Showing papers in "Journal of Geophysical Research in 1993"

Isoprene and monoterpene emission rate variability: Model evaluations and sensitivity analyses

Abstract: The temperature dependence of monoterpene emission varies among monoterpenes, plant species, and other factors, but a simple exponential relationship between emission rate (E) and leaf temperature (T), E = Es [exp (β(T - Ts))], provides a good approximation. A review of reported measurements suggests a best estimate of β = 0.09 K-1 for all plants and monoterpenes. Isoprene emissions increase with photosynthetically active radiation up to a saturation point at 700-900 μmol m-2 s-1. An exponential increase in isoprene emission is observed at leaf temperatures of less than 30°C. Emissions continue to increase with higher temperatures until a maximum emission rate is reached at about 40°C, after which emissions rapidly decline. This temperature dependence can be described by an enzyme activation equation that includes denaturation at high temperature. -from Authors

Precise U‐Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System

Abstract: Precise resolution of the timing of igneous activity is crucial to understanding the dynamic processes associated with continental rifting. Mafic intrusive rocks constitute a major portion of the exposed 1100 Ma (Keweenawan) Midcontinent Rift system in northeastern Minnesota; however, prior to this study, geochronological data were insufficient to allow rigorous interpretation of intrusive histories and their relationships to extrusive suites. Eight anorthositic and gabbroic intrusives were chosen to represent both the temporal and spatial ranges of plutonic activity that formed the Duluth Complex and related mafic intrusions. U-Pb isotopic analyses from zircons and baddeleyites result in U-Pb concordant ages with little or no ambiguity introduced by inherited components, Pb loss or common Pb. The earliest Keweenawan plutonism exposed in Minnesota occurs along the northeastern flank of the Duluth Complex as a series of layered gabbros (Nathan's layered series) emplaced at 1106.9 ± 0.6 Ma. This sequence of gabbro sheets shares temporal, spatial, and compositional similarities with the nearby Logan sills in Ontario. Four Duluth Complex anorthositic and troctolitic series samples from widely separated areas have unresolvable ages between 1099.3 ± 0.3 and 1098.6 ± 0.5 Ma, indicating a very short duration for peak intrusive activity (0.5–1 m.y.). The unresolvable ages between anorthositic and troctolitic plutons suggest that these two magma series are more closely related than previously modeled and argue strongly for the need to reexamine their fundamental petrogenetic relationships. These dates also imply that the major reverse-to-normal magnetic polarity switch, used throughout the rift system as an important correlation tool, occurred prior to 1099 Ma. This date is several million years earlier than previously suspected and emphasizes the need for further paleomagnetic and geochronological data from the overlying volcanics. Much of the hypabyssal intrusive suite within the volcanic pile overlying Duluth Complex plutons may be significantly younger than the main pulse of plutonic activity. Two hypabyssal bodies, the Sonju Lake intrusion and gabbro at Silver Bay, were emplaced at 1096.1 ± 0.8 Ma and 1095.8 ± 1.2 Ma, respectively. Dates reported here and in previous studies support the concept of episodic tectonomagmatic rift development where magmatism was apparently concentrated in episodes of short duration (<1–3 m.y.) interspersed with longer hiatuses (2–8 m.y.).

A model for the motion of the Philippine Sea Plate consistent with NUVEL‐1 and geological data

Abstract: We investigate angular velocity vectors of the Philippine Sea (PH) plate relative to the adjacent major plates, Eurasia (EU) and Pacific (PA), and the smaller Caroline (CR) plate. Earthquake slip vector data along the Philippine Sea plate are inverted, subject to the constraint that EU-PA motion equals that predicted by the global relative plate model NUVEL-1. The resulting solution fails to satisfy geological constraints along the Caroline-Pacific boundary: convergence along the Mussau Trench and divergence along the Sorol Trough. We then seek solutions satisfying both the CR-PA boundary conditions and the Philippine Sea slip vector data, by adjusting the PA-PH and EU-PH best fitting poles within their error ellipses. We also consider northern Honshu to be part of the North American plate and impose the constraint that the Philippine Sea plate subducts beneath northern Honshu along the Sagmi Trough in a NNW-NW direction. Of the solutions satisfying these conditions, we select the best EU-PH as 48.2 deg N, 157.0 deg E, 1.09 deg/my, corresponding to a pole far from Japan and south of Kamchatka, and PA-PH, 1.2 deg N, 134.2 deg E, 1.00 deg/my. Predicted NA-PH and EU-PH convergence rates in central Honshu are consistent with estimated seismic slip rates. Previous estimates of the EU-PH pole close to central Honshu are inconsistent with extension within the Bonin backarc implied by earthquake slip vectors and NNW-NW convergence of the Bonin forearc at the Sagami Trough.

Updated interpretation of magnetic anomalies and seafloor spreading stages in the south China Sea: Implications for the Tertiary tectonics of Southeast Asia

