T33C-1391 1340h
Quaternary Tectonism in a Collision Zone, Northwest Washington
Kinematic, geodetic, and geologic observations define a region with 6-8 mm/y of north-south contraction between the Columbia River and Vancouver Island. We attribute this contraction to differential forearc-block motion within the Cascadia subduction system where the Oregon Coast Range block is moving northward relative to Vancouver Island. The contraction is accommodated by a combination of distributed uplift in the Olympic Mountains, and faulting along the margins of the Coast Range and Vancouver Island blocks. The tide gauge at Neah Bay, which records one of the highest rates of uplift anywhere along the Cascadia subduction margin, suggests that a significant portion of this north-south contraction occurs between the northern Olympic Peninsula and Vancouver Island. The northwest-trending Calawah fault, extending from Makah Bay eastward to near Lake Crescent, appears to mark the modern boundary between the Olympic Mountains block and the Vancouver Island block in northwestern Washington. Onshore, the 80+ km-long Calawah fault displaces late Quaternary glacial sediments and geodetic uplift rates increase abruptly across the fault zone. Offshore in Makah Bay, new multibeam, sidescan-sonar, and high-resolution seismic reflection data image a complex, multi-strand fault zone that offsets the seafloor and moves Cape Flattery rocks seaward. Two parallel, northwest-trending fault strands bound a down-dropped block that in turn terminates along a northward-trending anticlinal fold and thrust fault. These data suggest that the Calawah fault zone currently accommodates contraction both by uplift and by seaward translation of rocks north of the forearc-block boundary. Our geologic mapping in the Cape Flattery area indicates that differential block motion is accommodated by a combination of crustal uplift, folding, and left-lateral, strike-slip faulting.
T33C-1392 1340h
A Three-Dimensional View of the Tualatin and Northern Willamette Basins, Oregon, from Inversion of New Gravity Data
A regional gravity study was initiated during the summer of 2004 over the Tualatin and northern Willamette basins in support of ongoing seismic hazard and ground-water studies. More than 1200 new gravity measurements were acquired over the basins and surrounding areas, increasing the average spacing of 1 station per 25 km$^{2}$ to 1 station per 5 km$^{2}$ over much of the lowlands. To first order, the gravity data reflect the substantial density contrast (400 to 600 kg/m$^{3}$) between dense Eocene oceanic basement rocks (e.g. Tillamook Volcanics and Siletz River Volcanics), exposed extensively in the Coast Range west of the valleys, and overlying Eocene to Miocene marine sedimentary rocks and Quaternary deposits. Isostatic gravity values as high as +70 mGal occur over Eocene basement rocks west of the valleys. Relatively high gravity values of $\sim$ +20 mGal coincide with a few scattered outcrops of the Tillamook Formation and Basalt of Waverly Heights (considered Eocene volcanic basement) on the eastern margin of the Tualatin basin, suggesting that Eocene basement extends across the Tualatin basin. However, the lower values on the eastern margin may indicate lower-density facies within the volcanic basement (i.e. breccias rather than flows) and/or a deeper oceanic slab. Both the Tualatin basin and the Willamette basin to the south are marked by gravity lows. The lowest gravity values (minus 45 mGal) of the entire region occur in the central part of the Tualatin basin, north of the intrabasinal, east-striking Beaverton Fault, whereas the gravity low over the northern Willamette basin reaches only minus 20 mGal. The two basins are separated by a broad, northeast-trending gravity high along the upthrown side of the Sherwood fault. The western margin of the Tualatin gravity low coincides with the Gales Creek fault and is marked by a steep gravity gradient, which broadens south of the Beaverton Fault. The southward continuation of the Gales Creek fault into the northern Willamette Valley, and the Mt. Angel fault farther southward, have little or no gravity expression, likely due to small vertical offsets. Gravity data filtered to enhance shallow sources (depths $<$ 2 km) show strands of the Portland Hills fault (along the northeast margin of the Tualatin basin) extending 15-20 km northwest of their mapped traces. The newly acquired gravity measurements were inverted to provide a preliminary three-dimensional view of Eocene basement and related tectonic structure. The Tualatin basin is characterized by a deep sub-basin surrounded by steep ($\sim$40$\deg$) margins. The southern margin of the Tualatin sub-basin corresponds to the concealed, east-striking Beaverton fault, where Eocene basement deepens abruptly by more than 2 km, consistent with a north-verging reverse fault. The northern Willamette basin is also characterized by a sub-basin, although smaller in horizontal and vertical dimensions. In contrast to the east-striking Beaverton fault, northwest- and northeast-trending strike-slip faults, like the Mt. Angel fault, show only modest vertical offsets. Pertinent future research should focus on obtaining detailed gravity profiles across the Beaverton, Canby-Molalla, Sherwood, Gales Creek, Sylvan, and Portland Hills faults for joint gravity and magnetic modeling and for extending regional coverage over the northern margin of the Tualatin basin and the eastern margin of the northern Willamette basin. A worthy goal is to develop a three-dimensional geologic model of the two basins, including Eocene basement, Columbia River basalt, interbasalt sedimentary units, and regional fault systems.
