Richard A. Bennett

Continuous GPS measurements of postglacial adjustment in Fennoscandia 1.Geodetic results

[1] Data collected under the auspices of the BIFROST GPS project yield a geographically dense suite of estimates of present-day, three-dimensional (3-D) crustal deformation rates in Fennoscandia [Johansson et al., 2002]. A preliminary forward analysis of these estimates [Milne et al., 2001] has indicated that models of ongoing glacial isostatic adjustment (GIA) in response to the final deglaciation event of the current ice age are able to provide an excellent fit to the observed 3-D velocity field. In this study we revisit our previous GIA analysis by considering a more extensive suite of forward calculations and by performing the first formal joint inversion of the BIFROST rate estimates. To establish insight into the physics of the GIA response in the region, we begin by decomposing a forward prediction into the three contributions associated with the ice, ocean, and rotational forcings. From this analysis we demonstrate that recent advances in postglacial sea level theory, in particular the inclusion of rotational effects and improvements in the treatment of the ocean load in the vicinity of an evolving continental margin, involve peak signals that are larger than the observational uncertainties in the BIFROST network. The forward analysis is completed by presenting predictions for a pair of Fennoscandian ice histories and an extensive suite of viscoelastic Earth models. The former indicates that the BIFROST data set provides a powerful discriminant of such histories. The latter yields bounds on the ( assumed constant) upper and lower mantle viscosity (nu(UM), nu(LM)); specifically, we derive a 95% confidence interval of 5 x 10(20) less than or equal to nu(UM) less than or equal to 10(21) Pa s and 5 x 10(21) less than or equal to nu(LM) less than or equal to 5 x 10(22) Pa s, with some preference for (elastic) lithospheric thickness in excess of 100 km. The main goal of the ( Bayesian) inverse analysis is to estimate the radial resolving power of the BIFROST GPS data as a function of depth in the mantle. Assuming a reasonably accurate ice history, we demonstrate that this resolving power varies from similar to 200 km near the base of the upper mantle to similar to 700 km in the top portion of the lower mantle. We conclude that the BIFROST data are able to resolve structure on radial length scale significantly smaller than a single upper mantle layer. However, these data provide little constraint on viscosity in the bottom half of the mantle. Finally, elements of both the forward and inverse analyses indicate that radial and horizontal velocity estimates provide distinct constraints on mantle viscosity.

Contemporary strain rates in the northern Basin and Range province from GPS data

We investigate the distribution of active deformation in the northern Basin and Range province using data from continuous GPS (CGPS) networks, supplemented by additional campaign data from the Death Valley, northern Basin and Range, and Sierra Nevada–Great Valley regions. To understand the contemporary strain rate field in the context of the greater Pacific (P)–North America (NA) plate boundary zone, we use GPS velocities to estimate the average relative motions of the Colorado Plateau (CP), the Sierra Nevada–Great Valley (SNGV) microplate, and a narrow north-south elongate region in the central Great Basin (CGB) occupying the longitude band 114–117°W. We find that the SNGV microplate translates with respect to the CP at a rate of 11.4 ± 0.3 mm yr^(−1) oriented N47 ± 1°W and with respect to NA at a rate of ∼12.4 mm yr^(−1) also oriented N47°W, slower than most previous geodetic estimates of SNGV-NA relative motion, and nearly 7° counterclockwise from the direction of P-NA relative plate motion. We estimate CGB-CP relative motion of 2.8 ± 0.2 mm yr^(−1) oriented N84 ± 5°W, consistent with roughly east-west extension within the eastern Great Basin (EGB). Velocity estimates from the EGB reveal diffuse extension across this region, with more rapid extension of 20 ± 1 nstr yr^(−1) concentrated in the eastern half of the region, which includes the Wasatch fault zone. We estimate SNGV-CGB relative motion of 9.3 ± 0.2 mm yr^(−1) oriented N37 ± 2°W, essentially parallel to P-NA relative plate motion. This rate is significantly slower than most previous geodetic estimates of deformation across the western Great Basin (WGB) but is generally consistent with paleoseismological inferences. The WGB region accommodates N37°W directed right lateral shear at rates of (1) 57 ± 9 nstr yr^(−1) across a zone of width ∼125 km in the south (latitude ∼36°N), (2) 25 ± 5 nstr yr^(−1) in the central region (latitude ∼38°N), and (3) 36 ± 1 nstr yr^(−1) across a zone of width ∼300 km in the north (latitude ∼40°N). By construction there is no net extension or shortening perpendicular to SNGV-CGB relative motion. However, we observe about 8.6 ± 0.5 nstr yr^(−1) extension on average in the direction of shear from southeast to northwest within the Walker Lane belt, comparable to the average east-west extension rate of 10 ± 1 nstr yr^(−1) across the northern Basin and Range but implying a distinctly different mechanism of deformation from extension on north trending, range-bounding normal faults. An alternative model for this shear parallel deformation, in which extension is accommodated across a narrow, more rapidly extending zone that coincides with the central Nevada seismic belt, fits the WGB data slightly better. Local anomalies with respect to this simple kinematic model may reveal second-order deformation signals related to more local crustal dynamic phenomena, but significant improvements in velocity field resolution will be necessary to reveal this second-order pattern.

