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At least 739 records · Page 41Linked to original sources
USGS1998

Episodic fluid flow in the Nankai accretionary complex: Timescale, geochemistry, flow rates, and fluid budget

Down-hole geochemical anomalies encountered in active accretionary systems can be used to constrain the timing, rates, and localization of fluid flow. Here we combine a coupled flow and solute transport model with a kinetic model for smectite dehydration to better understand and quantify fluid flow in the Nankai accretionary complex offshore of Japan. Compaction of sediments and clay dehydration provide fluid sources which drive the model flow system. We explicitly include the consolidation rate of underthrust sediments in our calculations to evaluate the impact that variations in this unknown quantity have on pressure and chloride distribution. Sensitivity analysis of steady state pressure solutions constrains bulk and flow conduit permeabilities. Steady state simulations with 30% smectite in the incoming sedimentary sequence result in minimum chloride concentrations at site 808 of 550 m M , but measured chlorinity is as low as 447 m M . We simulate the transient effects of hydrofracture or a strain event by assuming an instantaneous permeability increase of 3–4 orders of magnitude along a flow conduit (in this case the décollement), using steady state results as initial conditions. Transient results with an increase in décollement permeability from 10 −16 m 2 to 10 −13 m 2 and 20% smectite reproduce the observed chloride profile at site 808 after 80–160 kyr. Modeled chloride concentrations are highly sensitive to the consolidation rate of underthrust sediments, such that rapid compaction of underthrust material leads to increased freshening. Pressures within the décollement during transient simulations rise rapidly to a significant fraction of lithostatic and remain high for at least 160 kyr, providing a mechanism for maintaining high permeability. Flow rates at the deformation front for transient simulations are in good agreement with direct measurements, but steady state flow rates are 2–3 orders of magnitude smaller than observed. Fluid budget calculations indicate that nearly 71% of the incoming water in the sediments leaves the accretionary wedge via diffuse flow out the seafloor, 0–5% escapes by focused flow along the décollement, and roughly 1% is subducted.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Seismotectonics of the Loma Prieta, California, region determined from three-dimensional Vp, Vp/Vs, and seismicity

Three-dimensional V p and V p / V s velocity models for the Loma Prieta region were developed from the inversion of local travel time data (21,925 P arrivals and 1,116 S arrivals) from earthquakes, refraction shots, and blasts recorded on 1700 stations from the Northern California Seismic Network and numerous portable seismograph deployments. The velocity and density models and microearthquake hypocenters reveal a complex structure that includes a San Andreas fault extending to the base of the seismogenic layer. A body with high V p extends the length of the rupture and fills the 5 km wide volume between the Loma Prieta mainshock rupture and the San Andreas and Sargent faults. We suggest that this body controls both the pattern of background seismicity on the San Andreas and Sargent faults and the extent of rupture during the mainshock, thus explaining how the background seismicity outlined the along-strike and depth extent of the mainshock rupture on a different fault plane 5 km away. New aftershock focal mechanisms, based on three-dimensional ray tracing through the velocity model, support a heterogeneous postseismic stress field and can not resolve a uniform fault normal compression. The subvertical (or steeply dipping) San Andreas fault and the fault surfaces that ruptured in the 1989 Loma Prieta earthquake are both parts of the San Andreas fault zone and this section of the fault zone does not have a single type of characteristic event.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Deformation across the rupture zone of the 1964 Alaska earthquake, 1993–1997

A linear array of 15 geodetic monuments was installed in 1993 across the rupture zone of the 1964 Alaska earthquake ( M w = 9.2). The array extends from Middleton Island (at the edge of the continental shelf and 80 km from the Alaska‐Aleutian trench) to north of Palmer, Alaska (380 km from the trench), in the approximate direction of Pacific‐North American plate convergence (N15.5°W). The array was surveyed in June 1993, May 1995, and June 1997. The changes between surveys are a measure of the deformation of the continental margin across the subduction zone in southern Alaska. Measured relative to the interior of the North American plate, the horizontal velocities on the outer plate margin are parallel to the direction of plate convergence (N15.5°W ) and reach a maximum (58 mm yr −1 ) about 150 km from the trench. Beyond about 300 km from the trench the observed horizontal velocities are small. A narrow (halfwidth 50 km) zone of significant uplift (10 mm yr −1 maximum) is observed about 300 km from the trench, coinciding roughly with the locus of maximum coseismic subsidence associated with the 1964 Alaska earthquake. Although the deformation is roughly described by the conventional model of deformation at a subduction zone (deformation due to virtual back slip on the main thrust zone at the 55 mm yr −1 plate convergence rate), a better fit is given with a 65 mm yr −1 virtual back (normal) slip rate. This higher rate is attributed to continued postseismic relaxation. The model does not explain the relatively high uplift rate and low N15.5°W velocity observed at Middleton Island. That anomalous motion is attributed to continued thrusting on postulated upward trending splays from the subduction zone beneath the island.