T33C-1393 1340h
Neogene Tectonics in the Saratoga Valley, Wyoming: Preliminary Results From a Study Combining ASTER, Landsat, and Aerial Imagery With Field Work and GIS
The Laramide orogeny created many compressional structures and basement-cored uplifts in Wyoming and Colorado, including the Medicine Bow Mountains and the Sierra Madre/ Park Range. Within the uplifted core, the Rio Grande Rift along the Front Range in Colorado has given rise to several extensional basins. The Saratoga Valley lies on the border between Wyoming and Colorado, in line with the Rio Grand Rift, and its structural origin as a Laramide compressional basin is generally assumed. However, extensional features have been reported, leaving the tectonic history of the Saratoga valley subject to question. This study aims to identify the timing, dip, and offset of normal faults and erosional surfaces to constrain the tectonic history of the Saratoga Valley. To determine whether the northwest trending valley is primarily a Laramide syncline or is, instead, a Neogene half-graben, linear features such as faults, ridges, and rivers were mapped from ASTER, Landsat, and aerial imagery and confirmed in the field. Two sets of linear features trend N30W and roughly N20E. Visible offset of linear features indicates right-lateral movement along a northwest trending fault located south of Encampment. Faulting of unknown offset is also present along the southeast front of the Sierra Madre Mountains. Location of sedimentary units in the field has constrained a pre-Miocene paleosol in the eastern valley and post-Miocene normal faulting within the Saratoga Valley. Combining these results with multiple data-types in a GIS format allows display of both detailed surface topography and subsurface geology. The imagery and fieldwork lead to the conclusion that a composite of several tectonic episodes have created northeast and northwest trending, cross-cutting faults, some of which are extensional. Nevertheless, to confidently describe the Saratoga Valley as a Neogene half-graben thousands of feet of offset along a normal fault or series of faults would be required. This amount of normal fault motion is not yet verified.
T33C-1394 1340h
Results of the 2004 GPS Study of Extension Rates in the Eastern Snake River Plain, Idaho
Previous investigators suggest the eastern Snake River Plain (ESRP) is extending by dike intrusion, which enables it to keep pace with SW-NE extension that is occurring in the surrounding Basin and Range Province. Northwest-trending, linear eruptive fissures and aligned volcanic vents within ESRP volcanic rift zones provide observational evidence for dike intrusion in the ESRP as recent as 2000 yrs ago. To assess the amount of extension occurring in the ESRP, a campaign GPS survey was conducted in April of 2004 by Idaho State University and the Idaho National Engineering and Environmental Laboratory. Twenty benchmarks located within the ESRP and adjacent northwest Basin and Range (Lost River and Beaverhead Ranges, Idaho) were each occupied for 48 to 96 hours using Trimble 4000 dual frequency GPS receivers supplied by UNAVCO. The University of Utah occupied these same benchmarks in 1995 and 2000, resulting in three occupations over nine years. Preliminary results suggest that benchmarks on the ESRP consistently moved southwest relative to a fixed North American reference, and rates systematically increased from northeast (near Rexburg, Idaho) to southwest (near Twin Falls, Idaho) for the nine year time period. Benchmarks in the adjacent northwest Basin and Range also moved southwest at comparable rates, and an increase in displacement rate from NE to SW is also apparent, although it is not as systematic. The preliminary results suggest the strain measured over nine years is similar in the ESRP and adjacent northwest Basin and Range, and thus, no differential movement is indicated. To supplement the GPS results, an interferometric synthetic aperture radar (InSAR) study of the ESRP is also underway using European Resource Satellite (ERS-1 and ERS-2) images. The anticipated results of the InSAR study will provide more spatially detailed and coherent information about differential movements within the ESRP, beyond what is shown by the GPS study.
