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Tectonic setting of faulted Tertiary strata associated with the Catalina core complex in southern Arizona

William R. Dickinson
William R. Dickinson
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January 01, 1991

Stratigraphic and structural relations of syntectonic sedimentary sequences associated with Cordilleran metamorphic core complexes provide valuable information about the style and timing of extensional deformation related to tectonic denudation. Adjacent to the Catalina core complex, the San Pedro trough and other nearby depocenters contain multiple tilted half-grabens of conglomeratic mid-Tertiary strata partly buried beneath Neogene basin fill. Major episodes of local geologic history included mid-Proterozoic construction of continental crust, subsequent but intermittent platform sedimentation extending through Paleozoic time, mid-Mesozoic initiation of arc magmatism that persisted at intervals through mid-Tertiary time, complex Laramide orogenic deformation of latest Cretaceous to early Tertiary age, and Cenozoic extensional deformation involving both mid-Tertiary and basin-range phases of development.

Precambrian basement includes lower Proterozoic Pinal Schist intruded by voluminous lower to middle Proterozoic granitic plutons. Pre-Laramide stratigraphic cover includes middle Proterozoic sedimentary strata and intercalated diabase sills, Paleozoic carbonate and clastic units, and Mesozoic volcaniclastic and clastic successions. Laramide assemblages include metaluminous plutons and andesitic to rhyolitic volcanic fields, synorogenic nonmarine sedimentary sequences, and large bodies of peraluminous two-mica granite. Laramide structural features include both premetamorphic and ductile synmetamorphic thrusts within the Catalina core complex, brittle thrusts of uncertain vergence and overall configuration outside the Catalina core complex, and folds of varied geometry related in part to local thrusts exposed nearby. Paleogene erosion had stripped Laramide volcanic cover from wide areas by mid-Tertiary time.

Migratory Tertiary arc magmatism within the intermountain region gave rise to diachronous polymodal igneous suites, represented within and near the Catalina core complex by extensive volcanic fields and local granitic plutons of late Oligocene age. Across the whole Southwest Border region, analogous mid-Tertiary igneous activity was succeeded, following an intra-Miocene tectonomagmatic transition, by basaltic to bimodal suites erupted during subsequent block faulting.

Tertiary intermountain taphrogeny included a mid-Tertiary phase marked by listric or rotational normal faulting associated with tectonic denudation of core complexes along detachment systems, and a later basin-range phase of widespread block faulting. Mid-Tertiary extension was apparently promoted by reduction of interplate shear, kinematic rollback of a subducted slab, lateral spreading of overthickened crust, advective softening of arc lithosphere, and possibly by counterflow of asthenosphere. Basinrange extension was evidently initiated by shear coupling of Pacific and American lithosphere.

The Catalina core complex displays characteristic geologic features: mylonitic fabric overprinted near a detachment fault by brecciation and chloritic alteration, a brittle detachment surface abruptly separating rock masses derived from different crustal levels, cover strata broken into multiple tilted fault blocks forming a shingled imbricate array, and surrounding syntectonic sedimentary sequences containing intercalated megabreccia horizons. The domical uplift that controls the exposed extent of the Catalina core complex apparently reflects isostatic upwarp of the midcrust in response to tectonic denudation, coupled with rollover arching above listric faults cutting beneath the core complex and with later uplift of crustal masses along steep block faults.

A belt of mylonitic gneiss 10 km wide and 100 km long lies along the southwest flank of the Catalina core complex adjacent to the downdip segment of the detachment fault. Updip segments of the detachment system, across which cumulative displacement is estimated as 20 to 30 km, curve over and around the exposed core complex to merge with an inferred headwall rupture along the trend of the San Pedro trough. The detachment system probably involved a gently dipping aseismic slip surface beneath surficial tilted fault blocks; alternatively, faults now dipping gently may have originated as steep structures that rotated to shallow dips as slip proceeded. Field relations around the Catalina core complex are seemingly more compatible with the former interpretation, but insights gained from the alternate interpretation are useful for structural analysis of rotated tilt-blocks above the master detachment surface. Kinematic considerations imply that displacements within the local detachment system were diachronous during its evolution, but mylonitic deformation and detachment faulting both occurred during late Oligocene and early Miocene time.

