Search USGS⌕ Search

SEARCH · Search USGS

Results for “Journal of Geophysical Research--Solid Earth”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 127 records · Page 7Linked to original sources
USGS2012

State of stress and modern deformation of the northern Basin and Range Province

Constraints on the current stress regime of the actively extending northern Basin and Range province are provided by deformation data (focal mechanisms and fault slip studies), hydraulic fracturing in situ stress measurements, borehole elongation (“breakouts”) analyses, and alignment of young volcanic vents. The integrated data indicate significant variations both in principal stress orientations and magnitudes. An approximately E-W least principal stress direction appears to characterize both the eastern and western margins of the Basin and Range province, whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented NW to N60°W. The contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins. Active deformation along the southeastern and western province margins is characterized by a combination of strike-slip and normal faulting. Focal mechanisms along northeastern province margin (Wasatch front) and in central Nevada indicate a combination of normal and oblique-normal faulting. Temporal, regional, and depth-dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles. However, some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated. Evidence for significant temporal variation (or multiple cycles of variation) in relative stress magnitude comes from the Sierran front-Basin and Range boundary region where recent earthquakes are predominantly strike slip, whereas the profound relative vertical relief across the Sierra frontal fault zone in the last 9–10 m.y. implies a normal faulting stress regime. Using the best data on stress orientation, relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements. Analysis of well-constrained slip vectors in the Owens Valley, California, area indicate that large temporal variations in the magnitude of the approximately N-S oriented maximum horizontal stress are required to explain dominantly dip-slip and strike-slip offsets on subparallel faults. Similar faulting relations are observed throughout much of the boundary zone between the Basin and Range-Sierra Nevada (including the Walker Lane belt). Along the eastern province margin in the Wasatch front area in Utah, available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of <4–5 km. Earthquake focal mechanisms in this area suggest more variability in relative magnitude of the two horizontal stresses with depth. Furthermore, superimposed sets of young fault striae along a segment of the Wasatch fault also indicate temporal variations of relative stress magnitudes. Sources of regional and temporal variations in the stress field may be linked to variable shear tractions applied to the base of the brittle crust related to intrusion, thermally induced flow, and the influence of the San Andreas plate boundary. Although difficult to date accurately, the fault slip data suggest that the temporal variations in relative magnitudes stress may occur on the time scale of both a single major earthquake cycle (1000–5000 years) and multiple earthquake cycles (10,000+ years).

Journal of Geophysical Research Solid Earth↗
USGS2012

Compaction of basin sediments: Modeling based on time-temperature history

Porosity decrease of sandstones, carbonates, and shales during burial is modeled here in terms of time-temperature exposure rather than in terms of depth. Loss of porosity (ϕ) in the subsurface is represented by a power function, ϕ = A ( M ) B , where A and B are constants and M is a measure of integrated time-temperature history. Regression lines of carbonate and sandstone porosity upon Lopatin's time-temperature index of thermal maturity (TTI) generally fall within a rather narrow envelope whose axis is approximated by ϕ = 30(TTI) −0.33 . This equation is useful for regional modeling of carbonate and sandstone compaction. Dependence of porosity upon integrated time-temperature history implies that basin sediments are not in equilibrium but compact through geologic time. Calculations show that subsidence resulting from the loss of porosity with increasing time-temperature exposure (as opposed to deeper burial) can produce a second-stage, passively formed basin containing many hundreds of meters of sediments.