Alaska↗
USGS1998

Paleomagnetism of the Miocene intrusive suite of Kidd Creek: Timing of deformation in the Cascade arc, southern Washington

Paleomagnetic study of the intrusive suite of Kidd Creek in the southern Washington Cascades (23 sites in dikes and sills) was undertaken to help determine if these rocks are comagmatic and whether they postdate regional folding of the volcanic arc. Fission track and 40 Ar- 39 Ar age determinations indicate an age of ∼12.7 Ma (middle Miocene) for these rocks. The similarity of normal-polarity characteristic directions for most samples corroborate the available geochemical data indicating that these rocks are most likely comagmatic. Reversed-polarity directions for samples from four sites, however, show that emplacement of Kidd Creek intrusions spanned at least one reversal of the geomagnetic field. The paleomagnetic directions for the dikes and sills fail a fold test at the 99% confidence level indicating that the Kidd Creek rocks postdate regional folding. The mean in situ direction also indicates that the Kidd Creek and older rocks have been rotated 22°±6° clockwise about a vertical or near-vertical axis from the expected Miocene direction. Compression and regional folding of the Cascade arc in southern Washington therefore had ended by ∼12 Ma prior to the onset of deformation resulting in rotation of these rocks.

Journal of Geophysical Research B: Solid Earth↗
USGS1998

Poroelastic rebound along the Landers 1992 earthquake surface rupture

Maps of surface displacement following the 1992 Landers, California, earthquake, generated by interferometric processing of ERS-1 synthetic aperture radar (SAR) images, reveal effects of various postseismic deformation processes along the 1992 surface rupture. The large-scale pattern of the postseismic displacement field includes large lobes, mostly visible on the west side of the fault, comparable in shape with the lobes observed in the coseismic displacement field. This pattern and the steep displacement gradient observed near the Emerson-Camp Rock fault cannot be simply explained by afterslip on deep sections of the 1992 rupture. Models show that horizontal slip occurring on a buried dislocation in a Poisson's material produces a characteristic quadripole pattern in the surface displacement field with several centimeters of vertical motion at distances of 10–20 km from the fault, yet this pattern is not observed in the postseismic interferograms. As previously proposed to explain local strain in the fault step overs [ Peltzer et al ., 1996b], we argue that poroelastic rebound caused by pore fluid flow may also occur over greater distances from the fault, compensating the vertical ground shift produced by fault afterslip. Such a rebound is explained by the gradual change of the crustal rocks' Poisson's ratio value from undrained (coseismic) to drained (postseismic) conditions as pore pressure gradients produced by the earthquake dissipate. Using the Poisson's ratio values of 0.27 and 0.31 for the drained and undrained crustal rocks, respectively, elastic dislocation models show that the combined contributions of afterslip on deep sections of the fault and poroelastic rebound can account for the range change observed in the SAR data and the horizontal displacement measured at Global Positioning System (GPS) sites along a 60-km-long transect across the Emerson fault [ Savage and Svarc , 1997]. Using a detailed surface slip distribution on the Homestead Valley, Kickapoo, and Johnson Valley faults, we modeled the poroelastic rebound in the Homestead Valley pull apart. A Poisson's ratio value of 0.35 for the undrained gouge rocks in the fault zone is required to account for the observed surface uplift in the 3.5 years following the earthquake. This large value implies a seismic velocity ratio V p /V s of 2.1, consistent with the observed low V s values of fault zone guided waves at shallow depth [ Li et al ., 1997]. The SAR data also reveal postseismic creep along shallow patches of the Eureka Peak and Burnt Mountain faults with a characteristic decay time of 0.8 years. Coseismic, dilatant hardening (locking process) followed by post-seismic, pore pressure controlled fault creep provide a plausible mechanism to account for the decay time of the observed slip rate along this section of the fault.

