M. Meghan Miller

Error analysis of continuous GPS position time series

[1] A total of 954 continuous GPS position time series from 414 individual sites in nine different GPS solutions were analyzed for noise content using maximum likelihood estimation (MLE). The lengths of the series varied from around 16 months to over 10 years. MLE was used to analyze the data in two ways. In the first analysis the noise was assumed to be white noise only, a combination of white noise plus flicker noise, or a combination of white noise plus random walk noise. For the second analysis the spectral index and amplitude of the power law noise were estimated simultaneously with the white noise. In solutions where the sites were globally distributed, the noise can be best described by a combination of white noise plus flicker noise. Both noise components show latitude dependence in their amplitudes (higher at equatorial sites) together with a bias to larger values in the Southern Hemisphere. In the regional solutions, where a spatially correlated (common mode) signal has been removed, the noise is significantly lower. The spectral index of the power law in regional solutions is more varied than in the global solutions and probably reflects a mixture of local effects. A significant reduction in noise can be seen since the first continuous GPS networks began recording in the early 1990s. A comparison of the noise amplitudes to the different monument types in the Southern California Integrated GPS Network suggests that the deep drill braced monument is preferred for maximum stability.

Present day kinematics of the Eastern California Shear Zone from a geodetically constrained block model

We use Global Positioning System (GPS) data from 1993–2000 to determine horizontal velocities of 65 stations in eastern California and western Nevada between 35° and 37° N. We relate the geodetic velocities to fault slip rates using a block model that enforces path integral constraints over geologic and geodetic time scales and that includes the effects of elastic strain accumulation on faults locked to a depth of 15 km. The velocity of the Sierra Nevada block with respect to Nevada is 11.1±0.3 mm/yr, with slip partitioned across the Death Valley, (2.8±0.5 mm/yr), Panamint Valley (2.5±0.8 mm/yr), and Airport Lake/Owens Valley (5.3±0.7/4.6±0.5 mm/yr) faults. The western Mojave block rotates at 2.1±0.8°/My clockwise, with 3.7±0.7 mm/yr of left lateral motion across the western Garlock Fault. We infer 11±2 mm/yr of right lateral motion across the Mojave region of the Eastern California Shear Zone.

Refined kinematics of the Eastern California shear zone from GPS observations, 1993-1998

Global Positioning System (GPS) results from networks spanning the Eastern California shear zone and adjacent Sierra Nevada block, occupied annually between 1993 and 1998, constrain plate margin kinematics. We use an elastic block model to relate GPS station velocities to long-term fault slip rate estimates. The model accounts for elastic strain accumulation on the San Andreas fault, as well as faults of the Eastern California shear zone. South of the Garlock fault, 14 mm/yr of dextral shear is distributed across the Eastern California shear zone. Some of this slip penetrates eastward into the Basin and Range, and a collective budget of 13 mm/yr is observed to the north at the latitude of Owens Lake. Model slip rates for two important faults, the Garlock and Owens Valley faults, significantly misfit geologic estimates. By referencing station velocities to stable North America we observe northward-increasing deformation east of our regional GPS network. At the latitude of Mojave Desert, however, some of this deformation is ascribed to elastic strain accumulation due to a locked San Andreas fault and thus does not represent additional fault-related, permanent deformation.

GPS‐determination of along‐strike variation in Cascadia margin kinematics: Implications for relative plate motion, subduction zone coupling, and permanent deformation

High-precision GPS geodesy in the Pacific Northwest provides the first synoptic view of the along-strike variation in Cascadia margin kinematics. These results con- strain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of mag- nitude longer than the decades-long history of seismic moni- toring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station veloci- ties along the coast. After correction for a simple elastic dis- location model, important residual motions remain, especially south of the international border. The magnitude of northward forearc motion increases southward from western Washington (3-7 mm/yr)to northern and central Oregon (-9 mm/yr), con- sistent with oblique convergence and geologic constraints on permanent deformation. The margin-parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. Thus crustal faulting also contributes to seismic hazard. Farther south in southern Oregon, north- westward velocities reflect the influence of Pacific-North America motion and impingement of the Sierra Nevada block on the Pacific Northwest. In contrast to previous notions, some deformation related to the Eastern California shear zone crosses northernmost California in the vicinity of the Klamath Mountains and feeds out to the Gorda plate margin.

GPS deformation in a region of high crustal seismicity: N. Cascadia forearc

We estimate the rate of crustal deformation in the central and northern Cascadia forearc based on a combination of existing global positioning system (GPS) velocity data along the Cascadia subduction zone. GPS strain rates and velocities show that the northwestern Washington^southwestern British Columbia region is currently shortening at 3^ 3.5 mm yr 31 in a N^S direction, in good agreement with inference from crustal earthquake statistics. On the longterm, the shortening rate is 5^6 mm yr 31 , providing that the subduction-related interseismic loading of the margin is purely elastic. Compared to the velocity of the Oregon forearc with respect to North America (V 7m m yr 31 ), this indicates that most of the forearc motion is accommodated in the Puget^Georgia basin area, corresponding to the main concentration of crustal seismicity. The difference between the current and long-term shortening rates may be taken up during subduction megathrust earthquakes. Thus, these events could produce a sudden increase of N^S compression in the Puget sound region and could trigger major Seattle-fault-type crustal earthquakes. Published by Elsevier Science B.V.

