8 research outputs found
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Reconnaissance study of mylonitic fabrics in the Bellota Ranch area, eastern Santa Catalina Mountains, Arizona
The Santa Catalina Mountains in southeastern Arizona include extensive mylonitic fabrics developed within granitic and gneissic rocks that make up most of the range. These fabrics are strongest at the southern foot of the range where they dip south and project beneath the rangebounding Catalina – San Pedro detachment fault. Shear sense in the mylonitic rocks is primarily top-southwest, consistent with shearing down-dip from the detachment fault during early normal faulting and exhumation to form the metamorphic core complex. Two to three kilometers north of the foot of the Santa Catalina Mountains the mylonitic foliation is horizontal and farther north it dips to the north. The mylonitic fabrics thus dip outward from the axis of the Forerange arch with primarily top-northeast shear-sense indicators on the north side of the arch. This dominantly topnortheast fabric forms the Molino Basin shear zone, which continues eastward toward the Bellota Ranch area that is the subject of this study. We found that mylonitic fabrics in the Bellota Ranch area are generally subhorizontal, weak, and without clear shear-sense indicators in by far ost outcrops. Four days of field study yielded clear shear-sense indicators at only eight outcrops, with six of them indicating top-southwest shear sense. We were not able to divide mylonitic rocks into top-southwest and top-northeast zones as in a previous study by Bykerk-Kauffman (2008). Lineation trend gradually changes laterally from more northeasterly in the Molino Basin area to more northerly in the Bellota Ranch area. As with the Molino Basin area, lineations associated with top-northeast shearing generally trend more northerly than lineations associated with top-southwest shearing. We conclude that mylonitic lineation in the Bellota anch area is not a simple continuation of the Molino Basin shear zone as it is dominated by top-southwest shear-sense indicators rather than top-northeast as with the Molino Basin shear zone.Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact [email protected]
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Structural-Geologic Map Relationships in the Salcito Ranch Area, Rincon Mountains, Southern Arizona
Formerly known as the Martinez Ranch area (e.g., see Davis, 1975; Davis and others, 2004), the Salcito Ranch area is located at the southeastern-most corner of the Rincon Mountains (Figure 1) and contains a magnificent display of the structural characteristics of the Catalina-Rincon metamorphic core complex and superposed Basin and Range faulting. We carried out large-scale mapping of the geology of a part of this area in the mid-1990s. This mapping, which was initiated as a class project, expanded to become part of a larger study of geologic structures associated with extensional tectonics in a region centered on the Catalina and Rincon Mountains. Observations and conclusions based on this larger work were published in the Geological Society of America (GSA) Bulletin (Davis and others, 2004). The GSA Bulletin journal article contains a simplified, generalized version of the more detailed geological map of the Salcito Ranch area. Furthermore, practical page limits for the article prevented an elaboration on certain descriptive details that may be of interest. Thus we take this opportunity to release this more comprehensive accounting of our findings as an Arizona Geological Survey Contributed Report, with the expectation that the map (Plate 1) and this text will be useful to others.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact [email protected]
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Causes of Variable Shortening and Tectonic Subsidence During Changes in Subduction: Insights From Flexural Thermokinematic Modeling of the Neogene Southern Central Andes (28–30°S)
The Andes of western Argentina record spatiotemporal variations in morphology, basin geometry, and structural style that correspond with changes in crustal inheritance and convergent margin dynamics. Above the modern Pampean flat-slab subduction segment (27–33°S), retroarc shortening generated a fold-thrust belt and intraforeland basement uplifts that converge north of ∼29°S, providing opportunities to explore the effects of varied deformation and subduction regimes on synorogenic sedimentation. We integrate new detrital zircon U-Pb and apatite (U-Th)/He analyses with sequentially restored, flexurally balanced cross sections and thermokinematic models at ∼28.5–30°S to link deformation with resulting uplift, erosion, and basin accumulation histories. Tectonic subsidence, topographic evolution, and thermochronometric cooling records point to (a) shortening and distal foreland basin accumulation at ∼18–16 Ma, (b) thrust belt migration, changes in sediment provenance, and enhanced flexural subsidence from ∼16 to 9 Ma, (c) intraforeland basement deformation, local flexure, and drainage reorganization at ∼12–7 Ma, and (d) out-of-sequence shortening and exhumation of foreland basin fill by ∼8–2 Ma. Thrust belt kinematics and the reactivation of basement heterogeneities strongly controlled tectonic load configurations and subsidence patterns. Geo/thermochronological data and model results resolve increased shortening and combined thrust belt and intraforeland basement loading in response to ridge collision and Neogene shallowing of the subducted oceanic slab. Finally, this study demonstrates the utility of integrated flexural thermokinematic and erosion modeling for evaluating the geometries, rates, and potential drivers of retroarc deformation and foreland basin evolution during changes in subduction. © 2022. American Geophysical Union. All Rights Reserved.6 month embargo; first published: 05 August 2022This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]