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Metamorphic Core Complexes and Related Features Metamorphic Core Complexes 173 CHAPTER 13 Metamorphic Core Complexes and Related Features Continental and Oceanic Core Complexes A core complex “is a domal or arched geologic structure Cordilleran composed of ductilely deformed rocks and associated metamorphic intrusions underlying a ductile-to-brittle high-strain core complex zone that experienced tens of kilometers of normal- locations in the sense displacement in response to lithospheric American west. extension” (Whitney and colleagues, 2013, p. 274). Modified after Core complex structures occur in both oceanic and Coney (1973). continental crust in regions where extension of the crust has produced the exhumation of ductile portions of the deep crust and/or upper mantle. These regions include slow-spreading mid-ocean ridges, core complexes in the continents, and at continental rifted margins. Extension causes the brittle upper crust to pull apart along normal faults. If the extension occurs along a few faults, ductile lower crust or mantle material moves upward to shallow levels of the crust and is exhumed in the footwall of normal faults and exposed at the surface. characterized Oceanic core by complexesfault-bounded were domal first discoveredstructures in similarseafloor to images the metamorphic of the Mid-Atlantic core complexes Ridge; they of the were continents. Since then, oceanic core complexes to hundreds of meters. Corrugations may be called “megamullions” for oceanic core complexes. Antarctichave been Discordance. found in the Southwest Indian Ridge, the Typically, continental core complexes are elliptical Caribbean-NorthBoth continental American and oceanic Ridge, core and complexesthe Australian- in plan view and have a long axis between 6 and 24 may have corrugated or undulating detachment miles (10 and 40 km), although a few are larger such fault surfaces. In such cases, the corrugation axis is as the Shuswap complex. Also, the footwall tends to be elevated above surrounding rocks by as much as corrugations may have wavelengths of hundreds of metersaligned toparallel tens of to kilometers the direction and of amplitudes extension; of and tens the complexes are characterized by mylonite zones several hundred1.2 miles meters(2 km) thick.of relief. Most of the Cordilleran core 174 Idaho Geology Listric faults are normal faults with a concave- upwards fault plane that gradually flattens at depth. Detachment Faults The displacement of the brittle surface rocks or upper plate occurred along low-angle detachment n areas of extreme extension, such as the Basin faults. The deep, highly deformed middle crustal rocks of the lower plate are exposed by the removal metamorphic core complexes) have had the upper of the brittle relatively undeformed rocks along the layersand Range of brittle Province, rock strippedmany areas off (referredthe underlying to as ductile detachment fault. These detachment structures are rocks along detachment faults. The east-west extension characterized by parallel bands of mylonite formed occurs along parallel to subparallel listric normal faults along the shear zone in the lower plate. that merge at depth with the detachment fault. The In the detachment zone, structural fabric indicates detachment fault, which is commonly represented by the direction of transport of the upper plate, which a thick zone or layers of mylonite, occurs along the transition zone separating the ductile metamorphic core complexes are characterized by broad domal and plutonic core rocks below from the overlying upliftmay be presumably in response caused to regional by isostatic tectonics. adjustment Most to brittle rocks. The brittle rocks above are moved tens tectonic removal of the upper layers. The once planar of kilometers to the east or west along detachment detachment faults or layers of mylonite are now arched faults and the metamorphic and plutonic rocks that had previously been buried by as much as 12 miles of the upper plate rocks is consistent throughout theupwards; complex. however, Extension the direction of many metamorphic of tectonic transfer Core fault are generally represented by one or more layers of(20 mylonite km) are with exposed. gently Rocks dipping below foliation. the detachment In some plutonism but the role of these intrusions is somewhat cases the mylonitic rocks are deformed by cataclasis controversial.complexes appears to be influenced or facilitated by indicating a change from ductile to brittle deformation as the rocks were exposed to a lower pressure and temperature environment. There is a close association Development of Gneissic Domes of the magmatism and large-scale extension of the Again, there are two kinds of extensional faults in Cenozoic extension in the Cordilleran of North detachment