Rocking the Vote

New voting technologies are not subject to rigorous scientific testing. Caltech/MIT Voting Technology Project scientists hope to change that

Paleo- stress and strain rates in an intra-arc strike-slip fault, Sierra Nevada, California

Structures and microstructures of the Proto-Kern Canyon fault (PKCF), a 130-km-long dextral strike-slip shear zone of the southern Sierra Nevada batholith, provide constraints on displacement, flow stress, and strain rate during arc formation. Shear strain analyses of S-C mylonites indicate ~5 km of ductile dextral slip along the PKCF. But field mapping and measurements of individual plutons and metamorphic pendants show these bodies have much more elongated aspect ratios, of up to 1:17, within the shear zone than outside of it. This suggests significantly higher strain and dextral slip of up to 15 km along the highest-strain zone of the PKCF. Petrographic observations of high-strain igneous rocks near Lake Isabella indicate that deformation started at temperatures of 400-450° C and continued through cooling to ~300° C. Based on ^(40)Ar/^(39)Ar dating of hornblende, mica, and K-feldspar, early cooling (~20° C/m.y.) from 88-70 Ma was followed by very slow cooling (~1° C/m.y.). These data, combined with cross-cutting relationships, suggest that dextral ductile shear was active from 90-86 Ma. Grain sizes of dynamically recrystallized pure quartz mylonites in this part of the shear zone were used to estimate flow stresses of 20-40 MPa. Applying mylonitization temperature estimates of 400-350° C and lithostatic pressures of 350- 400 MPa (from Al-in-hbl barometry) yields paleo-strain rates along the PKCF of 10^(-13)-10^(-15) /s. Additional quartzite piezometry, as well as calcite piezometry on marble mylonites, should provide further constraints on stress and strain rates along the length and depth exposures of this intrabatholithic shear zone

Quaternary reactivation of the Kern Canyon fault system, southern Sierra Nevada, California

The Kern Canyon fault, the longest fault in the southern Sierra Nevada, is an active structure and has been reactivated at discrete times over the past 100 m.y. in response to changing lithospheric stresses. After initiation as a Cretaceous transpressional structure, the Kern Canyon fault transitioned into a dextral strike-slip shear zone that remained active as it was exhumed into the brittle regime during regional Late Cretaceous uplift of the Sierra Nevada batholith. The Kern Canyon fault was reactivated during Miocene regional extension as part of a transfer zone between two differentially extending domains in the southern Sierra Nevada. Subsequent normal displacement along the fault began in Pliocene time. New evidence for fault activity, which continued into late Quaternary time, includes its current geomorphic expression as a series of meters-high, west-side-up scarps that crop out discontinuously along the fault's 130-km length. Relocated focal mechanisms of modern earthquakes confirm ongoing normal faulting, and geodetic measurements suggest that the Sierra Nevada is uplifting relative to the adjacent valleys. This evidence for recent activity overturns a long-held view that the Kern Canyon fault has been inactive for more than 3.5 m.y. Its reactivation indicates that deformation repeatedly localized along a preexisting crustal weakness, a Cretaceous shear zone. We propose that a system of interrelated normal faults, including the Kern Canyon fault, is responding to mantle lithosphere removal beneath the southern Sierra Nevada region. The location of the active Kern Canyon fault within the Sierra Nevada–Great Valley microplate indicates that deformation is occurring within the microplate

Disruption of regional primary structure of the Sierra Nevada batholith by the Kern Canyon fault system, California