Tilted homoclines of syntectonic mid-Tertiary strata and less deformed beds of younger basin fill are both composed dominantly of alluvial fan and braidplain facies that grade laterally to finer-grained fluvial and lacustrine facies. Subordinate deposits include landslide megabreccia, algal limestone, lacustrine diatomite, and playa gypsum. The most voluminous strata are crudely bedded conglomerate and conglomeratic sandstone deposited by braided depositional systems. Clast imbrication is the most widespread and reliable paleocurrent indicator. Mid-Tertiary aggradation in half-graben basins gave rise to sedimentary onlap and overstep of evolving tilt-blocks, intricate local facies relations, and marked stratigraphic contrasts between sequences deposited within partly isolated subbasins. Stratigraphic successions that are concordant within basin depocenters are commonly broken by unconformities and intervals of nondeposition on basin flanks and across crests of bounding tilt-blocks.

Mid-Tertiary strata exposed as tilted homoclines along the flanks of the San Pedro trough and across broad uplands north of the Catalina core complex are assigned to the following formations, each of which includes informal local members and facies: (a) Mineta Formation, mid-Oligocene redbeds including both conglomeratic fluvial and finer-grained lacustrine deposits; (b) Galiuro Volcanics, including lavas and domes, air-fall and ash-flow tuffs, and intercalated volcaniclastic strata of late Oligocene to earliest Miocene age; (c) Cloudburst Formation, also of late Oligocene and earliest Miocene age but including a sedimentary upper member of conglomeratic strata as well as a volcanic lower member correlative with part of the Galiuro Volcanics; and (d) San Manuel Formation, composed of lower Miocene alluvial fan and braidplain deposits that display contrasting clast assemblages in different areas of exposure. Generally correlative Oligocene-Miocene strata exposed south of the Catalina core complex are assigned to the Pantano Formation, which contains similar lithologic components. Less-deformed Neogene strata of post-mid-Miocene basin fill are assigned to the Quiburis Formation along the San Pedro trough, but stratigraphic equivalents elsewhere lack adequate nomenclature. High benchlands mantled by paleosols mark the highest levels of Neogene aggradation. Successive stages of subsequent erosional dissection are recorded by multiple terrace levels incised into basin fill.

Key exposures of syntectonic mid-Tertiary sedimentary sequences in several local subareas reveal typical structural and stratigraphic relationships. Multiple fault blocks expose pre-Tertiary bedrock overlain by tilted mid-Tertiary strata confined to intervening half-grabens. Bounding syndepositional faults dip southwest and associated homo-clines dip northeast. Fanning dips and buttress unconformities reflect progressive tilt and burial of eroding fault blocks. Dips of block-bounding faults are inversely proportional to the ages of the faults. Steeper dips for younger faults suggest either progressive erosion of successive listric faults or progressive rotation of successive planar faults. Uniformly moderate to steep dihedral angles between fault surfaces and offset homoclinal bedding imply that the faults dipped more steeply near the surface when syntectonic mid-Tertiary strata were subhorizontal. Although the inference of listric faulting best links apparent strands of the Catalina detachment system, the alternate interpretation of rotational normal faulting is compatible with local structural relationships including tilt of porphyry copper orebodies.

Within the San Pedro trough, multiple homoclines of mid-Tertiary strata are exposed locally in tilt-blocks exhumed by Neogene erosion from beneath nearly flat-lying basin fill of the Quiburis Formation. Faults bounding the mid-Tertiary exposures include backtilted strands of the Catalina detachment system, somewhat younger listric or rotational normal faults, and steeper basin-range normal faults that display offsets both synthetic and antithetic to the flanks of the San Petro trough.

In Cienega Gap, flanking the Tucson Basin, multiple tilt-blocks of the Pantano Formation form part of the upper plate of the Catalina detachment system. Initial construction of alluvial fans by generally westward paleoflow was followed by ponding of lacustrine environments along the foot of secondary breakaway scarps that also generated massive megabreccia deposits.

In summary, syntectonic Oligocene to Miocene sedimentation succeeded a prominent pulse of polymodal mid-Tertiary volcanism and was coeval with mylonitic deformation and detachment faulting along the flank of the Catalina core complex. The headwall rupture for the detachment system migrated westward from an initial position along the range front of the Galiuro Mountains. After mid-Miocene time, accumulation and subsequent dissection of essentially undeformed basin fill was accompanied by basin-range block faulting. The most challenging structural issue is whether fault strands of the Catalina detachment system are interconnected or are disconnected rotational segments.

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