Abstract Extreme extension along the Colorado River has exposed the shallow to mid-crustal Spirit Mountain batholith and the roots of the roughly coeval Secret Pass Canyon volcanic center. Examination of the Spirit Mountain batholith reveals evidence for multiple replenishment and rejuvenation over a two million year period (ca. 17.5−15.3 Ma), with extensive coarse cumulate granites and leucogranite (high-silica rhyolites) sheets, mafic-felsic mingling and mixing, and a major dike swarm. The roots of the possibly related Secret Pass Canyon volcanic center comprise a large, very shallow, composite laccolith and smaller dikes, sills, and a volcanic neck. The volcanic sequence was emplaced within about a one million year period (ca. 18.5–17.3 Ma) and includes volcanogenic sediments, ignimbrites, domes, and block-and-ash flow deposits. An appended road log serves as a geologic guide to this magmatic region.

The Niğde Massif, south-central Turkey, experienced two complete cycles of burial and exhumation during orogenesis and is, therefore, an excellent example of yo-yo tectonics. We propose that burial and exhumation of the metamorphic basement and, in the second cycle, the basement and its sedimentary cover rocks, were driven largely by transpression and transtension in an intracontinental strike-slip zone. The eastern margin of the massif, where it is adjacent and subparallel to the sinistral Central Anatolian fault zone, is comprised of Upper Cretaceous basement that was the source of, and is unconformably overlain by, early Tertiary sedimentary rocks. The contact between the Tertiary rocks and basement is an unconformity that is locally sheared and characterized by a low-angle oblique-normal shear system with cataclasite in the basement and brittle-ductile shear zones in the sedimentary rocks. These relationships, documented by geo/thermochronology to encompass 80 million years, define the timing and magnitude of the yo-yo process: burial and heating of Mesozoic sedimentary rocks during Late Cretaceous transpression to form the high-grade metamorphic basement (peak metamorphism at 85–91 Ma); Late Cretaceous (ca. 80–60 Ma) unroofing by transtension and erosion, with early Tertiary deposition of massif-derived clastic material at the edge of a marine basin along the Central Anatolian fault zone; reburial of basement and cover rocks involving folding, shearing, and greenschist facies metamorphism of the sedimentary cover in late Eocene through early Miocene time (ca. 50–20 Ma); and final exhumation in the middle Miocene (17–9 Ma) along strike-slip and normal faults.

Field mapping and structural analysis indicate that the Snowcamp remnant of the Coast Range ophiolite, in the vicinity of Game Lake, southwest Oregon, was thrust to the northeast over mafic phyllonite and amphibolite correlated with the Chetco complex. The contact between the upper ophiolitic and lower mafic sections is marked by a ∼5- to 10-m-thick serpentinite unit and paralleled by mylonitic foliations in the lower mafic section. Syndeformational tonalites intrude the lower plate. The fabric-forming metamorphic mineral assemblages (amphibolite facies) in the mafic section suggest a temperature range of ∼550–600+ °C and medium pressure, whereas quantitative thermobarometry indicates 615–720 °C and 3.5–6 kb (11.5–20 km depth). A mylonitic greenschist-facies mineral assemblage in phyllonites below the thrust overprints the amphibolite-facies fabric, indicating that thrusting continued during cooling. The 40 Ar/ 39 Ar analyses indicate that thrusting and syntectonic intrusion occurred by at least 154.2 ± 2.0 and 149.1 ± 0.4 Ma, respectively. The character of this thrust strongly resembles that of the Madstone Cabin thrust that emplaced the Josephine ophiolite over the Rogue–Chetco arc complex. The proposed correlation between the two faults implies that either the Snowcamp remnant of the Coast Range ophiolite has undergone a similar emplacement history as the Josephine ophiolite or that the ophiolitic rocks near Game Lake may be an outlier of the Josephine ophiolite juxtaposed by postemplacement faulting against a Coast Range remnant around Snowcamp Mountain to the south.

Abstract The history of fault initiation and reactivation in the southern Rocky Mountains remains highly debated, as does the region’s exhumation history. Nowhere has the evidence been more contested than in the southern Sangre de Cristo Mountains, where major, 30+ km dextral separations of basement rocks and their aeromagnetic anomalies have been attributed to Proterozoic, Ancestral Rocky Mountain and Laramide orogenies. Since the sum of these dextral separations is in the range of 100 km, unambiguous determination of the age(s) of faulting would have major implications to Rocky Mountain tectonics. Likewise, the history of exhumation and stabilization of the western North American craton provides an important example of continental lithospheric evolution. This field trip will start by visiting excellent exposures of spectacularly brecciated yet indurated basement rocks and flanking Paleozoic sedimentary rocks along the Picuris-Pecos fault system, which has 37 km of dextral separation of Proterozoic contacts. Hypotheses for the age(s) of slip will be examined in light of stratigraphic and fault relationships, thin section petrography and isotopic analyses. The region’s history of fault reactivation and associated K-metasomatism will be discussed by combining thermochronology, largely based on new 40Ar/39Ar K-feldspar analyses, with recent seismic data across the Laramide front of the Sangre de Cristo Mountains. The regional tectonic implications of new geologic mapping, fault analyses, 40Ar/39Ar thermochronology and seismic studies will be discussed on the outcrop, with a full examination of all hypotheses.