Regional spatial variation patterns in igneous emplacement pressures, initial ^(87)Sr/^(86)Sr (Sr_i) values, zircon U/Pb ages, and pluton bulk compositions of the Sierra Nevada batholith are disrupted by the ~130-km-long proto–Kern Canyon fault, a Late Cretaceous ductile shear zone in the southern Sierra Nevada batholith. Vertical displacement and horizontal shortening across the proto–Kern Canyon fault in its early history are roughly constrained by the disruption of a regional primary batholithic structure that is recorded in petrologic and geochemical spatial variation patterns. The disruption of these patterns suggests that the proto–Kern Canyon fault underwent (1) subvertical west-directed reverse faulting that was instrumental in the exhumation and deep exposure of the southern part of the Sierra Nevada batholith, and (2) southward-increasing reverse/thrust displacement. The disruption of otherwise smoothly varying geobarometric gradients across the central part of the proto–Kern Canyon fault suggests up to ~10 ± 5 km of east-side-up reverse displacement across the shear zone. Southward from this area, the proto–Kern Canyon fault truncates, at an oblique angle, the petrologically distinct axial zone of the Sierra Nevada batholith, which suggests that up to ~25 km of normal shortening occurred across the southern part of the proto–Kern Canyon fault. Normal shortening is further supported by the coincidence of the Sr_i = 0.706 isopleth with the proto–Kern Canyon fault from the point of initial truncation southward. Zircon U/Pb ages from plutons emplaced along the shear zone during its activity indicate that this shortening and vertical displacement had commenced by 95 Ma and was abruptly overprinted by dominantly dextral displacement with small east-side-up reverse components by 90 Ma. Conventional structural and shear fabric analyses, in conjunction with geochronological data, indicate that at least ~15 km of dextral shear slip occurred along the zone between 90 and 86 Ma, and another 12 ± 1 km of dextral slip occurred along the northern segment of the zone between 86 and 80 Ma. This later 12 ± 1 km of dextral slip branched southwestward as the ductile-brittle Kern Canyon fault, abandoning the main trace of the shear zone near its central section. Dextral shearing in the ductile regime was replaced by brittle overprinting by 80 Ma. The timing of initiation and the duration of reverse-sense displacement along the proto–Kern Canyon fault correspond closely with the shallow flat subduction of the Franciscan-affinity Rand schist along the Rand fault beneath the southernmost Sierra Nevada batholith. In its southern reaches, the proto–Kern Canyon fault flattens into the Rand fault system, suggesting that it behaved like a lateral ramp. Post–90 Ma dextral shear along the proto–Kern Canyon fault is suggested to have partitioned at least part of the Farallon plate’s tangential relative displacement component during an increase in subduction obliquity. Late-stage dextral ductile shear and early phase brittle overprints on the Kern Canyon fault system are coeval with tectonic denudation of the southernmost Sierra Nevada batholith. Geometric relations of the system’s terminal ductile and early brittle history with orthogonal extensional structures pose the possibility that the southern segment of the proto–Kern Canyon fault, along with the younger Kern Canyon fault, behaved as a transfer system during the extensional phases of tectonic denudation of the southernmost Sierra Nevada batholith, leading to exposure of the oblique crustal section we see today

Rock and age relationships within the Talkeetna forearc accretionary complex in the Nelchina area, southern Alaska

Subduction-zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. We report results of field mapping and petrographic, geochemical, and geochronological analyses of McHugh Complex accretionary prism mlange in south-central Alaska that was recently exposed due to retreat of the Nelchina Glacier. Our new mapping and analyses of the mlange, as well as adjacent Talkeetna arc intrusives, suggests that the previously mapped trace of the Border Ranges fault should shift northward in this location. Detailed petrographic analysis places this mlange exposure with the Potter Creek assemblage of the McHugh Complex. Blocks of pillow lavas within the mlange have both MORB and intra-plate geochemical affinity, attesting to the complex relations of subduction-zone inputs in an alternating erosive–accretionary margin. A new zircon U-Pb age and geochemical analyses of a set of felsic dikes that crosscut the accretionary sequence provide constraints on the regional tectonic evolution, including near-trench plutonism associated with the migration of a subducting spreading ridge along the southern Alaska margin in Paleocene–Eocene time. The McHugh section and crosscutting dikes in this location are pervasively hydrothermally altered, which we attribute to elevated temperatures related to ridge subduction. Late-stage motion along the Border Ranges fault system, which is also recorded in the area, may also have contributed to the widespread alteration. Our data indicate that the Talkeetna volcanic arc and associated accretionary prism sediments were in their current configuration by 55 Ma.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Origin and evolution of the White Wolf Fault and the Maricopa Basin (MB), southernmost Great Valley (GV), California

