January
Environmental and Biotic Consequences of Major Cosmic Impact
over North America 12,900 Years Ago
James P. Kennett, Department of Earth Science, University of California, Santa Barbara, Calif., and Younger Dryas Boundary (YDB) Impact Group
Abstract: We (YDB impact group) have discovered remarkable, widespread geological and archaeological evidence for a major extraterrestrial collision over North America 12,900 years ago. Massive energy release from this impact caused continental-wide wildfires and other severe environmental changes. The YDB extraterrestrial impact hypothesis appears to be consistent in explaining at least three major events that have long puzzled the scientific community:
1) the massive, abrupt extinction over North America of many large mammal (e.g. mammoths, camels, sloths, saber tooth cat) and bird taxa;
2) the abrupt disappearance of the Clovis Culture, the first widely distributed peoples of North America; and
3) the triggering of abrupt cooling over broad areas of Earth and associated major change in ocean circulation.
February
Debris Flows in Colorado
Jeff Coe, U.S. Geological Survey, Golden, Colorado
Abstract: Debris flows are fast-moving landslides that are triggered by rapid snowmelt or intense or prolonged rainfall. Debris flows are extremely hazardous to anything in their path because they occur with little warning and often contain large boulders, trees, and other debris. Three primary types of debris flows affect areas of western North America: 1) flows that mobilize from discrete landslides (i.e., “slides” using the Varnes, 1978 classification) and travel over the surface of the hillslope, often flattening vegetation and leaving a thin veneer of deposits; 2) flows that mobilize from slides that then erode and entrain hillslope and channel materials; and 3) flows that initiate from surface-water runoff that erodes and entrains hillslope and channel materials.
In Colorado, recent debris flows along the Interstate 70 corridor, in the towns of Ouray and Telluride, along Highway 550 between Ouray and Silverton, and in the Cottonwood and Chalk Creek valleys near Buena Vista, indicate that Colorado is most susceptible to debris flows of type 3 (runoff-initiated flows). The runoff type is most common because Colorado has an abundance of steep, sparsely vegetated slopes that are covered by loose debris. The flanks of formerly glaciated valleys and areas burned by wildfire are especially susceptible to runoff-initiated debris flows.
To better understand initiation conditions and processes for runoff-initiated debris flows, the USGS began monitoring four debris-flow basins in central Colorado in 2004. One of these basins, at Chalk Cliffs near Buena Vista, had an average of two debris flows per year between 2004 and 2007. This high rate of occurrence led to an expanded monitoring effort atChalk Cliffs in 2008 when the University of Colorado and East Carolina University began collaborating with the USGS at the site. Current monitoring instrumentation is designed to capture flow stage, pore-fluid pressure, bed-normal stress, soil moisture, rainfall, and video and still photography during debris-flow events. Terrestrial laser scanning is used to monitor topographic changes caused by debris flows. Monitoring during the summers of 2008 and 2009 captured data from five debris-flow events.
Results from monitoring indicate that flows were initiated by surface-water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14° to 45°. The availability of channel material was essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry gravel processes. Rainfall exceeding I=6.61(D)-0.77, where I is rainfall intensity (mm/hr), and D is duration (hr), was required for the initiation of debris flows in the drainage basin. Soil moisture levels in hillslope and channel sediment were low (< 10 percent volumetric water content) immediately prior to debris-flow initiation. Observed flows consisted of multiple, steep-surge fronts of coarse-grained material without measurable pore-fluid pressure, pushed by more water-rich tails. Surges with the largest pore-fluid pressures, some two times greater than hydrostatic, were the most mobile and triggered mass movement into the channel by undercutting bounding hillslopes. The total depths of mapped debris-flow deposits were generated primarily by progressive vertical accretion of multiple surges, yet resulted in massive, vertically unstratified sedimentological textures.
Rincon Mountain Megaslide: La Conchita, Ventura County, California
Dr. Larry D. Gurrola, Consulting Geologist, Santa Barbara, Calif.
