Southern Basin and Range
Published:January 01, 1980
Metamorphic core complexes in southern Arizona may be subdivided into four elements: core, metamorphic carapace, decollement, and cover. Cores consist chiefly of mylonitic augen gneiss that is, for the most part, derived from Precambrian and Phanerozoic plutonic rocks of granitic composition. Foliation in the cores is characteristically low dipping and commonly defines large upright, doubly plunging foliation arches. The mylonitic augen gneisses everywhere display low-dipping mineral lineation of a cataclastic nature. Lineation within any given metamorphic core complex is generally remarkably systematic in orientation. In southeastern Arizona the lineation trends northeast to east-northeast; in south-central Arizona, it trends north-south. Ductile normal faults are locally abundant in the core rocks and always are oriented perpendicular to lineation. Core rocks in places are clearly transitional laterally or downward into undeformed protolith.
The metamorphic carapace consists of penetratively deformed younger Precambrian and Phanerozoic strata metamorphosed to greenschist-amphibolite grade. It forms a crudely tabular layer that locally overlies the crystalline rocks of the core. The contact is commonly so tight that the rocks of the carapace appear to be plated onto the crystalline rocks of the core. Within the metamorphic carapace, overturned to recumbent folds are ubiquitous, transposition is rampant, and boudins and pinch-and-swell features are commonplace. In spite of spectacular deformation, individual formations within the metamorphic carapace are arranged in normal stratigraphic order.
A decollement, marked by brittle low-angle faulting and shearing, separates carapace and cover or, where carapace is absent, core and cover. The surface thus separates rocks of remarkably contrasting deformational styles. Where the surface “caps” core rocks, a decollement zone is formed beneath the decollement and consists of a distinctive crudely tabular zone of crushed and granulated but strongly indurated mylonite, mylonitic gneiss, microbreccia, and chlorite breccia. Striking “younger-on-older” fault relations characterize the decollement. Decollements in this area typically separate orthogneisses, which were derived in part from Precambrian rocks, from unmetamorphosed upper Paleozoic, Mesozoic, or Tertiary strata. Overturned asymmetric folds, detached isoclinal folds, and unbroken cascades of recumbent folds are abundant in many of the cover sheets.
The metamorphic core complexes in southern Arizona are interpreted to be products of high-temperature extensional deformation, regional in extent, superseded by moderately ductile to moderately brittle tectonic denudation. Rocks in the augen gneissic core and metamorphic carapace were affected by profound ductile through brittle extension and flow in the direction of mineral lineation. Evolution of the decollement zones largely postdated the development of foliation and lineation.
Mechanics of strain are interpreted in the context of megaboudinage. Cores are pictured as parts of megaboudins imposed on heterogeneous crustal rocks. Profound thinning of younger Precambrian and Paleozoic sediments through flow during deformation had the effect of significantly decreasing the stratigraphic separation between individual Phanerozoic formations and the Precambrian basement. Intrafolial folds developed in the carapace as products of passive flow. The relatively brittle, massive crystalline basement responded to extension and flattening by ductile normal faulting and the development of penetrative foliation and lineation. The high-temperature extensional disturbance, which began in early Tertiary time and ended about 25 m.y. ago(?), was accompanied by moderately ductile to moderately brittle tectonic denudation and gravity-induced folding of cover rocks. Major listric normal faulting postdated the development of tectonite, shaped the internal fabric of the decollement zones, and effected final movements of the covers.