Journal of Geophysical Research Solid Earth↗
USGS2012

Dynamic rupture modeling with laboratory-derived constitutive relations

A laboratory-derived state variable friction constitutive relation is used in the numerical simulation of the dynamic growth of an in-plane or mode II shear crack. According to this formulation, originally presented by J. H. Dieterich, frictional resistance varies with the logarithm of the slip rate and with the logarithm of the frictional state variable as identified by A. L. Ruina. Under conditions of steady sliding, the state variable is proportional to (slip rate) −1 . Following suddenly introduced increases in slip rate, the rate and state dependencies combine to produce behavior which resembles slip weakening. When rupture nucleation is artificially forced at fixed rupture velocity, rupture models calculated with the state variable friction in a uniformly distributed initial stress field closely resemble earlier rupture models calculated with a slip weakening fault constitutive relation. Additional rupture models are calculated in which rupture nucleation is achieved naturally, with numerical simulations of the quasi-static response of the fault leading to the onset of unstable, dynamic rupture. When rupture nucleation with the state variable friction law takes place naturally, a large fraction of the fault accelerates before accelerating slip is concentrated in what ultimately becomes the rupture nucleation patch. The state evolution accompanying this accelerating slip leads to higher average rupture speeds or a more rapid rupture acceleration to near P wave rupture speeds. Rupture models are also calculated for the seismological asperity problem, that is, the failure of a highly stressed fault patch surrounded by a region of zero stress drop. Dynamic overshoot of slip into the region of zero stress drop roughly agrees with a simple energy balance analysis; the final size of the rupture is proportional to the square of the size of the high stress patch. Earlier frictional stability analyses have led to the definition of a critical fault patch size for rupture nucleation. This critical patch size is generally different from critical crack lengths determined from crack tip energy balance considerations applied to a simpler slip weakening law. In the model calculations, dynamic rupture does not nucleate if the starting patch size is less than the critical patch size. This is consistent with the frictional stability analyses. Thus these model calculations suggest that dynamic rupture following a state variable friction relation is similar to that following a simpler fault slip weakening law. However, when modeling the full cycle of fault motions, rate-dependent frictional responses included in the state variable formulation are important at low slip rates associated with rupture nucleation. The critical rupture nucleation dimension appropriate for a slip weakening fault does not predict the critical nucleation dimension for a state variable fault.

Journal of Geophysical Research Solid Earth↗
USGS2012

Synfolding magnetization in the Jurassic Preuss Sandstone, Wyoming- Idaho-Utah thrust belt

The Jurassic Preuss Sandstone, exposed in five thrust plates of the Wyoming-Idaho-Utah thrust belt, carries directions of remanent magnetization that group most tightly after only partial unfolding. Field, petrographic, and rock magnetic evidence indicates that the carrier of this magnetization is detrital, low-Ti titanomagnetite. The detrital titanomagnetite was remagnetized at low temperatures (75°–150°C) probably completely during folding. Anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain. We suggest that low-temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization, the acquisition of which was enhanced by stress, or a piezoremanent magnetization that involved stress-induced movement of domain walls during intracrystalline strain, or was a combination of the two mechanisms. Stress may promote remagnetization at temperatures much lower than predicted by current theoretical models. Other mechanisms, such as acquisition of chemical remanent magnetization during folding, deflection of a prefolding magnetization by internal strain, or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the Preuss. The locus of acquisition of synfolding magnetization in the Preuss migrated in conjunction with deformation in the thrust belt. A model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps. A lack of reverse-polarity directions remains a puzzling feature of the remanence. The remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in Late Cretaceous and early Tertiary (?) periods of predominantly normal polarity.

Journal of Geophysical Research Solid Earth↗
USGS2012

Spectral characteristics of chlorites and Mg‐serpentines using high‐resolution reflectance spectroscopy

The present laboratory study using high‐resolution reflectance spectroscopy (0.25–2.7 μm) focuses on two primary phyllosilicate groups, serpentines and chlorites. The results show that it is possible to spectrally distinguish between isochemical end‐members of the Mg‐rich serpentine group (chrysotile, antigorite, and lizardite) and to recognize spectral variations in chlorites as a function of Fe/Mg ratio (∼8–38 wt % Fe). The position and relative strength of the 1.4‐μm absorption feature in the trioctahedral chlorites appear to be correlated to the total iron content and/or the Mg/Si ratio and the loss on ignition values of the sample. Spectral differences in the 2.3‐μm wavelength region can be attributed to differences in lattice environments and are characteristic for specific trioctahedral chlorites. The 1.4‐μm feature in the isochemical Mg‐rich serpentines (total iron content ∼1.5–7.0 wt%) show marked spectral differences, apparently due to structural differences.

