Possible origin and significance of extension-parallel drainages in Arizona's metamorphic core complexes

The corrugated form of the Harcuvar, South Mountains, and Catalina metamorp hic core complexes in Arizona reflects the shape of the middle Tertiary ext ensional detachment fault that projects over each complex. Corrugation axes are approximately parallel to the fault-displacement direction and to the footwall mylonitic lineation, The core complexes are locally incised by eni gmatic, linear drainages that parallel corrugation axes and the inferred ex tension direction and are especially conspicuous on the crests of antiforma l corrugations. These drainages have been attributed to erosional incision on a freshly denuded, planar, inclined fault ramp followed by folding that elevated and preserved some drainages on the crests of rising antiforms, Ac cording to this hypothesis, corrugations were produced by folding after sub aerial exposure of detachment-fault footwalls. An alternative hypothesis, p roposed here, is as follows, In a setting where preexisting drainages cross an active normal fault, each fault-slip event will cut each drainage into two segments separated by a freshly denuded fault ramp, The upper and lower drainage segments mill remain hydraulically linked after each fault-slip e vent if the drainage in the hanging-wall block is incised, even if the stre am is on the flank of an antiformal corrugation and there is a large compon ent of strike-slip fault move ment, Maintenance of hydraulic linkage during sequential fault-slip events will guide the lengthening stream down the fa ult ramp as the ramp is uncovered, and stream incision mill form a progress ively lengthening, extension-parallel, Linear drainage segment. This mechan ism for linear drainage genesis is compatible with corrugations as original irregularities of the detachment fault, and does not require folding after early to middle Miocene footwall exhumation, This is desirable because man y drainages are incised into nonmylonitic crystalline footwall rocks that w ere probably not folded under low-temperature, surface conditions. rin alte rnative hypothesis, that drainages were localized by small fault grooves as footwalls mere uncovered, is not supported by analysis of a down-plunge fa ult projection for the southern Rincon Mountains that shows a Linear draina ge aligned with the crest of a small antiformal groove on the detachment fa ult, but this process could have been effective elsewhere. Lineation-parall el drainages now plunge gently southwestward on the southwest ends of antif ormal corrugations in the South and Buckskin Mountains, but these drainages must have originally plunged northeastward if they formed by either of the two alternative processes proposed here. Footwall exhumation and incision by northeast-flowing streams was apparently followed by core-complex archin g and drainage reversal.