Southward Change Research Articles

The influence of inherited continental margin structures on the stress and strain fields of the south-central Taiwan fold-and-thrust belt

SUMMARYIn this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (σ1, σ2 and σ3) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (SH) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of SH from north to south across the study area, with the direction of SH remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45° counter-clockwise. The direction of the horizontal maximum compression strain rate (εH) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the SH, εH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east–northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. SH in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role.

Mineralogical, isotopic (K-Ar) and structural-textural characteristics of the Jurassic terrigenous complex in different paleotectonic settings (Greater Caucasus, Chechnya - Georgia)

The variations in structural, textural, mineralogical and geochemical (radiometric) characteristics in the lower to middle Jurassic siliciclastic sediments of the transect across Chanty-Argun R., Mountain Chechnya and Georgia are considered. This transect crosses areas of different types of deformations from northern zone of weak deformation to southern zone of intensive deformation and cleavage. The southward change in clay mineral associations, polytypical modifications of mica minerals, their crystallinity index, a.o. are observed along the transect. The increase of intensity of secondary alternations of the rocks and dimensions of cleavage enabled the change in K-Ar-system. This is resulted in significantly younger values of measured radiologic age corresponding to older stratigraphic age.

Initial ionospheric observations made by the new Resolute incoherent scatter radar and comparison to solar wind IMF

The first Resolute incoherent scatter radar observations of the polar ionospheric F region show the fine control of the ionospheric plasma density and flow (electric field) by the solar wind interplanetary magnetic field (IMF). A summary of 8 days of observations is presented and 10 IMF Bz southward turning events during this period are analyzed in terms of the time delay of plasma density enhancements and ionospheric convection intensification with respect to the timing of Bz southward turning. We find that Ne enhancements are strongly tied to strong (≳5 nT) IMF Bz southward turnings; arrive 25–75 mins (depending on MLT) after the IMF pulse arrives at the bowshock nose; last as long as Bz stays southward; contain as small as ∼25 km horizontal substructures; are altitudinally smooth, a characteristic of a solar produced plasma. The most predictable response of ionospheric convection is anti‐sunward flow intensification on average ∼25 mins after Bz southward change.

Comparison of particle size characteristics of the Tertiary ‘red clay’ and Pleistocene loess in the Chinese Loess Plateau: implications for origin and sources of the ‘red clay’

AbstractThe origin of the Tertiary ‘red clay’ underlying the Pleistocene loess in the Chinese Loess Plateau remains controversial, although several lines of evidence have suggested a wind‐blown origin. This study examines the particle‐size parameters of the late Miocene and Pliocene ‘red clay’ by comparing it with those of the late Pleistocene loess. The particle‐size distribution of a total of 15 339 loess and 6394 ‘red clay’ samples taken from 12 loess sections along a north–south transect and two ‘red clay’ sequences at Lingtai and Jingchuan was systematically analysed. The median grain size, skewness and kurtosis of the late Pleistocene loess all show a systematic southward change and are principally influenced by distance from source region. The spatial and temporal differentiation of dust deposits is expressed in a skewness–kurtosis–median grain size ternary diagram, from which the distance to the source region can be inferred. The particle‐size characteristics of the Tertiary ‘red clay’ sediments are very similar to those of the palaeosols within the late Pleistocene loess deposits, suggesting an aeolian origin for the ‘red clay’. Based on the comparison of ‘red clay’ and loess in the ternary diagrams, it is inferred that the source–sink distance was greater in the Neogene than in the last and penultimate interglacials, and that the dust source region in north‐western China underwent a progressive expansion during the period from at least 7·0 Ma to the present.

Changes in Sand Content of Loess Deposits along a North–South Transect of the Chinese Loess Plateau and the Implications for Desert Variations

Geological records have shown that the deserts east of the Helan Mountains in northern China were covered by grass during the Holocene Optimum, whereas during marine oxygen isotope stages 2 and 4 distribution of the deserts was almost the same as at present. The wide advance–retreat cycles of the deserts may have exerted an important control on grain-size changes in the loess of the Loess Plateau by altering the distance between the source and the accumulation zone of the loess. This challenges the widely accepted model that winter monsoon winds were the sole factor responsible for spatial and temporal changes in loess texture. To observe spatial changes in sedimentological characteristics of loess during the last glacial–interglacial cycle, the texture of loess was measured along a north–south transect of the Loess Plateau. This transect consists of nine loess sections, starting at Yulin in the transitional region between the Loess Plateau and the Mu Us Desert and ending at Weinan in the southernmost part of the Loess Plateau. Southward changes in sand (>63 μm) content along the transect suggest that variations in desert extent have indeed played a significant role in loess grain-size distributions, particularly in the northern part of the Loess Plateau. It is proposed that sand content (>63 μm%) of loess in the loess–desert transitional zone may be used as a proxy indicator for proximity to the desert margin.