Abstract: We present the interpretation of a new set of closely spaced marine magnetic profiles that complements previous data in the northeastern and southwestern parts of the South China Sea (Nan Hai). This interpretation shows that seafloor spreading was asymmetric and confirms that it included at least one ridge jump. Discontinuities in the seafloor fabric, characterized by large differences in basement depth and roughness, appear to be related to variations in spreading rate. Between anomalies 11 and 7 (32 to 27 Ma), spreading at an intermediate, average full rate of ≈50 mm/yr created relatively smooth basement, now thickly blanketed by sediments. The ridge then jumped to the south and created rough basement, now much shallower and covered with thinner sediments than in the north. This episode lasted from anomaly 6b to anomaly 5c (27 to ≈16 Ma) and the average spreading rate was slower, ≈35 mm/yr. After 27 Ma, spreading appears to have developed first in the eastern part of the basin and to have propagated towards the southwest in two major steps, at the time of anomalies 6b-7, and at the time of anomaly 6. Each step correlates with a variation of the ridge orientation, from nearly E-W to NE-SW, and with a variation in the spreading rate. Spreading appears to have stopped synchronously along the ridge, at about 15.5 Ma. From computed fits of magnetic isochrons, we calculate 10 poles of finite rotation between the times of magnetic anomalies 11 and 5c. The poles permit reconstruction of the Oligo-Miocene movements of Southeast Asian blocks north and south of the South China Sea. Using such reconstructions, we test quantitatively a simple scenario for the opening of the sea in which seafloor spreading results from the extrusion of Indochina relative to South China, in response to the penetration of India into Asia. This alone yields between 500 and 600 km of left-lateral motion on the Red River-Ailao Shan shear zone, with crustal shortening in the San Jiang region and crustal extension in Tonkin. The offset derived from the fit of magnetic isochrons on the South China Sea floor is compatible with the offset of geological markers north and south of the Red River Zone. The first phases of extension of the continental margins of the basin are probably related to motion on the Wang Chao and Three Pagodas Faults, in addition to the Red River Fault. That Indochina rotated at least 12° relative to South China implies that large-scale “domino” models are inadequate to describe the Cenozoic tectonics of Southeast Asia. The cessation of spreading after 16 Ma appears to be roughly synchronous with the final increments of left-lateral shear and normal uplift in the Ailao Shan (18 Ma), as well as with incipient collisions between the Australian and the Eurasian plates. Hence no other causes than the activation of new fault zones within the India-Asia collision zone, north and east of the Red River Fault, and perhaps increased resistance to extrusion along the SE edge of Sundaland, appear to be required to terminate seafloor spreading in the largest marginal basin of the western Pacific and to change the sense of motion on the largest strike-slip fault of SE Asia.

Internal structure and weakening mechanisms of the San Andreas Fault

Abstract: New observations of the internal structure of the San Gabriel fault (SGF) are combined with previous characterizations of the Punchbowl fault (PF) to evaluate possible explanations for the low frictional strength and seismic characteristics of the San Andreas fault (SAF). The SGF and PF are ancient, large-displacement faults of the SAF system exhumed to depths of 2 to 5 km. These fault zones are internally zoned; the majority of slip was confined to the cores of principal faults, which typically consist of a narrow layer (less than tens of centimeters) of ultracataclasite within a zone of foliated cataclasite several meters thick. Each fault core is bounded by a zone of damaged host rock of the order of 100 m thick. Orientations of subsidiary faults and other fabric elements imply that (1) the maximum principal stress was oriented at large angles to principal fault planes, (2) strain was partitioned between simple shear in the fault cores and nearly fault-normal contraction in the damaged zones and surrounding host rock, and (3) the principal faults were weak. Microstructures and particle size distributions in the damaged zone of the SGF imply deformation was almost entirely cataclastic and can be modeled as constrained comminution. In contrast, cataclastic and fluid-assisted processes were significant in the cores of the faults as shown by pervasive syntectonic alteration of the host rock minerals to zeolites and clays and by folded, sheared, and attenuated cross-cutting veins of laumontite, albite, quartz, and calcite. Total volume of veins and neocrystallized material reaches 50% in the fault core, and vein structure implies episodic fracture and sealing with time-varying and anisotropic permeability in the fault zone. The structure of the ultracataclasite layer reflects extreme slip localization and probably repeated reworking by particulate flow at low effective stresses. The extreme slip localization reflects a mature internal fault structure resulting from a positive feedback between comminution and transformation weakening. The structural, mechanical, and hydrologic characteristics of the Punchbowl and San Gabriel faults support the model for a weak San Andreas based on inhomogeneous stress and elevated pore fluid pressures contained within the core of a seismogenic fault. Elevated fluid pressures could be repeatedly generated in the core of the fault by a combination of processes including coseismic dilatancy and creation of fracture permeability, fault-valve behavior to recharge the fault with fluid, post-seismic self-sealing of fracture networks to reduce permeability and trap fluids, and time-dependent compaction of the core to generate high pore pressure. The localized slip and fluid-saturated conditions are wholly compatible with additional dynamic weakening by thermal pressurization of fluids during large seismic slip events, which can help explain both the low average strength of the San Andreas and seismogenic characteristics such as large stress relief. In addition, such a dynamic weakening mechanism is expected only in mature fault zones and thus could help explain the apparent difference in strength of large-displacement faults from smaller-displacement, subsidiary seismogenic faults.

The Halogen Occultation Experiment

Abstract: The Halogen Occultation Experiment (HALOE) uses solar occultation to measure vertical profiles of O3, HCl, HF, CH4, H2O, NO, NO2, aerosol extinction, and temperature versus pressure with an instantaneous vertical field of view of 1.6 km at the earth limb. Latitudinal coverage is from 80 deg S to 80 deg N over the course of 1 year and includes extensive observations of the Antarctic region during spring. The altitude range of the measurements extends from about 15 km to about 60-130 km, depending on channel. Experiment operations have been essentially flawless, and all performance criteria either meet or exceed specifications. Internal data consistency checks, comparisons with correlative measurements, and qualitative comparisons with 1985 atmospheric trace molecule spectroscopy (ATMOS) results are in good agreement. Examples of pressure versus latitude cross sections and a global orthographic projection for the September 21 to October 15, 1992, period show the utility of CH4, HF, and H2O as tracers, the occurrence of dehydration in the Antarctic lower stratosphere, the presence of the water vapor hygropause in the tropics, evidence of Antarctic air in the tropics, the influence of Hadley tropical upwelling, and the first global distribution of HCl, HF, and NO throughout the stratosphere. Nitric oxide measurements extend through the lower thermosphere.