The White Wolf “fault” (WWF) is a long-lived polyphase structural zone that originated during the Late Cretaceous in continuity with the dextral Kern Canyon fault (KCF). Dextral displacement on the WWF-KCF zone increases southwards from zero at ~36.6°N to ~6 km at 36.2°N, ~12 km at ~35.7°N, and ~40 km adjacent to the MB at ~35.2°N. Southward increase in displacement arises from accumulated SSW directed extension along the zones east wall, which rode on a return flow channel in the shallowly underplated Rand schist subduction complex. Late Cretaceous east wall up displacement also increases from zero southwards to ~10 km at ~35.2°N reflecting greater Late Cretaceous exhumation southwards along the extended wall. The MB is an up to ~10 km deep wedge-shaped basin bounded on the southeast by the WWF, northeast by normal faults of the Kern range front (KRF), and to the west by the Coast Range fold belt. Subcrop mapping of the eastern slopes of the southern GV using basement core and geophysical data indicates that western Foothills belt ophiolitic, metaclastic and cross-cutting Sierran batholithic rocks extend southwards along the eastern half of the GV to the WWF. Southwest of the KRF and across the eastern slope of the MB, basement rocks of the Foothills belt are abruptly transposed into mylonites, chlorite breccias and cataclasites. The basement surface here is a W-dipping detachment fault that was nonconformably overlapped by lower Cenozoic marine strata. Thermochronological data along the KRF and WWF constrain the detachment to have been active in the Late Cretaceous. Southwest displaced upper plate Foothills belt rocks are exposed as an inlier beneath Paleocene marine strata in the western San Emigdio range, having emerged as a result of active north directed folding and thrusting. The WWF-KCF zone was remobilized in the early Neogene as a transfer system that again partitioned NE-SW directed extension, but of lower magnitude, between the MB and the zones east wall. Quaternary north directed thrusts rooted beneath the Tehachapi-San Emigdio Range, which are commonly regarded as the WWF, are ramping northward above the WWF basement structure. The unique configuration of the WWF-KCF zone and the MB arises from their position above a regional lateral ramp in the Rand-Franciscan subduction megathrust system

Global economic burden of unmet surgical need for appendicitis

Background There is a substantial gap in provision of adequate surgical care in many low- and middle-income countries. This study aimed to identify the economic burden of unmet surgical need for the common condition of appendicitis. Methods Data on the incidence of appendicitis from 170 countries and two different approaches were used to estimate numbers of patients who do not receive surgery: as a fixed proportion of the total unmet surgical need per country (approach 1); and based on country income status (approach 2). Indirect costs with current levels of access and local quality, and those if quality were at the standards of high-income countries, were estimated. A human capital approach was applied, focusing on the economic burden resulting from premature death and absenteeism. Results Excess mortality was 4185 per 100 000 cases of appendicitis using approach 1 and 3448 per 100 000 using approach 2. The economic burden of continuing current levels of access and local quality was US $92 492 million using approach 1 and$73 141 million using approach 2. The economic burden of not providing surgical care to the standards of high-income countries was $95 004 million using approach 1 and$75 666 million using approach 2. The largest share of these costs resulted from premature death (97.7 per cent) and lack of access (97.0 per cent) in contrast to lack of quality. Conclusion For a comparatively non-complex emergency condition such as appendicitis, increasing access to care should be prioritized. Although improving quality of care should not be neglected, increasing provision of care at current standards could reduce societal costs substantially

Global economic burden of unmet surgical need for appendicitis

Background There is a substantial gap in provision of adequate surgical care in many low- and middle-income countries. This study aimed to identify the economic burden of unmet surgical need for the common condition of appendicitis. Methods Data on the incidence of appendicitis from 170 countries and two different approaches were used to estimate numbers of patients who do not receive surgery: as a fixed proportion of the total unmet surgical need per country (approach 1); and based on country income status (approach 2). Indirect costs with current levels of access and local quality, and those if quality were at the standards of high-income countries, were estimated. A human capital approach was applied, focusing on the economic burden resulting from premature death and absenteeism. Results Excess mortality was 4185 per 100 000 cases of appendicitis using approach 1 and 3448 per 100 000 using approach 2. The economic burden of continuing current levels of access and local quality was US $92 492 million using approach 1 and$73 141 million using approach 2. The economic burden of not providing surgical care to the standards of high-income countries was $95 004 million using approach 1 and$75 666 million using approach 2. The largest share of these costs resulted from premature death (97.7 per cent) and lack of access (97.0 per cent) in contrast to lack of quality. Conclusion For a comparatively non-complex emergency condition such as appendicitis, increasing access to care should be prioritized. Although improving quality of care should not be neglected, increasing provision of care at current standards could reduce societal costs substantially