Tectonic Displacement Research Articles

A Discussion on the evolution of the Precambrian crust - Tectonic displacements and thermal activity in two contrasting Proterozoic mobile belts from Greenland

The Proterozoic Nagssugtoqidian and Ketilidian mobile belts are comparable in scale with those of the Phanerozoic rather than those of the Archaean. These two Proterozoic belts differ from one another both in the tectonic displacements which gave rise to them, and in their thermal activities as expressed by igneous and metamorphic characteristics. Similar differences between modern tectonic belts have been interpreted in terms of plate tectonics. The Nagssugtoqidian is characterized by considerable crustal shortening, very limited igneous activity, and high-pressure regional metamorphism which may be related to crustal thickening resulting from both ductile and brittle overthrusting of the Nagssugtoqidian rocks over the Archaean foreland. Evidence of crustal shortening in the Ketilidian is limited, but vertical and transcurrent movements are important. Widespread igneous activity throughout the active history of the belt resulted in the formation of mainly acid volcanic supracrustal rocks and widespread granite intrusion. The appinite suite is also well represented. Metamorphism is mainly of low-pressure type. A tentative comparison can be made between the Alpine and Nagssugtoqidian belts on the one hand, and Andean and Ketilidian belts on the other.

Structure of the Continental Margin and Tectonism at the Eastern Aleutian Trench

New seismic reflection data along closely spaced lines reveal deeper structures than did earlier records. The new records show sediment in the trench continuing as much as 12 km landward under the continental slope, that large slumps and extensional structures are common on the slope, and suggest that in earlier periods the sea floor now under the trench was receiving sediment from an eastern source rather than from the present western source. Large tectonic displacements appear to be absent from oceanic basement beneath the continental margin because undisturbed linear oceanic magnetic anomalies and a fracture zone continue from the abyssal plain at least to the continental shelf. However, since vertical displacements in the overlying sediment are greater than any in the basement, thrust faults are indicated even though they are not visible in the reflection records. No single zone of deformation marks the emergence of the Benioff zone, but instead the zone probably emerges in the whole complex of tectonic structures across the continental margin. There are enough data to estimate roughly the maximum amount of abyssal sediment that may have been plastered against the continent during the Tertiary.

Geological aspects of the May 31, 1970, Perú earthquake

abstract Geological effects of the destructive May 31, 1970, Perú earthquake (Ms = 7.7) extended over roughly 65,000 km2 of west-central Perú. Earthquake-triggered slope failures of all types that occurred throughout the mountainous parts of the region extensively damaged transportation routes and irrigation canals and temporarily dammed some rivers and lakes. The geologically most important and spectacular of these, a cataclysmic debris avalanche from between 5,500 and 6,400 m altitude on the north peak of Huascarán Mountain, buried the city of Yungay and part of Ranrahirca (elevation about 2,500 m) with a loss of more than about 18,000 lives. The debris avalanche involved 50-100 million m3 of rock, ice, snow, and soil that traveled 14.5 km from the source to Yungay at an average velocity of between 280 and 335 km/hr. Exceptionally rapid movement of the avalanche is indicated by eyewitness accounts, by topographic irregularities as high as 140 m that were overridden, and locally by boulders weighing several tons that were hurled as much as 1,000 m beyond its margins. A pulse of muddy water from the debris avalanche that swept down the Río Santa 160 km to the sea inundated farms and small settlements, buried highway and railroad routes, and destroyed the diversion dam and access bridge to a major hydroelectric plant. The extensive destruction to communities and an additional estimated 20,000 casualties resulted primarily from failure of adobe and masonry structures which had little shear resistance to lateral forces imposed by seismic shaking. The degree of damage to buildings and to transportation routes was aggravated in some areas of saturated unconsolidated deposits by differential compaction, downslope slumping or sliding, lateral spreading of liquefied sediments toward free faces, and possibly amplification of seismic vibrations. The absence of surface tectonic displacements or of a significant tsunami and the spatial distribution of the main shock and aftershocks suggest that the earthquake originated by movement on a fault, or faults, beneath the continental shelf at depths between 45 and 65 km.