Abstract: The 1995 and 2005 landslides in the 200-m high sea cliff above the community of La Conchita, California, are known to be part of a reactivated Holocene prehistoric landslide. We propose that the prehistoric Holocene slide is part of a much larger, several hundred million cubic meter late Pleistocene slide complex composed of upper slumps and lower flows, informally termed as the Rincon Mountain megaslide. An approximate age of 30 ka for the Rincon Mountain landslide is derived, based on a 25-m high fault scarp formed in earthflow deposits that overlie the megaslide deposits and a known rate of faulting (~0.8 m/ky). Geomorphic evidence for the megaslide includes a prominent 100-m-high amphitheater-shaped head scarp, back-tilted landslide benches, hummocky topography, and numerous smaller landslides and earthflow deposits. Geologic evidence includes deposits composed of slide breccia with fragments of the late Pleistocene (45 ka) emergent marine platform and terrace deposits displaced several tens of meters.
Isolated parts of the Rincon Mountain landslide are active in the La Conchita area, but no evidence exists that the entire slide mass is moving as a unit. Landslides from the 200-m high slope behind La Conchita will reoccur and future development on the proposed Rincon Mountain slide should be very carefully evaluated to avoid reducing slope stability and reactivation of the megaslide.
March
A Bad Day in the Field: Things To Do and Not To Do During a Field Emergency;
A Talk in Honor and Memory of Terry Hiester
Emmett Evanoff, University of Northern Colorado
Abstract: I was involved in a major accident in Badlands National Park, South Dakota, on the last day of the summer field season of 2009. While walking out of the field, I was on a narrow game trail above a steep badlands drainage when a gust of wind of over 60 miles per hour hit me on the side, picked me up, and I fell about 60 feet into the drainage. I ended up in a hole formed by a pipe, an underground drainage system that forms at the headwaters of gullies in the badlands. I incurred a broken left arm, several cracked left ribs, a cracked left lower pelvis, and several broken right ribs. I was not able to get out of the drainage unaided.
My field assistant, Terry Heister, helped me get to a stable position in the gully and then went for help. He returned two hours later with over 50 rescuers from Badlands National Park and three local search-and-rescue teams. Because of the steepness of the topography in the area, the rescuers decided to call in a helicopter from the South Dakota Army National Guard to pick me up and fly me the half mile to near where the Flight-For-Life helicopter was waiting. Five hours after the accident, and only three hours after the rescue teams arrived, I was in the hospital at Rapid City, South Dakota. The efficiency of the rescue parties was quite remarkable, and the rescue was one of the most involved in the history of Badlands National Park.
My perspective on the accident and rescue was quite limited while it was happening, but during my recovery, I was able to learn about the details of what Terry experienced and what was the Park Services’ role in the rescue. From this experience, Terry and I will give you our perspectives on not only what happened, but what you need to think about and do during an emergency. I will also give you the rescuers’ perspective on what information they need and what you should do to help the professionals during a rescue in the field.
I extend my greatest appreciation to all those members of the Colorado Scientific Society who contacted me in the weeks following the accident. Your cards and e-mails of support gave me great comfort during the two weeks I was in the hospital at Rapid City and the month I was recovering at home. Thankfully, my recovery has been relatively swift, and this is in large part from the support from all the many people who contacted me. Thank you for your support!
A Talk in Honor and Memory of Terry Hiester
The April 5, 2010, joint CSS-WIPS talk entitled “A Bad Day in the Field” was presented by Emmett Evanoff, in memory of his field partner, Terry Hiester, who got Emmett out of the hole he landed in while traversing the Badlands. The talk was originally going to be presented by both Emmett and Terry, but sadly, Terry passed away on March 24 after a long illness. The talk was dedicated to Terry, whose fast actions and clear thinking made the rescue go quickly and saved Emmett’s life. Terry was a great field assistant and a very good friend. He is greatly missed.
