Main Structural Trends Research Articles

Palaeomagnetic constraints on formation of the Mianwali reentrant, Trans-Indus and western Salt Range, Pakistan

Successions of Lower to lower Middle Cambrian, Upper Permian to Upper Triassic and Lower Tertiary carbonates and arenites have been sampled in five sections, representative of the three main segments of the Mianwali reentrant in the (Trans-Indus) Salt Range (northern Pakistan), i.e.: the southern Khisor Range, the northern Surghar Range and the western Salt Range. Comparison of primary and secondary magnetization directions with the Indian APWP demonstrates the secondary origin of the Mianwali reentrant and shows a pattern of rotations which varies in sense and magnitude along the reentrant with the main structural trends. Data from the Trans-Indus and western Salt Range and published Early Cambrian, Early Permian and Late Tertiary palaeomagnetic results from the southern Salt Range and the Potwar Plateau show that the Hazara Arc underwent a 20–45° counterclockwise rotation relative to the Indian Shield. A contrasting clockwise rotation over about 45° has recently been established for thrust sheets in the opposing eastern limb of the Western Himalayan Syntaxis, i.e. for the Panjal Nappe [1] and the Riasi thrust sheet [2]. These palaeomagnetically established rotations conform with the about 75° azimuthal change in structural trend along the Syntaxis, and support Crawford's [3] suggestion that the Salt Range was originally in line with the northwestern Himalaya. The Salt Range front prograded and moved southwards as part of the Hazara Arc thrust sheet, detached from basement along the evaporitic Salt Range Formation. The Mianwali reentrant originated through obstruction of the southwards advancing thrust sheet by moulding around basement topography of the northwest oriented Sarghoda Ridge.

Tectonically deformed pebbles in the Brazeau and Paskapoo formations, central Alberta Foothills, Canada

Characteristic features of pebbles in the Entrance and High Divide Ridge conglomerates, of late Maastrichtian and mid-Paleocene age, respectively, are the common occurrence of pock marks, fractures and polished surfaces with striations. These features are due to tangentially oriented tectonic movements that affected the highly clast supported conglomerate composed of well-rounded quartzite pebbles. The stress was transmitted through the points of contact between the pebbles, causing solution of the silica and producing pock marks and tensile fractures extending through whole pebbles. The remobilized silica tended to migrate and recrystallize, forming cement in some parts of the conglomerate. Many pebbles have been reoriented to occupy the most stable position in relation to the maximum tectonic stress. Their componental movements within the matrix produced striations on their surfaces and lineations on their imprints. The resulting preferred orientation is expressed on most of the striated surfaces of the elongated pebbles. The direction of the maximum compressive stress, measured as a line parallel to the major tensile fractures joining two opposite pock marks, also shows a preferred orientation normal to the main NW-SE structural trend of the Rocky Mountains. Layer-parallel tectonic movements have also produced slickensides on the planes of sandstone interbeds in the Entrance and High Divide Ridge conglomerates. The occurrence of the deformed pebbles in the Brazeau-Paskapoo Formations of the Alberta Foothills may indicate repeated deformation accompanying the Laramide Orogeny in an area of the compressional foreland basin in the Cordillera. Deformed pebbles may be useful in dating Laramide deformation in the area of the Foothills and the Plains.

The Calabrian Arc and the Ionian Sea in the dynamic evolution of the Central Mediterranean

