Growth Of Normal Faults Research Articles

Three-dimensional geometry and growth of conjugate normal faults

Good quality 2-D and 3-D seismic reflection data from the Timor Sea are used to determine the threedimensional geometry, displacement patterns and development of intersecting conjugate normal faults. These data are supplemented by data from previous physical modelling studies. Conjugate structures, which comprise two intersecting opposed-dipping normal faults or fault sets, form synchronously on a geological time scale and develop due to the incidental intersection of the faults, factors which affect both the formation and imaging of these structures include: the fault density, the spatial distribution of opposed-dipping faults, the seismic resolution and the vertical extent of the imaged fault data. Large conjugate structures grow from smaller ones; larger conjugates are associated with more numerous and larger faults than small structures. On the scale of the seismic data (fault throws range from ca 10–400 m). synchronous fault movements are accommodated by a reduction of displacements on discrete fault surfaces towards the fault intersection zone, and a corresponding increase in ductile strain of this region. High strains in the volume proximal to the fault intersection zone are expressed as thinning of stratigraphic units between the conjugate faults, and are believed to be accommodated by numerous small sub-seismic faults. Intersection of two opposed-dipping faults does not prevent their continued synchronous movement and does not result in mechanical locking of the system.

Overlapping Faults, Intrabasin Highs, and the Growth of Normal Faults

Normal fault systems bounding extensional basins are typically adjoined by a series of subbasins separated by intrabasin highs. The strata within these basins form syndepositional anticlines and synclines whose axes are transverse to the strike of the main bounding fault. One possible explanation for these intrabasin highs is that they result from persistent along-strike deficits in fault displacement. Such deficits are incompatible with scaling relationships observed between fault displacement and length based on large populations of faults. We present data from active normal faults within the Basin and Range province and from inactive normal faults of the Newark basin of eastern North America demonstrating a clear correlation between the along-strike position of overlapping splay faults and the location of intrabasin highs as well as syndepositional transverse folds. Summed displacements for all faults within an intrabasin high are comparable to the displacements on faults bounding flanking subbasins. Older synextensional deposits exhibit localized tilt maxima within subbasins flanking an intrabasin high whereas younger units exhibit uniform tilt patterns across the entire region. Footwall elevation profiles, used as a proxy for fault displacement, define uniform arcuate patterns independent of along-strike position of intrabasin highs. These characteristics of hanging walls and footwalls suggest that intrabasin highs do not represent locations of long-term fault-displacement deficits, but rather are the location of anastomosing fault segments, which upon linking together, rapidly compensate for initial displacement deficits by increased displacement distributed over several splay faults.

Formation and growth of normal faults at the divergent plate boundary in Iceland

ABSTRACTThe divergent plate boundary in Iceland is characterized by 40–80 km long and 5–10 km wide swarms of tension fractures (∼102 m long) and normal faults (∼103 m long). The upper part of the crust is mainly composed of lava flows, with abundant columnar joints that are mostly perpendicular to the lava contacts. The lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1° for every 150 m depth in the crust. At the surface of the rift zone the joints are vertical and parallel to the vertical principal stress. Because of tilting of the lava pile, the columnar joints become oblique to this stress, hence becoming potential shear fractures, and form echelon sets at greater depths in the crust. Theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and/or large‐scale tension fractures at crustal depths of 0.5–1.5 km. The width (depth) must be the smallest (controlling) dimension of many faults. Nevertheless, there is a positive linear relation (r = 0.91) between maximum throw and length of the Holocene faults. If the faults grow as self‐similar structures, the throw‐length relationship can be explained by a similar relation between fault length and width.

On the growth of normal faults and the existence of flats and ramps along the El Asnam active fold and thrust system

The combination of detailed topographic leveling on the southwest segment of the El Asnam thrust fault with existing seismic and geologic data implies that the geometry of this fault involves shallow dipping flats and steep ramps. The fault appears to be growing along strike toward the southwest end, where the main shock initiated in 1980. From a depth of about 10 km, the main thrust appears to ramp to the basement‐Cenozoic cover interface on a plane striking N40°E and dipping 50°–55° to the northwest. Along the southwest segment where folding has not yet developed, the thrust continues steeply through the Cenozoic cover to the near surface where it flattens, causing normal faulting. Along the central and northeast segments, which display a more evolved fold structure, the deep thrust probably flattens at a depth of 5–6 km, into a decollement along the Cenozoic‐Jurassic interface before ramping to the surface. The Sara El Marouf and Kef El Mes anticlines have thus formed as fault propagation folds. Normal faults at Beni Rached probably branch with the thrust to maintain kinematic compatibility between the deep ramp and decollement. The greater separation (∼7 km) between the normal faults at Beni Rached and the thrust where it crosses Oued Cheliff than along the southwest segment (∼1 km) reflects the greater depth of the ramp to flat bend. We infer that the September 9, 1954, earthquake activated only the central deep segment of the main thrust together with the Beni Rached normal faults, while that of October 10, 1980, activated the whole system of flat decollements, ramp thrusts and compatibility normal faults. Further complexities of the faulting in map view are related to changes of strike of the thrust (in particular north of Oued Cheliff).

Structure of Tertiary and Pleistocene normal faults in Iceland

Normal faults were measured in three selected areas of the late Tertiary and Pleistocene lava pile in Iceland. The arithmetic mean throw of 315 faults is 8.2 m, the minimum being 0.5 m and the maximum 150 m. About 93% of the faults have throws of less than 20 m. The mean strike of the faults is N31°E, but the mode is N45°E. The faults dip from 42° to 89°, nearly 80% dip between 65° and 79°, and the mean dip is 73°. Fault breccia is common and varies in thickness from 0.05 m to 2 m. There is a positive linear relation between breccia thickness and throw, the linear correlation coefficient being 0.78. It is proposed that the normal faults in Iceland form by two basic mechanisms. One is the development of faults from sets of inclined joints in the lava pile. The initially vertical columnar joints in the lavas gradually decrease in dip with depth in the crust because the regional dip of the lavas increases by about 1° for every 150–170 m depth. The inclined joints form sets of en echelon cracks that may link up through tension cracks that propagate from near the ends of the joints. The other mechanism involves formation and growth of normal faults by coalescence of smaller tension fractures. The great majority of fractures within the active rift zone are tension fractures, but when they attain the length of several hundred meters they normally change into normal faults. It is concluded that normal fault development in Iceland commonly starts at depths of about 1 km in the crust.

Emperor Fracture Zone Morphology by Multi-Beam Sonar

U.S. Naval Oceanographic Office surveys have updated bathymetry for 1300 km of the Emperor Fracture Zone (EFZ). Multi-beam swath mapping sonar provides up to total coverage on survey lines of various spacings. The feature's accurate position is obtained with LORAN-C data plus navigational satellite data. Data are presented at a 200 fm (366 m) contour interval on a Mercator projection. The EFZ apparently was a spreading rift along a paleo-plate boundary of a microplate some 70 m.y. ago. At present, it is a relict feature. The EFZ is a series of secondary and tertiary linear ridges and troughs. The overall trend is from NW to SE. There are three principal secondary segments: from 46°15'N, 172°50'E to 41°30'N, 178°E to 40°30'N; and 180° to 36°45'N, 175°30'W. Apparent pull-apart basins are found within each segment. The growth of listric normal faults may also explain the growth of the intrafracture zone basins. The most complex bathymetry is found where the Chinook Fracture Zone intersects the EFZ.