Journal of Geophysical Research: Solid Earth↗
USGS2012

Teleseismically recorded seismicity before and after the May 7, 1986, Andreanof Islands, Alaska, earthquake

The May 7, 1986, Andreanof Islands earthquake ( M w 8.0) is the largest event to have occurred in that section of the Aleutian arc since the March 9, 1957, Aleutian Islands earthquake ( M w 8.6). Teleseismically well-recorded earthquakes in the region of the 1986 earthquake are relocated with a plate model and with careful attention to the focal depths. The data set is nearly complete for m b ≥4.7 between longitudes 172°W and 179°W for the period 1964 through April 1987 and provides a detailed description of the space-time history of moderate-size earthquakes in the region for that period. Additional insight is provided by source parameters which have been systematically determined for M w ≥5 earthquakes that occurred in the region since 1977 and by a modeling study of the spatial distribution of moment release on the mainshock fault plane. A technically significant component of oblique convergence in the central Aleutian arc results in its breakup into clockwiserotating and westward translating blocks. The western part of the Andreanof block is distinct from and stronger than flanking regions. The greater strength of this block segment and strong coupling along the main thrust zone result in the accumulation of high levels of shear stress, which give rise to great earthquakes near its eastern boundary. The occurrence of the 1986 rupture only 29 years after the 1957 earthquake may indicate that in the central Aleutians M 7+ earthquakes ordinarily do represent the predominant mode of strain release. Segmentation of the main thrust zone into upper and lower planes is supported by spatial and temporal patterns of seismicity and by focal mechanism data. This and other lines of evidence indicate a downdip increase in fault strength and possibly in heterogeneity within the main thrust zone in subduction zone environments. Aftershocks of the 1986 earthquake were bounded to the west by the Andreanof block boundary (Adak Canyon) and to the east by an aseismic segment of the main thrust zone near the subducted extension of the Amlia fracture zone. The aftershock distribution was bounded to the south by a forearc shear zone and to the north by the base of the main thrust zone in the Hawley Ridge segment and by the downdip edge of the upper plane of the main thrust zone in the eastern segment. Aftershocks which occurred near the volcanic line at shallow crustal depths in the upper plate were triggered by the mainshock and manifest a partial decoupling of oblique slip in this region along a west-striking right-lateral fault with low shear strength. Aftershock clustering along the main thrust zone was very similar to the distribution of prior seismicity, suggesting a continuation of long-term processes and the existence of areas with distinct mechanical properties. Interconnecting regions of low seismicity during both the premainshock and aftershock periods coincided with areas of major moment release during the mainshock. Seismicity data prior to the 1986 mainshock gave few clues about the location of the mainshock nucleation point, the mainshock size, and its time of occurrence. However, a large part of the mainshock moment release did coincide with a zone of seismic quiescence monitored by the Adak local seismograph network.

Journal of Geophysical Research Solid Earth↗
USGS2012

Acceleration spectra for subduction zone earthquakes

We estimate the source spectra of shallow earthquakes from digital recordings of teleseismic P wave groups, that is, P + p P + s P , by making frequency dependent corrections for the attenuation and for the interference of the free surface. The correction for the interference of the free surface assumes that the earthquake radiates energy from a range of depths. We apply this spectral analysis to a set of 12 subduction zone earthquakes which range in size from M S = 6.2 to 8.1, obtaining corrected P wave acceleration spectra on the frequency band from 0.01 to 2.0 Hz. Seismic moment estimates from surface waves and normal modes are used to extend these P wave spectra to the frequency band from 0.001 to 0.01 Hz. The acceleration spectra of moderate subduction zone earthquakes, that is, earthquakes whose seismic moments are less than 10 27 dyn cm, exhibit ω-square or Brune-type spectra, while the acceleration spectra of large subduction zone earthquakes, that is, earthquakes whose seismic moments are greater than 10 27 dyn cm, exhibit intermediate slopes where ü(ω) ∝ ω 5/4 for frequencies from 0.005 to 0.05 Hz. For this set of earthquakes, spectral shape appears to be a discontinuous function of seismic moment. Using reasonable assumptions for the phase characteristics, we transform the spectral shape observed for large earthquakes into the time domain to fit Ekström's (1987) moment rate functions for the M S = 8.1 Michoacan earthquake of September 19, 1985, and the M S = 7.6 Michoacan aftershock of September 21, 1985.

Journal of Geophysical Research Solid Earth↗
USGS2012

Active faulting and deformation of the Coalinga anticline as interpreted from three-dimensional velocity structure and seismicity