FAULT DISPLACEMENTS AND MOTION RELATED TO NUCLEAR EXPLOSIONS.

abstract Studies of some of the geologic effects of underground nuclear explosions that have been carried on during the last 3 years by the U. S. Geological Survey on behalf of the U. S. Atomic Energy Commission are summarized. Movement has been triggered on some faults within a few thousand feet of explosions in Nevada and in Amchitka Island, Alaska. The permanent relative vertical displacement on the faults always has been in the same direction as the last recognizable natural tectonic displacement and has been as much as 1 m (meter); horizontal displacement of as much as 15 cm (centimeters) has occurred on a few faults although no previous horizontal displacement had been apparent. The strike length of triggered faulting ranges from about 1 to 8 km (kilometers) and seems to correlate linearly with yield and magnitude of the explosions. High-speed motion picture photography of two faults from both the air and the ground provides the following data on the motion of the faults. 1 Fault motion started at the first arrival of seismic energy. 2 Rupture velocity along triggered faults, measured over a distance of 0.210 km, was 2.1 km/sec (kilometers per second) in alluvium of Hot Creek Valley, Nevada, which has a compressional velocity that ranges from about 2 km/sec to 3.3 km/sec and which has a density of 2.3 g/cc. 3 The velocity of displacement of alluvium at the Yucca Fault plane in Yucca Flat was a little more than 1 m/sec (meters per second). Velocity of displacement of alluvium at the fault plane in Hot Creek Valley was between 6 and 30 m/sec.

Mechanism of the May 21–22, 1960, Chilean Earthquakes as derived from residual vertical displacements

The great earthquakes of 21–22 May, 19 in west‐central Chile were accompanied tectonic displacements and intense, predominantly shallow (h≤70 km), aftershock activity. They affected a region more than 2 km wide and about 1000 km long that along the continental margin between 37° 46° S latitude. Studies of shorelines levelling data in this region indicate that vertical deformation involved a broad downwarp and two flanking upwarps. The structures lie roughly between and parallel the Peru‐Chile Trench and Andeanchain. Vertical tectonic displacements to sea level resulted in: (1) uplift of at least 5 m on offshore islands, at some points along mainland coast, and on at least part of contiguous continental shelf; (2) subsidence as much as 2.7 m in a zone 75–110 km that extends eastward from the vicinity of outer coast almost to the western margin of borders the downwarp on the east.

Late Cenozoic Structural Movement, Northern Florida: ABSTRACT

Pleistocene and Plio-Pleistocene sediments in northern Florida have been studied to detect tectonic displacement. Previous studies in southeastern Georgia defined six prominent former coastlines which are essentially horizontal and have altitudes (oldest to youngest) of 95-100 ft, 70-75 ft, 40-45 ft, and approximately 24, 13, and 4.5 ft above present sea level. The heights of the former sea levels were determined from the upper limit of littoral trace fossils and the altitude of salt-marsh sediments associated with each coastline. Each coastline is marked by a series of barrier-island deposits consisting of well-sorted, fine-grained, angular sand, and by lagoonal salt-marsh sediments of fine-grained sand, silt, and clay which accumulated landward of the barriers. The former coastlines have been traced southward from Georgia into northern Florida where the lagoonal salt-marsh sediments of the Wicomico Formation (sea level 95-100 ft in Georgia) are found at progressively higher altitudes. Maximum uplift is east of Starke along the northeast flank of the Ocala uplift where salt-marsh sediments have an altitude of approximately 166 ft or 65-70 ft higher than those in Georgia. The Penholoway salt-marsh sediments (sea level 70-75 ft in Georgia) reach an altitude of 95 ft in Florida. The Talbot salt-marsh sediments (sea level 40-45 ft in Georgia) are not well preserved but appear to be displaced 5-10 ft in the area of maximum uplift in Florida. Pamlico salt-marsh sediments (sea level approximately 24 ft in Georgia) are as high as 32 ft south of St. A gustine; thus, there is progressively less warping of the lower, younger coastlines. South of the Ocala uplift the five lower Pleistocene coastlines are approximately the same altitudes that they are in Georgia. The oldest coastline, which may be of either Pliocene or Pleistocene age, maintains an altitude of 140-150 ft for more than 150 mi south of the area of maximum uplift. End_of_Article - Last_Page 2038------------

Spores, Pollen, and Microplankton from the Horizon Beta Outcrop

Palynology was used for dating a pre-Pleistocene deep-sea organic lutite layer situated stratigraphically near seismic horizon beta, below horizon A. The spores and pollen are closely identified, quantitatively, with nonmarine and marine Middle Cretaceous assemblages (Albian-Cenomanian) on the continents, an age designation that is confirmed by the occurrence of dinoflagellate cysts, acritarchs, foraminifers, and coccoliths in the investigated cores. The abundance of these well-preserved, land-derived assemblages in an area far removed from a source suggests some tectonic displacement since their deposition.