April
#1: Driving Mechanism and 3-D Circulation of Plate Tectonics
#2: Before Plate Tectonics — Earth’s First 4 Billion Years
Warren Hamilton, USGS Emeritus and Distinguished Senior Scientist, Department of Geophysics, Colorado School of Mines
#1: Driving Mechanism and 3-D Circulation of Plate Tectonics
Abstract: Popular explanations of plate tectonics are not based on knowledge of the behavior of plates but rather on cartoon misconceptions combined with geochemical speculations that the lower mantle is unfractionated and the upper mantle has been progressively depleted by continental growth. From these conjectures, geodynamicists conjure whole-mantle convection, plates driven from below, and deep-mantle subduction and plumes, all of which are refuted by powerful evidence but hang on as zombie science (www.mantleplumes.org/Zombie.html). The Atlantic spreads very slowly and is rimmed by separating continents, whereas the Pacific spreads very rapidly, is rimmed by subduction, and is getting smaller: hinges roll back. Leading edges of overriding plates mostly are unshortened. Spreading ridges are pulled apart, not pushed, no ridges or trenches are “fixed,” and spreading ceases when ridges meet trenches. Plates and subduction are driven by density inversions produced by cooling from the top. Seismological and mineral-physics evidence best accords with the 650 km discontinuity, the base of the upper mantle, as a barrier to material going either up or down. Almost none of this is considered in popular models.
Young subducting oceanic lithosphere becomes marblecaked into the mid-upper mantle beneath overriding plates, but older lithosphere is laid down on the 650 as hinges roll back. Slabs sink steeply—they do not slide down slots—and push all sublithospheric upper mantle back toward fast-spreading ridges (e.g., Pacific). Only subducted lithosphere is transferred to slow-spreading passive oceans for recycling (e.g., Atlantic, and back-arc basins). Subducting plates are driven by their mass toward their only exits from the surface, and overriding plates are sucked toward sinking slabs. Motions are self-organized from the top.
#2: Before Plate Tectonics — Earth’s First 4 Billion Years
Abstract: From the late Cambrian on, indicators of modern-style plate interactions are abundant in the geologic record — ophiolites, subduction melanges, blueschists, magmatic arcs. There are sniffs of these in the Neoproterozoic, but none in the older Precambrian. Nevertheless, most Precambrian specialists force interpretations into non-actualistic plate-tectonic frameworks.
Archean igneous rocks are strikingly different from modern ones as rock types, lithologic associations, and structural assemblages. The lower crust is everywhere dominated by TTG (tonalite-trondhjemite-granodiorite) gneisses, igneous zircons in which can scatter from 4.4 to 2.5 Ga, which display long histories of extreme mobility. Only TTG has ever been seen depositionally beneath supracrustal greenstone belts, including voluminous mafic and ultramafic lavas. The characteristic structural style of the upper crust is of diapiric and batholithic domes that rose as denser greenstone sank in synforms, while simultaneously the crust underwent stretching and complementary shortening as it rode atop flowing lower crust. These and other features, including isotopics, can be explained by thorough fractionation of the Earth to produce a thick, global mafic/ultramafic protocrust by 4.4 Ga, delamination and recycling of which generated most subsequent Archean magmatism.
Seekers of Archean plate tectonics largely ignore geology, geochronology, and descriptive and phase petrology, and turn to ratios of trace elements taken on faith, from quite different modern rocks, to define ancient tectonic environments. Wild pseudo-plate settings and impossible structures are postulated by armchair spreadsheet manipulators.
Abbreviate the long story of Proterozoic orogens to note that their dominant igneous rocks are bimodal (which rules out popular island-arcs analogies), sedimentary rocks are mostly terrigenous, and there is no evidence for subduction nor proof of ensimatic rocks. Increasing evidence for exposed and contaminating Archean basement permits the inference that Proterozoic orogens typically formed from basins, filled thickly by sediments and volcanics, formed on extensionally thinned Archean lithosphere.
The upper mantle has evolved opposite to popular theory: it was severely depleted very early, and became progressively more enriched during geologic time by delamination and, later, subduction.
