Sarah J. Titus

Thirty-Five-Year Creep Rates for the Creeping Segment of the San Andreas Fault and the Effects of the 2004 Parkfield Earthquake: Constraints from Alignment Arrays, Continuous Global Positioning System, and Creepmeters

We present results from differential Global Positioning System (gps) surveys of seven alignment arrays and four continuous gps sites along the creeping segment of the San Andreas fault. Surveys of four alignment arrays from the central creeping segment yield 33- to 36-year average minimum slip rates of 21–26 mm/yr. These rates are consistent with previous alignment array surveys spanning a 10-year period and with rates determined by creepmeters, indicating approximate steady- state creep along the central creeping segment for at least 35 years. Motion between continuous gps sites that span the central creeping segment is 28.2 ± 0.5 mm/yr for two sites that are 1 km apart and 33.6 ± 1 mm/yr for two sites that are 70 km apart. Slip rates therefore increase with distance from the creeping segment of the San Andreas fault. All rates reported here are significantly slower than the 39 ± 2 mm/yr rate predicted for motion between the Sierra Nevada–Great Valley block and the Pacific plate. Repeat surveys of three alignment arrays following the 2004 Parkfield earthquake demonstrate that its coseismic and short-term postseismic offsets decrease rapidly with distance from the epicenter, from 150 mm to 15 mm to <5 mm at respective distances of 9, 36, and 54 km to the northwest. Continuous gps data confirm that little coseismic and postseismic slip occurred along the central creeping segment. Geodetic and geologic slip rates are compared and different models for the accommodation of transcurrent deformation across the creeping segment are discussed.

How Geoscientists Think and Learn

Decades ago, pioneering petroleum geologist Wallace Pratt pointed out that oil is first found in the human mind. His insight remains true today: Across geoscience specialties, the human mind is arguably the geoscientists most important tool. It is the mind that converts colors and textures of dirt, or blotches on a satellite image, or wiggles on a seismogram, into explanatory narratives about the formation and migration of oil, the rise and fall of mountain ranges, the opening and closing of oceans. Improved understanding of how humans think and learn about the Earth can help geoscientists and geoscience educators do their jobs better, and can highlight the strengths that geoscience expertise brings to interdisciplinary problem solving.

Geologic versus geodetic deformation adjacent to the San Andreas fault, central California

We combine geologic and global positioning system (GPS) data to characterize the style and magnitude of off-fault deformation across the San Andreas fault system in central California. Geologic structures record ∼12 km of both fault-parallel and fault-perpendicular displacements across creeping and locked portions of the San Andreas fault. Analysis of 150 GPS site velocities suggests that the borderlands record 4–6 mm/yr of fault-parallel and 3–5 mm/yr of fault-perpendicular motion alongside the creeping segment, where elastic strain is minimized. The distribution of both long-term geologic and short-term geodetic deformation is affected by basement type, where more deformation is concentrated northeast of the San Andreas fault on Franciscan basement. We suggest that at least half the fault-parallel GPS deformation measured by GPS bordering the creeping segment must be accommodated by geologic structures; this permanent deformation needs to be incorporated into dynamic models of the fault system. Elastic modeling of the San Andreas fault in central California, which incorporates its well-known transition from locked to creeping behavior near Parkfield, predicts first-order variations in the GPS velocity field along the fault and corresponding variations in dilatational strain rates. The strain rate pattern is dominated by a large contractional region northeast of the transition from locked to creeping behavior and a large extensional region southwest of the transition. The former coincides with the Coalinga and Kettleman Hills anticlines, the growth and development of which seem to have occurred under at least two kinematic conditions. We suggest that the onset of fault creep in central California promoted the growth of these folds. By implication, fault creep has been active over geologic time scales.

New slip rate estimates for the creeping segment of the San Andreas fault, California

New continuous and differential global positioning system (GPS) measurements of recent slip rates and 30 yr alignment-array offsets from the central creeping segment of the San Andreas fault yield a maximum right-lateral slip rate of 25 ′ 1 mm/yr. This slip rate is 20% slower than the 30 mm/yr slip rate accepted for this segment of the fault and 35% slower than the 39 mm/yr slip rate predicted between the Sierra Nevada-Great Valley block and the Pacific plate. New continuous GPS measurements between pairs of sites that flank the creeping segment at intersite distances of 1.0 km and 70 km give relative fault-parallel slip rates of 28 ′ 2 and 30 ′ 2 mm/yr, respectively. These observations indicate that right-lateral deformation rates increase with distance from the fault. Possible explanations for the gradient observed in the geodetic data are elastic strain accumulation along the creeping segment or significant distributed deformation on off-fault structures.