This work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake. Interpretation of hypocenters and fault plane solutions, from the 1983 Coalinga, Coast Range California, earthquake sequence, in combination with the three-dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher-velocity material. Above the main listric blind thrust there is a conjugate fault, steeply northeast dipping, that provides the western limit of the aftershocks within the Coalinga Anticline and that corresponds in location and spatial extent with the adjacent Pleasant Valley syncline. The character of the seismicity varies with the degree of previous deformation on each section of the anticline. Where the previous uplift was largest, the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure. Diffuse seismicity characterizes the area with the least previous deformation. The mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features. The upward extent of the mainshock rupture ended at the approximate boundary between Franciscan and Great Valley Sequence rocks. Above that depth the main thrust appears to splay into a steeper segment and a near-horizontal segment. Thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material. With the three-dimensional velocity model each individual hypocenter moved slightly (0–2 km) in accord with the details of the surrounding velocity structure, so that secondary features in the seismicity pattern are more detailed than with a local one-dimensional model and station corrections. The overall character of the fault plane solutions was not altered by the three-dimensional model, but the more accurate ray paths did result in distinct changes. In particular, the mainshock has a fault plane dipping 30° southwest instead of the 23° obtained with the one-dimensional model.

Journal of Geophysical Research Solid Earth↗
USGS2012

Tectonic setting of the Yukon-Koyukuk basin and its borderlands, western Alaska

The Yukon-Koyukuk basin of western Alaska is composed of an arcuate belt of Jurassic and Early Cretaceous subduction-related volcanic and plutonic rocks (Koyukuk terrane) flanked by deep subbasins filled with mid-Cretaceous terrigenous sedimentary rocks. The basin is bordered on three sides by metamorphosed Proterozoic and Paleozoic continental rocks (Seward, Arctic Alaska, and Ruby terranes) and is separated from the metamorphic borderlands by a narrow, highly tectonized belt of oceanic crust and mantle rocks (composite Angayucham-Tozitna terrane). The oceanic and mantle rocks, which dip inward beneath the basin and are thrust outward onto the borderlands, are divided into three separate thrust panels: (1) a structurally lowest panel (Slate Creek) composed of phyllite and metagraywacke of probable Devonian age, (2) a middle panel (Narvak) composed of imbricated basalt, chert, and gabbro of Devonian to Early Jurassic age, and (3) a structurally highest panel (Kanuti) composed of gabbro and peridotite of probable Middle and Late Jurassic age. The three thrust panels appear to represent a reversely stacked sequence that progresses from continental slope deposits in the lower panel to cumulus and mantle peridotites in the upper. Metamorphic mineral K-Ar ages from garnet amphibolite on the sole of the upper panel suggest that the upper panel was emplaced on the middle panel in the Middle to Late Jurassic. Subsequent accretion of all three panels to the continental rocks of the borderlands occurred in the latest Jurassic and Early Cretaceous, synchronous with arc volcanism within the basin. Arc volcanism waned and the accretionary phase ended in the middle of Early Cretaceous time. Uplift and erosion of the metamorphic borderlands and the obducted oceanic rocks began in late Early Cretaceous and was accompanied by the rapid filling of two flanking subbasins with turbiditic sediments. In the latest Early Cretaceous and early Late Cretaceous, shallow marine and nonmarine conglomerates were deposited around the margins of the basin, and a prograding delta was built out from the southeast margin of the basin across the turbiditic subbasins and the remnant volcanic arc. In the Late Cretaceous, western Alaska was subjected to strong east-west compression which severely deformed both the Yukon-Koyukuk basin and the borderlands. Several widespread magmatic episodes in the mid- and Late Cretaceous and in early Tertiary transgress the basin boundaries and stitch together the accreted arc and oceanic terranes and the continental borderlands.