Tectonic heredity and tectonic wedging along an oblique hanging wall ramp: The southern termination of the Misty thrust sheet, southern Canadian Rocky Mountains

The Misty Range is a homoclinal, southwest-dipping panel of Paleozoic carbonate rocks in the Canadian Rocky Mountains southwest of Calgary that has been juxtaposed against underlying Mesozoic siliciclastic rocks along the northeast-verging Misty thrust fault. At the south end of the Misty Range, a steep, oblique ramp marks an abrupt southward change along the hanging wall of the Misty thrust, from a detachment zone in the Upper Devonian Palliser Formation to another detachment zone in the Jurassic Fernie Group. This 2-km-high ramp is expressed, in the hanging wall rocks, as a large, oblique, southeast-plunging fold. A west-verging back-thrust that rises from a detachment zone in the Mississippian Mount Head Formation extends across the hanging wall ramp fold and is truncated by the Misty thrust. The Misty thrust evidently propagated southward as an oblique tectonic wedge into this detachment zone, forming a “triangle zone” structure with the back-thrust as the roof thrust. As displacement on the Misty thrust increased, and the hanging wall ramp of the Misty thrust moved into the triangle zone, the back-thrust was progressively back-folded and became convex. Subsequently, the lower part of the back-thrust was partly overridden by the Misty thrust and became overturned, before becoming detached and incorporated in the Misty thrust sheet. The configuration and evolution of the tectonic wedge and the oblique hanging wall ramp provide a natural prototype for the interpretation of ramps that mark the lateral terminations of oil and gas reservoir structures in the Foothills belt, but which are concealed by overlying deformed Mesozoic rocks.

Regional Distribution of Microdolomite Inclusions in Mississippian Echinoderms from Southwestern New Mexico

ABSTRACT Crinoid grains from Mississippian skeletal packstones and grainstones from the Lake Valley and Kelley Formations, New Mexico, have retained most of their original Mg, which now resides in 1 to 10 µm microdolomite inclusions. The mean microdolomite content of these crinoids is about 9.3% ± 1.1% (95% confidence limit) by volumes, as determined by petrography. The microdolomite contents show regional variations being lowest in the northern part of the study area (about 6%), and highest (about 11%), 170 km to the south. There are no consistent stratigraphic gradients in microdolomite contents. The average apparent Mg content of the Mississippian crinoids is estimated from the 9.3% microdolomite by assuming 50% original intraskeletal porosity. This estimate of 10.4 mole % MgCO3 is lower than the average 14.5 mole % of modern low-latitude crinoids. In spite of this, the higher values of Mg contents of Mississippian crinoids fall within the range of Mg contents of modern, low-latitude crinoids. We interpret these data to mean that original Mg contents of Mississippian crinoids were comparable to those of modern crinoids and that the relatively low mean Mg content of Mississippian crinoids is due to diagenetic loss of Mg. We rule out lower temperatures or Mg contents of Mississippian sea waters and biological differences of modern and Mississippian crinoids as causin the low-Mg contents of Mississippian crinoids. Mg loss from Mississippian crinoids is supported by the decrease in microdolomite content toward rims of grains, and by the fact that intragranular variation is lower than intergranular variations of microdolomite content. On the basis of the above evidence we suggest that, on average, Mississippian crinoids have retained about 75% of their Mg. The southward increase in Mg content of Mississippian crinoids we interpret to reflect a greater diagenetic Mg loss in the north than in the south, rather than a southward increase in sea-water temperature or a southward change in taxa. This interpretation is supported by the southward decrease in intensity of chemical compaction and by the covariance of Mg and 18O contents of crinoids. Our diagenetic model, based on these data and on previous work, invokes regional, fresh, phreatic groundwater systems that formed in response to late Mississippian-early Pennsylvanian emergence of the shelf. The groundwaters were recharged in the north and flowed, generally, southward. Northern areas contained diagenetically more aggress ve groundwaters which leached more Mg from crinoid grains than did groundwaters farther south.

