Abrupt Change In Dip Research Articles

A Unified Framework for Earthquake Sequences and the Growth of Geological Structure in Fold‐Thrust Belts

AbstractObservations of fold growth in fold‐thrust belt settings show that brittle deformation can be localized or distributed. Localized shear is associated with frictional slip on primary faults, while distributed brittle deformation is recognized in the folding of the bulk medium. The interplay of these processes is clearly seen in fault‐bend folds, which are folds cored by a fault with an abrupt change in dip (e.g., a ramp‐décollement system). While the kinematics of fault‐bend folding were described decades ago, the dynamics of these structures remain poorly understood, especially the evolution of fault slip and off‐fault deformation over different periods of the earthquake cycle. In order to investigate the dynamics of fault‐bend folding, we develop a numerical modeling framework that combines a long‐term elasto‐plastic model of folding in a layered medium with a rate‐state frictional model of fault strength evolution in order to simulate geologically and mechanically consistent earthquake sequences. In our simulations, slip on the ramp‐décollement fault and inelastic fold deformation are mechanically coupled processes that build geologic structure. As a result, we observe that folding of the crust (like fault slip) does not occur steadily in time but is modulated by earthquake cycle stresses. We suggest combining seismological and geodetic observations with geological fault models to uncover how elastic and inelastic crustal deformation generate fault‐bend folds. We find that distinguishing between the elastic and inelastic response of the crust to fault slip is possible only in the postseismic period following large earthquakes, indicating that for most fault systems this information currently remains inaccessible.

Sequence-stratigraphic architectures and sand-body distribution in Cenozoic rifted lacustrine basins, east China

Cenozoic rifted lacustrine basins in east China display three main basin types: (1) basins with steeply dipping boundary fault, whose hanging walls tilt along pivot points; (2) basins with listric boundary faults, whose hanging walls bend along flexural bending points; and (3) basins formed by earlier extensional rifting with later strike-slip movement superimposed. The sequence development is intimately linked to the tectonic movements in the area, where second-order sequences are regionally correlatable from basin to basin and relate to the large-scale tectonic movements in the region. Third-order sequences are related to local tectonic activity and are correlatable within basins, between subbasins, and sometimes, between neighboring basins. Detailed sequence-stratigraphic analysis and mapping of depositional systems demonstrate that sand-body distribution patterns are related to sequence-stratigraphic frameworks. For the three kinds of basins, the positions of pivot point zones, flexural bend zones, and strike-slip faults plus the syndepositional faults all control the distribution of depositional systems, systems tract, and sand bodies. These controlling factors can be attributed to different structural and stratigraphic features that change the accommodation. Structural elements include boundary faults, syndepositional faults, and abrupt changes in dip. Stratigraphic controls include preexisting surfaces with local channelization, paleobathymetric lows, and onlap onto clinoform slopes. The lowstand sand bodies deposited at the downdip end of these controlling factors constitute the current and future exploration targets for conventional reservoirs.

New insights into the mechanics of sill emplacement provided by field observations of the Njardvik Sill, Northeast Iceland