Spatio-temporal complexity of slip on a fault

Abstract: Three-dimensional analyses are reported of slip on a long vertical strike-slip fault between steadily driven elastic crustal blocks. A rate- and state-dependent friction law governs motion on the fault; the law includes a characteristic slip distance L for evolution of surface state and slip weakening. Because temperature and normal stress vary with depth, frictional constitutive properties (velocity weakening/ strengthening) do also. Those properties are taken either as uniform along-strike at every depth or as perturbed modestly from uniformity. The governing equations of quasi-static elasticity and frictional slip are solved on a computational grid of cells as a discrete numerical system, and a viscous radiation damping term is included to approximately represent inertial control of slip rates during earthquakelike instabilities. The numerical results show richly complex slip, with a spectrum of event sizes, when solved for a grid with oversized cells, that is, with cell size h that is too large to validly represent the underlying continuous system of equations. However, in every case for which it has been feasible to do the computations (moderately large L only), that spatio-temporally complex slip disappears in favor of simple limit cycles of periodically repeated large earthquakes with reduction of cell size h. Further study will be necessary to determine whether a similar trhnsition occurs when the elastodynamics of rupture propagation is treated more exactly, rather than in the radiation damping approximation. The transition from complex to ordered fault response occurs as h is reduced below a theoretically derived nucleation size h* which scales with L but is 2 x 104 to 105 larger in cases considered. Cells larger than h* can fail independently of one another, whereas those much smaller than h* cannot slip unstably alone and can do so only as part of a cooperating group of cells. The results contradict an emergent view that spatio-temporal complexity is a generic feature of mechanical fault models. Such generic complexity does apparently result from models which are inherently discrete in the sense of having no well-defined continuum limit as h diminishes. Those models form a different class of dynamical systems from models like the present one that do have a continuum limit. Strongly oversized cells cause the model developed here to mimic an inherently discrete system. It is suggested that oversized cells, capable of failing independently of one another, may crudely represent geometrically disordered fault zones, with quasi-independent fault segments that join one another at kinks or jogs. Such geometric disorder, at scales larger than h*, may force a system with a well-defined continuum limit to mimic an inherently discrete system and show spatio-temporally complex slip at those larger scales.

Stratospheric aerosol optical depths, 1850–1990

Abstract: A global stratospheric aerosol database employed for climate simulations is described. For the period 1883-1990, aerosol optical depths are estimated from optical extinction data, whose quality increases with time over that period. For the period 1850-1882, aerosol optical depths are more crudely estimated from volcanological evidence for the volume of ejecta from major known volcanoes. The data set is available over Internet.

The solar flare myth

Abstract: Many years of research have demonstrated that large, nonrecurrent geomagnetic storms, shock wave disturbances in the solar wind, and energetic particle events in interplanetary space often occur in close association with large solar flares. This result has led to a pradigm of cause and effect - that large solar flares are the fundamental cause of these events in the near-Earth space environmemt. This paradigm, which I call 'the solar flare myth,' dominates the popular perception of the relationship between solar activity and interplanetary and geomagnetic events and has provided much of the pragmatic rationale for the study of the solar flare phenomenon. Yet there is good evidence that this paradigm is wrong and that flares do not generally play a central role in producing major transient disturbances in the near-Earth space environment. In this paper I outline a different paradigm of cause and effect that removes solar flares from their central position in the chain of events leading from the Sun to near-Earth space. Instead, this central role is given to events known as coronal mass ejections.

A model for the hydrologic and climatic behavior of water on Mars

Abstract: An analysis is carried out of the hydrologic response of a water-rich Mars to climate change and to the physical and thermal evolution of its crust, with particular attention given to the potential role of the subsurface transport, assuming that the current models of insolation-driven change describe reasonably the atmospheric leg of the planet's long-term hydrologic cycle. Among the items considered are the thermal and hydrologic properties of the crust, the potential distribution of ground ice and ground water, the stability and replenishment of equatorial ground ice, basal melting and the polar mass balance, the thermal evolution of the early cryosphere, the recharge of the valley networks and outflow, and several processes that are likely to drive the large-scale vertical and horizontal transport of H2O within the crust. The results lead to the conclusion that subsurface transport has likely played an important role in the geomorphic evolution of the Martian surface and the long-term cycling of H2O between the atmosphere, polar caps, and near-surface crust.

Active thrusting and folding along the northern Tien Shan and Late Cenozoic rotation of the Tarim relative to Dzungaria and Kazakhstan

Abstract: We have studied geometries and rates of late Cenozoic thrust faulting and folding along the northern piedmont of the Tien Shan mountain belt, West of Urumqi, where the M= 8.3 Manas earthquake occurred on December 23, 1906. The northern range of the Tien Shan, rising above 5000 m, overthrusts a flexural foredeep, filled with up to 11,000 m of sediment, of the Dzungarian basement. Our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front, within a 200-km-long zone of Neogene-Quaternary anticlines. Fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range. In all the valleys, the scarps offset vertically the highest terrace surface by the same amount (10.2±0.7 m). Inferring an early Holocene age (10±2 kyr) for this terrace, which is continuous with the largest recent fans of the piedmont, yields a rate of vertical throw of 1.0±0.3mm/yr on the main active thrust at the surface. A quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5.5±2.5 m^2/kyr. A rather fresh surface scarp, 0.8±0.15 m high, that is unlikely to result from shallow earthquakes with 6 < M < 7 in the last 230 years, is visible at the extremities of the main fold zone. We associate this scarp with the 1906 Manas earthquake and infer that a structure comprising a deep basement ramp under the range, gently dipping flats in the foreland, and shallow ramps responsible for the formation of the active, fault propagation anticlines could have been activated by that earthquake. If so, the return period of a 1906 type event would be 850 ±380 years. The small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth (≈2/3) is taken up by incremental folding near the surface. Comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising Quaternary ranges such as the San Gabriel mountains in California or the Mont Blanc-Aar massifs in the Alps. We estimate the finite Cenozoic shortening of the folded Dzungarian sediments to be of the order of 30 km and the Cenozoic shortening rate to have been 3 ± 1.5 mm/yr. Assuming comparable shortening along the Tarim piedmont and minor additional active thrusting within the mountain belt, we infer the rate of shortening across the Tien Shan to be at least 6 ± 3 mm/yr at the longitude of Manas (≈85.5°E). A total shortening of 125±30 km is estimated from crustal thickening, assuming local Airy isostatic equilibrium. Under the same assumption, serial N-S sections imply that Cenozoic shortening across the belt increases westwards to 203±50 km at the longitude of Kashgar (≈ 76 °E), as reflected by the westward increase of the width of the belt. This strain gradient implies a clockwise rotation of Tarim relative to Dzungaria and Kazakhstan of 7±2.5° around a pole located near the eastern extremity of the Tien Shan, west of Hami (≈96°E, 43.5°N), comparable to that revealed by paleomagnetism between Tarim and Dzungaria (8.6° ± 8.7°). A 6 mm/yr rate of shortening at the longitude of Manas would imply a rate of rotation of 0.45°/m.y. and would be consistent with a shortening rate of 12 mm/yr north of Kashgar. Taking such values to be representative of Late Cenozoic rates would place the onset of reactivation of the Tien Shan by the India-Asia collision in the early to middle Miocene (16 +22/−9 m.y.), in accord with the existence of particularly thick late Neogene and Quaternary deposits. Such reactivation would thus have started much later than the collision, roughly at the time of the great mid-Miocene changes in tectonic regimes, denudation and sedimentation rates observed in southeast Asia, the Himalayas and the Bay of Bengal, and of the correlative rapid change in seawater Sr isotopic ratio (20 to 15 Ma). Like these other changes, the rise of the Tien Shan might be a distant consequence of the end of Indochina's escape.

