Minor Normal Faults Research Articles

Seismic Reflection Models of Rift-Related Structures: ABSTRACT

Published field data from several rifted basins indicate that normal faults and associated secondary structures (i.e., minor normal faults, folds produced by drag on fault surfaces, forced folds above faults) are common in rifts. The dip and curvature of the fault surfaces, the fault displacements, the dip of the strata within the fault blocks, and the position and size of the folds vary considerably. Our two-dimensional, seismic-reflection models systematically show how each of these variables, as well as rock velocity, influence the seismic expression of rift-related structures. These seismic models reveal several pitfalls of seismic interpretation common to rifts, many of which we have recognized on actual seismic data. The observed dip and curvature of any fault surface on our unmigrated seismic models depend, not only on the dip and curvature of the actual fault surface, but also on the dip and velocity of the adjacent beds. The observed dip of a fault decreases as the angle between the actual fault surface and strata decreases. For example, normal faults dipping in the opposite direction as the strata appear to have greater dips than identical normal faults dipping in the same direction as the strata. Also, normal faults active during deposition (with beds on the downthrown side having increasing dip toward the faults with depth) appear to steepen Figure End_Page 571------------------------------ with depth more so than identical normal faults active after deposition. The observed dip of a fault decreases as the velocity of the adjacent rock increases. For example, normal faults in rocks whose velocities increase with depth (e.g., most clastic sedimentary rocks) appear to flatten with depth more so than identical normal faults in rocks with more uniform velocities. The appearance of secondary structures associated with normal faulting on our unmigrated seismic models depends on the position and size of the secondary structures. The increased thickness of low-velocity rock on the downthrown side of normal faults disrupts and bends the reflections on the upthrown side. Depth, fault displacement rock velocity distribution, and the angle between the fault surface and adjacent beds affect the severity of the distortion. This distortion obscures any secondary structures present on the upthrown side of faults (i.e., minor faults, anticlines produced by drag) and can erroneously be interpreted as secondary faulting and folding. Synclines produced by drag on the downthrown side of normal faults have small radii of curvature relative to their burial depth . This relationship makes these synclines difficult to identify on unmigrated seismic sections. Many forced folds in rifts are gentle shallow structures overlying normal faults. These folds are the most easily identifiable because they are unaffected by the distortion beneath faults and the synclines have large radii of curvature compared to their burial depths. End_of_Article - Last_Page 572------------

Displacement distribution along minor fault traces

The variation of displacement along fifteen traces of minor normal faults was measured in the multilayered Quaternary sediments of Kyushu, Japan. In the diagrams of distance along a fault trace ( L) vs displacement ( D) two distinct types of faults, a cone-shaped L- D pattern (C-type) and mesa-shaped one (M-type), were detected. Because the L- D pattern is subject to slip-parallel strain in the wall rocks, a D-constant pattern is ascribed to the competent (rigid) material and a D-variable pattern is found in the incompetent material. Therefore, C-type faults are characteristic of homogeneous incompetent materials, whereas M-types are representative of faults that cut through a rigid unit. However, the steep slopes in the flanking sections of M-type patterns indicate that the faulting of a rigid unit should terminate in a strain absorber of incompetent materials. The concept of lithologic control in the L- D pattern is important for the better understanding of faulting processes as well as the localization of faults.

Study of Microearthquake Activity in Four Geothermal Areas of Taiwan: ABSTRACT

Detailed microearthquake surveys were conducted in four geothermal areas of Taiwan, namely, Tatun volcanic region, Chingshui- Tuchang geothermal area, Lushan hot spring area, and Hungyeh hot spring area, during the past few years. It is found that most of them have microearthquake activity. Based on the microearthquake data, fracture zones permitting deep circulation of water are inferred. Microearthquakes in the Tatun volcanic region are concentrated in a 1.9 mi (3 km) wide and 4.3 mi (7 km) long, northeast-striking elongated zone, where conspicuous geothermal manifestations are present. The focal depths of these events are mostly less than 0.6 mi (1 km). The occurrence of these microearthquakes may be related to the minor normal faults in that region. In the Chingshui- uchang geothermal area, microearthquakes are located in a northeast-striking zone, 25 mi (40 km) long and 4.3 mi (7 km) wide. The foci clearly define a northwest-dipping fracture zone with a thickness of about 1.9 mi (3 km). Three composite fault plane solutions are all of the normal fault type. No microearthquake activity was found within a 3 mi (5 km) radial range around the Lushan hot spring area, during a 34-day recording period. A highly active microearthquake zone was found in a region 1.25 mi (2 km) to the west of the Hungyeh hot spring area and the foci define an east dipping fracture zone. However, the causative faults of these events are of the thrust fault type. End_of_Article - Last_Page 987------------

The Chief Oxide-Burgin area discoveries, East Tintic District, Utah, A case history--Part 1, U. S. Geological Survey studies and exploration, by T. S. Lovering, D. R. Cook, and H. T. Morris; Part 2, Bear Creek Mining Company studies and exploration, by J. B. Bush and D. R. Cook