Journal of Geophysical Research Solid Earth↗
USGS2012

The Kanuti ophiolite, Alaska

The Kanuti ophiolite is a mafic-ultramafic thrust sheet of probable Jurassic age, formerly considered to be the upper part of the Yukon-Koyukuk ophiolite belt (Angayucham terrane). It is here called the Kanuti ophiolite after the Kanuti River region on the southeastern flank of the Yukon-Koyukuk Basin. The thrust sheet crops out discontinuously for a distance of more than 900 km along the northern and southeastern margins of the basin. It is probably correlative with similar ophiolite thrust sheets to the north in the western Brooks Range and to the south in the Ruby geanticline. Technically, the ophiolite is considered to be the Kanuti thrust panel of the Angayucham-Totzitna terrane. The Kanuti consistently overlies another extensive thrust sheet, consisting mostly of pillow basalt and radiolarian chert of Devonian to Jurassic age (Narvak thrust panel). This sheet is thrust over a third sheet consisting of probable Devonian phyllite and metagraywacke, which is in turn thrust over older metamorphic rocks (Slate Creek thrust panel). The Kanuti ophiolite is a partial ophiolite that consists of a lower residual mantle suite and an upper magmatic suite, but dikes, extrusives, and sediments are absent. The residual mantle suite is composed of harzburgite and dunite with refractory mineral compositions. The harzburgite is attributed to partial melting and extraction of basaltic magma; residual dunite is attributed to partial melting or to reaction of orthopyroxene out of harzburgite in contact with ascending melt diapirs. The magmatic suite consists of layered ultramafic and gabbroic rocks, containing minerals having limited iron enrichment. The absence of large volumes of magmatic rocks intermediate in composition between cumulus ultramafics and evolved gabbros favors periodic introduction of magma, rather than closed system fractional crystallization. The ultramafic rocks of both the residual mantle and magmatic suites are tectonites, which have undergone high-temperature deformation involving isoclinal folding on all scales and related syntectonic recrystallization. The olivine fabric is consistent with the glide system {0kl} [100], which has been produced experimentally at 800°–1190°C at 20 kbar. Olivine Z axes and subparallel isoclinal fold axes have consistent, northeast trends throughout the Kanuti region (>100 km NE-SW) and may be close to the original upper mantle flow direction, despite later low-angle thrust faulting. The order of crystallization in the cumulus ultramafic rocks of the magmatic suite is olivine, clinopyroxene, plagioclase, and orthopyroxene. The high Mg numbers of clinopyroxene (0.85–0.93) coexisting with olivine suggest that the cumulus ultramafic rocks crystallized at relatively high pressures (>10 kbar). The effects of parental magma composition cannot be evaluated, but the small difference in Mg numbers of coexisting olivine and clinopyroxene in the cumulus ultramafic rocks and in residual harzburgite suggests that regardless of absolute pressures, the pressure difference between the melting that produced the basalt magma and the initial fractional crystallization of the magma is small. Because of the limited range in rock types in the ophiolite, the tectonic environment cannot be interpreted unambiguously. However, the structural and petrological data are best reconciled with an origin in a volcanic arc tectonic setting.

Journal of Geophysical Research Solid Earth↗
USGS2012

Historic surface slip along the San Andreas Fault near Parkfield, California

The Parkfield Earthquake Prediction Experiment is focusing close attention on the 44-km-long section of the San Andreas fault that last ruptured seismically in 1966 ( M s 6.0). The 20-km-long central segment of the 1966 Parkfield rupture, extending from the mainshock epicenter at Middle Mountain southeastward to Gold Hill, forms a 1- to 2-km salient northeastward away from the dominant N40°W strike. Following the 1966 earthquake afterslip, aseismic slip has been nearly constant. Moderate Parkfield earthquakes have recurred on average every 21 years since 1857, when a great earthquake ( M ≈8) ruptured at least as far north as the southern Parkfield segment. Many measurements of slip have been made near Parkfield since 1966. Nevertheless, much of the history of surface slip remained uncertain, especially the total amount associated with the 1966 event. In 1985 we measured accumulated slip on the four oldest cultural features offset by the fault along the 1966 Parkfield rupture segment. We interpret net slip on each feature as a sum of event slip (sum of coseismic and rapid preseismic and postseismic slip) from Parkfield earthquakes and steady interseismic slip as measured over the last 20 years on nearby alinement arrays, creep meters, and trilateration lines. We assumed for each site that event slip was identical for the 1922, 1934, and 1966 Parkfield events and that long-term average rate of interseismic slip was constant between all events. Two fences on the southern segment, southeast of Gold Hill, indicate event slip of 13 and 15 cm and interseismic slip rate of 0.36 and 0.30 cm/yr since 1959 and 1908, respectively. At these sites, redundant independent data support our assumption that both event and interseismic slip occur uniformly. On the central segment, near Parkfield, both the 1934 and the 1966 ruptures offset a bridge built in 1932. Interseismic slip rate near the bridge has been about 1.1 cm/yr since 1966; thus we deduce an average event slip of 31 cm for the 1934 and 1966 earthquakes. On a parallel fault trace, 1 km to the southwest, slip was about 8 cm in 1966; thus total event slip summed across the entire fault zone near Parkfield was nearly 40 cm. On Middle Mountain, 4 km north of the 1966 mainshock epicenter, an offset fence indicates 17 cm of slip in 1966 and a 2.26-cm/yr interseismic slip rate since circa 1946. Thus the central segment of the 1966 rupture is characterized by much larger event slip (∼40 cm) than both distal segments (∼15 cm). This amount of surface slip per event is about twice what had been previously assumed. Larger 1966 surface slip in the central part of the rupture is geodetically compatible with a coseismic slip of 65±10 cm slip on a narrow, buried asperity between Middle Mountain and Gold Hill that has been inferred from the depth distribution of early aftershocks. Assuming our characteristic surface slip model, one can further deduce a deficit in slip since the great 1857 earthquake. Taking the long-term slip rate as 3.3 cm/yr, the surface slip deficit is 3±0.2 m south of Gold Hill but only 0.3±0.3 m northward from Parkfield.