Reaction of N2O5 on tropospheric aerosols: Impact on the global distributions of NO x , O3, and OH

Abstract: Using a three-dimensional global model of the troposphere, we show that the heterogeneous reactions of NO 3 and N205 on aerosol particles have a substantial influence on the concentrations of NOx; 03, and OH. Due to these reactions, the modeled yearly average global NOx burden decreases by 50% (80% in winter and 20% in summer). The heterogeneous removal of NOx in the northern hemisphere (NH) is dominated by reactions on aerosols; in the tropics and southern hemisphere (SH), with substantial smaller aerosol concentrations, liquid water clouds can provide an additional sink for N205 and NO 3. During spring in the NH subtropics and at mid-latitudes, O3-concentrations are lowered by 25%. In winter and spring in the subtropics of the NH calculated OH concentrations decreased by up to 30%. Global tropospheric average 03 and OH burden (the latter weighted with the amount of methane reacting with OH) can drop by about 9% each. By including reactions on aerosols, we are better able to simulate observed nitrate wet deposition patterns in North America and Europe. 03 concentrations in springtime smog situations are shown to be affected by heterogeneous reactions, indicating the great importance of chemical interactions resulting from NOx and SO2 emissions. However, a preliminary analysis shows that under present conditions a change in aerosol concentrations due to limited SO2 emission control strategies (e.g., reductions by a factor of 2 in industrial areas) will have only a relatively minor influence on 03 concentrations. Much larger reductions in SO2 emissions may cause larger increases in surface 03 concentrations, up to a maximum of 15%, if they are not accompanied by a reductio!a in NOx or hydrocarbon emission.

Effect of Saltation Bombardment on the Entrainment of Dust by Wind

Abstract: Saltation is the wind-driven, hopping motion of sand-sized particles across an erodible surface. This mode of motion not only transports sand and similar materials in its own right but can also initiate (through bombardment of the surface) the entrainment and subsequent transport by suspension of smaller dust particles. In this paper, we report a wind tunnel study of the effect of saltation bombardment on dust entrainment. The technique is to allow sand grains to saltate from an upwind sand source onto a bed of dust particles. The experiment confirms that the ejection of dust particles by saltation bombardment (as opposed to detachment of dust particles by aerodynamic forces) is the principal mechanism for the natural entrainment of dust by wind. The data are used to examine the dependence of the dust emission flux Fa (mass per unit ground area per unit time) upon the friction velocity u,; it is found that Fa is closely proportional to the streamwise flux of saltating sand grains, which in turn is approximately proportional to u, 3. At a given u,, Fa increases as the size of the bombarding sand grains increases. On the basis of the hypothesis that Fa is proportional to the kinetic energy flux of the saltating sand grains, we derive theoretically the result that Fa scales with the streamwise saltating sand grain flux and thence approximately with u, 3, as observed in this experiment.

Thermal constraints on the zone of major thrust earthquake failure: The Cascadia Subduction Zone

Abstract: Constraints on the seismogenic portion of the subduction thrust zone along the Cascadia margin are provided by the thermal regime. The zone of stick-slip “locked” behavior where earthquakes can nucleate may be limited downdip by a temperature of about 350°C, and the transition stable sliding zone into which coseismic displacement can extend by a temperature of approximately 450°C. The seaward limit of the stick-slip zone may be associated with the dehydration of stable sliding clays at 100 to 150°C and dissipation of high pore pressures in the area of the deformation front. Temperatures on the thrust have been estimated by numerically modelling the thermal regimes along three profiles crossing the margin with constraints provided by surface heat flow and detailed structural information, particularly at southern Vancouver Island. The models that best fit the heat flow data have negligible shear strain heating. The Cascadia subduction margin is unusually hot as a consequence of the very young plate age and the thick insulating sediment section on the incoming plate; the temperature at the top of the oceanic crust at the deformation front is about 250°C. As a result, the modelled zone of stick-slip seismogenic behaviour is restricted to a narrow zone beneath the continental slope and outer shelf, with the transition zone extending to the inner shelf. The seismogenic zone is wider off the Olympic Peninsula compared to off southern Vancouver Island because of the much shallower thrust dip angle and the slightly older incoming plate. The profile off Oregon is found to have intermediate width zones. An important assumption, well justified only off southern Vancouver Island, is that the thrust detachment is located at the top of the downgoing oceanic crust. The same modelling technique shows that more typical subduction zones with older incoming oceanic lithosphere such as central Chile have thermally restricted seismogenic zones that are much wider, commonly extending well beneath the coast. Support for the position of the Cascadia locked zone from the thermal results is provided by a comparison of the horizontal and vertical interseismic deformation predicted by simple dislocation models with the observed rates from tide gauge and geodetic surveys on adjacent coastal regions. The general agreement indicates that any seismic “locked zone” must be located offshore where the subduction thrust fault is less than about 20 km deep and where the contact is between the oceanic crust and the accreted sedimentary wedge, not between the oceanic and continental crusts. The restriction to an offshore zone provides an important limit to the maximum magnitude and to the ground motion and seismic hazard from subduction megathrust earthquakes in southwestern British Columbia, Washington, and Oregon.