Comparison of geodetic and geologic data from the Wasatch region, Utah, and implications for the spectral character of Earth deformation at periods of 10 to 10 million years

The Wasatch fault and adjacent fault zones provide an opportunity to compare present-day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows, ranging from the last millennium up to 10 Myr. The three easternmost GPS sites of the Basin and Range Geodetic Network (BARGEN) at this latitude define a ∼130-km-wide region spanning three major normal faults extending east-west at a total rate of 2.7 ± 0.4 mm/yr, with an average regional strain rate estimated to be 21 ± 4 nstrain/yr, about twice the Basin and Range average. On the Wasatch fault, the vertical component of the geologic displacement rate is 1.7 ± 0.5 mm/yr since 6 ka, <0.6 mm/yr since 130 ka, and 0.5–0.7 mm/yr since 10 Ma. However, it appears likely that at the longest timescale, rates slowed over time, from 1.0 to 1.4 mm/yr between 10 and 6 Ma to 0.2 to 0.3 mm/yr since 6 Ma. The cumulative vertical displacement record across all three faults also shows time-variable strain release ranging from 2 to 4 mm/yr since 10 ka to <1 mm/yr averaged over the past 130 kyr. Conventional earthquake recurrence models (“Reid-type” behavior) would require an accordingly large variation in strain accumulation or loading rate on a 10-kyr timescale, for which there appears to be no obvious geophysical explanation. Alternatively, seismic strain release, given a wide range of plausible constitutive behaviors for frictional sliding, may be clustered on the 10-kyr timescale, resulting in the high Holocene rates, with comparatively low, uniform strain accumulation rates on the 100-kyr timescale (“Wallace-type” behavior). The latter alternative, combined with observations at the million-year timescale and the likelihood of a significant contribution of postseismic transients, implies maxima of spectral amplitude in the velocity field at periods of ∼10 Myr (variations in tectonic loading), ∼10 kyr (clustered strain release), and of 100 years (postseismic transients). If so, measurements of strain accumulation and strain release may be strongly timescale-dependent for any given fault system.

Global Positioning System constraints on fault slip rates in southern California and northern Baja, Mexico

We use Global Positioning System (GPS) estimates of horizontal site velocity to constrain slip rates on faults comprising the Pacific-North America plate boundary in southern California and northern Mexico. We enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary. We estimate a Pacific-North America relative plate motion rate of 49 ± 3 mm/yr (one standard deviation), consistent with NUVEL-1A estimates. We are able to resolve robust slip rate estimates for the southernmost San Andreas, San Jacinto, and Elsinore faults (26 ± 2, 9 ± 2, and 6 ± 2 mm/yr, respectively) and for the Imperial and Cerro Prieto faults (35 ± 2 and 42 ± 1 mm/yr, respectively), accounting for about 86% of the total plate motion. The remaining 14% appears to be accommodated to the west of these fault systems, probably via slip along the San Clemente fault and/or the San Miguel, Vallecitos, Rose Canyon, and Newport-Inglewood fault systems. These results are highly consistent with paleoseismic estimates for slip rates implying that off-fault strain accumulation within the deforming zone of the plate boundary is largely elastic. We estimate that the seismically quiescent, southernmost San Andreas fault has incurred about 8.2 m of slip deficit over the last few hundred years, presumably to be recovered during a future large earthquake.