faults associated with metamorphic core Americaupper crust has in revealed the Basin metamorphic and Range Province.core complexes complexesthe Basin and where Range horizontal Province: extension low-angle may normal exceed 200 in several locations in Idaho. Core complexes typically percent and the more typical high-angle normal faults have a window to a lower plate of high-grade metamorphic and igneous rocks separated by a low- angle detachment fault from an upper plate of less complexesthat caused develop the Basin along and detachment Range Province. faults. The deformed rocks. In the detachment fault system, the detachment Metamorphic fault mergesdomes ininto the a metamorphicductile shear zonecore upper layers are characterized by brittle deformation in the crust. Above the shear zone, curved or listric producing breccia and the deeper more ductile portions normal faults merge with the detachment fault. The fault blocks are tilted because they move down the same extensional event. curved listric faults. produce mylonites; however, they are all related to the Metamorphic Core Complexes 175 As the extension faulting thins and removes the upper plate, the lower plate rises from thermal expansion and from isostatic adjustment. Consequently, exposedplate of Precambrian in the metamorphic gneisses core. dated An at upper 2 Ga andplate the footwall is at least partially exposed at the surface Cretaceous to Early Paleogene granitic rocks are having risen from 6 to 12 miles (10 to 20 km) below the overlain by Challis volcanic rocks surrounds the surface by a process called tectonic exhumation. Finally, metamorphicof unmetamorphosed core. The Paleozoic Wild Horse sedimentary detachment rocks system a dome of high-grade metamorphic rocks is exposed at separates the upper plate from the lower plate. the surface. Timing of Metamorphic Core Complex produced Through east-west the Mesozoic, compression the subduction and thickening of the of the Pacific plate and the intrusion of the Idaho batholith Extension extension produced structures such as the Wildhorse detachmentcrust. By the fault. early ThisPaleogene detachment (50 Ma) fault the involved transition to by the Siletzia accretion and then the Cascadia about 10 miles (17 km) of separation and the upper subduction,The termination gravitational of the flat collapse slab subduction of the thickened followed plate moved northwest relative to the lower plate. The crust could cause extension. Immediately after the Siletzia accretion, Laramide orogeny compression was extension related to the trans-Challis fault system was changed to metamorphic core complex extension and alsoChallis part volcanic of this episodeextension. that Low-angle occurred faults 50 to removed40 Ma and the brittle sedimentary cover of the uppercrustal rocks ignimbrite flare-up. This ignimbrite volcanism was most likely caused by the flat-slab removal and slab upperfrom the crust Pioneer exposed Mountains high-grade about metamorphic 40 to 50 Ma. rocks From up rollback. The ignimbrite flare-up propagated from 45 to37 2.3 to 34billion Ma, finalyears uplift old. and removal of the overlying Ma in the north to 21 Ma in the south. Most of the core complexes started 50 to 15 Ma—well before Basin and Range faulting. Bitterroot Metamorphic Core Complex Clearwater and Priest River Metamorphic The Bitterroot metamorphic core complex is an Core Complexes unroofed metamorphic complex straddling the Idaho- Vervoort and colleagues (2016) have examined the core complexes, the area experienced crustal thickening North American Cordillera in northern Idaho at duringMontana the border Cordilleran in east-central east-west Idaho. compression Like the followed other Precambrian crystalline basement underlying the associated with extension led to exhumation of the gneissesthe Clearwater that represent and Priest two River discrete metamorphic periods in core the complex.by extension starting about 50 Ma. Granitic intrusions complexes. They identified and dated two basement that these gneisses were generated by two periods of METAMORPHIC CORE COMPLEX OF Neoarchean and Paleoproterozoic. They determined THE ALBION MOUNTAINS Ga and a later addition to the crust at 1.86 Ga involving amagmatism: mix of pre-existing and initial Neoarchean formation crust of the and crust juvenile at 2.66 mantle. These Neoarchean basement rocks represent the The Albion Mountains are a representative portion of Thisa larger metamorphic region referred core complexto as the hasRaft resulted River-Grouse in diverse exposed in the northwest excluding the Wyoming igneous,Creek-Albion metamorphic Mountains and metamorphic sedimentary core rocks complex. with ages largest area of Paleoproterozoic and Archean crust depths of 20 to 30 km during the early Eocene as a together in
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