Sizes and locations of coronal mass ejections - SMM observations from 1980 and 1984-1989

Abstract: A statistical description of the sizes and locations of 1209 mass ejections observed with the SMM coronagraph/polarimeter in 1980 and 1984-1989 is presented. The average width of the coronal mass ejections detected with this instrument was close to 40 deg in angle for the entire period of SMM observations. No evidence was found for a significant change in mass ejection widths as reported by Howard et al. (1986). There is clear evidence for changes in the latitude distribution of mass ejections over this epoch. Mass ejections occurred over a much wider range of latitudes at the times of high solar activity (1980 and 1989) than at times of low activity (1985-1986).

The genesis of oceanic crust - Magma injection, hydrothermal circulation, and crustal flow

Abstract: In this study we construct a thermal and mechanical model for the genesis of oceanic crust. Magma is halted in its ascent within the oceanic crust when it reaches a freezing horizon, where the dilational volume change associated with magma freezing leads to viscous stresses that favor magma ponding near the freezing horizon. To model the steady state thermal impact of crustal accretion via dike injection and pillow flows, we treat all crustal accretion in rocks cooler than a magma "solidus" to occur in a narrow 250-m-wide dike-like region centered about the ridge axis. The rest of the oceanic crust is modeled to be emplaced as a steady state magma lens directly beneath the "solidus" freezing horizon where the steady state emplacement rate is determined by the constraint that this lens supply all crust that is not emplaced through diking/extrusion above the magma lens. If hydrothermal heat transport within crustal rocks cooler than 600oC removes heat 8 times as efficiently as heat conduction, then we find that a steady state magma lens will only exist within the crust for ridges spreading faster than a 25 mm/yr half rate. The depth dependence of the magma lens with spreading rate is in good agreement with seismic observations. These results suggest that a fairly delicate balance between magmatic heat injection during crustal accretion and hydrothermal heat removal leads to a strongly different crustal thermal structure at fast and slow spreading ridge axes. Our results support the hypothesis that median valley topography is due to extension of strong ridge axis lithosphere; it is the difference in thermal regime that is directly responsible for the striking difference between the typical median valley seen at slow spreading ridges (e.g., Mid-Atlantic Ridge) and the axial high seen at fast spreading ridges (e.g., East Pacific Rise). This paradigm for the origin of a median valley at a slow spreading ridge predicts that along-axis variations in median valley topography of a slow spreading center reflect variations in recent magmatic heat input along a segment, that is, that the axial topography is a good time-averaged indicator of the relative importance of hydrothermal cooling and magmatic injection along a given section of a ridge segment. We determine the accumulated crustal strain associated with lower crustal flow which supports the hypothesis that the Oman Ophiolite crust was created at a paleo-analogue to a fast spreading ridge and also suggests that crustal strain, and not cumulate layering, may be the dominant physical process that generates "layered gabbros" within the Oman Ophiolite.

Emplacement of mantle rocks in the seafloor at mid-ocean ridges

Abstract: This paper discusses the geological and geophysical data available on mid-ocean ridges with outcrops of serpentinized mantle peridotites, with the objective of better constraining the modes of emplacement of these rocks in the seafloor. Ridges with serpentinized peridotites outcrops are in most cases characterized by slow-spreading rates, and in every case by deep axial valleys. Such deep axial valleys are thought, based on geophysical constraints and on mechanical modelling results, to characterize ridges with a thick axial lithosphere. A predictable effect of a thick axial lithosphere is that it should prevent magmas from pooling at crustal depths in a long-lasting magma chamber: gabbroic magmas should instead form shortlived dike or sill-like intrusions. Samples from axial outcrops of serpentinized peridotites are often cut by dikelets of evolved gabbros which are interpreted as apophyses of such dike and sill-like intrusions. This observation leads to a discontinuous magmatic crust model, in which mantle-derived peridotites form screens for numerous gabbroic intrusions. This discontinuous magmatic crust is expected to form in magma-poor ridge regions, where there is not enough magma to produce a 4-to 7-km-thick magmatic crust, and where the uppermost kilometers of oceanic lithosphere therefore have to be at least partially made of tectonically uplifted mantle material. Because the dimensions of individual mantle-derived ultramafic screens may be smaller than seismic experiments detection limits, the discontinuous magmatic crust model discussed in this paper may produce a layer 3-type seismic signature, even without extensive serpentinization of its ultramafic component. It therefore provides an alternative to Hess's [1962] serpentinite layer 3 model, for the geological interpretation of seismic data from oceanic areas with frequent outcrops of deep crustal and mantle-derived rocks.

Coastal Geomorphology of the Martian northern plains

Abstract: The paper considers the question of the formation of the outflow channels and valley networks discovered on the Martian northern plains during the Mariner 9 mission. Parker and Saunders (1987) and Parker et al. (1987, 1989) data are used to describe key features common both in the lower reaches of the outflow channels and within and along the margins of the entire northern plains. It is suggested, that of the geological processes capable of producing similar morphologies on earth, lacustrine or marine deposition and subsequent periglacial modification offer the simplest and most consistent explanation for the suit of features found on Mars.