Present-day pattern of Cordilleran deformation in the western United States

We present the first detailed geodetic image of the entire western United States south of lat 42°N, merging both campaign and continuous Global Positioning System (GPS) and very long baseline interferometry (VLBI) data sets in a combined solution for station velocities having a single, uniform reference frame. The results are consistent with a number of features previously observed through local geodetic studies and very sparse space geodetic studies, including a dominant pattern of right-lateral shear associated with the San Andreas fault, rates of the westernmost sites (along the California coast) of 46–48 mm/yr relative to a North America reference frame, and some 11–13 mm/yr of deformation accommodated east of the Sierra Nevada in the Basin and Range province north of lat 36°N. South of 36°N, the solution also shows that the southernmost San Andreas fault system accommodates effectively all interplate motion and that the southern Basin and Range is not deforming significantly. At lat 37°N, the eastern California shear zone appears to exhibit simple shear oriented between ∼N20°W and ∼N40°W relative to North America, with a fairly well defined transition zone from localized shear to diffuse spreading in the Basin and Range. Enigmatically, this transition involves a significant component of contraction normal to the overall shear-zone trend; sites in the Great Basin move southwestward at up to ∼5 mm/yr toward sites within the eastern California shear zone. To the north, in contrast, there appears to be a relatively smooth transition from east-west spreading within the eastern Great Basin to northwest-southeast shear across the westernmost Basin and Range.

Codependent histories of the San Andreas and San Jacinto fault zones from inversion of fault displacement rates

Department of Geosciences, Pennsylvania State University, 542 Deike Building, University Park,Pennsylvania 16082, USAABSTRACTThe displacement histories of the San Jacinto and southernmost San Andreas fault zonesare constrained by offset data with ages in the range of 5 Ma to 5 ka. Apparent discrep-ancies between long- and short-term average displacement rates can be reconciled with atime-variable rate model. In this model, the displacement rate on the San Andreas decel-erated from ;35 mm/yr at 1.5 Ma to as low as 9 6 4 mm/yr by 90 ka. Over this sametime period, the rate on the San Jacinto fault zone accelerated from an initial value ofzero to a rate of 26 6 4 mm/yr. The data also imply that the rate of the San Andreasfault accelerated since ca. 90 ka, from ;9 mm/yr to the modern rate of 27 6 4 mm/yr,whereas the San Jacinto decelerated from 26 6 4 mm/yr to the modern rate of 8 6 4mm/yr. The time scale of these changes is significantly longer than the earthquake cycle,but shorter than time scales characteristic of lithospheric-scale dynamics. The emergenceof the San Jacinto fault zone ca. 1.5 Ma coincided with the development of a majorrestraining bend in the San Andreas fault zone, suggesting that the formation of newsubparallel faults could be driven by conditions that inhibit displacement on preexistingfaults.Keywords: fault displacement rates, lithospheric rheology, continental dynamics, crustaldeformation.INTRODUCTIONContinental plate boundary zones are oftencharacterized by systems of faults spanningbroadly deforming regions (Fig. 1). Determin-ing how these faults accommodate relativeplate motions and the nature of their spatialand temporal interactions remains an elusivegoal. In particular, whether fault-displacementrates are constant or time variable—and if so,on what time scales—is difficult to ascertainbecause most geologically determined dis-placement rates represent averages since theage of an offset geologic or geomorphic fea-ture. In the case of time-varying rates, this av-eraging effect can lead to apparently inconsis-tent estimates depending on the ages of theoffset markers.The partitioning of Pacific–North Ameri-ca relative plate motion among the manynorthwest-oriented dextral strike-slip faults insouthern California has been the subject of in-tensive research for decades (Table 1). In thelatitude band 328Nto348N, ;70% of the totalrelative motion between these plates is accom-modated by the San Jacinto and southernmostSan Andreas faults (Fig. 1). Together theytransfer ;35 mm/yr of motion from the Im-perial to the Mojave segment of the San An-dreas fault (north of the San Jacinto–San An-dreas intersection). Previous studies havetended to concentrate either on displacementrates of individual faults or on the steady-statepartitioning of deformation between faults.However, these data can also be used to assesswhether the rate of deformation is conservedbetween these two fault zones over time, as iswidely accepted.Here we apply a technique that allows us touse time-averaged estimates of displacementrate to infer the time history of instantaneousdisplacement rates on the San Andreas andSan Jacinto fault zones. This history allows usto investigate interactions between these twofault zones and, in particular, to assess wheth-er they act in a coupled manner. As we show,interactions between these faults could haveimportant implications for geodynamics, bear-ing on issues such as the relative strengths ofupper mantle and lower- and upper-crustal lay-ers, and the degree to which the deformationfield within the upper crust is coupled to thatwithin the upper mantle. Investigations suchas ours, which use data representing signifi-cantly different periods of time, may thus pro-vide a new approach to investigating somelong-standing problems in geodynamics.DATATable 1 summarizes the geologic, geomor-phic, and geodetic observations constrainingaverage displacement rates on these two faultsover a diverse range of time scales. Each ratelisted in Table 1 represents an average over afinite interval of time. The duration of eachaveraging interval is determined by the age ofa measured offset marker, such as a distinctiverock unit, a geomorphic feature, or a geodeticmonument. With the exception of the geodeticinferences, the averaging intervals are quitelong relative to the earthquake cycle, which ison the order of hundreds of years for the SanAndreas and San Jacinto fault zones. Becausethese data record averages over long periodsof time, they should be insensitive to temporalirregularities in earthquake recurrence.Age-dependent variation in the rate data isappreciable for both faults (Table 1). For theSan Andreas, rate estimates range from ashigh as 30 mm/yr to as low as 14 mm/yr.Rates for the San Jacinto fault zone also varyby nearly a factor of two; long-term averagesare significantly larger than estimates repre-senting shorter, more recent, intervals. In or-der to compare the different observations andto constrain the history of fault displacements,we need to consider the uncertainty in eachdatum. In the geologic literature, error bars donot typically represent statistical measures ofuncertainty (e.g., standard deviations of a nor-mal distribution), but rather they usually rep-resent upper and lower bounds on possibledisplacement rates, based on complex age andoffset relationships among various markersacross the faults. Error estimates often resultin asymmetric plus-or-minus ranges. Our anal-yses of these data are based on more tradi-tional statistical measures of uncertainty. Wehave thus had to make some assumptionsabout how best to represent the reported errorbounds in terms of Gaussian error distribu-tions. Table 1 lists the standard deviations thatwe chose to best represent the allowable slipranges derived from geologic observations. Inall cases we conservatively chose error distri-butions with 95% confidence regions coincid-ing roughly with the absolute bounds reportedin the literature.ANALYSIS AND RESULTSTo explore the history of displacement rateson the San Andreas and San Jacinto faultzones implied by the averages of Table 1, weemployed the theory of smoothing splines(e.g., Matthews and Segall, 1993). The basicgoal is to use the rate averages, which repre-

