normal fault had emplaced the Schneeberg Complex with only low Variscan metamorphism (no Variscan garnet) on an amphibolite-facies metamorphic Variscan basement. The original normal fault dipped south or southeast, like the Grassi detachment in the Southern Alps. As the most deeply subducted units of the Eoalpine orogen (e.g. Koralpe, Saualpe, Pohorje) are also the ones showing the strongest Permian rift-related magmatism, we hypothesize that the Eoalpine subduction was localized in a deep Permian rift system within continental crust.</p>

Geometric and temporal evolution of an extensional detachment fault, Hohhot metamorphic core complex, Inner Mongolia, China

Geology ◽  
2002 ◽  
Vol 30 (11) ◽  
pp. 1003 ◽  
Author(s):  
Gregory A. Davis ◽  
Brian J. Darby ◽  
Zheng Yadong ◽  
Terry L. Spell
Keyword(s):  
Inner Mongolia ◽  
Temporal Evolution ◽  
Detachment Fault ◽  
Metamorphic Core Complex ◽  
Core Complex ◽  
Metamorphic Core

The Xiaoqinling Detachment Fault and Metamorphic Core Complex of China: Structure, Kinematics, Strain and Evolution.

2018 ◽  
pp. 158-172
Author(s):  
Jinjiang Zhang ◽  
Yadong Zheng ◽  
Quanzeng Shi ◽  
Xiangdong Yu ◽  
Liangwei Xue
Keyword(s):  
Detachment Fault ◽  
Metamorphic Core Complex ◽  
Core Complex ◽  
Metamorphic Core

Miocene slip history of the Eagle Eye detachment fault, Harquahala Mountains metamorphic core complex, west-central Arizona

Tectonics ◽  
2016 ◽  
Vol 35 (8) ◽  
pp. 1913-1934 ◽  
Author(s):  
Michael G. Prior ◽  
Daniel F. Stockli ◽  
John S. Singleton
Keyword(s):  
Detachment Fault ◽  
Metamorphic Core Complex ◽  
Core Complex ◽  
West Central ◽  
Slip History ◽  
History Of ◽  
Central Arizona ◽  
Metamorphic Core

MICROSTRUCTURAL ANALYSIS OF METASEDIMENTARY AND QUARTZOFELDSPATHIC ROCKS WITHIN THE WILDHORSE DETACHMENT FAULT IN THE PIONEER MOUNTAINS METAMORPHIC CORE COMPLEX, IDAHO

2019 ◽  
Author(s):  
Alex Senjem ◽  
◽  
Rory R. McFadden ◽  
Jennifer M. Taylor ◽  
Nicholas C.A. Seaton ◽  
...  
Keyword(s):  
Microstructural Analysis ◽  
Detachment Fault ◽  
Metamorphic Core Complex ◽  
Core Complex ◽  
Metamorphic Core

scholarly journals Compressional origin of the Naxos metamorphic core complex, Greece: Structure, petrography, and thermobarometry

2019 ◽  
Vol 132 (1-2) ◽  
pp. 149-197 ◽  
Author(s):  
Thomas N. Lamont ◽  
Michael P. Searle ◽  
David J. Waters ◽  
Nick M.W. Roberts ◽  
Richard M. Palin ◽  
...  
Keyword(s):  
Regional Metamorphism ◽  
Shear Zones ◽  
Metamorphic Core Complex ◽  
Crustal Thickening ◽  
Normal Faults ◽  
Crustal Extension ◽  
Core Complex ◽  
The Core ◽  
History Of ◽  
Metamorphic Core

Abstract The island of Naxos, Greece, has been previously considered to represent a Cordilleran-style metamorphic core complex that formed during Cenozoic extension of the Aegean Sea. Although lithospheric extension has undoubtedly occurred in the region since 10 Ma, the geodynamic history of older, regional-scale, kyanite- and sillimanite-grade metamorphic rocks exposed within the core of the Naxos dome is controversial. Specifically, little is known about the pre-extensional prograde evolution and the relative timing of peak metamorphism in relation to the onset of extension. In this work, new structural mapping is presented and integrated with petrographic analyses and phase equilibrium modeling of blueschists, kyanite gneisses, and anatectic sillimanite migmatites. The kyanite-sillimanite–grade rocks within the core complex record a complex history of burial and compression and did not form under crustal extension. Deformation and metamorphism were diachronous and advanced down the structural section, resulting in the juxtaposition of several distinct tectono-stratigraphic nappes that experienced contrasting metamorphic histories. The Cycladic Blueschists attained ∼14.5 kbar and 470 °C during attempted northeast-directed subduction of the continental margin. These were subsequently thrusted onto the more proximal continental margin, resulting in crustal thickening and regional metamorphism associated with kyanite-grade conditions of ∼10 kbar and 600–670 °C. With continued shortening, the deepest structural levels underwent kyanite-grade hydrous melting at ∼8–10 kbar and 680–750 °C, followed by isothermal decompression through the muscovite dehydration melting reaction to sillimanite-grade conditions of ∼5–6 kbar and 730 °C. This decompression process was associated with top-to-the-NNE shearing along passive-roof faults that formed because of SW-directed extrusion. These shear zones predated crustal extension, because they are folded around the migmatite dome and are crosscut by leucogranites and low-angle normal faults. The migmatite dome formed at lower-pressure conditions under horizontal constriction that caused vertical boudinage and upright isoclinal folds. The switch from compression to extension occurred immediately following doming and was associated with NNE-SSW horizontal boudinage and top-to-the-NNE brittle-ductile normal faults that truncate the internal shear zones and earlier collisional features. The Naxos metamorphic core complex is interpreted to have formed via crustal thickening, regional metamorphism, and partial melting in a compressional setting, here termed the Aegean orogeny, and it was exhumed from the midcrust due to the switch from compression to extension at ca. 15 Ma.

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