The Burgin shaft is in the Chief Oxide area of the E. Tintic district, Utah, and is about a mile E. of any previously known ore bodies; workings from it are currently developing a substantial amount of commercial Pb-Zn ore in several blind ore bodies that lie in folded Paleozoic carbonate rock concealed beneath a blanket of Eocene lava. This area was mapped by Tower and Smith of the U.S. Geological Survey in 1897 and again by Lindgren and Loughlin in 1911, but no detailed work was done until after 1943 when a field party headed by T.S. Lovering began a study of the entire E. Tintic district. The history of the development of exploration concepts is summarized under Historical Summary. The E. Tintic mining district is in the E.-central part of a N.-trending fault-block mountain range near the eastern margin of the Great Basin; dominant structures of the range are a N.-trending syncline on the W. and a less well exposed anticline on the E. Both folds are cut by an intersecting system of transcurrent strike-slip faults and by minor thrust faults and normal faults of moderate displacement. A strong W.-dipping thrust fault cuts the anticline a short distance E. of the fold axis, but is hidden by Eocene lava throughout the E. Tintic district. The pre-Tertiary rocks range from Lower Cambrian to Upper Mississippian and exceed 7,000 ft. in total thickness; they are dominantly marine limestone and dolomite except for a thick basal Cambrian quartzite. The Tertiary rocks are chiefly dacitic lavas and pyroclastic deposits that are intruded by moderately persistent dikes and irregular bodies of monzonite and quartz porphyry. Nearly all the faulting and folding took place before the extrusion of the lavas on a rugged Eocene erosion surface. At about the time of the intrusion of monzonitic magma, many of the faults in the Paleozoic rock were re-opened and in the overlying lava some fracturing took place which was later accented by hydrothermal alteration. Most of the ore mined in the E. Tintic district has come from Pb-Zn-Ag replacement bodies in shattered Jasperoidized hydrothermal dolomite at the intersection of low angle faults and steep mineralized NE.-trending cross fractures. The U.S. Geological Survey studies of 1943 to 1957 concentrated on detailed mapping of geology and alteration in the E. Tintic district, together with field and laboratory studies of the relation of alteration to stages of mineralization and ore deposition. Trenching and core drilling were carried on after World War II to aid in interpreting the subrhyolite geology, and the Chief Oxide area was 1 of 4 localities tested by drilling. Study of the fossils, lithology, and alteration shown here in a deep drill core, together with the knowledge of the regional geology, led to an essentially correct interpretation of subrhyolite structure in the strongly discordant underlying Paleozoic rocks in which a mineralized tear fault cuts a strong thrust fault, and to the conclusion that ore stage mineralization was present in substantial amounts in the Paleozoic rocks below the Chief Oxide alteration patch in the quartz latite lava. Subsequent geothermal and geochemical work strengthened this conclusion, and the recent development work of the Bear Creek Mining Company, which sank the Burgin shaft, has shown the presence of Pb-Zn-Ag ore of commercial grade in substantial amount in blind ore bodies below the lava blanket in the Chief Oxide area.

Reported Late Tertiary Thrusting in Northeastern Nevada

In 1950 Wheeler and McNair reported low-angle thrust faulting involving late Tertiary Humboldt lacustrine sediments in two areas in eastern Elko County, Nevada. Since this concept has an important bearing on petroleum exploration, an examination was made of the areas. Spruce Mountain area: Along U. S. Highway 93 in T. 31 N., R. 62 E., about 40 miles south of Wells, a low escarpment paralleling the highway on the east is capped by a 50-75-foot limestone breccia which was apparently believed the basal part of a thrust plate. Study of the lithologic and structural features of the breccia and of its distribution and regional topographic relations shows that it is an old alluvial fan deposit related in origin to the present fan at the mouth of a large canyon draining the western slope of Spruce Mountain. This breccia rests on the eroded surface of gently tilted Humboldt beds, with complete absence of structural features indicative of low-angle thrust faulting. A dissected piedmont fault scarp about 2 miles east of the highway probably accounts for the slight eastern tilt of the lower part of the fan and the exposure by erosion of what is undoubtedly its basal breccia. About 10 miles south, a thrust was reported at the western base of Phalen Mountain in T. 29 N., R. 63 E. Excellent exposures of the contact between the Humboldt beds and the Paleozoic rocks occur along the southwest side of the mountain. Here the basal maroon conglomeratic silt unit of the Humboldt is in depositional contact with the Paleozoic. Minor post-Humboldt normal faulting has occurred. Again, there is complete absence of structural features indicative of major low-angle faulting. Thousand Springs Valley area: No specific localities are given by Wheeler and McNair for this area but the observations made throw grave doubt on the possibility of low-angle thrusts which involve the Humboldt beds. East of Thousand Springs Creek along the western base of Nine Mile Ridge, in Secs. 27 and 34, T. 42 N., R. 66 E., the Paleozoic-Humboldt contact is well exposed. For approximately 3/4 mile along the range front, steeply west-dipping Humboldt beds rest in depositional contact on the Paleozoic. The basal depositional breccia and conglomerate show no evidence of low-angle deformation. West of Thousand Springs Creek along the western edge of T. 42 and 43 N., R. 66 E., ridges of Paleozoic rocks stand above the gravel-covered pediment which cuts across gently to steeply dipping Humboldt beds. The Paleozoic-Humboldt contact was studied at two localities. In Sec. 29, T. 43 N., R. 66 E., Paleozoic limestone occurs as an upfaulted wedge between two intersecting normal faults, one of which continues into the Humboldt sediments. In Sec. 6, T. 42 N., R. 66 E., a high-standing area of Paleozoic rocks has Humboldt beds in normal depositional contact dipping away from it on at least three sides. The Paleozoic is anticlinally folded, and relations indicate that these rocks and the Humboldt beds were folded at the same time. Directly south of the anticlinal area, the lake beds ar faulted against the older rocks. No features indicative of low-angle faulting were found.