Middle Miocene extension in the Gulf Extensional Province, Baja California: Evidence from the southern Sierra Juarez

New geologic mapping, structural studies, and geochronology of Miocene volcanic and sedimentary rocks in the southern Sierra Juarez, Baja California, shed light on the extensional history of the Gulf Extensional Province prior to sea-floor spreading in the Gulf of California. The southern Sierra Juarez is underlain by lower–middle Miocene rocks including fluvial strata, intermediate composition volcanic deposits, basalt lava flows and cinder cones, and dacite pyroclastic deposits and lavas that nonconformably overlie the Cretaceous Peninsular Ranges batholith. The 40 Ar/ 39 Ar geochronology indicates that basaltic rocks are 16.90 ± 0.05 Ma and dacite pyroclastic deposits are between 16.69 ± 0.11 Ma and 15.98 ± 0.13 Ma. These strata were subsequently cut by two generations of faults. First generation faults comprise a dominant set of north-south–striking, west-dipping normal faults, a secondary set of north-south–striking, east-dipping normal faults, and a lesser set of variably oriented strike-slip faults. All three fault sets are temporally and spatially related and were produced by east-west extension. The dominant west-dipping faults, which are antithetic to and oblique to the east-dipping Main Gulf Escarpment, may have been a precursor or an early phase accommodation zone along the escarpment. West-dipping normal faults are cut by a 10.96 ± 0.05 Ma dacite hypabyssal intrusion, thus bracketing the age of east-west extension between 15.98 ± 0.13 Ma and 10.96 ± 0.05 Ma. Hence, this faulting event clearly indicates a period of extension that predates the onset of oceanic rifting and even predates other dated Miocene extension within Baja California. Second generation faults, which are comprised of east-west–striking strike-slip faults that cut first generation faults and associated northwest-striking, northeast-dipping normal faults, may be related to early development of the Transpeninsular Strike-slip Province. Global plate reconstructions suggest that transtensional motion between the North American and Pacific plates along the western margin of Baja California began during middle Miocene time, coeval with east-west extension in the southern Sierra Juarez. This observation supports a hypothesis that middle Miocene transtensional plate motion was partitioned into two components: a strike-slip component parallel to active faults along the western offshore margin of Baja California, and an extensional component normal to the margin, but located in what is now the Gulf Extensional Province. Hence, the onset of extension within the circum-gulf region was in response to plate boundary processes.

Tectonic implications of detrital zircon data from Paleozoic and Triassic strata in western Nevada and Northern California

U-Pb analyses of detrital zircons from various allochthonous assemblages of Paleozoic and early Mesozoic age in western Nevada and northern California yield new constraints on the sediment dispersal patterns and tectonic evolution of western North America. During early Paleozoic time, a large submarine fan system formed in slope, rise, basinal, and perhaps trench settings near the continental margin, west of continental shelf deposits of the Cordilleran miogeocline. Our detrital zircon data suggest that most of the detritus in this fan system along the western U.S. segment of the margin was derived from the Peace River Arch region of northwestern Canada, and some detritus was shed from basement rocks of the southwestern United States or western Mexico. In most cases, the detritus in the allochthonous assemblages was recycled through platformal and/or miogeoclinal sedimentary units prior to accumulating in offshelf environments. Lower Paleozoic rocks of the Roberts Mountains allochthon, Shoo Fly Complex, and Yreka terrane are interpreted to have been parts of this fan complex that accumulated along the central U.S. segment of the continental margin, probably within 1000 km of the miogeocline. During the mid-Paleozoic Antler orogeny, parts of the lower Paleozoic fan complex were deformed and uplifted, and strata of the Roberts Mountains allochthon were tectonically emplaced onto the continental margin. This orogeny was apparently driven at least in part by convergence of the Sierra-Klamath arc with the continental margin, as has been proposed by many previous workers, because these arc terranes are overlain by Mississippian clastic strata derived from the Roberts Mountains allochthon. Our data are not sufficient, however, to determine the polarity of the arc, or whether the arc formed along the continental margin or was exotic to western North America. Detrital zircon data indicate that following the Antler orogeny, clastic sediments derived from the Roberts Mountains allochthon were deposited both on the continental margin to the east and within intra-arc and backarc basins to the west. The occurrence of this detritus in terranes of western Nevada and northern California indicates that they were proximal to each other and to the continental margin during late Paleozoic time. The presence of upper Paleozoic volcanic and plutonic rocks and arc-derived detrital zircons in strata of the northern Sierra, eastern Klamath, and Black Rock terranes records the existence of a west-facing magmatic arc near the continental margin during late Paleozoic time. Our data are not supportive of scenarios in which these arc terranes were located farther north or thousands of kilometers offshore of the Nevada continental margin during late Paleozoic time. Following a second phase of uplift, erosion, and allochthon emplacement during the Permian-Early Triassic Sonoma orogeny, Middle and Upper Triassic strata now preserved in west-central Nevada accumulated in a backarc basin. Our data indicate that the basinal assemblages contain detritus from arc terranes to the west as well as the craton to the east.