Biography: Warren Hamilton had a long career with the U.S. Geological Survey, punctuated by visiting professorships. He made field-based structural and petrologic studies in various parts of North America and Antarctica. The latter got him into continental drift when that was widely considered impossible, and enabled him to lead the way into plate-tectonic explanations of Phanerozoic continental geology. His monograph and wall maps on Indonesia and surrounding regions remain the most comprehensive integration of marine geophysics and onshore geology to reach new understanding of a huge active region. He worked with increasingly broad problems of crustal evolution and tectonics, early in USGS regional geology branches, later in a geophysics branch. After retirement, he joined the Department of Geophysics, Colorado School of Mines, where he continues research. His research honors include the Distinguished Service Medal of the Department of the Interior, the Penrose Medal of the Geological Society of America, and membership in the National Academy of Sciences.
PDFs of recent papers on the topics he will be presenting are available on his website, www.mines.edu/~whamilto ; further work is ongoing.
May
Brittle Structures and Inheritance in the Central Front Range of Colorado
Jonathan Saul Caine, U.S. Geological Survey
Abstract: A long history of mining and geologic mapping in the Front Range of the central Colorado Rocky Mountains has resulted in an exceptionally rich dataset for geologic structures and epithermal ore deposits. These regional-scale data were among the first to lead geologists to ponder the role of Precambrian structural inheritance in the localization of Tertiary mineral deposits. Of particular significance was the idea that localization of epithermal, polymetallic fault veins in this region was controlled by a pre-existing crustal “weakness,” the Proterozoic Idaho Springs-Ralston ductile shear zone (ISRZ) being one prime example. However, recent compilation of structural and mineral deposit data from existing 1:24,000 geologic maps, reports, argon geochronology on fault and hydrothermally altered rocks, and new structural data from outcrop in the Front Range result in five key observations:
1) There is little correlation between the locations of major ductile shear zones, inferred mineral deposit-related plutons, and major brittle fault zones.
2) Mapped features suggest that myriad directions of potential permeability structures in Proterozoic basement rocks existed during the Tertiary and that metalliferous hydrothermal fluids may have flowed in many directions at any given time during the evolution of the Colorado Mineral Belt.
3) Small displacement fault veins with striated and cataclasized margins that carried ore bearing fluids show steep dips and either preferential ENE trends well correlated with model paleostress directions for the Laramide orogeny or radial trends around Late Cretaceous to Tertiary igneous intrusions. These relationships hold regardless of co-planarity with preexisting foliations in metasediments or in massive unfoliated metaigneous plutons.
4) The total gas 40Ar/39Ar age of alteration is Laramide and the brittle faults are younger.
5) There are only minor differences in orientation and intensity of potential structures that may have controlled permeability from within the ISRZ compared with similar structures outside the ISRZ.
These observations suggest that Proterozoic inheritance in the Front Range is not the primary control of mineral deposit permeability structure, location, or orientation. Rather, responsible processes likely include: a) proximity to shallowly emplaced plutons; b) self-generated, hydro-fracture-like permeability due to thermally driven pore fluid pressure changes associated with pluton emplacement; and c) competition between varying magnitudes and orientations of shallow regional horizontal principal stresses, overburden load, and local stress perturbations related to pluton emplacement.
