Footwall rocks to the mid-Tertiary Chemehuevi detachment fault: A window into the middle crust in the southern Cordillera

Journal Article · Wed Jan 10 04:00:00 EST 1990 · Journal of Geophysical Research; (United States)

DOI:https://doi.org/10.1029/JB095iB01p00463· OSTI ID:5073979

John, B E ^[1]; Mukasa, S B ^[2]

---

1. Univ. of Cambridge (England)
2. Univ. of Michigan, Ann Arbor (United States)

Crystalline rocks unroofed from the middle crust during Tertiary crustal extension by the Chemehuevi detachment fault record a complex history of comprises both subhorizontal and subvertically foliated mylonitic gneisses sharing a common gently SW-plunging stretching lineation, which are intruded by members of the voluminous laccolith-shaped Chemehuevi Mountains plutonic suite. A belt of steeply foliated mylonitic gneiss up to 5 km across, traceable for 12 km along strike (NE-SW) crops out in the northern part of the range. This belt passes abruptly downward into the subhorizontal shear zone exposed in the easter Chemehuevi Mountains. A preliminary shear sense study indicates NE-directed shear within the sub-horizontal mylonitic rocks, and sinistral hear parallel to the steeply foliated mylonites. Four zircon fractions from this unit are discordant, but define a chord with a lower intercept of around 90 Ma and an upper intercept of {approximately} 1,900 Ma. Thirteen zircon fractions from two separate undeformed members of the suite (biotite granodiorite and porphyritic biotite granodiorite) are variably discordant, but fractions for each unit define chords that yield latest Cretaceous crystallization ages with upper intercepts of {approximately} 1,600 Ma. Together these data constrain the timing of plutonism and ductile deformation to the Late Cretaceous. Superimposed on this fabric and involving all rocks types in the footwall, are the localized brittle and ductile effects of the mid-Tertiary extensional deformation. Movement direction and the timing of mylonitic deformation in the Chemehuevi Mountains suggest that the effects of the Cordilleran fold and thrust belt extends into the Chemehuevi Mountains region, where it in part overlapped with, and was heated by, plutonism along the eastern margin of the Mesozoic magmatic arc during late Cretaceous time.

• OSTI ID: 5073979
• Journal Information: Journal of Geophysical Research; (United States), Journal Name: Journal of Geophysical Research; (United States) Vol. 95:B1; ISSN 0148-0227; ISSN JGREA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ARIZONA
CALIFORNIA
CENOZOIC ERA
CONTINENTAL CRUST
CRETACEOUS PERIOD
CRYSTALLIZATION
DEFORMATION
DEVELOPED COUNTRIES
EARTH CRUST
FEDERAL REGION IX
GEOLOGIC AGES
GEOLOGIC FAULTS
GEOLOGIC FORMATIONS
GEOLOGIC FRACTURES
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
GNEISSES
IGNEOUS ROCKS
MESOZOIC ERA
METAMORPHIC ROCKS
MINERALS
MOUNTAINS
NORTH AMERICA
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
ROCKS
SHEAR
SILICATE MINERALS
TECTONICS
TERTIARY PERIOD
USA
ZIRCON