Cordilleran Metamorphic Core Complexes — Historical Perspectives on Evolution of Concepts in the Basin and Range Keith Howard United States Geological Survey

The Basin and Range seeded many of the concepts.…about core complexes as domed windows of the middle crust denuded by extensional faulting and isostatically uplifted,.…and their significance for understanding how the crust stretches. Coney (1980) DEATH VALLEY TURTLEBACKS

This talk will highlight * A sampling of advances that came from work in the Basin and Range. * And a few of the key players. The list could be much longer……

K Howard C Thorman Misch, Hazzard and students, Gr. Basin (e.g. Snelson, Thorman). Lanphere et al., Death Valley. Young K-Ar dates: Damon & colleagues in AZ. Armstrong & Hansen in Gr. Basin. Metamorphic Complexes Ruby Mountains

Proposed the metamorphic complexes in the Great Basin record a ductile infrastructure separated from a brittle suprastructure by a shearing-off zone (Armstrong and Hansen, 1966). Fold in the Raft River Range Richard Armstrong

Low-angle normal faults were recognized long ago in the Basin & Range. Longwell ‘45 Hunt and Mabey ‘66 Wright and Troxell ‘69 RE Anderson ’71 Proffett ‘77 Armstrong ’72: low-angle younger-on- older faults are extensional, NOT thrusts! DEATH VALLEY TURTLEBACKS Longwell 1945

How did we get the term “detachment fault”? W Pierce (Heart Mtn Fault) Hunt and Mabey 1966 Death Valley R Campbell et al Santa Monica Mts. W Carr & D Dickey 1976 Whipple Mts. Whipple Mts Detachment Fault

Richard Armstrong (in 1975): ”I think the mylonite fabrics are actually Tertiary, related to extension, rising domes and arches leading to exposure of the rocks from depth…a rapid sequence of events that “quenches” the geochron at the time of denudation…penetrative fabrics are a related feature.. Best regards, Dick A.” Mapped MCCs (Armstrong, 1982)

The “Arizona School” led by Coney and Davis tied brittle detachments to mylonitic shear zones and extension Steve Reynolds documented Miocene mylonitization Rehrig & Reynolds (1980) George Davis

Integrated studies of core- complex anatomy and evolution Arthur Snoke documented Miocene denudation & a series of detached mylonitic fault slivers in the Ruby- East Humboldt Range. He spurred a long series of collaborative studies of structure, PT, and timing there. Sponsored a 1981 Penrose Conference on fault rocks that brought new players to the Cordilleran MCCs. G Lister

PTt studies in the Ruby-East Humboldt Range by Al McGrew) Clockwise PTt path Art Snoke K Hodges

Other core complexes revealed further insights on complex, polydeformational histories for the metamorphic cores and their unroofing. V Todd R Compton Raft River Range (and Albion, Grouse Cr.) Armstrong Compton Todd D Miller M Wells & T Hoisch E Miller & students

E Miller Elizabeth Miller and colleagues and students, following earlier work by Misch and by Coney, led a comprehensive series of studies in the Snake Range.

Whipple and Rawhide Mountains Gregory Davis J Lawford Anderson Davis et al. (1980) “Spectacular examples of Cenozoic denudation.”

Whipple Mountains Greg Davis & Gordon Lister

The Whipple and Chemehuevi core complexes are part of the 100-km-wide Colorado River extensional corridor.

Fault rocks progress from ductile to brittle upon unroofing Barbara John Chemehuevi detachment fault Barbara John—Chemehuevi Greg Davis-Whipple George Davis—Catalina-Rincon

Corrugated faults Down-dip ridges of Harcuvar complex B John—Corrugations down the slip direction in the Chemehuevi detachment are grooves cut in the brittle part of the section. J Spencer— Detachment-fault corrugations down the slip direction are molded by continuous casting. Jon Spencer Sacsayhumán, Peru

Do MCCs record gravitational collapse of overthickened crust? (Armstrong 1982) --Spencer and Reynolds tested this concept in the Maria fold-thrust belt. B Wernicke Coney & Harms 1984 map of restored crustal thickness. --Wernicke et al. & Sonder et al. ‘87 related delay of collapse to thermal state of crust.