Journal of Geophysical Research B: Solid Earth↗
USGS1999

Coseismic slip resolution along a plate boundary megathrust: the Nankai Trough, southwest Japan

Geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 Tonankai (M w =8.1) and 1946 Nankaido (M w =8.3) earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust. Data used include 798 angle changes from triangulation surveys, 328 leveling section differences, and 5 coseismic tidal gage offsets. Many of the nominally coseismic triangulation data span ∼50 years, nearly half the earthquake cycle, and correction for interseismic deformation using post-1950 observations is applied. Microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous, multisegment fault model. Because the onshore geodetic data have very limited resolving power for offshore fault segments, offshore coseismic slip was constrained by Satctke's [1993] estimation based on tsunami data. The majority of the coseismic slip occurs between 15 and 25 km depth. Although resolution declines toward the trench axis, it is sufficiently good to define two distinct high-slip regions, one off southeastern Shikoku Island (11 m maximum) and the other offshore of Kii Peninsula (3 m maximum). The slip magnitude off southeastern Shikoku, coupled with the plate convergence rate, would imply an recurrence interval of about 270 years, much-longer than the average repeat time of ∼120 years for historical great earthquakes on the Nankai Trough. However, the maximum coseismic slip is sensitive to the assumed fault geometry, and slippage on trough-parallel splay faults could significantly decrease the maximum slip to about 6 m.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Strain accumulation on the San Andreas Fault near Palmdale, California

Precise distance measurements of a 10×25 km 15‐station trilateration network that spans the San Andreas fault west of Palmdale, California, have been repeated annually in the period 1971–1975. The network appears to be deforming under simple uniform tensor shear of about 0.21±0.03 μstrain/yr with the direction of maximum right‐lateral shear parallel to the local strike of the San Andreas fault. Comparison of trilateration with triangulation surveys of the same network shows that the rate of strain accumulation has been constant over the past 40 yr. The strain accumulation can be explained by conventional dislocation models (i.e., slip at depth beneath a locked section) of the San Andreas fault with 30‐ to 50‐mm/yr slip. Leveling surveys along a 16‐km line that crosses the fault at Palmdale indicate significant changes in tilt but with frequent reversals, so that no net tilt has accumulated in the overall period 1935–1975.

California↗
USGS2012

Paleomagnetic results from the Lassiter Coast, Antarctica, and a test for oroclinal bending of the Antarctic Peninsula

Paleomagnetic results from 17 magnetically stable units of Upper Cretaceous (‘Andean’) plutons and dikes of the Lassiter Coast, on the southern Antarctic Peninsula, define a mean paleomagnetic pole at 87°S, 131°W (α 95 = 11.5°). This indicates that little latitudinal movement of the southern Antarctic Peninsula has occurred during the past 100 m.y. All magnetically stable intrusives are normally polarized and are believed to have been emplaced during the Late Cretaceous epoch of predominantly normal polarity. There is no evidence of postemplacement remagnetization. The uncertainty in declination at the 95% confidence level is computed for both the Lassiter Coast data and those data available from other Andean sites in the Antarctic Peninsula. Within the limits of uncertainty, data from four localities north of 68°S support the contention of Dalziel et al. (1973) that there has not been any apparent post‐Late‐Cretaceous oroclinal bending in the northern half of the peninsula. For sites to the south in the Lassiter Coast, the uncertainty in declination, due to steep inclinations, is too large to support reliably or deny any large‐scale structural bending.

Antarctic Peninsula, Lassiter Coast↗
USGS2012

Geodimeter measurements of strain during the Southern California Uplift

A review of geodimeter measurements made along the ‘big‐bend’ section of the San Andreas fault in southern California indicates no significant increment in strain during the period of major uplift (late 1959 to mid‐1963). Specifically, no evidence of an increment in compressional strain normal to the San Andreas fault at the time of the uplift was found. Geodolite measurements at four networks along the big bend independently indicate that the strain rate during the 1974–1977 episode of subsidence was essentially a uniaxial north‐south compression at the rate of about ⅓ μstrain/yr. Whether the 1974–1977 rate is significantly different from earlier rates determined by triangulation is not clear owing to a rather large variability in the earlier determinations.