In the Central Mediterranean area it is possible to recognize three main realms: the Tyrrhenian Sea, the Pelagian block and the Ionian block. These domains are limited by important fracture zones that are easily recognizable on land and offshore. These fracture zones can be grouped into four main trends: (1) the NW-SE trend which is characterized by dextral horizontal movements; (2) the NE-SW trend which is generally characterized by sinistral lateral horizontal movements; (3) the E-W trend which also shows dextral shear movements; and (4) the N-S trend which is characterized by normal faults only. The Tyrrhenian Sea realm is characterized by a thinned crust of oceanic type and is connected to the Apenninic chain by means of listric faults. From a geophysical point of view, the area shows a high heat flow which follows the main structural trends, and positive gravimetric anomalies which are compatible with seismic refraction data and can be explained by the proposed model. The Pelagian block shows a continental crust, about 20 km thick, which progressively increases beneath the Sicilian chain. Here an abrupt interruption with the Tyrrhenian block takes place. In the Pelagian realm, rifting processes developed according to the NW-SE and N-S trend. The N-S trend characterizes the westernmost zone of the realm where volcanic processes and a relatively high heat flow can also be observed. The Ionian block is characterized by the following: a thinned crust (17–20 km) which increases (40 km) towards the Southern Calabrian chain; a thick sedimentary sequence; high positive Bouguer anomalies (+300 mGal); and very low heat flow (50 mW m −2). Geophysical and structural analyses allowed us to evaluate the post-Tortonian evolution of the whole area under consideration. This evolution resulted in continental crust formation according to a rigid—plastic deformation model. According to our model, the Ionian block can be considered as a domain, characterized by a thinned continental crust and affected by intrusive material derived from the upper mantle. Furthermore, this domain, of probably Jurassic age, can be considered as a pelagic basin bordered by the Pelagian block to the west and by the Apulian block to the east-northeast. This basin, according to its position with respect to the Apenninic and Sicilian chains, has not been affected by tectogenesis and still represents a site of continuous sedimentation. From a kinematic point of view, the Ionian realm can be linked with the Pelagian block shifting towards the east-northeast, whereas it is limited towards the north by an E-W fracture zone which regulates the opening of the Tyrrhenian Sea and the counterclockwise rotation of the Apenninic chain. According to the proposed model, the opening of the Tyrrhenian Sea can be considered as a megaextension-fracture which developed parallel to the maximum principal stress direction and evolved as a triangular-shaped structural feature. In preparing the model the authors have taken into account a large set of data collected both on land and offshore.

Evolving miogeanticlines of the East Mediterranean (Hellenic, Calabrian and Cyprus Outer Ridges)

Extensive surveys with long range side-scan sonar, as well as an air-gun sub-bottom profiler and a narrow beam echo-sounder, are described for the eastern half of the Mediterranean Sea. The main structural trends are shown in plan view to follow the curve of the Hellenic Outer Ridge (previously known as the Mediterranean Ridge, East Mediterranean Ridge or Mediterranean Rise), and suggest a structural continuation into the Ionian Islands west of Greece. To the west a similar but smaller feature, the Calabrian Outer Ridge (external to the Calabrian Arc) is described. This is partly welded to the Hellenic Outer Ridge along a narrow suture zone. To the east the Hellenic Outer Ridge is shown to merge into the Cyprus Outer Ridge (external to the Cyprus Arc). The Hellenic Outer Ridge is clearly asymmetrical in cross section, with its steeper slope facing towards the interior of its Arc System. Folds and strike faults have been recognized on sonographs, particularly those of the Hellenic Outer Ridge. Cross-faults (possibly strike-slip) are numerous on the northern slope of this Outer Ridge. Cross-faults are especially well developed where the Ridge is narrowest and highest between Crete and North Africa, and where it may have been thickened by thrusting. In general the intensity of deformation decreases southwards across the Hellenic Outer Ridge. Slumping is probably responsible for progressively reducing the height of the relief produced by folding and faulting. The Hellenic, Calabrian and Cyprus Outer Ridges are interpreted as miogeanticlines related to the Plio-Quaternary phase of the continuing southwards outgrowth of the Hellenic, Calabrian and Cyprus Arc Systems. The large and small scale structures are of particular interest because they show the surface relief of some early evolutionary stages of dominantly compressional submarine mountain ranges before they are subject to subaerial erosion or modified by later tectonism. The driving force of the continuing orogeny is seen as resulting from local mantle diapirs spreading outwards from the Tyrrhenian, Aegean and Turkish regions, rather than from a simple closure of the Eastern Mediterranean due to the supposed convergence of the Eurasian and African ‘Plates’.

