Susanne U. Janecke

Chronology of Miocene–Pliocene deposits at Split Mountain Gorge, Southern California: A record of regional tectonics and Colorado River evolution

Late Miocene to early Pliocene deposits at Split Mountain Gorge, California, preserve a record of basinal response to changes in regional tectonics, paleogeography, and evolution of the Colorado River. The base of the Elephant Trees Formation, magnetostratigraphically dated as 8.1 ± 0.4 Ma, provides the earliest well-dated record of extension in the southwestern Salton Trough. The oldest marine sediments are ca. 6.3 Ma. The nearly synchronous timing of marine incursion in the Salton Trough and northern Gulf of California region supports a model for localization of Pacifi c‐North America plate motion in the Gulf ca. 6 Ma. The fi rst appearance of Colorado River sand at the Miocene-Pliocene boundary (5.33 Ma) suggests rapid propagation of the river to the Salton Trough, and supports a lake-spillover hypothesis for initiation of the lower Colorado River.

GEOMETRY, MECHANISMS AND SIGNIFICANCE OF EXTENSIONAL FOLDS FROM EXAMPLES IN THE ROCKY MOUNTAIN BASIN AND RANGE PROVINCE, U.S.A.

Abstract Geologic mapping and structural analysis in Paleogene half graben of Idaho and Montana have revealed over 70 extensional folds that form orthogonal sets with mostly NE- and SE-trending axes. On a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults. Transverse folds, at a high angle to the fault, and folds oblique to the fault, comprise more than half of the folds and reflect the highly three-dimensional nature of the bulk strain. Detailed geometric analysis of 15 folds in the Salmon, Horse Prairie and Medicine Lodge basins shows that they typically have an upright to steeply inclined bisecting surface, an interlimb angle of 141°, and a plunge of 19°. Based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics, we were able to determine how some of the investigated folds formed. Fault-bend folding above both fault-parallel and fault-perpendicular bends in the underlying normal fault produced most of the folds in the study area, but displacement gradients along normal faults, fault-drag, isostatic adjustments, and transtension were also factors in the deformation. Most of the compound folds, which result from more than one mechanism of folding, are oblique to all adjacent normal faults. Recognition of such extensional folds is critical, because they might be misinterpreted as contractional structures, they influence sediment thickness patterns and dispersal in rift basins, and may control the migration and trapping of petroleum and groundwater resources.

Feldspar-Influenced Rock Rheologies

Progressive fracture, subsequent cataclasis, and syntectonic alteration of feldspar to phyllosilicates transformed a massive granite into a quartz-mica phyllonite during early Tertiary deformation in a 20-30-m-wide shear zone in southeastern Arizona. The ductile deformation proceeded at temperatures and pressures approximating, middle to upper crustal levels. Most of the deformation was taken up by the feldspar and its alteration products. Quartz deformed by both brittle fracture and crystal plastic mechanisms. These results show that under certain temperature-pressure-strain rate conditions feldspars are weaker than quartz. The observed deformation mechanisms indicate that the brittle to ductile transition in fault zones is not simply a function of the onset of temperature-activated plastic deformation in quartz nor of the frictional behavior of rocks, as inferred in the familiar strength vs. depth curves. It is more likely that the transition is complex and depends on a variety of parameters. One such factor delineated in this study is the fracture and fracture-facilitated syntectonic alteration of feldspar under hydrated conditions.

Stratigraphic record of basin development within the San Andreas fault system: Late Cenozoic Fish Creek–Vallecito basin, southern California

The Fish Creek–Vallecito basin contains a 5.5-km-thick section of late Miocene to early Pleistocene sedimentary rocks exposed in the hanging wall of the West Salton detachment fault. These deposits preserve a high-fidelity record of late Cenozoic subsidence and basin filling that resulted from deformation in the San Andreas fault system of southern California. Existing and new paleomagnetic data, combined with new U-Pb zircon ages of two tuffs high in the section, show that the section ranges in age from ca. 8.0 ± 0.4 Ma at the base to ca. 0.95 Ma at the top. Geohistory analysis reveals: (1) moderate subsidence (0.46 mm/yr) from ca. 8.0 to 4.5 Ma; (2) rapid subsidence (2.1 mm/yr) from 4.5 to 3.1 Ma; (3) moderate subsidence (0.40 mm/yr) from 3.1 to 0.95 Ma; and (4) rapid uplift and erosion that has exhumed the section since ca. 1 Ma. Onset of sedimentation at ca. 8.0 ± 0.4 Ma records earliest extension or transtension in the area, possibly related to localization of the Pacific–North America plate boundary in the Salton Trough and Gulf of California. Alternatively, marine incursion at 6.3 Ma may be the earliest record of plate-boundary deformation in the Gulf of California–Salton Trough region. A thick interval higher in the section records progradation of the Colorado River delta into and across the basin starting ca. 4.9 Ma. Progradation continued during an abrupt increase in subsidence rate at 4.5 Ma, and fluvial-deltaic conditions persisted for 1.4 m.y. during the rapid-subsidence phase, indicating that delta progradation was driven by a large increase in rate of sediment input from the Colorado River. Uplift and inversion of the basin starting ca. 1.0 Ma record initiation of strike-slip faults that define the modern phase of dextral wrench tectonics in the western Salton Trough.