California↗
USGS2012

Revised geomagnetic polarity time scale for the interval 0–5 m.y. B.P.

A change in the constants used in K‐Ar dating and a significant increase in new data have made a recompilation and recomputation of data used to define the Late Cenozoic K‐Ar polarity time scale highly desirable at this time. All available data in the range 0–5 m.y. have been recalculated using the refined constants, with 354 data points in this time interval now meeting the minimum criteria for acceptability. Recalculation of the major polarity epoch boundaries has yielded ages of 0.73 m.y. for the Brunhes‐Matuyama, 2.48 m.y. for the Matuyama‐Gauss, and 3.40 m.y. for the Gauss‐Gilbert boundaries. A revised polarity time scale has been constructed based on available K‐Ar data and information obtained from marine magnetic anomalies and deep‐sea sedimentary cores.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Deformation across the Salton Trough, California, 1973-1977

A trilateration network extending across the San Andreas, San Jacinto, and Elsinore faults in the vicinity of the Salton Sea, California, has been surveyed to very high precision several times in the 5‐year interval 1973–1977. The average strain across the entire network is essentially a uniaxial north‐south contraction at the rate of about 0.3 μstrain/a. There is no substantial strain perpendicular to the Salton trough, indicating no tendency to either open or close that rift. The observed uniaxial north‐south contraction differs from a pure shear parallel to the major faults by a uniform dilatation of about −0.25 μstrain/a that is only partly explained. The shear strain across the network reaches a maximum near the San Jacinto fault and could be explained by right lateral slip at depth on that fault. The displacement pattern also suggests right lateral slip at depth on the San Jacinto fault with total right lateral relative movement near 50 mm/a across the 120‐km breadth of the network. The deformation appears to be uniform in time in the 1973–1977 interval. A dislocation model of the deformation suggests about 50±15 mm/a of relative right lateral slip at depth distributed between the San Andreas, San Jacinto, and Elsinore faults and demonstrates that a significant average dilatation can be generated by pure strike slip on several subparallel faults.

California↗
USGS2012

Comparison of the TRM of the Yellowstone Group and the DRM of some Pearlette ash beds

Air fall ash beds (Pearlette) originating from rhyolitic eruptions in the Yellowstone‐Island Park region of Wyoming and Idaho are discontinuous but widespread throughout the western United States. Accumulation and deposition of ashes occurred in low‐energy fluvial and lacustrine environments. These ash beds have been correlated, according to their chemistry and remanent magnetism, with specific eruptions of tuffs of the Pleistocene Yellowstone Group. Type O Pearlette ash beds are normally magnetized and correlate with the Lava Creek Tuff (0.60 m.y.), whereas type S Pearlette ash beds are reversely magnetized and correlate with the Mesa Falls Tuff (1.22 m.y.). The mean direction of detrital remanent magnetization (DRM) of 11 type O Pearlette ash beds is identical to the mean direction of thermal remanent magnetization (TRM) of the Lava Creek Tuff after correcting for differences in site latitude and longitude. Considered separately, only 2 of the 13 Pearlette ash beds studied possess mean inclinations significantly shallower than, and only 4 have mean declinations significantly different from, those of their correlative tuffs. Identical directions of magnetization measured from evenly laminated and highly contorted type O ash suggest that a readjustment of magnetic grains in water‐saturated ash shortly after deposition may account for the close agreement of TRM and DRM.