Marine geophysical study off western India

Seismic reflection and total magnetic field intensity observations were made off the western coast of India as part of the 1967 global scientific expedition of the USS Oceanographer. Linear fault block fragments appear to form the base of the northern segment of the platform that comprises the Laccadive-Maldive-Chagos trend. The NW continuation of these linear fault block fragments creates a basement ridge that extends to at least 20°N and is interpreted to be the NW continuation of the main Laccadive structural trend. This basement ridge may be a relict margin of the Indian continent. No obvious magnetic lineations were found in the study area. Short-wavelength variations of the magnetic field along the Laccadive trend suggest a relatively shallow source. Long-wavelength high-amplitude magnetic anomalies suggest the presence of a broad ridge composed of relatively high magnetic material near the western shelf edge of the Indian peninsula.

Brevard zone compared to Alpine root zones

The Urseren zone of Switzerland is a long linear zone of deformed rocks paralleling the main structural trends in the Alps. This zone clearly is the root zone for the Helvetic nappes and is not a major strike-slip fault. The Pusteria-Insubric line in Austria also appears to be a root zone rather than a major strike-slip fault. Like the Urseren zone it is a long linear zone of deformed rocks, but unlike the Urseren zone it has undergone important late tectonic movements, which have strongly modified its root-zone characteristics. In both of these root zones, significant volumes of crustal material seem to have disappeared; this fact appears to demand a significant amount of crustal shortening. The Brevard zone in the southern Appalachians appears to have the characteristics of the linear root zones described from the Alps.

SOME GEOPHYSICAL DATA BEARING ON DEEP STRUCTURE OF THE CENTRAL KAMCHATKA TROUGH

The central Kamchatka trough, a large intermontane structure, extends for over 800 km along the main structural trend of the peninsula. Surface and aerial magnetic data indicate thick Paleogene-Lower Miocene volcanics. Gravity anomalies are associated with Upper Cretaceous structures. The origin of the Klyuchosk hollow's lava upland and younger volcanic structures is associated with the central Kamchatka graben on the site of an early Paleogene trough. The marginal parts of this trough were affected by vigorous volcanism at the late stage of the Kamchatka geosyncline. C. E. Sears