Journal of Geophysical Research Solid Earth↗
USGS2012

Pb, Sr, and Nd isotopes in seamount basalts from the Juan de Fuca Ridge and Kodiak-Bowie seamount chain, northeast Pacific

Pb, Sr, and Nd isotopic ratios and their parent/daughter element concentrations for 28 basalts from 10 hotspot and nonhotspot seamounts are reported. Nd and Sr isotopic compositions ( 143 Nd/ 144 Nd = 0.51325–0.51304; 87 Sr/ 86 Sr = 0.70237–0.70275) plot in the envelope for Juan de Fuca-Gorda ridge basalts with tholeiitic basalts showing more depleted sources and a better negative correlation than transitional to alkalic basalts. Pb isotopic ratios in tholeiitic and alkalic basalts overlap ( 206 Pb/ 204 Pb = 18.29–19.44) and display a trend toward more radiogenic Pb in alkalic basalts. The isotopic data for hotspot and nonhotspot basalts are indistinguishable and correlate broadly with rock composition, implying that they are controlled by partial melting. The isotopic variation in the seamount basalts is about 60% (Nd-Sr) to 100% (Pb) of that in East Pacific Rise basalts and is interpreted as a lower limit for the magnitude of mantle heterogeneity in the northeast Pacific. The data indicate absence of a chemically distinct plume component in the linear seamount chains and strongly suggest an origin from mid-ocean ridge basalt-like east Pacific mantle.

Journal of Geophysical Research Solid Earth↗
USGS2012

Late Cretaceous paleomagnetism and clockwise rotation of the Silver Bell Mountains, south central Arizona

Late Cretaceous ash flow volcanism in the Silver Bell Mountains of southern Arizona (32.3°N, 248.5°E) was associated with caldera formation and porphyry copper mineralization. Oriented samples from 34 sites in volcanic, volcaniclastic, and intrusive units related to this episode of igneous activity (73–69 Ma) yield a mean paleomagnetic direction of I = 61.2°, D = 24.0°, α 95 = 7.6°. Primary remanent magnetizations are indicated by the presence of both normal and reversed polarities and by the significantly improved grouping (95% confidence level) of site-mean directions for a widespread tuff unit after structural correction. A comparison of this direction with a direction at Silver Bell calculated from contemporaneous units for stable North America ( I = 58.8°, D = 342.3°, α 95 = 7.7°) indicates a Late Cretaceous paleolatitude anomaly of −2.4°±7.6° (not significant at the 95% confidence level) and a declination anomaly of 41.7°±14.3° for the Silver Bell Mountains. Previously determined paleomagnetic data for southeastern Arizona suggest that this apparent clockwise rotation in the Silver Bell Mountains is a local phenomenon. Although preliminary, the average paleomagnetic direction for Oligocene and lower Miocene rocks in the Silver Bell area ( I = 43.8°, D = 357.3°, α 95 = 16.5°) is similar to that calculated for stable North America ( I = 50.2°, D = 352.2°, α 95 = 3.9°), implying that the observed rotation in the Silver Bell Mountains occurred before 26 Ma and was most likely associated with dextral strike-slip movement along the Ragged Top and related WNW trending faults bounding the Silver Bell Mountain block. These data, in conjunction with plate reconstructions and other paleomagnetic data from southwestern North America, imply that WNW trending strike-slip faults may have played an important role during Late Cretaceous to early Tertiary deformation in southern Arizona.