T33C-1395 1340h
Gravity Study of the Fish Lake Valley Area, Nevada.
The regional releasing bend within the central Walker Lane Belt (WLB), know as the Mina Deflection, is characterized by rhomboidal pull-apart basins and associated NE-trending normal faults that transfer the strike-slip displacement between the right-lateral transcurrent northern and southern portions of the WLB. A prominent example can be found at the northern end of Fish Lake Valley (FLV), where right-lateral displacement along the Fish Lake Valley-Furnace Creek fault zone feeds northward into a large right-stepping pull-apart structure, bounded by the Emigrant Peak normal fault system. Existing regional gravity archives, available from federal agencies, are adequate for characterizing general features of FLV and other nearby basins. Regional coverage is very sparse within surrounding ranges, however. Over 900 new gravity data points have been acquired in the FLV and surrounding ranges in support of ongoing geothermal studies of the area. The new data coverage is detailed enough to constrain the subsurface geometry of the range bounding faults and the projection of outcropping lithologies in the ranges into the basins. In conjunction with an ongoing program at KU of detailed geological field mapping to refine the lithologies and tectonic history of the ranges, a detailed gravity modeling project is also being undertaken to extend the new interpretations into the subsurface and to allow quantitative estimates of basement depth, fault offsets, and sediment volumes within the FLV. Prominent features seen in preliminary results include a steep range-bounding fault at the north end of the FLV with an apparent depth to basement of over one km immediately to the south of the fault, and large gravity highs due to metasedimentary rocks within the Paleozoic section.
T33C-1396 1340h
Using Receiver Functions to Analyze Rapid Transitions in Crustal Structure and Deformation in Northern California
The Coast Ranges of Northern California have formed in association with the passage of the Mendocino triple junction through the region over the past 10-15 million years. Present day crustal structure reflects the way triple junction processes have driven crustal evolution in this area. Receiver function analyses at a suite of broadband stations in the Coast Ranges allow us to map a rapid change in crustal thickness over a relatively short distance: a decrease from 35+km to $<$25km thick over a distance of only 50km. This rapid transition coincides with a sharp variation in topography, and is also near an area of proposed lower crustal melts (Levander et. al, Geology, 1998). One of our broadband stations (FREY) is located in the vicinity of this sharp transition. Station FREY has been in operation since October 1999, and has produced receiver functions for approximately 20 events with good azimuthal coverage from the NW, W, SW, S and SE and reasonable ray parameter variation. Although there may be complications from dipping layers in this area, a careful analysis can help constrain this rapid change in crustal structure. Initial results at FREY suggest a distinct difference in crustal thickness imaged from the southeast ($\sim$22km) compared to that from the northwest to southwest ($\sim$25km) associated with this transition, and potentially constraining where the crustal thinning takes place. Crustal structure characteristics (such as Poisson's Ratio) for these directions also differs, a phenomenon that may be associated with the presence (or lack of) melt in the lower crust.