Coupled surface-atmosphere reflectance (CSAR) model: 2. Semiempirical surface model usable with NOAA advanced very high resolution radiometer data

Abstract: A new semiempirical model to describe the bidirectional reflectance of arbitrary natural surfaces using only three parameters has been developed. This model successfully accounts for the observed variability of reflectance measurements in laboratory and field conditions, ranging from bare soil to full canopy cover, in both the visible and the near-infrared bands. Coupled with a simple atmospheric radiation transfer model, this model has been inverted against actual NOAA/advanced very high resolution radiometer (AVHRR) data from several desert sites in northern Africa. This procedure allows the retrieval of surface properties and average amounts of atmospheric constituents (aerosol optical thickness and water vapor) for the duration of the measurement period. Further work is required to expand the usability of the coupled model to other locations and shorter periods of time, but the paper demonstrates the feasibility of inverting a coupled surface-atmosphere model against existing AVHRR data and documents the current limits of this approach.

Model of the solar wind interaction with the local interstellar medium: Numerical solution of self‐consistent problem

Abstract: A self-consistent gasdynamic model of the solar wind interaction with the local interstellar medium (LISM), which took into account the mutual influence of the plasma component (electrons and protons) of the LISM and the LISM H atoms that penetrate into the heliosphere was constructed by Baranov et al. (1981) in the approximation of axial symmetry. This model, however, had a number of defects. In particular, the motion of the H atoms was described by hydrodynamical equations, although the mean free path of the H atoms and the characteristic length of the problem were comparable. An iterative method, that used a Monte Carlo simulation of H atom motion in the field of the plasma component hydrodynamic parameters, was suggested by Baranov et al. (1991) and only the first step of the iteration was realized (non-self-consistent problem solution). In this paper the results of the self-consistent problem solution for a single set of the undisturbed solar wind and LISM parameters are presented. The structure of the upwind as well as wake regions of the flow is calculated. The geometrical pattern of the flow (bow shock, heliopause, termination shock, Mach disc, etc), the bulk velocity and the number densities of H atoms and plasma component are obtained and analyzed as a function of the distance from the Sun for different values of the polar angle. The effects of resonance charge exchange of the LISM H atoms as well as energetic H atoms “born” in the solar wind are taken into account. It is interesting to note that the effect of H atoms penetrating the solar wind results in the disappearance of the complicated flow structure as well as the supersonic regions between the heliopause and termination shock in the downwind region. In future we are going to compare our theoretical results with the results of Voyager 1/2, Pioneer 10/11, Ulysses spacecraft, and other experiments.

Quaternary uplift of southern Italy

Abstract: Dramatic coastline changes demonstrate rapid Quaternary uplift of Calabria in southern Italy. Because most of the west (Tyrrhenian Sea) coast is normal fault bounded, previous work has asserted that its uplift is local footwall uplift related to extension. However, the east (Ionian Sea) coast is also uplifting but is not normal fault bounded. This reanalysis, based on original fieldwork and reinterpretation of the literature, reaches the following conclusions. First, although radiometric dating of only one marine terrace sequence in Calabria is available, on the east coast at Crotone, terraces occur at similar elevations, and can thus be correlated, throughout this coast and the west coast of northern and central Calabria. Uplift rate at both coasts and across the region in between is the same, 1.0 ± 0.1 mm yr−1, provided my correlations are correct. Second, the oldest raised shorelines date from the 0.9 Ma marine highstand and have uplifted ∼700 m since ∼0.7 Ma. Regional uplift rate earlier was minimal, possibly zero. Third, localities on this west coast show marginally (up to ∼5%) higher uplift rates, indicating some local footwall uplift (at ∼0.05 mm yr−) associated with slow extension (at ∼0.1 mm yr−1). Fourth, the 12-m Holocene marine terrace formed around 7 ka. Allowing for a 3-m range of wave and tidal action, it is interpreted as the result of 7 m of tectonic uplift and 2 m of eustatic sea level fall during ∼6–4 ka. Finally, the west coast of southern Calabria shows significant elevation changes caused by active normal faulting, as well as regional uplift. Footwall localities have uplifted up to 1300 m since 0.9 Ma. The estimated maximum present-day uplift rate of 1.67 mm yr−1 is interpreted as 1 mm yr of regional uplift plus 0.67 mm yr of local footwall uplift, indicating much faster extension than farther north. It is suggested that the Tyrrhenian Benioff zone detached from beneath Calabria shortly before 0.7 Ma. The regional uplift of the overlying landmass is thus explained as its transient isostatic response to removal of this load. Order-of-magnitude calculations suggest that 1 mm yr−1 uplift rate is reasonable, with ∼2 km total uplift expected, indicating that uplift will continue for > ∼1 Myr into the future. Extension of Calabria appears to have begun at ∼11 Ma, at the same time as formation of the Tyrrhenian Sea and the related subduction at its Benioff zone. Extension rate in southern Calabria abruptly increased from ∼0.1 to ∼1 mm yr−1 around 0.9–0.7 Ma., when the slab detached and the regional uplift began; extension in northern and central Calabria continues at the same ∼0.1 mm yr−1 rate as before.

Teleseismic b values; Or, much ado about 1.0

Abstract: In this paper we investigate the value of b in the Gutenberg-Richter relation for four teleseismic catalogs of earthquakes: Abe's historical catalog, the Harvard Centroid Moment Tensor (CMT) catalog, the catalog of the International Seismological Centre (ISC), and the Blacknest catalog. An unfortunate result is that b differs by 30% or more when determined in different magnitude ranges, in different catalogs, or using different methods. For global catalogs separated into shallow, intermediate, and deep earthquake groups, all values determined for b lie between 0.72 and 1.34. We can identify no systematic global variation of b with depth. For teleseismic catalogs it is difficult to believe measured geographic variations in b because systematic errors cause problems of earthquake detection, earthquake location, aftershock identification, and magnitude determination. However, some variations in b are so persistent and large that they must be real. For deep earthquakes in Tonga-Fiji, for example, various measurements of b He between 1.06 and 1.57, comparable to b for shallow earthquakes, whereas measurements of b for deep earthquakes in the rest of the world are much lower, between 0.53 and 0.96. For shallow earthquakes in the Harvard CMT catalog, earthquakes with thrust and strike slip focal mechanisms have significantly lower b values (0.86 and 0.77) than earthquakes with normal faulting mechanisms (1.06). When we separate the ISC catalog into primary events (mainshocks and earthquakes with no aftershocks or foreshocks) and secondary events (aftershocks and foreshocks), we observe that b for secondary events is nearly always significantly higher than b for mainshocks. However, we show that the difference has no physical significance, as it arises simply from the act of choosing mainshocks as the largest earthquake in a foreshock-mainshock-aftershock sequence. When we correct for this systematic effect by comparing the real catalogs to identical catalogs with randomly reassigned magnitudes, we find that b for secondary events in the real catalog is actually lower than expected. Thus among aftershocks large earthquakes are relatively more common than expected, perhaps because the mainshock rupture loads asperities in adjacent regions.

