Coupled Basin Evolution and Late-Stage Metamorphic Core Complex Exhumation in the Southern Basin and Range Province, Southeastern Arizona

Records of lithospheric extension and mountain-range uplift are most continuously contained within syntectonic sedimentary rocks in basins adjacent to large structural culminations. In southeastern Arizona, metamorphic core complexes form mountain ranges with the highest elevations in the region and supposedly much less extended terranes lie at lower elevations. Adjacent to the Santa Catalina-Rincon metamorphic core complex, within the Tucson Basin, stratigraphic-sequence geometries evident in a large suite of 2-D seismic reflection data suggest a two-phase basin-evolution model controlled by the emplacement and subsequent uplift of the core complex. In its earliest stage, Phase I of basin formation was characterized by extensive faults forming relatively small-scale proto-basins, which coalesced with the larger basin-bounding detachment fault system. Synextensional sedimentation within the enlarging basin is evidenced by sediment-growth packages, derived from adjacent footwall material, fanning into brittle hanging-wall faults. During this phase, volcanism was widespread, and growth packages contain interbedded sediments and volcanic products but, importantly, no mylonitic clasts from the adjacent metamorphic core complex. Phase II of basin evolution began after a significant tectonic hiatus and consisted of a symmetric deepening of the central basin with the introduction of mylonitic clasts in the basin fill. This phase of deformation is coupled with the activation of a series of high-angle normal faults ringing the mountain range. These observations suggest a two-phase model for metamorphic core-complex evolution, with an initial stage of isostatic core-complex emplacement into the upper crust during detachment faulting that resulted in little topographic expression. Core-complex emplacement was followed, after a significant tectonic hiatus, by late-stage exhumation and flexural uplift of the Santa Catalina-Rincon metamorphic core complex through younger high-angle faulting. Moreover, the geometry of upper basin-fill units suggests an extremely low effective elastic thickness in the region and that flexural uplift of the core complex induced asymmetric transfer of ductile mid-crustal rocks from beneath the subsiding Tucson Basin to the uplifting mountain range.