Geophysical Research Letters
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Kinematics of Mass Transport Deposits revealed by magnetic fabrics Abstract The internal deformation and movement directions of Mass Transport Deposits (MTDs) are key factors in understanding the kinematics and dynamics of their emplacement. Although these are relatively easy to recover from well‐bedded sediments, they are more difficult to deduce from massive beds without visible strain markers. In order to test the applicability of using anisotropy of magnetic susceptibility (AMS) to determine MTD movement, we compare AMS fabrics, with structural measurements of visible kinematic indicators. Our case study involves the structural analysis of slumped lake sediments extensively exposed in MTDs within the Dead Sea Basin. Structural analyses of MTDs outcropping for >100 km reveal radial transport directions toward the basin depocenter. We show that the AMS fabrics display the same transport directions as inferred from structural analyses. Based on this similarity, we outline a robust procedure to obtain the transport direction of slumped MTDs from AMS fabrics. Variations in the magnetic fabrics and anisotropies in fold‐thrust systems within the slumps match the various structural domains. We therefore suggest that magnetic fabrics and anisotropy variations in drill cores may reflect internal deformation within the slumps rather than different slumps. Obtaining magnetic fabrics from MTDs provides a viable way to infer the transport directions and internal deformation of MTDs and reconstruct the basin depocenter in ancient settings. The present results also have implications beyond the kinematics of MTDs, as their geometry resembles fold‐thrust systems in other geological settings, scales, and tectonic environments.
Geophysical Research Letters - Tập 44 Số 15 - Trang 7743-7749 - 2017
Epicenter distribution and magnitude of earthquakes in fold‐thrust belts: Insights from Sandbox Models Scaled analogue models are used to illustrate the effect of basal friction and erosion on fault activity and hence on epicentre distribution and magnitude of earthquakes in the sedimentary cover of active fold‐thrust belts. Model results suggest that in fold‐thrust belts shortened above low‐friction ductile decollements (rock salt or over‐pressured mudstone), low‐ to moderate‐magnitude earthquakes (Mw = 5.3–5.6), distributed over a wide area, occur along several long‐lived thrust faults. Conversely, in areas shortened above high‐friction decollements large‐magnitude earthquakes (Mw = 6.6–6.8), distributed over a narrow zone are likely to occur along few short‐lived thrust ramps. Calculated magnitude of earthquakes from models and their distribution are in agreement with recorded earthquake pattern from the Zagros mountain belt, which is partially shortened above a ductile decollement of Hormuz salt formation. Model results also showed that erosion reactivates older inactive thrusts and promotes formation of out‐of‐sequence thrusts.
Geophysical Research Letters - Tập 27 Số 2 - Trang 273-276 - 2000
Crustal xenolith magnetic properties and long wavelength anomaly source requirements Granulite xenoliths, probable components of the lower continental crust, are a primary source of information about the magnetization of the lower crust. Magnetization values for lower crustal xenoliths from three tectonic settings (converging plate margin, rift valley, and continental intraplate region) demonstrate that metabasic rocks in the granulite facies have magnetization values consistent with magnetizations inferred for modeled sources of long wavelength anomalies. Measured Curie points for granulite xenoliths are near 560‐570°C except for those from rift zones and other regions where anhydrous, probably reducing lower crustal conditions exist in a steep geothermal gradient. In samples from the reducing environments Curie points <300°C are measured. The lower crust could be the most magnetic crustal layer. Satellite magnetic anomalies may serve to delineate magnetization provinces which may be related to the tectonic and chemical evolution of continents.
Geophysical Research Letters - Tập 9 Số 4 - Trang 329-332 - 1982
The Moho as a magnetic boundary Mantle derived rocks — peridotites from St. Pauls rocks, dunite xenoliths from the Kaupulehu flow in Hawaii, as well as peridotite, dunite, and eclogite xenoliths from Roberts Victor, Dutoitspan, Kilbourne Hole, and San Carlos diatremes — are weakly magnetic at room temperature. Saturation magnetization values range from 0.013 emu/gm to <0.001 emu/gm equivalent to ∼0.01 to 0.001 wt % Fe3 O4 . A review of pertinent literature dealing with analysis of the minerals in mantle xenoliths provides evidence that metals and primary Fe3 O4 are absent, and that complex Cr, Mg, Al, Fe spinels dominate the oxide mineralogy. These spinels would be non‐magnetic at mantle temperatures. The crust/mantle boundary can be specified as a magnetic mineralogy discontinuity. Curie depth estimates from aeromagnetic anomalies do not require a source of magnetization in the mantle. All of the available evidence supports the new magnetic results, indicating that the seismic Moho is a magnetic boundary. The source of magnetization is in the crust and the maximum Curie isotherm depends on the type of magnetic mineralogy and is located at depths which will vary with the regional geothermal gradient.
Geophysical Research Letters - Tập 6 Số 7 - Trang 541-544 - 1979
Premature mortality in India due to PM<sub>2.5</sub> and ozone exposure Abstract This bottom‐up modeling study, supported by new population census 2011 data, simulates ozone (O3 ) and fine particulate matter (PM2.5 ) exposure on local to regional scales. It quantifies, present‐day premature mortalities associated with the exposure to near‐surface PM2.5 and O3 concentrations in India using a regional chemistry model. We estimate that PM2.5 exposure leads to about 570,000 (CI95: 320,000–730,000) premature mortalities in 2011. On a national scale, our estimate of mortality by chronic obstructive pulmonary disease (COPD) due to O3 exposure is about 12,000 people. The Indo‐Gangetic region accounts for a large part (~42%) of the estimated mortalities. The associated lost life expectancy is calculated as 3.4 ± 1.1 years for all of India with highest values found for Delhi (6.3 ± 2.2 years). The economic cost of estimated premature mortalities associated with PM2.5 and O3 exposure is about 640 (350–800) billion USD in 2011, which is a factor of 10 higher than total expenditure on health by public and private expenditure.