Journal of Geophysical Research Solid Earth↗
USGS2012

Crustal strain near the Big Bend of the San Andreas Fault: Analysis of the Los Padres-Tehachapi Trilateration Networks, California

In the region of the Los Padres-Tehachapi geodetic network, the San Andreas fault (SAF) changes its orientation by over 30° from N40°W, close to that predicted by plate motion for a transform boundary, to N73°W. The strain orientation near the SAF is consistent with right-lateral shear along the fault, with maximum shear rate of 0.38±0.01 μrad/yr at N63°W. In contrast, away from the SAF the strain orientations on both sides of the fault are consistent with the plate motion direction, with maximum shear rate of 0.19±0.01 μrad/yr at N44°W. The strain rate does not drop off rapidly away from the fault, and thus the area is fit by either a broad shear zone below the SAF or a single fault with a relatively deep locking depth. The fit to the line length data is poor for locking depth d less than 25 km. For d of 25 km a buried slip rate of 30 ± 6 mm/yr is estimated. We also estimated buried slip for models that included the Garlock and Big Pine faults, in addition to the SAF. Slip rates on other faults are poorly constrained by the Los Padres-Tehachapi network. The best fitting Garlock fault model had computed left-lateral slip rate of 11±2 mm/yr below 10 km. Buried left-lateral slip of 15±6 mm/yr on the Big Pine fault, within the Western Transverse Ranges, provides significant reduction in line length residuals; however, deformation there may be more complicated than a single vertical fault. A subhorizontal detachment on the southern side of the SAF cannot be well constrained by these data. We investigated the location of the SAF and found that a vertical fault below the surface trace fits the data much better than either a dipping fault or a fault zone located south of the surface trace.

Journal of Geophysical Research Solid Earth↗
USGS2012

Order and diversity in the modes of circum-Pacific earthquake recurrence

Recurrence characteristics of great circum-Pacific earthquakes and determinations of spatial distribution of seismic moment release are surveyed in order to delineate their general features and relate earthquake slip distribution to models of recurrent rupture. As noted by others, the pattern of moment release is typically very irregular, with strong concentrations in a few isolated regions of a much larger aftershock zone. Despite this complexity, rupture nucleation is notably systematic, with mainshock epicenter showing a strong tendency to locate in or immediately adjacent to identified regions of high moment release. This generalization suggests that earthquake recurrence is more likely to be controlled by maximum rather than average fault slip. Well-documented case histories from twelve plate boundary regions demonstrate that seismic strain release tends to be temporally well-ordered, while source dimensions, slip, and cumulative moment release vary considerably from cycle to cycle. On none of the segments studied do earthquakes consistently recur in nearly identical events. Instead, individual great earthquakes differ significantly from cycle to cycle or rerupture takes place in a sequence of two or more smaller events. Despite these differences the duration of the seismic cycle is approximately uniform. Furthermore, when strain release occurs in a sequence of large earthquakes, these events take place towards the end of the cycle and occur with increasing rupture length and magnitude. These ordered and irregular features of earthquake recurrence argue for the existence of corresponding elements on plate boundary faults. The ordered characteristics are identified with zones of concentrated moment release that slip comparable amounts in each cycle and have high shear strength. The irregular features are associated with intervening weaker regions that move in response to the stress concentration of dynamic rupture and slip by differing amounts in each event. Although these mechanistic associations are indirect and tentative, the features of recurrent behavior documented here have implications for long-term earthquake hazard assessment that are not dependent on the models proposed to explain them. Chief among these are the cycle-to-cycle differences among gap-filling events and the absence of shocks that fill major slip-deficient regions early in the seismic cycle.

Journal of Geophysical Research Solid Earth↗
USGS2012

Cooling rate and thermal structure determined from progressive magnetization of the dacite dome at Mount St. Helens, Washington

Our study of a magnetic anomaly associated with the recently active dacite dome at Mount St. Helens suggests that the dome consists of a hot, nonmagnetized core surrounded by a cool, magnetized carapace and flanking talus. The talus does not contribute to the anomaly because its constituent blocks are randomly oriented. Temporal changes in the magnetic anomaly indicate that the magnetized carapace thickened at an average rate of 0.03±0.01 m/d from 1984 to 1986. Petrographic and rock magnetic properties of dome samples indicate that the dominant process responsible for these changes is magnetization of extensively oxidized rock at progressively deeper levels within the dome as the rock cools through its blocking temperature, rather than subsequent changes in magnetization caused by further oxidation. Newly extruded material cools rapidly for a short period as heat is conducted outward in response to convective heat loss from its surface. The cooling rate gradually declines for several weeks, and thereafter the material cools at a relatively constant rate by convective heat loss from its interior along fractures that propagate inward. The rate of internal convective heat loss through fractures varies with rainfall, snowmelt, and large-scale fracturing during subsequent eruptive episodes. In accordance with a model for solidification of the 1959 lava lake at Kilauea Iki, Hawaii, we picture the dome's magnetized carapace as being a two-phase, porous, convective zone separated from the nonmagnetized core of the dome by a thin, single-phase conductive zone. As a consequence of the heat balance between the conductive and convective zones, the blocking-temperature isotherm migrates inward at a relatively constant rate. If the dome remains inactive, the time scale for its complete magnetization is estimated to be 18–36 years, a forecast which can be refined by shallow drilling into the dome and by continuing studies of its growing magnetic anomaly.