WINDII, the wind imaging interferometer on the Upper Atmosphere Research Satellite

Abstract: The WIND imaging interferometer (WINDII) was launched on the Upper Atmosphere Research Satellite (UARS) on September 12, 1991. This joint project, sponsored by the Canadian Space Agency and the French Centre National d'Etudes Spatiales, in collaboration with NASA, has the responsibility of measuring the global wind pattern at the top of the altitude range covered by UARS. WINDII measures wind, temperature, and emission rate over the altitude range 80 to 300 km by using the visible region airglow emission from these altitudes as a target and employing optical Doppler interferometry to measure the small wavelength shifts of the narrow atomic and molecular airglow emission lines induced by the bulk velocity of the atmosphere carrying the emitting species. The instrument used is an all-glass field-widened achromatically and thermally compensated phase-stepping Michelson interferometer, along with a bare CCD detector that images the airglow limb through the interferometer. A sequence of phase-stepped images is processed to derive the wind velocity for two orthogonal view directions, yielding the vector horizontal wind. The process of data analysis, including the inversion of apparent quantities to vertical profiles, is described.

A geodynamic model of mantle density heterogeneity

Abstract: Using Cenozoic and Mesozoic plate motion reconstructions, we derive a model of present-day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle. The thermal buoyancy of these slabs is estimated from the observed thermal subsidence (cooling) of oceanic lithosphere. Slab velocities in the upper mantle are computed from the local convergence rate. We assume that slabs cross the upper/lower mantle interface and continue sinking into the lower mantle with a reduced velocity. For a velocity reduction factor between 2 and 5, our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other. We have also computed a synthetic geoid from our density model. For a viscosity increase of about a factor of 40 from the upper to lower mantle, our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95% and explains 84% of the observed geoid assuming that the model C21 and S21 terms are absent due to a long relaxation time for Earth's rotational bulge. Otherwise, 73% of the geoid variance is explained. The viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle, since downwelling velocities are expected to scale roughly as the logarithm of viscosity (loge 40 = 3.7). These results show that the history of plate tectonics can explain the main features of the present-day structure of the mantle. The dynamic topography induced by this heterogeneity structure consists mainly of about 1-km amplitude lows concentrated along the active continental margins of the Pacific basin. Our model can also be used to predict the time variation of mantle heterogeneity and the gravity field. We find that the “age” of the geoid, defined as the time in the past before which the geoid becomes uncorrelated with the present geoid, is about 50 m.y. Our model for the history of the degree 2 geoid, which is equivalent to the history of the inertia tensor, should give us a tool to study the variations in Earth's rotation pole indicated in paleomagiietic studies.

The effect of roughness elements on wind erosion threshold

Abstract: A theory is developed to describe the dependence upon roughness density of the threshold friction velocity ratio Rt, the ratio of the threshold friction velocity of an erodible surface without roughness to that of the surface with nonerodible roughness present The roughness density is quantified by the frontal area index λ The prediction is Rt = (1 − mσλ)−½(1 + mβλ)−½, where β is the ratio of the drag coefficient of an isolated roughness element on the surface to the drag coefficient of the substrate surface itself; σ is the basal-to-frontal area ratio of the roughness elements; and m (< 1) is a parameter accounting for differences between the average substrate surface stress and the maximum stress on the surface at any one point The prediction is well verified by four independent data sets

Dynamics of fault interaction: parallel strike‐slip faults

Abstract: We use a two-dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures. Our results indicate that a strike-slip earthquake is unlikely to jump a fault step wider than 5 km, in correlation with field observations of moderate to great-sized earthquakes. We also find that dynamically propagating ruptures can jump both compressional and dilational fault steps, although wider dilational fault steps can be jumped. Dilational steps tend to delay the rupture for a longer time than compressional steps do. This delay leads to a slower apparent rupture velocity in the vicinity of dilational steps. These “dry” cases assumed hydrostatic or greater pore-pressures but did not include the effects of changing pore pressures. In an additional study, we simulated the dynamic effects of a fault rupture on ‘undrained’ pore fluids to test Sibson's (1985, 1986) suggestion that “wet” dilational steps are a barrier to rupture propagation. Our numerical results validate Sibson's hypothesis by demonstrating that the effect of the rupture on the ‘undrained’ pore fluids is to inhibit the rupture from jumping dilational stepovers. The basis of our result differs from Sibson's hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments.