Dynamics of the interaction of human apurinic endonuclease (Ape1) with its substrate and product.

We investigated the interaction dynamics of human abasic endonuclease, the Ape1 protein (also called Ref1, Hap1, or Apex), with its DNA substrate and incised product using electrophoretic assays and site-specific amino acid substitutions. Changing aspartate 283 to alanine (D283A) left 10% residual activity, contrary to a previous report, but complementation of repair-deficient bacteria by the D283A Ape1 protein was consistent with its activity in vitro. The D308A, D283/D308A double mutant, and histidine 309 to asparagine proteins had 22, 1, and ∼0.02% of wild-type Ape1 activity, respectively. Despite this range of enzymatic activities, all the mutant proteins had near-wild-type binding affinity specific for DNA containing a synthetic abasic site. Thus, substrate recognition and cleavage are genetically separable steps. Both the wild-type and mutant Ape1 proteins bound strongly to the enzyme incision product, an incised abasic site, which suggested that Ape1 might exhibit product inhibition. The use of human DNA polymerase β to increase Ape1 activity by eliminating the incision product supports this conclusion. Notably, the complexes of the D283A, D308A, and D283A/D308A double mutant proteins with both intact and incised abasic DNA were significantly more stable than complexes containing wild-type Ape1, which may contribute to the lower turnover numbers of the mutant enzymes. Wild-type Ape1 protein bound tightly to DNA containing a one-nucleotide gap but not to DNA with a nick, consistent with the proposal that substrate recognition by Ape1 involves a space bracketed by duplex DNA, rather than mere flexibility of the DNA.