Magmatism “The association of magmatism and core complexes leads to the deduction that magmatism is a necessary precondition to the formation of metamorphic core complexes, because it leads to elevated geotherms and a consequent weakening of the entire lithosphere so that extension may occur.” (Armstrong and Ward, 1991) Phillip Gans et al. (1989) plotted extension & magmatism ages up and down the B & R Dick Armstrong age latitide

Isostatic doming S Reynolds J Spencer Rehrig and Reynolds ‘80 Hyndman ‘80 Spencer ‘84

Rolling hinge? Buck’s model (1988) B Wernicke -Wernicke and Axen -Hamilton

Scientists mentored by Burchfiel have had a huge impact B Wernicke Clark Burchfiel B Wernicke K Hodges E Miller J Spencer examples ….and J Bartley, D Walker, others.

Flat Moho! COCORP results No Moho fault offsets. But there are huge lateral variations in upper crustal extension…..(Gans, 1987) So the upper & lower crust are decoupled, and there must be lower crust and igneous modifications to keep the crustal thickness constant. Phillip Gans

J McCarthy et al.’s (1991) seismic model across the Whipple core complex showed a domed but flat-bottomed middle crust—consistent with lower crustal return flow. Block & Royden model

Heat-flow constraints (Lachenbruch, Sass, & Morgan) Core complexes in the southern Basin and Range turned out to be heat-flow lows (low-heat-producing lower crust is domed up). This implies return flow occurred in the deep crust, not in the mantle.

Thermochronology Techniques: Cooling, Uplift, and Stretching Rates Cooling paths and rates (e.g. Dokka’s). From Foster & John (1999)

Cooling, Uplift, and Stretching Rates Extension onset and lateral denudation rates (e.g. Gans, Foster). David Foster Foster & John (1999)

Cooling, Uplift, and Stretching Rates Unroofing age of tilt blocks (e.g. Stockli, Colgan). Thermal structure of tilted blocks (e.g. Blackwell, Foster, Fitzgerald, Reiners). Joe Colgan Reiners— Thermochronometers in Gold Butte block. E Miller

To sum up Basin and Range contributions include: Low-angle extensional faults and their quantification of extension.

To sum up Basin and Range contributions include: Low-angle extensional faults and their quantification of extension. Integration of extensional processes in the shallow and deep crust.

To sum up Basin and Range contributions include: Low-angle extensional faults and their quantification of extension. Integration of extensional processes in the shallow and deep crust. Progressive rise and unroofing of middle crust by ductile- to-brittle faulting.

To sum up Basin and Range contributions include: Low-angle extensional faults and their quantification of extension. Integration of extensional processes in the shallow and deep crust. Progressive rise and unroofing of uplifted middle crust by ductile-to-brittle faulting. Gravitational collapse and magmatism as drivers.

To sum up Basin and Range contributions include: Low-angle extensional faults and their quantification of extension. Integration of extensional processes in the shallow and deep crust. Progressive rise and unroofing of uplifted middle crust by ductile-to-brittle faulting. Gravitational collapse and magmatism as drivers. Lateral intracrustal return flow compensates for denudation and updoming.

To sum up Basin and Range contributions include: Low-angle extensional faults and their quantification of extension. Integration of extensional processes in the shallow and deep crust. Progressive rise and unroofing of uplifted middle crust by ductile-to-brittle faulting. Gravitational collapse and magmatism as drivers. Lateral intracrustal return flow compensates for denudation and updoming. Methods of determining cooling and denudation rates.

Other B&R contributions not explored here include: Slip on gently dipping faults. Dike inflation. Polymodal extensional histories. Corrugated faults. Basin analysis & megabreccia sheets. Transverse accommodation zones. and on and on………. Domino faulting

So, The excellent exposures and a lot of great field-based science in the Basin and Range have led a remarkable revolution in understanding crustal extension. Continuing progress since the 1970s has resulted in major advancements …….and still they come.

CONCLUSION Grooved footwall, Atlantis Transform, Mid-Atlantic Ridge (Cann et al., 1997) The Basin and Range has been an amazingly fertile spawning ground for concepts about metamorphic core complexes and what they say about processes of crustal extension. The concepts have been found to be broadly applicable to the other continents, to mid- Ocean Ridges, maybe even another planet.