Structural Analysis of the Idaho Springs-Ralston Ductile Shear Zone: Reinvestigating Hypotheses of Inheritance from Proterozoic Structures in the Central Front Range of Colorado
Zachary Wessel, Department of Geosciences, Colorado State University;
John Ridley, Department of Geosciences, CSU; and
Jonathan Saul Caine, U.S. Geological Survey
Abstract: The Idaho Springs-Ralston ductile shear zone (IRSZ) has been described and mapped as a regionally important, long-lived and reactivated crustal structure in the Front Range and Central Rocky Mountain regions of Colorado. It has been interpreted as one of several persistent zones of weakness in the continental crust. The IRSZ has also been associated with the Proterozoic assembly of North America and with the later development of the Colorado Mineral Belt (CMB). Previous research has only partially characterized the style, timing, and sense of displacement of the IRSZ. Characterization at an appropriate scale for robust evaluation of influence on CMB mineralization has in particular been lacking. In view of the proposed weakness of the IRSZ and its control on the localization of Late Cretaceous–Tertiary mineral deposits, we have reinvestigated the structural history of the zone and adjacent rocks. Two fundamental questions were addressed: 1) is the description and interpretation of the IRSZ as a major through-going crustal-scale shear zone accurate?, and 2) is there evidence for repeated reactivation within the shear zone which would support the interpretation that it has formed a long-lasting zone of weakness? Structural domain analysis of compiled USGS map data and newly collected outcrop foliation and lineation data was used to determine if any large-scale fabric patterns exist that indicate the presence of a major shear zone. Multiple tests were performed to determine how regional-scale structural fabrics are influenced by deformation along and adjacent to the IRSZ. A kilometer-scale structure defined by parallel regional, fold-related foliation is mapped and supports the existence of the shear zone. However, geologic units do not show any significant variation or offset in rock types adjacent to and across the shear zone as would be expected at a major through-going crustal structure. The IRSZ does, however, run adjacent to a km-scale synform and is coincident with the south limb of this fold. A change in regional foliation across the fold-shear zone pair, from NE-SW to NW-SE strikes, suggests the zone marks a boundary between structural fabric domains. This boundary indicates that either pre-IRSZ structures had different orientations or that the folding, shearing, or both caused regional realignment of structures. Kinematic analyses of rare meter-scale, discontinuous mylonites in the study area show no evidence of strike-slip movement along the fold limb or the IRSZ, but the kinematics are consistent with both north-sideup and south-side-up sense of shear. The domain analyses indicate the presence of a regional, foliation-parallel structure consistent with the existence of the IRSZ, although mapping, petrologic, and kinematic evidence suggest movement along the IRSZ was not of a crustal-scale magnitude.
Biography: Jonathan Saul Caine is a Research Geologist with the U.S. Geological Survey. His work is focused on characterization of fault zones, fracture networks, and fluid flow in the Earth’s upper crust. He combines structural geology, hydrogeology, geochemistry, petrophysics, and detailed field studies to understand fault-zone architecture and permeability structure; fault-rock textures, deformation mechanisms, weakening mechanisms, kinematics, and reactivation; direct fault-rock dating; and fault- and fracture-network related fluid flow as it pertains to groundwater supply, mineral deposits, hydrocarbon migration, and environmental geochemistry of hydrothermally altered and complexly deformed crystalline-rock and sedimentary basin aquifer systems. Caine also works closely with graduate students and colleagues at the University of Colorado, Boulder, the Colorado School of Mines, and Colorado State University. He received his B.A and M.A. in Geology from S.U.N.Y, New Paltz (1986 and 1991) and his Ph.D. from the University of Utah (1999).
Biography: Zachary Wessel is a Ph.D. candidate at Colorado State University. His dissertation is focused on a detailed study of the Idaho Springs-Ralston Shear Zone (IRSZ). The study includes performing local scale and regional scale structural domain analyses of metamorphic and deformational fabrics, petrographic and petrologic analyses of metamorphic suites within and bounding the shear zone, and a comparison of the structural variance and compatibilities between the IRSZ and the Moose Mountain Shear Zone. Zachary completed a B.S. and M.S. in Geology at Ohio University (2001 and 2004). His M.S. project focused on the structural and temporal relationship between low- and high-grade metamorphic suites, igneous intrusives, and an associated mylonite zone in the Creignish Hills of Cape Breton, Nova Scotia. His research interests include the interaction of igneous and metamorphic suites, ductile deformation and deformation mechanisms, and continental-scale deformational processes. He also has an extensive knowledge and understanding of sedimentary processes, structural analysis, and geophysical data interpretation as related to natural resource exploration. After completion of his Ph.D., Zachary will begin work for El Paso Oil & Gas in Houston, Texas.