Detrital zircon geochronologic study of upper Paleozoic strata in the eastern Klamath terrane, northern California

U-Pb analyses have been conducted on individual detrital zircon grains from Upper Devonian(?)-Carboniferous strata of the Bragdon Formation and Upper Carboniferous-Lower Permian strata of the Baird Formation. These are the two most important clastic units in the Redding section of the eastern Klamath terrane of northern California. There are 31 grains from the Bragdon Formation that yield mainly concordant to slightly discordant ages ranging from ca. 363 Ma to ca. 3.12 Ga. Grains with ages of ca. 363-572 Ma in this sample were apparently shed primarily from the nearby Trinity complex, which contains igneous rocks of these ages. Older grains, ranging from 1.07 to 3.12 Ga, but mostly between 1.76 and 1.99 Ga, were likely recycled from lower Paleozoic strata of the Yreka terrane or the Roberts Mountains allochthon. Six detrital zircons from the Baird Formation are all apparently concordant, with interpreted ages between 320 and 326 Ma. These grains are locally derived and probably record igneous activity represented by the widespread volcanic and volcaniclastic rocks within the Baird Formation.

U‐Pb geochronology of detrital zircon from Upper Jurassic synorogenic turbidites, Galice Formation, and related rocks, western Klamath Mountains: Correlation and Klamath Mountains provenance

Synorogenic turbidites of the Upper Jurassic Galice Formation overlie a variety of basement terranes within the western Klamath Mountains along the Oregon-California border, including the early Late Jurassic ophiolite assemblages, ensimatic arc deposits, and sedimentary terranes. U-Pb analyses of 68 multiple grain fractions from 11 samples of detrital zircon support the correlation of Galice Formation on these various basement terranes, although some new complexities in provenance are revealed. With one exception, upper intercept ages range from 1509^(+3)_(-3) to 1675^(+8)_(-8) Ma. Least squares regression of all fractions yields an upper intercept age of 1583±1 Ma, indicating the importance of an ultimately continental, recycled, and generally well-mixed sedimentary source. Early Mesozoic lower intercept ages range between 183^(+2)_(-2) and 263^(+4)_(-3) and average 215±1 Ma. Results from Galice cover on sedimentary basement show significantly older 2.1 Ga Precambrian component, however, that may be locally derived from pre-Late Jurassic basement rocks that are rich in recycled sedimentary debris. Existing isotopic data from older, zircon-bearing Klamath units further indicate that Galice detritus was derived from immediate source terranes within the Klamath Mountains. Reworking of fragile limestone clasts from the biogeographically distinctive eastern Klamath terrane (McCloud Limestone) into Galice Formation substrate also supports early paleogeographic ties between terranes. Thus the tectonic setting of the Late Jurassic Nevadan orogeny in the Klamath Mountains is tightly constrained by original paleogeographic ties between subterranes of the western belt and by provenance ties to terranes to the east. Ultimately continent-derived clastic debris and other distinctive tracers were recycled within this long-lived ensimatic convergent margin system.

GPS determination of current Pacific–North American plate motion

Global Positioning System (GPS) data, collected by campaign-style GPS experiments at five sites along the Gulf of California in 1996 and 1998, determine a locally based estimate for current relative motion between the Pacific and North American plates. At the mouth of the Gulf of California, the Pacific plate moves 50.4 ± 3.4 mm/yr, along an azimuth of N59.0°W ± 2.7°, relative to mainland Mexico. These estimates substantiate and refine previous locally based GPS-determined rates, and agree with GPS determinations of global plate motion. A reexamination of magnetic anomalies in the gulf used in the widely accepted NUVEL-1A global plate model has yielded an average Pacific–North American relative velocity from 0.78 Ma to the present of 51.1 ± 2.5 mm/yr. The new GPS-determined velocity agrees with this estimate, supporting the ideas that the transfer of Baja California to the Pacific plate continued until ca. 1 Ma, and that the current Pacific–North American rate is greater than the 3.16 Ma average. The azimuth determination is ∼5° west of the NUVEL-1A results calculated from earthquake slip vectors and azimuths of gulf transforms offsetting both oceanic and continental crust. The Tamayo fracture represents the only fault zone used in the NUVEL-1A model that offsets solely oceanic crust. This fault zone trends N60°W, consistent with the GPS-determined azimuth at the mouth of the gulf.