Magnetotail response to sudden changes in the interplanetary magnetic field

The effect of southward or northward changes in the interplanetary magnetic field is examined statistically in the nightside magnetosphere over the range of 6.6 to 80RE from the Earth. After southward changes, the deformation of the magnetosphere toward a greater antisunward extension of field lines occurs at 6.6RE with ≲10 min delay and spreads down the tail to 80RE in ≲30 min. Around the onset of the field-line collapse that occurs 1–2 hr later, the southwarddirected field is observed briefly in the distant tail. The effect of northward changes could not be recognized in the lobe region of the tail.

Relationship of interplanetary magnetic field structure with development of substorm and storm main phase

Development of a polar magnetic substorm and the main phase of a magnetic storm is compared with magnetic variation in interplanetary space. It is confirmed, using AE indices, that a polar substorm is well correlated with the southward component, B zs , of the interplanetary magnetic field. The hourly-value AE shows the highest correlation with B zs , of the preceding hour. It is also found in the ground magnetic data that the twin-vortex current system without enhancement of auroral zone current, called DP 2 or SP, develops with south ward changes in the interplanetary magnetic field before the expansion phase of magnetic substorm. In cases of magnetic storms following the ssc's, caused by interplanetary shock waves, developments of substorm and D st are found to be highly correlated with B zs . When B zs is small for about one day after the interplanetary shock no appreciable geomagnetic activity follows the ssc even in cases where the increase in kinetic energy flux of the solar wind is fairly large. In cases of large magnetic storms, large B zs changes are usually observed during several hours or more within a day after the ssc, accompanying the development of geomagnetic disturbances.

Polar substorm and interplanetary magnetic field

By means of the IMP 3 magnetic data, a study is made on the relationships between the interplanetary magnetic field and the development of polar substorm. It is found that an explosive growth of the westward electrojet tends to appear after the interplanetary magnetic field has directed southward for 1–2 hr. Examinations of ground magnetic data indicate that the magnetic variation of a twin vortex mode are enhanced with the southward changes of the interplanetary magnetic field in the pre-explosive phase of the westward electrojet. In the early stage of this phase sudden geomagnetic impulses are often observed in association with tangential discontinuities in the solar wind. An explosive appearance of auroral electrojet often corresponds to changes in direction and/or magnitude depression of the interplanetary magnetic field. Such a typical sequence is also found at the main-phase beginning of magnetic storms. It is suggested that the polar magnetic substorm is a two-stage phenomenon lasting 4–6 hr. The twin current vortices in the polar region develop with the southward interplanetary magnetic field during the pre-breakup phase of 1–2 hr duration. This is probably to be a manifestation of the energy storage process in the magnetotail.

Petrology of the Vindhyan Rocks Around Rohtasgarh, India

ABSTRACT The Vindhyans constitute a sequence of about 7000 feet of unfossiliferous Precambrian (Algonkian) sedimentary rocks which form the Vindhyan ranges of central India. The Kheinjua, Kaimur and Rohtas formations are exposed in the northeast portion of the area. The orthoquartzite-limestone association indicates a stable tectonic phase, as do uniform grain-size distributions, low 1 values, stable mineralogical composition, monomineralic stable heavy mineral suites, and high roundness of the grains constituting the sandstones. The sandstone bodies of the Amarkha, Akbarpur and Amjhora beds are wedge shaped, thickening to the south-southwest. In the Amarkha Formation a southwesterly paleocurrent is indicated by ripple marks and a regular variation of the frequency of shale fragments. Although the size distribution and mineral composition of the Akbarpur quartzites do not give any clue regarding the paleocurrent direction, they show cyclic variation of the skewness values as was observed by Folk and Ward. In the Murli Sandstone for which no thickness data is available, a regular southward change in the type of quartz is noted, indicating an approach towards a point of mixing of two sediments, one borne by a other probably flowing current, the other probably flowing in the general direction of south-southwest. In the absence of any evidence of bevelling due to erosion, the zero isopach of the beds is believed to roughly indicate the position of the shoreline. Examination of the position of the shoreline at different times reveals a migration of shoreline roughly towards the northeast.

Jurassic Overlap in Western New Mexico

Southward changes in Jurassic stratigraphy along the east side of the Cebolleta Mesa give a clear picture of the nature of the southern margin of Jurassic deposition in western New Mexico. Results of the measurement of nine sections are shown graphically and discussed in terms of the structural and stratigraphic relations of the units. The Wingate sandstone is divided into three members and the Morrison formation into four. These members thin and become coarser southward and progressively overlap a positive area in that direction. The wedge of Jurassic rocks lies between the Chinle shale (Upper Triassic) and the Dakota (?) sandstone (Upper Cretaceous). Its disappearance southward is due (1) to thinning against a positive area and (2) to truncation by the pre-Dakota erosion surface.