September
Brief Overview of the Quaternary in the Roaring Fork River Valley, Central Colorado
Lucille A. Piety, Seismotectonics and Geophysics Group, Bureau of Reclamation,
Abstract: The Roaring Fork River heads in the Sawatch Range and flows about 100 km northwestward to join the Colorado River at Glenwood Springs. During the Quaternary, the Roaring Fork River and its tributaries, which head in both the Sawatch Range and Elk Mountains to the south, were filled periodically with glacial ice. The position and extent of moraine deposits indicate that the glaciers along the Roaring Fork River and three of its tributaries at times coalesced near Aspen, but later remained 2 to 7 kilometers upstream. The plentiful supply of water and sediment from glacial outwash helped form four extensive terraces about 10 to 200 meters above the Roaring Fork River downstream of Aspen. Terrace formation was likely influenced by easily erodible sedimentary rocks, such as Mancos Shale, that underlie the valley. The Maroon Formation gives parts of the valley and the moraine and terrace deposits their distinctive reddish color.
The terraces continue downstream and are extensive between the towns of Basalt and Glenwood Springs. In this section, basalt flows that erupted periodically beginning in the Laramide and continuing into the Quaternary are preserved on some of the highest terraces. Eagle Valley Evaporite crops out along the margins of the valley and underlies the terraces in the Roaring Fork valley and in the lower Crystal River and Cattle Creek valleys. Tilting of the older Pleistocene terraces and their underlying deposits toward the valley walls and anomalous drainages on these terraces suggest that deformation related to salt tectonics has continued into the Quaternary.
Late Pleistocene-Holocene Stratigraphy of Rock Glacier Debris Mantles, Mt. Sopris, Near Aspen
Peter W. Birkeland, Emeritus, Department of Geological Sciences, University of Colorado
Abstract: Over 40 years ago I asked Gerry Richmond where I should do Quaternary stratigraphic work in the Colorado mountains. He suggested Mt. Sopris, so I spent several years pounding rocks and digging soils as I struggled over the unstable, bouldery rock glacier surfaces. I was hoping to emulate his La Sal work by doing all deposits on the mountain (slope deposits, etc.), but decided to concentrate on the rock glaciers.
My focus was to define mapping units and estimate ages of rock glacier debris mantles. Map units were defined using a variety of data: lichens, rock weathering parameters, soils, and the presence of loess. The first subdivision of mantles is Pleistocene vs. Holocene, best made with rock weathering, soil, and loess data. Within the Holocene, various lichen parameters help delineate three ages of mantles. At the time of publication I used ages of other workers elsewhere to estimate these ages for the mantles: Gannett Peak, present to several centuries; Audubon, 1-2 ka; early Neoglacial, 3-5 ka; and Pinedale (includes Younger Dryas), >10 ka. These still seem to be adequate ballpark ages. Once the rocks hit the surface at the upper end of the rock glacier, most travel piggyback down the valley, picking up lichens and weathering features. There was always a debate about the age of the Temple Lake, and years later, Gerry hiked into the Wind Rivers to help sort it out.
The rock glaciers are confined to a NW to NE aspect. Cirque basins characterize the headwaters of NE to N rock glaciers, but headwaters facing NW have a non-glacial look. Most rock glaciers have mantles of all ages, and the actively moving part of the rock glaciers does not coincide with a consistent stratigraphic boundary; in fact, the active-nonactive boundary decreases in altitude from NE to NW. I speculate that these are talus-fed rock glaciers with a core of ice cementing clasts and fines. Stratigraphic evidence suggests that they moved downvalley in the latter Holocene at rates of roughly 8-32 cm/yr, well within Bryant’s Colorado rates of 5-60 cm/yr.