T33C-1397 1340h
Late Pleistocene to Holocene extension along the southern Sierra Nevada frontal fault zone, California
Quaternary vertical slip rate estimates along the southern Sierra Nevada frontal fault zone in California are not well constrained due to the absence of absolute ages on offset Quaternary geologic markers. This study presents new geologic and geochronologic evidence for late Pleistocene to Holocene extension across the southern Sierra Nevada frontal fault zone from Oak Creek to just south of Lubken Creek by incorporating geologic field mapping, tectonic geomorphology and beryllium-10 cosmogenic radionuclide geochronology. The southern escarpment of the Sierra Nevada exposes numerous NNW-striking, east-facing, en echelon normal fault scarps that offset three distinct Quaternary surfaces. The oldest alluvial fan surface, Qf1, is generally smooth and overlain with scarce, typically strongly weathered granitic boulders that are well-embedded into the alluvial surface. Qf2 surfaces are incised into Qf1 surfaces and characterized by ridge and ravine topographic patterns, and moderately weathered granite boulders. Qf2 surfaces are, in turn, incised by Qf3 surfaces, which are distinguished by a bar and swale surface morphology and large abundant unweathered granite boulders. The youngest fan surfaces are Qf4, which are defined by active or recently abandoned channels. Beryllium-10 cosmogenic radionuclide surface exposure dating of twenty-nine granitic boulders from these surfaces provide the age of deposition and abandonment: 140 ± 34 ka for Qf1, 64 ± 16 ka for Qf2, 23 ± 7 ka for Qf3a, 5 ± 1, and 4 ± 1 ka for Qf4. These ages are consistent, within error, of published ages for surfaces elsewhere in the region. Topographic profiles measured across normal fault scarps that cut and offset these surfaces yielded minimum vertical surface offsets of 40.8 ± 8.2 m for Qf1 surfaces, 21.9 ± 4.4 m for Qf2 surfaces, 10.2 ± 2.0 m for Qf3a, and 6.4 ± 1.3 m for Qf3c surfaces. Qf4 surfaces did not show evidence of offset. These data suggest late Pleistocene vertical slip rates of 0.3 to 0.4 ± 0.1 mm/yr since ~140 ka to ~23 ka and 1.4 mm/yr since ~5 ka. If we assume a fault dip of 60°, then the late Pleistocene and Holocene horizontal extension rate across this part of the Sierra Nevada frontal fault zone is 0.2 ± 0.1 and 0.8 ± 0.2 mm/yr. Our slip rate estimates are the same as or somewhat slower than slip rate estimates elsewhere along the Sierra Nevada frontal fault zone and are comparable to late Pleistocene slip rate estimates across the Basin and Range Province.
T33C-1398 1340h
2-D Lithospheric Density Modeling in Southern California
We report results from density modeling of the crust and uppermost mantle along the LARSE I and II transects across southern California. A linear gravity inversion technique was used to calculate crustal and mantle densities along a two-dimensional profile. We used borehole measurements, seismic velocities, and petrologic information to constrain the model where possible. We further assumed that the lithosphere is close to isostatic equilibrium in the deep ocean and east of the Mojave Desert. We used the linear equation $\rho$=a+bVp (where $\rho$ is density, Vp is seismic P-wave velocity) to approximate the mantle density-velocity ratio. A value of b=0-0.2 corresponds to a purely thermal effect on density in the mantle, whereas a coefficient of b $>$ 0.3 implies that petrological or metamorphic changes play an important role in determining density. Solutions with b=0.2-0.4 are considered optimal. It was noted that solutions without mantle density variations required an abnormally dense lower crust (3.1 g/ccm) beneath the Los Angeles basin. Analysis of the isostatic balance of the models and the average density of the consolidated crust did not allow us to clearly define a preferred density model. With this approach, we cannot distinguish if thermal or petrological and/or metamorphic processes cause observed density variations in the study area.