Hydrological signatures of earthquake strain

Abstract: The character of the hydrological changes that follow major earthquakes has been investigated and found to be dependent on the style of faulting. The most significant response is found to accompany major normal fault earthquakes. Increases in spring and river discharges peak a few days after the earthquake, and typically, excess flow is sustained for a period of 6–12 months. In contrast, hydrological changes accompanying pure reverse fault earthquakes are either undetected or indicate lowering of well levels and spring flows. Strike-slip and oblique-slip fault movements are associated with a mixture of responses but appear to release no more than 10% of the water volume of the same sized normal fault event. For two major normal fault earthquakes in the western United States (those of Hebgen Lake on August 17, 1959, and Borah Peak on October 28, 1983), there is sufficient river flow information to allow the magnitude and extent of the postseismic discharge to be quantified. The discharge has been converted to a rainfall equivalent, which is found to exceed 100 mm close to the fault and to remain above 10 mm at distances greater than 50 km. The total volume of water released in these earthquakes was around 0.3 km3 (Borah Peak) and 0.5 km3 (Hebgen Lake) Qualitative information on other major normal fault earthquakes, in both the western United States and Italy, indicates that the size, duration, and range of their hydrological signatures have been similar. The magnitude and distribution of the water discharge for these events are compared with deformation models calibrated using seismic and geodetic information. The quantity of water released over a time period of 6–12 months suggests that crustal volume strain to a depth of at least 5 km is involved. The rise and decay times of the discharge are shown to be critically dependent on crack widths, and it is concluded that the dominant cracks have a high aspect ratio and cannot be much wider than 0.03 mm. Using the estimated depth to which water is mobilized, the modeled crack size, and the measured volumes of water expelled, it is concluded that even at distances of 50 km from the earthquake epicenters, cracks must be separated by no more than 10 or 20 m. In regions of highest discharge nearer the earthquake epicenters, separations of 1 or 2 m are required. These results suggest that water-filled cracks are ubiquitous throughout the brittle continental crust and that these cracks open and close throughout the earthquake cycle. The existence of tectonically induced fluid flows on the scale that we demonstrate has major implications for our understanding of the mechanical and chemical behavior of crustal rocks.

Nature of seismic coupling along simple plate boundaries of the subduction type

Abstract: The downdip width of the seismogenic zone is defined for 19 subduction zones. This width is measured from the base of the accretionary prism to the maximum depth of nucleation of thrust events along the plate boundary. Those two points are taken to define the upper and lower depth transitions from stable to unstable frictional sliding. The lower depth transition is found to be between 35 and 70 km. The dip angle of the thrust zone is also reevaluated. We find a linear increase in the dip angle as a function of depth, the slope of which varies between 0.2° and 0.6° km−1. The downdip width obtained, which is generally narrower than previously determined by most other authors, varies from about 50 to 150 km. We also determine the ratio of the rate of slip that occurs in earthquakes to the rate of relative plate motion. This ratio is defined as the seismic coupling coefficient (a). We obtain two different estimates of the seismic coupling coefficient: an average value from 90 years of seismicity and a value obtained using the slip-predictable recurrence model for large earthquakes. We find a large variation in the computed values of a along and among subduction zones. For most of the subduction zones a is much less than 1.0; for several it is less than a few percent. Worldwide, we find no significant correlation between either the seismic coupling coefficient or the width of the seismogenic zone and subduction parameters such as the age of the oceanic lithosphere that is being subducted, plate convergence rates or absolute velocity of the upper plate in the hot spot reference frame. Such correlation exists only for a few individual subduction zones where other parameters do not vary as much. The observed variations in seismic coupling could be explained as differences in the frictional behavior of materials at the plate interface. Some of these differences may be attributed to the subduction of large bathymetric features, the roughness of topography, the presence of unstable triple junctions and active-spreading ridges, and sediment composition.

Depth of seismic coupling along subduction zones

Abstract: Underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth. The maximum depth of the seismically coupled zone (or seismogenic zone) is a transition from unstable to stable sliding along the plate interface. We have determined the depth of this stability transition for the circurn-Pacific subduction zones of: Honshu, Kuriles, Kamchatka, Aleutians, Alaska, Mexico, and Chile. These subduction zones have experienced great interplate earthquakes and the aftershock regions are well-located. Depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling. For an average P wave velocity of 6.7 km s−1 above the plate interface, we find that for most subduction zones the stability transition occurs at 40 ± 5 km depth. There are, however, several exceptions. At the Hokkaido trench junction, where the Japan trench and the Kurile trench intersect, seismic coupling is deep and extends down to 52–55 km. Deep coupling was also found in the Coquimbo region in central Chile. The Mexico subduction zone has shallow coupling: the transition occurs at 20–30 km depth. Previous studies of micro-earthquakes in Honshu, Hokkaido, the Aleutians, and Alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find. Given our measurements of seismic coupling depth, we then explore the mechanism that may determine coupling depth. The concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments, thus we first test whether a critical temperature can explain our results. Temperatures at the plate interface are dependent on many variables; but two that are poorly determined are shear stress and radiogenic heat generation. Shear stress has been constrained by inversion of heat flow data. Assuming a crustal radiogenic heat production rate of 3.1 exp−z/8.5 μWm−3 and a constant coefficient of friction, we find two critical temperatures of about 400 ° C and 550 ° C. The lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust. On the other hand, one single critical temperature of about 250 ° C can explain the coupling depths if shear stresses are constant with depth.

Measurement of the gas phase concentration of H2SO4 and methane sulfonic acid and estimates of H2SO4 production and loss in the atmosphere

Abstract: Measurements of gas phase sulfuric and methane sulfonic acid (MSA) have been performed using a relatively new atmospheric pressure selected ion chemical ionization mass spectrometric technique at two field sites. Both gas phase acids are photooxidation products, and their concentrations are seen to qualitatively follow solar flux. While sulfuric acid concentrations typically decline in conjunction with declining solar radiation, they sometimes level off in the mid 105–106 molecules cm−3 range after dark, even in relatively clean air. The reason for this quasi-stable nighttime sulfuric acid concentration is not well understood but may be a result of a steady state exchange of sulfuric acid between particles and the gas phase. Measurements of OH, H2SO4, and SO2 concentrations in conjunction with aerosol number and size distribution also made possible the independent calculation of gas phase sulfuric acid production and loss rates. Calculated production and loss rates are seen to agree well in relatively clean air during the daylight hours. At night, however, the sulfuric acid concentrations and its calculated loss rate often have a nonzero value. In more polluted air masses, calculated gas phase sulfuric acid losses significantly exceed calculated production if H2SO4/aerosol reaction probabilities of 1.0 or 0.5 are assumed.