Geophysical Research Letters - Tập 43 Số 9 - Trang 4650-4658 - 2016
Electron Vorticity Indicative of the Electron Diffusion Region of Magnetic Reconnection Abstract While vorticity defined as the curl of the velocity has been broadly used in fluid and plasma physics, this quantity has been underutilized in space physics due to low time resolution observations. We report Magnetospheric Multiscale (MMS) observations of enhanced electron vorticity in the vicinity of the electron diffusion region of magnetic reconnection. On 11 July 2017 MMS traversed the magnetotail current sheet, observing tailward‐to‐earthward outflow reversal, current‐carrying electron jets in the direction along the electron meandering motion or out‐of‐plane direction, agyrotropic electron distribution functions, and dissipative signatures. At the edge of the electron jets, the electron vorticity increased with magnitudes greater than the electron gyrofrequency. The out‐of‐plane velocity shear along distance from the current sheet leads to the enhanced vorticity. This, in turn, contributes to the magnetic field perturbations observed by MMS. These observations indicate that electron vorticity can act as a proxy for delineating the electron diffusion region of magnetic reconnection.
Geophysical Research Letters - Tập 46 Số 12 - Trang 6287-6296 - 2019
Intermittent energy dissipation by turbulent reconnection Abstract Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the “diffusion region” at the sub‐ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O‐lines) and the separatrices. Inside each current filament, kinetic‐scale turbulence is significantly increased and the energy dissipation, E′ ⋅ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X‐lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O‐lines but not X‐lines.
Geophysical Research Letters - Tập 44 Số 1 - Trang 37-43 - 2017
Electron Diffusion Regions in Magnetotail Reconnection Under Varying Guide Fields Abstract Kinetic structures of electron diffusion regions (EDRs) under finite guide fields in magnetotail reconnection are reported. The EDRs with guide fields 0.14–0.5 (in unit of the reconnecting component) are detected by the Magnetospheric Multiscale spacecraft. The key new features include the following: (1) cold inflowing electrons accelerated along the guide field and demagnetized at the magnetic field minimum while remaining a coherent population with a low perpendicular temperature, (2) wave fluctuations generating strong perpendicular electron flows followed by alternating parallel flows inside the reconnecting current sheet under an intermediate guide field, and (3) gyrophase bunched electrons with high parallel speeds leaving the X‐line region. The normalized reconnection rates for the three EDRs range from 0.05 to 0.3. The measurements reveal that finite guide fields introduce new mechanisms to break the electron frozen‐in condition.
Geophysical Research Letters - Tập 46 Số 12 - Trang 6230-6238 - 2019
On the electron diffusion region in asymmetric reconnection with a guide magnetic field Abstract Particle‐in‐cell simulations in a 2.5‐D geometry and analytical theory are employed to study the electron diffusion region in asymmetric reconnection with a guide magnetic field. The analysis presented here demonstrates that similar to the case without guide field, in‐plane flow stagnation and null of the in‐plane magnetic field are well separated. In addition, it is shown that the electric field at the local magnetic X point is again dominated by inertial effects, whereas it remains dominated by nongyrotropic pressure effects at the in‐plane flow stagnation point. A comparison between local electron Larmor radii and the magnetic gradient scale lengths predicts that distribution should become nongyrotropic in a region enveloping both field reversal and flow stagnation points. This prediction is verified by an analysis of modeled electron distributions, which show clear evidence of mixing in the critical region.
Geophysical Research Letters - Tập 43 Số 6 - Trang 2359-2364 - 2016
Time domain structures: What and where they are, what they do, and how they are made Abstract Time domain structures (TDS) (electrostatic or electromagnetic electron holes, solitary waves, double layers, etc.) are ≥1 ms pulses having significant parallel (to the background magnetic field) electric fields. They are abundant through space and occur in packets of hundreds in the outer Van Allen radiation belts where they produce magnetic‐field‐aligned electron pitch angle distributions at energies up to a hundred keV. TDS can provide the seed electrons that are later accelerated to relativistic energies by whistlers and they also produce field‐aligned electrons that may be responsible for some types of auroras. These field‐aligned electron distributions result from at least three processes. The first process is parallel acceleration by Landau trapping in the TDS parallel electric field. The second process is Fermi acceleration due to reflection of electrons by the TDS. The third process is an effective and rapid pitch angle scattering resulting from electron interactions with the perpendicular and parallel electric and magnetic fields of many TDS. TDS are created by current‐driven and beam‐related instabilities and by whistler‐related processes such as parametric decay of whistlers and nonlinear evolution from oblique whistlers. New results on the temporal relationship of TDS and particle injections, types of field‐aligned electron pitch angle distributions produced by TDS, the mechanisms for generation of field‐aligned distributions by TDS, the maximum energies of field‐aligned electrons created by TDS in the absence of whistler mode waves, TDS generation by oblique whistlers and three‐wave‐parametric decay, and the correlation between TDS and auroral particle precipitation, are presented.
Geophysical Research Letters - Tập 42 Số 10 - Trang 3627-3638 - 2015
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