Quaternary Tephrochronology in Northeast Honshu, Japan

The volcanoes of Quaternary age in Northeast Honshu are distributed along two belts which are named the Nasu and Chokai Volcanic Belts. The Nasu Volcanic Belt is situated along the Backbone Ranges of Northeast Honshu and the Chokai in the Dewa Hilly Lands. Northeast Houshu is divided into five sections, each has its own geologic and geomorphic characteristics. There are, from east to west, Kitakami-Abukuma Massif, lowland on the western side of the massif. Backbone Ranges, lowland on the western side of the ranges, and Dewa Hillylands. The Kitakami-Abukuma Massif consists largely of pre-Tertiary and granitic rocks. Along the eastern foot of the Abukuma Massif and in the areas between the Kitakami and Abukuma Massifs, and between the Kitakami Massif and Cape Shiriya, there are hillylands mainly consisting of Cenozoic sediments. The lowlands between the Kitakami-Abukuma Massif and the Backbone Ranges comprise several basins which are distributed along the main valleys of the Kitakami, and other trunk rivers. The Backbone Ranges consist mainly of Tertiary rocks and comprise the central belt of the Tertiary of the region. The lowlands between the Backbone Ranges and the Dewa Hilly Lands comprise several basins. The Dewa Hilly Lands consist largely of Tertiary rocks which rest upon the pre-Tertiary and granitic rocks with unconformity. Kitamura (1959) summarized the Tertiary orogenesis in this region as shown in Figure 2.Most of the Quaternary sediments and terraces of Northeast Honshu are developed in the mentioned two lowlands and coastal areas and distributed in the north to south direction in belts parallel to the main structural trend of the region. An analogous trend is found in the distribution of the volcanoes which are located near the tectonic lines. They have a northeast trend, obliquely crossing the main structural trend. The tectonic lines with a NW-SE trend were important throughout the pre-Quaternary history of the region, and at least, during the Pleistcene time, they controlled the northeastwards tilting blocks and determined the sitesof the basins within the above cited two lowland areas. For determination of the Quaternary tephrochronology in Notheast Honshu, available are such data as shown by the structural control on the distribution of the volcanoes, the sediments and terraces of the Quaternary age.In the middle latitudes of Japan, ash is generally spread eastwards from the volcanoes because it was transported by the prevailing westerly wind. In Northeast Honshu, the Quaternary sediments are developed more or less continuously and the terraces with a N-S trend are crossed longitudinally by the distribution of the volcanic ashes. The volcanic ash layers which originated from the Bandai-Azuma Volcanoes are distributed in the area between Koriyama and Fukushima along the Abukuma River, and between Hisanohama and Soma along the Pacific Coast. The ashes from the Zao Volcano are distributed between Watari and Shiogama along the Pacific Coast and are separated from those of the Bandai-Azuma Volcanoes. The ash layers derived from the Kurikoma Volcano are distributed east and northeastward and remote from those of the Iwate Volcano. Except for the most extensively distributed ash layers originated from the Towada-Hakkoda Volcanoes whch extend southward to the northern area of ash distrisbuted from the Iwate Volcano, and covered in part by the eastern part of the ash derived from the Iwaki Volcano, the ash layers from one volcano or group of volcanoes in Northeast Honshu are distributed in areas separated and at some distance from those of the neighbouring volcanoes.A discontinuity in the distribution of the Quaternary sediments and terraces is also found in this region.

Observations sur la chromite du mont Ahito (Togo)

Abstract The chromite mineralization of Ahito, one of a series of small serpentine massifs in French Togo, is shown to have been emplaced in discontinuous lenticular veins, under pneumatolytic-hydrothermal conditions, along north-south fractures paralleling the main structural trend of the massif.

Racetrack Hill Anticlinal Trend, Kern County, California: ABSTRACT

The Racetrack Hill anticlinal trend, approximately 7 miles east of Bakersfield, Kern County, consists of an area 10 miles long and one mile wide. On it are three oil fields of moderate economic importance--Racetrack Hill, Graham, and Jeppi--that have produced approximately 10,000,000 barrels of high-gravity oil from lower Miocene sands. The Racetrack Hill anticlinal trend plunges approximately 500 feet per mile southwest which is almost perpendicular to the main structural trends in this part of California. It can be traced in the lower Miocene sediments as far as North Mountain View. Seismic data suggest that it probably extends southwest. The anticlinal structure, limited in general to the lower Miocene sediments, upward becomes less pronounced, and, in one place is displaced by a broad syncline in the younger sediments. Production along it is caused by actual domal closure in the Racetrack Hill oil field. Graham and Jeppi have been caused by cross-faulting in conjunction with nosing. This structure is bounded on the east by a major, possibly pre-middle Miocene fault having maximum vertical displacement of 2,000 fe t. The lower Miocene in this area is represented by a series of sediments ranging in thickness from 1,500 feet at outcrop to at least 4,000 feet in the vicinity of the North Mountain View area. The Nozu in this area reaches a maximum thickness of more than 400 feet in the Graham area where it consists mostly of a series of very coarse conglomerates. The underlying Freeman is a dark brownish gray siltstone. Beneath this the lower Miocene productive zones occur--the Jewett consisting of a series of siltstones and permeable sands having maximum thickness of 250 feet, the Pyramid Hill sand, a 20-foot ash bed beneath this characterized by a basal grit believed to be the base of the Saucesian stage, and the underlying probably Zemorrian stage sediments that are referred to as Vedder except whe e they grade into the non-marine Walker series. The Walker is non-productive. End_of_Article - Last_Page 217------------