T33C-1399 1340h
Petrogenesis of Quaternary Potassic Volcanism (Big Pine Volcanic Field) Along the Owens Valley Fault Zone in the Eastern California Shear Zone
The Big Pine Volcanic Field (BPVF) is situated in the northern part of the Owens Valley, a fault-controlled basin that has evolved between the Sierra Nevada and the White-Inyo Mountains during the last 7 m.y. The Owens Valley is part of the Eastern California Shear Zone (ECSZ), which is a broad trans-tensional zone of right-Iateral shear that defines an oblique rift zone in the wake of the NW-moving Sierra Nevada-Great Valley block. The Quaternary BPVF occupies 400 square miles of the basin straddling the dextral Owens Valley Fault Zone (with long-term horizontal slip rate of 2-3 mm/yr) and consists of scattered volcanic centers dominated by cinder cones, lava flows, and domes spatially associated with normal faults bounding the Sierra Nevada and White-Inyo Mountains and with across-basin oblique-normal fault systems. The lavas, which are 1.18 m.y. and younger in age, consist of moderately potassic olivine basalt, hawaiite and trachyandesite. One dome west of Poverty Hills consists of rhyolite. Most basaltic samples contain 1-4 mol% normative nepheline but a few have small amounts of normative hypersthene. All analysed samples have similar chondrite-normalized rare-earth element patterns with strong light rare-earth enrichment. La is up to 200 times chondrite and La$_{n}$/Sm$_{n}$ ratios average 3.5. Two hawaiite samples from the Red Hill cone west of Bishop are distinct from the other samples. They have relatively high K$_{2}$O (3.5 wt%), low Na$_{2}$O (2.5 wt%), high FeOt (9.8 wt%) and very high Zr (350-400 ppm). The distinct geochemistry of the hawaiites suggests that they originated from a separate parental magma. Likewise, the geochemistry of the Poverty Hills rhyolite and the lack of intermediate samples between it and the basalts suggest that its felsic magmas too were derived from a separate parental melt. The Big Pine volcanic rocks are the youngest of the eastern Sierra Nevada potassic province. They have undergone relatively little fractionation and thus were probably erupted shortly after formation of the parental liquids. Development of the BPVF corresponds to the evolution of the Owens Valley Fault Zone (OVFZ) as a major transtensional dextral fault system during the Pleistocene-Holocene. We predict that volcanism in the BPVF became progressively younger both westward and northward as the OVFZ established itself as the most recently active fault system in this part of the ECSZ. The plumbing system of the Big Pine basaltic volcanism is likely to have been controlled by lithospheric-scale extension developed in part as a result of displacement-transfer mechanism at the intersection of the NW-trending, dextral OVFZ and the NE-striking, NW-dipping Deep Springs normal fault.
T33C-1400 1340h
Crustal Structure of Southern Baja California Peninsula, Mexico, From Magdalena Microplate to Farallon Basin
Wide-angle seismic data were used to investigate the crustal structure of a transect between the Magdalena Microplate to the Farallon Basin in the Gulf of California, crossing the southern Baja California Peninsula to the north of La Paz (Mexico). This is the first deep seismic study in the area, providing information of the fossil subduction zone of the Magdalena Microplate under Baja California. We have also obtained results of the seismic structure of major fault zones in the area such as Tosco-Abreojos and Santa Margarita. Seismic data were recorded by Ocean Bottom Seismometers (OBS) in the Pacific and the Gulf of California and by portable seismic instruments onshore. More than 5000 offshore high-volume air gun shots were used as energy source in both sides of the Peninsula. Wide-angle data were processed to enhance the signal to noise ratio to help in the identification of the seismic energy arrivals. We used a direct method of interpretation, including ray tracing, travel times and synthetic seismograms calculation. The availability of a number of recording instruments allows multiple coverage of the crustal structure.
T33C-1401 1340h
Continental Rifting Across the Alarcon Basin, Gulf of California
In Fall 2002 seismic refraction data and multi-channel seismic reflection data were collected in the Gulf of California as part of the Margins Rupturing Continental Lithosphere (RCL) initiative. Across the Alarcon Basin 56 Ocean-Bottom Seismometers and 11 onshore Ref-Teks collected wide-angle refraction data over an 880 km transect, coincident with 660 km of reflection data. The refraction data has been used to construct an initial velocity model across the entire rifted margin. Our results show ~150 km of new oceanic crust of normal thickness (6.5- 7 km) and lower crustal velocity (~6.7 km/s). The continent-ocean transition seems relatively abrupt occurring over less than 50 km on both conjugate margins. The rifted margin across the Alarcon Basin seems symmetric: There is roughly 300 km of extended continental crust on both margins- characterized by normal faulting at the surface and crust thinned to an average of 15 km with an average velocity of 6.2 km/s. At the furthest extents of our model, underneath the Baja peninsula crust seems to be about 28km thick, and under mainland Mexico about 26 km. Assuming an initially uniform crust of 30 km thickness, slightly greater than the observed maximum thicknesses, and 2-D extension there has been 100% extension across both margins. This predicts a total tectonic separation of around 470 km, which agrees with previous estimates for the Gulf of California of 450-500 km.