We determined U-Pb ages for detrital zircons from 26 samples of Paleozoic sandstone from the Grand Canyon. Cambrian strata yield mainly ca. 1.44 and 1.7-1.8 Ga ages that indicate derivation from nearby basement rocks of the Yavapai Province. Devonian strata contain zircons of 1.6-1.8 Ga, 1.34-1.40 Ga, and ca. 520 Ma, suggesting derivation from the Mazatzal and Yavapai Provinces, midcontinent region, and the Amarillo-Wichita uplift, respectively. Mississippian strata record a major change in provenance, with predominantly 415-475 Ma and 1030-1190 Ma grains interpreted to have been shed from the central Appalachian orogen. Pennsylvanian strata contain subequal proportions of 1.4-1.8 Ga grains derived from basement rocks exposed in the Ancestral Rocky Mountains and 409-464 and ca. 1070 Ma grains derived from the Appalachians. Permian strata contain abundant Appalachian zircons, including 270-380 Ma grains, and a lesser proportion of grains derived from the Ancestral Rocky Mountains. Transcontinental transport during Mississippian through Permian time is interpreted to have occurred in large river systems, facilitated by northeasterly trade winds during low sea level and by coastal currents. A compilation of young ages from all Upper Paleozoic strata yields age peaks of Ma, an excellent match for Alleghanian, Acadian, Taconic, and Neoproterozoic (peri-Gondwanan) episodes of magmatism along the Appalachian margin. Lag times of the youngest grains in these Upper Paleozoic strata average ~25 m.y., suggesting relatively rapid exhumation and erosion of Appalachian source regions.
The Unkar Group of the Grand Canyon Supergroup is one of the best-preserved remnants of Mesoproterozoic sedimentary rocks in the southwestern United States. It provides an exceptional record of intracratonic basin formation and associated tectonics kinematically compatible with protracted "Grenville-age" NW-directed shortening. New U/Pb age determinations from an airfall tephra at the base of the Unkar Group dates the onset of deposition at ca. 1255 Ma, and 40 Ar/ 39 Ar K-feldspar thermochronology in the Grand Canyon indicates that basement rocks cooled through 150 °C between ca. 1300 and 1250 Ma, refi ning exhumation rates of basement rocks just prior to Unkar deposition. Abrupt thickness and facies changes in conglomerate and dolomite of the Bass Formation (lower Unkar Group) associated with NE-striking monoclinal fl exures indicate NW-directed synsedimen-tary contraction at ca. 1250 Ma. A large disconformity (~75 m.y. duration) is inferred between the lower and upper Unkar Group and is located below the upper Hakatai Shale, as documented by detrital zircons. A second style of Unkar Group deformation involved the development of half grabens and full grabens that record NE-SW extension on NW-striking, high-angle normal faults. Several observations indicate that NW-striking normal faulting was concurrent with upper Unkar deposition, mafi c magmatism, and early Nankoweap deposition: (1) intraformational faulting in the Bass Formation, (2) intraformational faulting in the 1070 Ma (old Rb/Sr date) Cardenas Basalt and lower Nankoweap Formation, (3) syntectonic relationships between Dox deposition and 1104 Ma (new Ar/Ar date) diabase intrusion, and (4) an angular unconformity between UnkarGroup and Nankoweap strata. The two tectonic phases affecting the Unkar Group (ca. 1250 Ma and ca. 1100 Ma) provide new insight into tectonics of southern Laurentia:(1) Laramide-style (monoclines) deformation in the continental interior at ca. 1250 Ma records Grenville-age shortening; and (2) ca. 1100 Ma detrital muscovite (Ar/Ar) and zircon (U/Pb) indicate an Unkar Group source in the Grenville-age highlands of southwestern Laurentia during development of NWstriking extensional basins. We conclude that far-fi eld stresses related to Grenville-age orogenesis (NW shortening and orthogonal NE-SW extension) dominated the sedimentary and tectonic regime of southwestern Laurentia from 1250 to 1100 Ma.
The Rocky Mountains, Colorado Plateau, and Midcontinent, regions of the North American cratonic platform, display similar styles and patterns of Phanerozoic deformation. In these regions, movement on basement-penetrating faults during the late Paleozoic Ancestral Rockies event and/or during the Mesozoic-Cenozoic Laramide event generated flat-topped uplifts bordered by outward-verging, monoclinal forced folds. We suggest that these structures, divided into two sets on the basis of orientation (west to northwest and north to northeast), formed by inversion of Proterozoic extensional-fault systems. In this model, Proterozoic rifting events formed weak faults in the cratonic platform crust, and these faults were reactivated by stress transmitted during Phanerozoic compressional orogenies. If this model is correct, the pattern of Ancestral Rockies and Laramide contractional structures reflects the trends of Proterozoic extensional faults, and regional variation in forced-fold vergence reflects the control of antecedent fault dips on fault-propagation fold geometry during inversion. Late Proterozoic rifts formed throughout Rodinia, so inversion tectonics likely occurred in cratonic platforms worldwide.
Aeromagnetic maps from north-central New Mexico show east-northeast-trending linear features that are offset dextrally across the north-striking Picuris-Pecos, Tusas-Picuris, and (possibly) Nacimiento fault systems. Geologic structures that correspond to these regional aeromagnetic lineaments, where exposed in Phanerozoic uplifts, are major ductile shear zones of Proterozoic age that juxtapose folded metasedimentary rocks associated with areas of low aeromagnetic value with metavolcanic and metaplutonic rocks associated with areas of high aeromagnetic value. Regional aeromagnetic lineaments serve as piercing lines that suggest at least ~55 km and perhaps as much as ~90 km of net dextral separation since 1.4 Ga. The restored aeromagnetic lineaments are interpreted to represent a series of east-northeaststriking, dominantly north-vergent, thrustsense shear zones that formed initially during the ca. 1.65 Ga Mazatzal orogeny and were variably reactivated during the ca. 1.4 Ga magmatic and deformational event. There is no direct evidence for Proterozoic strike or dip slip on the Picuris-Pecos fault, although such slip is possible. The lack of mylonites and other ductile deformation along the Picuris-Pecos fault indicates that it is not older than ca. 1.2-0.9 Ga, the age when the basement rocks of the Sangre de Cristo Mountains last cooled through temperatures characteristic of the lower limits of ductile deformation (300-250 ºC). The ~55-90 km net dextral separation on major north-striking faults in northern New Mexico is the cumulative result of numerous tectonic events, not all of them dextral. The directions of horizontal shortening and/or extension were analyzed for the six major deformations that have affected the region since peak metamorphism at ca. 1.4 Ga. For each of these tectonic episodes, the resolved lateral shear sense on north-striking faults in northern New Mexico was inferred from regional deformation patterns. Grenville (ca. 1.1 Ga) and Neoproterozoic (ca. 0.7 Ga) slip potentially had sinistral components. Cambrian slip accompanying the opening of the southern Oklahoma aulacogen potentially had a small dextral component (a few kilometers). Lateral slip during the late Mississippian-Early Permian Ancestral Rocky Mountain orogeny was probably dextral and possibly of large magnitude. Laramide fault slip was dextral and probably of large magnitude (tens of kilometers). The lateral slip component during the main phase of Rio Grande rifting (Miocene) was sinistral, but of small magnitude. The Ancestral Rocky Mountain and Laramide events thus appear to have been largely responsible for the dextral separations seen today. The relative importance of dextral contributions by these two orogenies, however, has not yet been determined.
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