Journal of Geophysical Research Solid Earth↗
USGS2012

Reflected and mode-converted seismic waves within the shallow Aleutian subduction zone, southern Kenai Peninsula, Alaska

Pronounced secondary phases observed in local recordings of quarry shots and earthquakes on the southern Kenai Peninsula, Alaska, are identified as reflected P and S and converted S-to-P phases originating within four depth ranges: in the upper few kilometers of the Cook Inlet Tertiary basin, at midcrustal depths within the overthrust North American plate, at about 35 km depth near the top of the Wadati-Benioff seismic zone in proximity to the inferred interplate megathrust, and at about 5–10 km below the megathrust in the subducted Pacific plate. Beneath the landward margin of the accreted Chugach terrane, the mid-upper plate (MUP) discontinuity dips as steeply as 20°–30° to the west-northwest over the depth range 12–18 km. At shallower depths it flattens and possibly arches with the crest at about 10 km depth. Similar midcrustal reflectors have been imaged about 125 km to the southwest and about 350 km to the northeast along the structural trend of the Chugach terrane. The extensive reflectors may have a common origin, possible caused by the presence of underplated rocks. Relative amplitudes and polarities of the secondary phases originating at MUP depths provide few constraints on the nature of the discontinuity. It appears that the MUP discontinuity is seismically inactive and does not represent a brittle-ductile transition zone within the upper plate. The two converted S -to- P phases generated near the top of the subducted plate could indicate a low-velocity zone associated with subducted oceanic crust.

Journal of Geophysical Research Solid Earth↗
USGS2012

A tomographic glimpse of the upper mantle source of magmas of the Jemez lineament, New Mexico

The 800-km-long Jemez lineament is the most active volcanic feature in the southwestern United States. It is the southeastern tectonic boundary of the Colorado Plateau and crosses the Rio Grande rift at the Jemez Mountains. The primary volcanism of the lineament is basaltic and has occurred in the last 4.5 m.y. To infer spatial distributions of partial melt in the upper mantle source zones for the Rio Grande rift and the Jemez lineament, we investigated the lateral variations of P wave velocity in the upper mantle beneath these features. We used teleseismic P wave delays recorded at a 22-station network to perform a damped least squares, three-dimensional inversion for these lateral variations. Our technique employed velocity interpolation within a three-dimensional grid of points, rather than using blocks of constant P wave velocity. This method allows highly realistic computation of seismic ray paths as well as accurate computation of the matrix elements in our system of equations. Determinations of resolution of results were done in two independent ways, both of which gave consistent estimates of resolution. In our best resolved volume the inversion showed no significant concentration of relative low velocity for P waves beneath the Rio Grande rift. However, directly beneath the Jemez lineament there is a ∼100-km-wide, 1–2% low-velocity feature in the depth range of 50–160 km. Because of the association of the low P wave velocity with the Jemez volcanic lineament but not with the Rio Grande rift, because lowered P wave velocity can be associated with increased partial melt, and because the volume of recent volcanism at the lineament greatly exceeds that at the rift, we infer that a large magmatic source zone exists beneath the Jemez lineament but not beneath the Rio Grande rift. This implies that the volcanic potential of the Jemez lineaments continues to greatly exceed that of the Rio Grande rift. The mantle source zones for volcanics of the Jemez lineament are not overridden by, but rather track, the motion of the North American plate; this implies that these sources are within the lithospheric plate, as is clarified in the discussion. The magmatic source zones of the Jemez lineament are modeled as due to clockwise rotation of the Colorado Plateau about a pole in northeastern Colorado. This rotation caused extension of the lithosphere beneath the Jemez lineament, permitting concentration there of partially melted rock in the upper mantle.

Journal of Geophysical Research Solid Earth↗