November
The EarthScope Rio Grande Rift GPS Experiment: Measuring Active Tectonics in Colorado and New Mexico
Anne Sheehan, University of Colorado at Boulder
Abstract: Using high precision GPS measurements from 25 new GPS monuments in Colorado and New Mexico combined with data from EarthScope-Plate Boundary Observatory, we seek to determine the present-day crustal strain rates associated with the Rio Grande Rift and southern Rocky Mountains. The factors that drive the Rio Grande rift are not well understood, and an accurate set of measurements of crustal motions are a first step towards an improved understanding of earthquake and volcanic hazards within rift zones and why tectonic plates undergo stretching. The low rates of motion in the region make this project especially challenging, and the geodetic monumentation, observing time, and post-processing strategies are designed to optimally resolve deformation at the sub-mm/yr scale. Velocity gradients from 2006–2010 for our east-west profiles across Colorado and New Mexico suggest that deformation may not be concentrated in a narrow zone but distributed broadly across a region spanning from the western edge of the Colorado Plateau to several hundred km east of the Rio Grande Rift. The unexpected broadly distributed deformation has implications for lithospheric strength, fault mechanics, and earthquake hazards in the region.
Quaternary Life and Times of Pluvial Lake Manix, Mojave Desert, California: Climatic and Geologic Controls on a Desert Lake
Marith Reheis, U.S. Geological Survey, Denver
Abstract: During the middle and late Pleistocene, Lake Manix in south-central California was the terminus for the Mojave River during periods of enhanced runoff from the Transverse Ranges, until it drained east to form Lake Mojave at about 25 ka. The long sedimentary record in outcrop and sediment cores reveals a complex history of fluctuations influenced by paleoclimate, basin configuration (multiple sub-basins), drainage integration events, faulting, and out-of-basin diversions. The relative effects of all these influences must be evaluated using detailed geologic mapping (this is the most fun part), stratigraphy, and sedimentology in order to isolate the paleoclimate signal contained in a 45–m core from the thickest remaining section of lake sediments. Core analyses included particle size, ostracode assemblages, stable isotopes (δ18O and δ13C) on ostracodes, inorganic carbonate, and magnetic properties. Mapping and dating of freshwater Anodonta shells (14C) and lake tufa (U-series) from exposures supplemented these analyses. Our studies show that the core is about 500 ka at the base, and it contains a record of lake fluctuations that extends from Marine Isotope Stage (MIS) 12 to early in MIS 2. If the age model is correct, then sedimentologic, ostracode, and isotope data suggest that some episodes of moderately deep water occurred during interglacials and interstadials as well as glacial periods. Further, a well-dated lake-level curve based on outcrop stratigraphy shows multiple highstands during MIS 3 and early MIS 2 (~45–25 ka), at times when the Laurentide ice sheet was well below its maximum height and geographic extent and long before the Great Basin lakes to the north. Thus, the Lake Manix data suggest complex responses to climatic drivers other than jet stream position.
December
What’s Up, Down, and Sideways about Fault Slip on the Santa Barbara Coastal Plain and Channel Islands,
Southern California: Tests of Neogene-Quaternary Rotation Model for the Western Transverse Ranges
Colorado Scientific Society President’s Address
Scott Minor, U.S. Geological Survey, Denver, Colorado
Abstract: It is generally accepted that crustal blocks underlying the western Transverse Ranges (WTR) of coastal southern California have rotated ~75-90 degrees clockwise since about 16 Ma, based largely on seminal paleomagnetic studies done in the late 1980s by Bruce Luyendyk and his students at UC Santa Barbara. Most believe that these rotations resulted from the unique geometric and geologic configuration and evolution of this part of the dextral transform tectonic plate boundary during the Neogene and Quaternary. Such large block rotations are expressed by the anomalous east-west topographic and structural grain of the province, which encompasses (from south to north) the Channel Islands, Santa Barbara Channel, Santa Barbara coastal plain, and Santa Ynez Mountains. Analog transrotational geometric models and tectonic reconstructions predict that slip on the set of parallel faults bounding and accommodating the rotating blocks should have evolved from normal and oblique normal-sinistral movement to sinistral, oblique sinistral-reverse, and reverse movement as the rotations proceeded. In this talk I present kinematic data (slip surface orientation, slickenline rake, and slip sense) from exposed faults north and south of the Santa Barbara Channel to test these rotational models and to reveal the kinematics of neotectonic deformation. The latter provide clues concerning the causes of spatially variable Quaternary uplift rates, strain partitioning of potentially seismogenic structures, and relationships between fault kinematics and landscape change.