Idaho, Montana, Wyoming↗
USGS2012

Strain accumulation rates in the western United States between 1970 and 1978

The rate of dilatation and the rate and direction of shear have been determined from trilateration data for 23 Geodolite networks in the western United States. Sixteen nets are located along the San Andreas fault system between Point Reyes, California, and the United States‐Mexico border. Other locations are across the Garlock fault in California; across Puget Sound near Seattle, Washington; near Hanford in eastern Washington; near Hebgen Lake in Montana; across the Wasatch fault at Ogden, Utah; across the Rio Grande rift at Socorro, New Mexico; and Dixie Valley in Nevada; and at the northern end of Owens Valley on the California‐Nevada border. Implicit in the treatment are the assumptions that the strain was accumulating at a constant rate over the time period (within the interval 1970–1978) and over the local area (usually about 50‐km diameter) covered by the surveys. Of the nets located away from the San Andreas fault, only Ogden and Hebgen show significant strain accumulation. At Ogden the deformation is principally an east‐west compression of 0.23±0.05 μstrain/yr and at Hebgen Lake a northeast‐southwest extension of 0.17±0.03 μstrain/yr. Along the San Andreas fault system the rate of shear is 0.2 to 0.4μ/yr. The direction of shear agrees very well with the surface strike of nearby faults. This agreement is maintained even in regions like the ‘big bend,’ where both the fault strike and the observed shear direction are more westerly than they are elsewhere. Shear strain in northern California appears to be concentrated more closely on the faults, whereas in southern California the strain is a broader, smoother feature. In the San Francisco Bay area the strain data indicate slip at depth on both the San Andreas and the Calaveras faults. In addition to the observed shear the nets in California indicate a negative dilatation (areal decrease) of about 0.2 μstrain/yr. This dilatation is unexplained, but the following sources appear unlikely: (1) systematic survey error, (2) an association with the southern California uplift, (3) an association with the big bend in the San Andreas fault in Southern California, or (4) the result of the superposition of a uniaxial strain on the Pacific‐North American plate boundary shear.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Regional deformation of the Sierra Nevada, California, on conjugate microfault sets

Strike slip microfaults are pervasive throughout the granitic rocks of the eastern Sierra Nevada. Offsets typically range from less than a millimeter to several tens of centimeters but exceed 100 m in some places. The spacing between microfaults varies from a few tens of centimeters to a few tens of meters throughout much of the high Sierra Nevada. Many of these microfaults are loci of slickensided, compact fault gouge, and they are commonly mineralized by quartz veinlets with minor amounts of epidote, chlorite, and rare sulphide minerals. The microfaults are oriented in two nearly vertical conjugate sets; a north to northeast striking set showing right lateral offset and an east to northeast striking set showing left lateral offset. Microfaults with left lateral offset are more common than microfaults with right lateral offset. Most lineaments visible on aerial photographs are microfaults. The age of this microfaulting is not precisely known. It developed after consolidation of the youngest granitic plutons in the Sierra (79 m.y. B.P.) and is known to cut a late Miocene volcanic dike in one area. Slickensides along the microfaults are subhorizontal but show a slight (about 3°) westward plunging average inclination, suggesting that much of the deformation occurred prior to the westward tilting of the Sierran block in late Tertiary time. The direction of maximum horizontal extensional strain (determined as the bisector of average microfault trends) changes systematically from north to south (WNW at 38.5°N; NW at 36.5°N). A pure shear constant volume solution based on a detailed study of microfaults at 37°20′N indicates a maximum extension of 2.3% in a N61°W direction. These extension directions are remarkably parallel to late Mesozoic to present‐day tectonic extension directions in the Basin and Range province. The pattern of microfaulting demonstrates that the supposedly monolithic Sierran terrane was also affected by the late Cenozoic and possibly earlier regional extension of western North America and provides an independent criterion for determining extensional strain directions.

California↗
USGS2012

Geodolite measurements of deformation near Hollister, California, 1971-1978

A 24‐station trilateration network spanning the San Andreas and Calaveras faults near Hollister, California, has been surveyed each year between 1971 and 1978, inclusive. Two moderate ( M L = 5) earthquakes have occurred within the network during the interval. No convincing preseismic or coseismic anomalies associated with those earthquakes have been identified. The deformation of the network can be described roughly by rigid body motion of the three blocks bounded by the two faults with accommodation occurring by right‐lateral strike slip on the San Andreas (13±2 mm/a) and Calaveras (17±2 mm/a) faults. The required slip rates are within the range of the observed fault creep on those faults. A more detailed analysis of the deformation indicates appreciable strain accumulation (0.4 μstrain/a tensor shear) within the block lying between the San Andreas and Calaveras faults. Many of the features of the observed deformation can be produced by an elementary dislocation model, indicating that most of the deformation is associated directly with slip on the major faults. The network is not extensive enough to define uniquely the relative motion across the San Andreas fault system, but the data are consistent with a value of about 38 mm/a. The rate of deformation in 1971–1978 was not uniform but rather appears to have been higher than normal in 1973–1974 and lower than normal in 1975–1976.