Eocene to present subduction of southern Adria mantle lithosphere beneath the Dinarides

We modeled global positioning system measurements of crustal velocity along a N13°E profile across the southern Adria microplate and south-central Dinarides mountain belt using a one-dimensional elastic dislocation model. We assumed a N77°W fault strike orthogonal to the average azimuth of the measured velocities, but we used a constrained random search algorithm minimizing misfit to the velocities to determine all other parameters of the model. The model fault plane reaches the surface seaward of mapped SW-verging thrusts of Eocene and perhaps Neogene age along the coastal areas of southern Dalmatia, consistent with SW-migrating deformation in an active fold-and-thrust belt. P-wave tomography shows a NE-dipping high-velocity slab to ∼160 km depth, which reaches the surface as Adria, dips gently beneath the foreland, and becomes steep beneath the Dinarides topographic high. The thrust plane is located directly above the shallowly dipping part of the slab. The pattern of precisely located seismicity is broadly consistent with both the tomography and geodesy; deeper earthquakes (down to ∼70 km) correlate spatially with the slab, and shallower earthquakes are broadly clustered around the geodetically inferred thrust plane. The model fault geometry and loading rate, ages of subaerially exposed thrusts in the fold-and-thrust belt, and the length of subducted slab are all consistent with Adria-Eurasia collision involving uninterrupted subduction of southern Adria mantle lithosphere beneath Eurasia since Eocene time.

The Saccharomyces cerevisiae ETH1 Gene, an Inducible Homolog of Exonuclease III That Provides Resistance to DNA-Damaging Agents and Limits Spontaneous Mutagenesis

ABSTRACT The recently sequenced Saccharomyces cerevisiae genome was searched for a gene with homology to the gene encoding the major human AP endonuclease, a component of the highly conserved DNA base excision repair pathway. An open reading frame was found to encode a putative protein (34% identical to the Schizosaccharomyces pombe eth1 + [open reading frame SPBC3D6.10] gene product) with a 347-residue segment homologous to the exonuclease III family of AP endonucleases. Synthesis of mRNA from ETH1 in wild-type cells was induced sixfold relative to that in untreated cells after exposure to the alkylating agent methyl methanesulfonate (MMS). To investigate the function of ETH1, deletions of the open reading frame were made in a wild-type strain and a strain deficient in the known yeast AP endonuclease encoded by APN1. eth1strains were not more sensitive to killing by MMS, hydrogen peroxide, or phleomycin D1, whereas apn1 strains were ∼3-fold more sensitive to MMS and ∼10-fold more sensitive to hydrogen peroxide than was the wild type. Double-mutant strains (apn1 eth1) were ∼15-fold more sensitive to MMS and ∼2- to 3-fold more sensitive to hydrogen peroxide and phleomycin D1 than wereapn1 strains. Elimination of ETH1 inapn1 strains also increased spontaneous mutation rates 9- or 31-fold compared to the wild type as determined by reversion to adenine or lysine prototrophy, respectively. Transformation ofapn1 eth1 cells with an expression vector containingETH1 reversed the hypersensitivity to MMS and limited the rate of spontaneous mutagenesis. Expression of ETH1 in adut-1 xthA3 Escherichia coli strain demonstrated that the gene product functionally complements the missing AP endonuclease activity. Thus, in apn1 cells where the major AP endonuclease activity is missing, ETH1 offers an alternate capacity for repair of spontaneous or induced damage to DNA that is normally repaired by Apn1 protein.

Continuous GPS measurements of contemporary deformation across the Northern Basin and Range Province

We have acquired and analyzed data from the northern Basin and Range (NBAR) continuous GPS network since July 1996. The RMS residual with respect to the best fitting lines through the individual station position estimates is 2–3 mm in the horizontal and 6–10 mm in the vertical. After the first 395 days of operation, uncertainties in horizontal velocity estimates are 1–2 mm/yr (1-σ). Relative motion among NBAR sites located in eastern Nevada and in Utah is small, but east-west extension is significant assuming uniform strain accumulation across the whole network. The relative motion observed across the Wasatch fault zone is 2 ±2 mm/yr, east-west. Relative motions among stations in western Nevada and California, in contrast, are dominated by northwest, right-lateral shear. We infer an integrated deformation across the northern Basin and Range of 11 ± 2 mm/yr, northwest. These rates are consistent with previous geodetic measurements. Our GPS velocity estimates, however, reveal a possibly abrupt transition from east-west extension in eastern Nevada and Utah to right-lateral shear in western Nevada. This transition is roughly coincident with the central Nevada seismic belt and is consistent with the right-oblique focal mechanisms of the 1954 Dixie Valley and Fairview Peak earthquakes. The transition also appears to correlate spatially with a transition in upper mantle structure.