Mid-Atlantic Ridge between 22° and 23° north latitude and the tectonics of mid-ocean rises

A detailed geological and geophysical study of the crest and upper western flank of the mid-Atlantic ridge between 22° and 23°N and 44°30′ to 46°20′W reveals that: (1) Several markedly different physiographic zones parallel the ridge axis, but only the crest shows strong topographic linearity. (2) Metamorphosed basalts exposed on the flanks of the median valley indicate regional metamorphism of the lower part of a gradually thickening crust, followed by uplift and tectonic displacement of the overburden. (3) The distribution of sediments and sediment ages suggest that the relief of the upper flank is late Miocene and the crestal relief is Quaternary. (4) The small free-air gravity anomalies over the ridge indicate that the ridge is nearly in isostatic equilibrium. Topography accounts for most of these anomalies. The slope of the regional Bouguer anomaly curve gradually increases toward the ridge crest, indicating a decreasing depth of compensation or an increasing slope of the boundary of the material providing the mass deficit or both toward the crest. (5) Magnetic profiles can be interpreted by the sea-floor spreading hypothesis, and they give a spreading rate of 1.4 cm/year for the crestal part of the ridge. The mid-Atlantic ridge and some other rises are characterized by high relief with a well developed median valley, a thick crust, exposures of greenstones and peridotites, and a slow spreading rate. Others, most notably the East Pacific rise, have a fairly smooth relief and a thin crust, lack a median valley and greenstone and peridotite exposures, and spread much more rapidly. The large topographic differences probably indicate behavioral differences of only the shallow levels of the crust and upper mantle. The thickness of the material that deforms by brittle fracture may determine the behavior of the surficial rocks and hence the topography. Material that deforms by brittle fracture may be thin at sites of fast spreading because higher rock temperatures might occur at shallower depths. This may account for the correlation of rifting and greater topography where spreading rates are less than 2.0 to 2.5 cm/yr and the occurrence of less extensive surficial structural rearrangement where spreading rates exceed these values.

Paleomagnetism and the Alpine tectonics of Eurasia U the magnetism of the Permian porphyries near Lugano (Northern Italy, Switzerland)

The magnetic properties of 25 oriented samples of the Permian porphyries from the Lugano district are investigated. After a.c. demagnetization (up to 900 Oe peak value) eleven samples produce a characteristic direction of magnetization, declination ( D) = N 143.5°E, inclination ( I) = −17°. The other samples show streaking towards the direction of the present-day geomagnetic field; their recent magnetic components are very hard and could not be removed by our a.c. demagnetization procedures. The paleomagnetic pole derived from the Lugano rocks is situated at 41.5°N 119.5°W, and is equally divergent from the other Permian poles from extra-Alpine European rocks as are the ones traced in other parts of the Alps. This divergency is best explained by assuming large-scale tectonic displacements of the south Alpine unit before and during the Alpine orogenesis.

FINITE STRAIN IN TECTONIC DEFORMATIONS

An analysis is made of the mathematical means that are available to describe finite tectonic displacements. It is shown that, from field observations, the principal axes of a tensor can be found which is related to the strain tensor in finite strain theory. The trajectories of this tensor yield a graphic picture of the local deformations; if certain averages are calculated for the local deformation, the over-all regional displacement pattern can be established.

Seismological evidence for the tectonics of the Northwest Pacific Ocean

Abstract From a series of fault-plane solutions of earthquakes in any one area, it is possible to determine the tectonic displacement of that area by a least squares solution. This method has been applied to the areas marginal to the Northwest Pacific Ocean. It turns out that the directions of the tectonic displacement in the Kurile Islands, Honshu, the Marianas Islands, and the Philippines are more or less parallel: the direction is approximately N 14° W. This is a rather startling result: it appears that the whole Northwest Pacific Ocean is moving en bloc with regard to its margin. A determination of the motion for Sakhalin gives an entirely different direction (N 36° E), which is an indication that the tectonic motion of Sakhalin is unrelated to that of the Pacific Ocean.

Submarine Sliding in Western Venezuela

The Paleocene and Eocene shales of the state of Lara, northwestern Venezuela, contain boulders and slabs (more than 1 kilometer in length) of pre-Cretaceous, Cretaceous, and Paleocene rocks. It is believed that this allochthonous material originated from the breaking-off of submarine escarpments caused by vertical tectonic displacements along the edges of the Barquisimeto trough. This material is thought to have moved by gravitational sliding toward the center of the trough; it has been encountered at distances of at least 30 kilometers from its nearest possible source. Calcareous sandstones with graded bedding, resulting from turbidity currents, are associated with the shales containing the boulders.