California↗
USGS2012

Early acquisition of characteristic magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona

Four discrete magnetizations associated with events occurring during deposition, early diagenesis, recent weathering, and cloud to ground lightning have been identified in rocks of the Moenkopi Formation at Gray Mountain, Arizona. A paleomagnetic study of more than 800 samples tied closely to the physical stratigraphy of the Moenkopi indicates that most of the magnetization remaining after partial alternating field and thermal demagnetization was acquired during and shortly after deposition. Directions of this remaining (characteristic) magnetization, in both normally and reversely magnetized rock, are closely similar to directions reported for the Moenkopi elsewhere on the Colorado Plateau. Sampling across the Moenkopi Formation at Gray Mountain has revealed two reversed magnetozones separating three normal magnetozones. Generally sharp boundaries of the magnetozones, their close concordance with physical stratigraphic units, and the rarity of anomalous directions of magnetization within the magnetozones indicate that most of the secondary components of the characteristic magnetization were acquired within 10 4 –10 5 years after deposition. A conglomerate test based on siltstone clasts in sandstone shows highly scattered but not completely random directions of magnetization following partial thermal demagnetization. A secondary Triassic normal magnetization appears to be superimposed on some of the clasts, as suggested by the approximate coincidence of the resultant magnetic vector from the clasts with a Triassic normal direction. A fold test based on samples from highly deformed silty clay stone and sandstone in an intraformational landslide shows that these rocks acquired most of their stable magnetization before folding. A component of Triassic normal magnetization acquired after deformation also occurs in some of these rocks.

Arizona↗
USGS2012

Late Cenozoic volcanism, geochronology, and structure of the Coso Range, Inyo County, California

The Coso Range lies at the west edge of the Great Basin, adjacent to the southern part of the Sierra Nevada. A basement complex of pre‐Cenozoic plutonic and metamorphic rocks is partly buried by ∼35 km 3 of late Cenozoic volcanic rocks that were erupted during two periods, as defined by K‐Ar dating: (1) 4.0–2.5 m.y., ∼31 km 3 of basalt, rhyodacite, dacite, andesite, and rhyolite, in descending order of abundance, and (2) ≤1.1 m.y., nearly equal amounts of basalt and rhyolite, most of the rhyolite being ≤0.3 m.y. old. Vents for the volcanic rocks of the younger period are localized on and near a horst of basement rocks within a concavity defined by the distribution of vents of the older period. The alignment of many vents and the presence of a considerable number of roughly north‐trending normal faults of late Cenozoic age reflect basin and range tectonics dominated by roughly east‐west lithospheric extension. Fumaroles, intermittently active thermal springs, and associated altered rocks occur within and immediately east of the central part of the field of Quaternary rhyolite, in an area characterized by various geophysical anomalies that are evidently related to an active hot‐water geothermal system. This system apparently is heated by a reservoir of silicic magma at ≥8‐km depth, itself produced and sustained through partial melting of crustal rocks by thermal energy contained in mantle‐derived basaltic magma that intrudes the crust in response to lithospheric extension.

California↗
USGS2012

Aeromagnetic and gravity surveys in the Coso Range, California

The effect of an underlying magma reservoir cannot be identified within the complex gravity pattern in the Coso Range, California. Rather, linear gravity contours, which suggest a regional tectonic origin, enclose the location of most of the volcanic activity of the Coso Range. Faults along the edges of northwest trending, magnetic blocks probably provided paths of minimum resistance to the ascending viscous magma that was extruded as rhyolite domes. Dense, magnetic rocks associated with a complex mafic pluton 9 km in diameter form a relatively impermeable north border of the Pleistocene volcanic field. A heat flow high nearly coincides with the west half of a 6‐km‐diameter magnetic low. A 2‐km‐diameter outcrop of a pre‐Cenozoic silicic pluton, which has low magnetization compared to the surrounding metamorphic rocks, presumably typifies the rocks that underlie the magnetic low and heat flow high. Hydrothermal fluids may have destroyed some magnetite in the more magnetic wall rock, further reducing the magnetic intensity.

California↗