Stratigraphic record of Pleistocene faulting and basin evolution in the Borrego Badlands, San Jacinto fault zone, Southern California

Sedimentary rocks in the Borrego Badlands, Southern California, contain a record of Pleistocene crustal deformation during initiation and evolution of the San Jacinto fault zone. We used detailed geologic, stratigraphic, and paleomagnetic analysis to determine the age and geometry of the deposits and reconstruct the history of fault-controlled sedimentation in this area. The base of the ~300 to 500 m thick Ocotillo Formation is a paraconformity to abrupt conformable contact that records a brief hiatus followed by rapid progradation of coarse alluvial sediment over lacustrine facies of the Borrego Formation at 1.05 ± 0.03 Ma. This coincides with regional-scale progradation of Ocotillo Formation sand and gravel, and appears to record initiation of strike-slip faults in the southwestern Salton Trough at ca. 1.1 Ma. Thickness trends, clast compositions, paleocurrents, and distribution of paleosols provide evidence for initiation of the East Coyote Mountain fault at ca. 1.05 Ma, followed by onset of NNE-ward basin tilting obliquely toward the Santa Rosa segment of the Clark fault at ca. 1.0 Ma. Stratigraphic omission of the Ocotillo Formation and progressively older units southwest of the Coyote Creek fault beneath the Fonts Point Sandstone provides evidence that tilting to the northnortheast was related in part to growth of the San Felipe anticline during deposition of the Ocotillo Formation. Map and stratigraphic data suggest that the Coyote Creek fault in the western Borrego Badlands postdates Ocotillo deposition, and thus appears to have propagated southeast into the study area at ca. 0.6 Ma. The Fonts Point Sandstone is a thin, sheetlike alluvial deposit that records the end of deposition and onset of transpressive deformation in the Borrego Badlands. The base of the Fonts Point Sandstone changes from a conformable contact in a narrow belt southeast of the Inspiration Point fault, where it is dated at 0.6 ± 0.02 Ma, to an angular unconformity on the folded Ocotillo Formation northwest of the fault. The pattern of stratal truncation records initiation of the Inspiration Point fault at ca. 0.6 Ma. This coincides with a major structural reorganization in the San Jacinto fault zone that initiated the modern phase of north-south shortening and erosion in the southwestern Salton Trough.

Pleistocene Brawley and Ocotillo Formations: Evidence for Initial Strike‐Slip Deformation along the San Felipe and San Jacinto Fault Zones, Southern California

We examine the Pleistocene tectonic reorganization of the Pacific–North American plate boundary in the Salton Trough of southern California with an integrated approach that includes basin analysis, magnetostratigraphy, and geologic mapping of upper Pliocene to Pleistocene sedimentary rocks in the San Felipe Hills. These deposits preserve the earliest sedimentary record of movement on the San Felipe and San Jacinto fault zones that replaced and deactivated the late Cenozoic West Salton detachment fault. Sandstone and mudstone of the Brawley Formation accumulated between ∼1.1 and ∼0.6–0.5 Ma in a delta on the margin of an arid Pleistocene lake, which received sediment from alluvial fans of the Ocotillo Formation to the west‐southwest. Our analysis indicates that the Ocotillo and Brawley formations prograded abruptly to the east‐northeast across a former mud‐dominated perennial lake (Borrego Formation) at ∼1.1 Ma in response to initiation of the dextral‐oblique San Felipe fault zone. The ∼25‐km‐long San Felipe anticline initiated at about the same time and produced an intrabasinal basement‐cored high within the San Felipe–Borrego basin that is recorded by progressive unconformities on its north and south limbs. A disconformity at the base of the Brawley Formation in the eastern San Felipe Hills probably records initiation and early blind slip at the southeast tip of the Clark strand of the San Jacinto fault zone. Our data are consistent with abrupt and nearly synchronous inception of the San Jacinto and San Felipe fault zones southwest of the southern San Andreas fault in the early Pleistocene during a pronounced southwestward broadening of the San Andreas fault zone. The current contractional geometry of the San Jacinto fault zone developed after ∼0.5–0.6 Ma during a second, less significant change in structural style.

