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Description
The Riverside Mountains of southeastern California are perhaps the clearest example of the superposition of Mesozoic compressional features with mid-Tertiary extensional phenomena in the lower Colorado River region. Warren Hamilton described isoclinally folded complexes of thrust sheets involving Precambrian basement interlayered with Mesozoic and Paleozoic metasediments of the Grand Canyon sequence, which are overprinted by a Tertiary low-angle fault on the southern side of the Riversides. Will Carr and Del Dickey labeled the low-angle fault the Whipple detachment fault and also show Mesozoic and Paleozoic thrust stacks on Precambrian units in the lower plate. The Riverside Mountains resemble a horseshoe morphology open to the north. The eastern Riversides define a major southwest-plunging antiform with several minor antiforms and synforms forming the rest of the range as a result of mid-Tertiary crustal warping. The detachment fault is locally exposed and flanks the southern margin of the Riversides conforming to the folded geometry of the range. The detachment fault separates a brittley distended upper plate consisting of Tertiary volcanics, breccias, fanglomerates, fluvial-lacustrine deposits and Precambrian schist and gneiss from a non-mylonitic, relatively unaffected lower plate consisting of interleaves of upright and overturned Paleozoic and Mesozoic sequences between Precambrian crystalline units that reflect deep-seated Mesozoic compressional deformation. Kinematic indicators suggest upper-plate transport to the northeast above a lower plate that shows partial K-Ar resetting to 37 myBP just below the fault. Lower-plate metamorphic mineral assemblages suggest greenschist-grade metamorphism and physical conditions of formation of 350-450 and 2-7kb. As a result, Paleozoic sequences display a high ductile deformation fabric, yet remain relatively intact. The lower-plate Mesozoic deformational history has been interpreted by palinspastically removing the mid-Tertiary deformation. Two distinct episodes of Mesozoic folding and thrusting (f1, f2) have been recognized on the basis of superposed large-and small-scale folds and their associated structural elements. Northeast-verging folds (f2) are most conspicuous in all lower-plate rocks as they refold previously formed f1 folds and thrusts and produce a locally developed northeast-trending lineation. F1 folds are east to southeast vergent, generally recumbent and fold inverted thrust limbs. Although geochronologic data is poorly constrained, the f1 east-to southeast-vergent folds seemingly reflect elements of the late Mesozoic Sevier foreland fold and thrust belt. The thick Precambrian section and interleaved metasedimentary rocks appear to be structurally higher than rocks below the detachment fault in the adjacent Whipple and Buckskin Mountains. This may suggest that the coeval mylonitic fabric and northeast-trending lineation that are so widely exposed in ranges northeast and east of the Riversides are of simple-shear origin. If so, the origin of the northeast-trending lineation associated with northeast-vergent folds (f2) in the Riversides may be explained as a result of antithetic overthrusting in conjunction with late Mesozoic synthetic underthrusting. This underthrusting appears to have produced the mylonitic horizons so widely exposed as tectonic windows throughout southeastern California and western Arizona. These tectonic windows are products of the mid-Tertiary crystal warping associated with the development of detachment-related antiforms and synforms.