The Santa Barbara coastal plain north of the Channel is structurally dominated by the Santa Barbara fold and fault belt (SBFFB) and the Santa Ynez Mountains uplift overlapping it to the north. The SBFFB is a WNW-trending zone of potentially active folds and partly blind oblique-slip reverse and thrust faults that spans the entire coastal plain and continues into the lower southern flank of the Santa Ynez Mountains. On the coastal plain several folds of this zone deform older alluvial and marine terrace deposits and have subtle to strong geomorphic expression that is consistent with a youthful age of deformation. Kinematic data were collected from numerous small-scale (<5 m displacement) and map-scale (5 to >100 m) fault surfaces exposed in the coastal plain area within sedimentary rocks and deposits ranging in age from middle Eocene to late Pleistocene. WNW- to NW-striking faults cutting Miocene and older rocks on the lower flanks of the Santa Ynez Mountains exhibit multiple generations of slickenlines, many indicating older normal- and oblique normal-slip movement and younger oblique strike-slip and reverse movement. Structural restorations suggest that much of the folding and associated reverse faulting in the SBFFB were preceded by normal- and strike-slip faulting. Some WNW- to NNW-striking faults show evidence of both dextral and sinistral strike-slip movement, and for several of these dextral slip postdates sinistral slip. Some reverse faults in Miocene and older rocks exhibit progressive shifts in slickenline rakes that are consistent with clockwise rotation of the faults. Faults in middle and upper Pleistocene marine and alluvial sediments on the coastal plain lack evidence of early normal slip, but otherwise have slip histories similar to faults in the older rocks and show abundant evidence of late reverse and oblique reverse movement. Structural and stratigraphic age relations observed in the area imply that late Cenozoic uplift and related transpressional deformation was most pronounced during the Plio-Pleistocene, and was preceded by a possibly widespread episode of normal faulting and transtensional deformation during the middle to late Miocene. Kinematic measurements were also obtained for small- to large-displacement (<1m to >1000m) faults on Santa Rosa Island (SRI), one of four E-W-aligned islands on the south side of the Santa Barbara Channel. Most of these faults have W-WNW strikes, and many displace Quaternary (mostly ~80-120 ka) marine terraces and overlying deposits. One such fault, the large SRI Fault spanning the length of the island, sinistrally deflects stream channels and separates differentially uplifted and dissected blocks. The SRI fault kinematic observations suggest that normal-sinistral and sinistral strike-slip movement prevailed on ~W-striking faults in Pliocene and earlier Quaternary time. Later Quaternary slip on these faults was characterized by greater components of reverse and, locally, dextral slip.
Fault-slip patterns in the Santa Barbara coastal plain area and on the Channel Islands broadly support the clockwise transrotational model for the WTR involving late Miocene transtension and Plio-Quaternary transpression. Not predicted by the model, however, are the observed large components of dextral slip that commonly overprint sinistral-reverse slip on some WNW-striking faults. Such overprinting may reflect increased accommodation of the regional, transform-related NW-trending dextral shear couple by dextral slip along faults as they are rotated into strikes approaching the NW trend. This study indicates that strongly oblique contractional (transpressional) deformation has prevailed in this part of the WTR throughout the Quaternary, and based on earthquake focal mechanisms, such deformation is ongoing. Despite the similarities in fault slip north and south of Santa Barbara Channel, Quaternary uplift rates (based on precise ages of elevated marine terraces) differ significantly between the two areas (i.e., larger on the coastal mainland). Thus, fault slip rates may have a greater influence on uplift rates than slip geometries.