Characterizing and Improving Spatial Visualization Skills

Three-dimensional spatial visualization is an essential skill for geoscientists. We conducted two evaluations of students’ spatial skills to examine whether their skills improve after enrollment in a geology course or courses. First, we present results of pre- and post-course survey of abstract visualization skills used to characterize the range of spatial abilities in the student population at Carleton College. In Introductory Geology, there was a correlation between those who score very poorly on the spatial survey and those who receive a grade of C or lower. Students in higher-level courses had better developed visualization skills than those in Introductory Geology. Gender differences disappeared in upper-level courses except for the spatial relations (mental rotation) task, where male students consistently outperformed females. Second, we describe the efficacy of instructional materials designed for a Structural Geology course at the University of Wisconsin. This study included a qualitative controlled experiment investigating whether frequent use of stereographic projections affected student performance on exam questions requiring spatial skills. The results of both the survey-based quantitative study and materials-based qualitative study suggest that students’ spatial abilities can improve through practice provided in geology courses.

Geologic and geophysical investigation of two fine-grained granites, Sierra Nevada Batholith, California: Evidence for structural controls on emplacement and volcanism

The Johnson Granite Porphyry and the Silver Pass Porphyry are two late-stage porphyritic intrusions in the Cretaceous Sierra Nevada Batholith, California. Both intrusions are characterized by fine-grained textures, miarolitic cavities, pegmatite veins, and abundant xenoliths. Microstructures in the Johnson Granite Porphyry record magmatic flow. Magnetic fabrics in the Johnson Granite Porphyry, determined by anisotropy of magnetic susceptibility, show that the poles to magnetic foliations define a NNE-SSW girdle and magnetic lineations plunge shallowly to moderately to the WNW and ESE. A gravity survey and three-dimensional inversion indicate that the Johnson Granite Porphyry is shallow and tabular, extending to 1.5 km depth beneath the southwestern margin. A second gravity anomaly was delineated southeast of the Johnson Granite Porphyry, suggesting the intrusion continues to depth. The Silver Pass Porphyry is deformed by the dextral Rosy Finch shear zone and records a gradient of high- to low-temperature, solid-state fabrics. Field and magnetic foliations in the Silver Pass Porphyry are subvertical and lineations are shallowly plunging. We propose that regional shear zones controlled the emplacement of these intrusions. The Johnson Granite Porphyry was emplaced in a three-dimensional tension gash associated with the termination of the Bench Canyon shear zone. The Rosy Finch shear zone facilitated the emplacement of the Silver Pass Porphyry, probably as a tension fracture associated with the shear zone. The field observations and magnetic fabrics suggest that the Johnson Granite Porphyry may record an extrusive event, suggesting that volcanism may be related to deformation along active shear zones.

Pull-apart formation and strike-slip partitioning in an obliquely divergent setting, Leka Ophiolite, Norway

Abstract The Leka Ophiolite Complex (LOC) is located on the island of Leka, Norway, and belongs to the Uppermost Allochthon of the Scandinavian Caledonides. The rocks of the adjacent mainland and most of the surrounding islands are basement gneisses and supracrustal rocks not related to the ophiolite complex. Paleostress analysis, gravity inversion, and regional geology support a fault-bounded rhombochasm geometry for the LOC. The paleostress inversions revealed two types of tensors, interpreted as small strains: (1) horizontal extension, generally E–W to NE–SW, and (2) horizontal extension in the same direction with an added component of perpendicular horizontal contraction. A strong positive gravity anomaly (25 mGal) is centered on Leka, and gravity inversion indicates that the LOC lies directly below its surface exposures with steep-sided walls and a flat bottom located at ∼7 km depth. The faults bounding the LOC probably initiated during postorogenic extension in the Scandinavian Caledonides. The faults are regional in scale and are parallel to other NE–SW trending en echelon faults along the Norwegian coastline and on the adjacent mainland. A pull-apart structure explains the down-dropping and subsequent preservation of the LOC, as it is surrounded by rocks from lower structural positions within the nappe stack. The paleostress directions from Leka support a sinistral component of shear along these faults. The gravity inversion is consistent with a fault-bounded geometry. This pull-apart structure, as uniquely recorded by the dense ophiolitic rocks, suggests that strike-slip partitioning was active in an obliquely divergent setting.