Kinematics and timing of three superposed extensional systems, east central Idaho: Evidence for an Eocene tectonic transition

Cenozoic crustal extension in east central Idaho began about 50 Ma and continues at present. Three distinct episodes characterize one of the longest intervals of Cenozoic extension yet documented in the North America Cordillera. Crosscutting relationships between NE striking normal faults and volcanic rocks, regional dike trends, and slickenline data indicate NW-SE extension during peak Eocene volcanism about 49–48 Ma (episode 1). NE striking normal faults, with at most a few kilometers of offset, formed in an intraarc setting during rapid NE subduction of oceanic plates under the Pacific Northwest. North to NNW striking and west dipping normal faults, with offsets up to 10–15 km, formed during a younger middle Eocene to Oligocene basin-forming event (episode 2). This newly documented episode was the most important extensional event in east central Idaho and began during the waning phases of Challis volcanism. WSW-ENE to SW-NE extension during episode 2 was nearly perpendicular to the extension direction during episode 1 and perpendicular to the grain of the Idaho-Montana fold and thrust belt. The flip in extension direction between episode 1 and episode 2 is tightly constrained by 40Ar/39Ar age determinations to have taken place at the end of Eocene Challis magmatism about 46–48 Ma. I infer that plate boundary forces controlled the geometry of normal faults and dikes during episode 1, whereas internal stresses within previously thickened crust drove major SW to WSW directed extension during episode 2. A drop in convergence rates between the North American and Farallon plates between 59 Ma and 42 Ma (Stock and Molnar, 1988) may coincide with the onset of gravitational spreading during episode 2 and may also explain the abrupt end of Eocene magmatism in the Pacific Northwest. Miocene and younger SW dipping Basin and Range faults (episode 3) extended the region in a NE-SW direction. Although faults formed during episode 2 and episode 3 are not parallel, slickenlines indicate only small changes in slip vector trends, suggesting little rotation of the extension direction in east central Idaho since 46 Ma.

Timing and episodicity of Middle Eocenevolcanism and onset of conglomerate deposition

Seventeen

Paleogeographic reconstruction of the Eocene Idaho River, North American Cordillera

^{40}Ar/^{39}Ar

Sedimentation and paleogeography of an Eocene to Oligocene rift zone, Idaho and Montana

incremental release mineral ages from lava flows and tuffs in the Lost River and Lemhi Ranges, Idaho, (1) demonstrate deposition of thick sequences of syntectonic conglomerate in middle Eocene and Oligocene time and (2) reveal two temporally and compositionally distinct phases of the Challis Volcanic Group. Thick sections of coarse conglomerates are preserved in two NNW-trending half graben. These were previously assigned to the Pliocene ( ? ) but are reassigned to the Eocene and Oligocene based on ages from four tuffs interbedded with conglomerates. Andesitic and dacitic lava flows and tuffs of the Challis Volcanic Group, up to 2.5 km thick, were deposited between about 49 and 48 Ma, whereas rhyolite tuffs, only tens of meters thick, accumulated between about 46 and 45.5 Ma. The rhyolite tuffs are intercalated within or underlie the syntectonic conglomerates. Lithologic, stratigraphic, and geochronologic evidence correlate these tuffs with the tuff of Challis Creek, which erupted at the end of Challis volcanism from the Twin Peaks caldera in central Idaho. This correlation expands the lateral extent of this tuff southeast of the caldera to at least 100 km. The distribution of the tuff of Challis Creek suggests that it was unimpeded by topographic relief across numerous synvolcanic NE-striking normal faults. Displacements on these faults either were too small to block the tuff, or depressions in the hanging wall had been filled by volcanic rocks before tuff emplacement. In the central part of the Lost River and Lemhi Ranges a magmatic hiatus of about 1-2 m.y. separates deposition of older intermediate-composition lava flows from deposition of younger rhyolite tuffs that may record rhyolite-dominated bimodal magmatism. Previous K-Ar ages of volcanic and intrusive rocks in the northern half of the Challis volcanic field suggest that this hiatus may be widespread. We speculate that the limited intermixing of mafic magmas during younger silicic volcanism, and abrupt decline of Challis volcanism after 48 Ma, reflect to a decrease in rates of magma input from the mantle when convergence between the Farallon plate and North America slowed.