Fabric development in the mantle section of a paleotransform fault and its effect on ophiolite obduction, New Caledonia

The Bogota Peninsula shear zone has been interpreted as a paleotransform fault in the mantle section of the New Caledonia ophiolite. New, detailed field measurements document the rotation of foliation, lineation, and pyroxenite dikes across a 50-km-wide region. Deformation intensity recorded by folding and boudinage of dikes increases toward a central, 3-km-wide mylonitic zone. We used several additional methods to characterize fabric patterns across the shear zone. The shape-preferred orientation of orthopyroxene grains, computed from outcrop tracings, closely parallels field fabrics, with increased alignment and flattening near the center of the shear zone. The lattice-preferred orientations of olivine are consistent with high-temperature fabrics; the a axes within the mylonitic core were used to constrain the orientation of shear zone boundaries. Seismic anisotropy calculations, based on the lattice-preferred orientation of olivine, indicate 5%–11% shear-wave anisotropies, with increased values in the center of the shear zone. The magnetic silicate fabric in the rocks, determined from anisotropy of magnetic susceptibility techniques, broadly matches field fabrics but provides less consistent information across the shear zone than other fabric methods. This suite of field and laboratory data provides a unique and detailed view of strain and fabric patterns across a shear zone in oceanic mantle lithosphere. Because the primary mantle fabrics seem to be related to the present distribution of ophiolitic rocks in New Caledonia, we propose that ophiolite obduction and Neogene extension may have been controlled by preexisting fabrics and structures in the oceanic lithosphere.

Using vertical axis rotations to characterize off-fault deformation across the San Andreas fault system, central California

We use vertical axis rotations from new paleomagnetic data to constrain off-fault deformation within the San Andreas fault system in central California. Samples were collected from 177 sites in the Miocene Monterey Formation adjacent to the Rinconada fault. Reliable means from 57 sites have 3 prominent patterns: (1) the largest clockwise rotations are close to the Rinconada and Nacimiento faults; (2) no significant rotation is observed near the San Andreas fault; and (3) counterclockwise rotations are observed at several sites northwest of Paso Robles. These paleomagnetic results are compared to two other measures of off-fault deformation where rotation can be calculated. Results from a fold-based kinematic model show increasing clockwise rotations toward the Rinconada fault, consistent with pattern 1. Few folds are observed in rocks on Salinian basement near the San Andreas fault, suggesting that little deformation has occurred, and providing an explanation for the negligible paleomagnetic rotations in pattern 2. Rotations calculated from the global positioning system velocity field predict counterclockwise rotations that coincide with those observed from paleomagnetic data in pattern 3. Broad patterns in the velocity field appear to be controlled by the transition from creeping to locked behavior along the San Andreas fault, and the region of counterclockwise rotation is linked to this transition. Thus, we suggest that the creeping segment has been aseismic over geologic time scales in order to produce the observed paleomagnetic rotations. The integration of all three data sets demonstrates that the San Andreas fault borderlands record an important portion of fault-parallel plate motion over geologic and geodetic time scales.

Using field data to constrain a numerical kinematic model for ridge-transform deformation in the Troodos ophiolite, Cyprus

The Troodos ophiolite in Cyprus provides a unique opportunity to examine spatially varying patterns of deformation near a ridge-transform intersection. We focus on the paleo–inside corner defined by the E-W–striking, dextral Arakapas transform fault and the N-S–striking Solea graben. Rocks within the inside corner are primarily sheeted dikes and gabbros. The strikes of dikes vary with proximity to the Arakapas fault, changing from NW- to N- to E-striking with increasing proximity to the fault. We report new paleomagnetic results from 24 stations in the gabbroic rocks. When augmented with data from several previous studies, the combined paleomagnetic data set indicates that vertical-axis rotations increase from 5° to 90° with distance from the Solea graben. Rotations are also largest near the transform fault. We develop numerical kinematic models for deformation within the inside corner based on these field data. First, we fit an interpolation function to the two-dimensional field of vertical-axis rotations. This field is then used to undeform dikes, assuming that dikes were either part of rigid blocks or passive markers within a continuum. We find that dikes return to a consistent NW to NNW strike throughout much of the inside corner. This initial orientation is not ridge-parallel and therefore different from most common assumptions of dike behavior in Cyprus. However, the orientation is consistent with predictions from dynamic models of heterogeneous stress directions that develop near ridge-transform intersections.