Sills are concordant sheet-like bodies of magma. Their mechanics of emplacement is an important but still not fully understood topic. The well-exposed basaltic Njardvik Sill in the extinct Tertiary Dyrfjöll Volcano in Northeast Iceland offers exceptionally clear insights into the mechanism of sill emplacement. The sill is multiple and consists of at least 7 units (sills) all of which were emplaced along a sharp contact between a rhyolitic intrusion and adjacent basaltic lava flows. Each sill unit was supplied with magma from an inclined sheet. The contacts between the sheets and the sill units are very clear and show that the sill units are much thicker than their feeder sheets. Since the Njardvik Sill consists of separate units, it obviously did not evolve into a homogeneous magma body. Nevertheless, the abrupt change in dip and thickness from inclined sheets to horizontal sills at this particular locality indicates that the earlier sills were influencing the stress field in their vicinity during the subsequent sheet injections. The local stresses around the newly formed sill units forced each of the subsequently injected sheets to change into sills. The Njardvik Sill can be followed laterally in a coastal section for 140 m until it ends abruptly at a fault that cuts the sill. Using these field observations as a basis, a numerical model shows how an inclined sheet opens up the contact between the felsic intrusion and the basaltic lava pile, along which the sill emplacement takes place. The results suggest that sill emplacement is primarily the result of stress rotation at contacts between layers of contrasting mechanical properties. There, the orientation of the maximum principal compressive stress σ 1 is horizontal. Hence, such contacts can represent interfaces along which sill emplacement is encouraged. Once a sill has been emplaced, it extends the stress field with a horizontal orientation of σ 1. Consequently, inclined sheets and dykes injected near the sill will be deflected into sills. The injection frequency of further sill units controls if the sill can grow into a larger magma body by mixing of the newly supplied with the initially injected magma. In case of the Njardvik Sill, the injection frequency was low, so subsequently emplaced sill units can be distinguished.

The Three-Dimensional Dynamics of a Nonplanar Thrust Fault

Advances in computational methodology have made it possible to ex- plore the dynamics of earthquake rupture on nonplanar faults. Using a method that allows geometrical flexibility, we simulate in three dimensions the dynamics of a fault that has an abrupt change in dip with depth. Using a homogeneous prestress on both fault segments, we find that while the resultant final stress field is strongly influenced by the fault bend, the fault slip and low-frequency ground motion are relatively insensitive to the pure dynamic effects of the nonplanar fault. The ground velocity from the nonplanar fault is qualitatively quite similar to that of a planar fault with the same dip angle as the nonplanar fault's shallow segment. As the effects of multiple earthquakes accumulate on this fault, stress concentrations at the fault bend are compounded, but the effect of the free surface on the stress appears to approach a steady state. The results of this study imply that for the prediction of peak ground motion from faults that intersect the surface of the Earth, a bend in the fault at depth may not be a significant factor. The very long term effects of the fault bend are not fully determined, but could lead to complexity in the rupture and slip process in future events.

A new insight on the geometry of subducting slabs in northern Luzon, Philippines

We used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern Luzon and propose a model for describing the geometry of the subducted slab of the Eurasian plate beneath the northern segment of the Manila Trench. We took into consideration some of the observed bathymetric features (i.e. the bend in the trench line of the Manila Trench at 20°N lat. and the collision and subduction of an extinct mid-oceanic-ridge at 16–17°N lat.) and the intraplate deformation pattern in the North Luzon Ridge region in depicting a model that is consistent with most of the observed features and phenomena in the study area. An earthquake catalog covering the period from 1619 to 1997 was used in studying the distribution of strain energy released, while earthquakes covering the instrumental years (1963–1997) were used to analyze vertical distributions of earthquakes in sections. We refined the location of some of the historical earthquakes using macroseismic data. The inclusion of historical records despite their inaccurate locations could prevent misidentification of some regions as aseismic zones or gaps in seismicity especially along structures with long return periods. A focal mechanism database that covers the period from 1963 to 1997 was used to decipher intraplate deformation pattern and subduction process of the slab. The focal mechanism database consists of first motion solutions from previous authors and CMT solutions from Harvard University. Seismicity, strain energy release, and focal mechanism maps and cross-sections were drawn in order to have a three-dimensional visualization of the geometry and stress regimes of seismogenic zones. A new model of the subducted slab of the Eurasian plate beneath the Manila Trench is proposed. The model suggests the collision and subsequent partial subduction of a buoyant plateau at around 20°N lat. to explain the sharp bend in the trench line, the complicated deformation pattern on the overriding plate fronting the bend and the shallow dip of the subducted slab beneath this zone. A tear in the slab is also inferred to be present as evidenced by the observed gap in strain energy release and the abrupt change in dip from shallow to steep south of 18°N lat. The gap in seismicity and strain energy release (65–300 km depth) at around 17°N lat. may be used to infer the trajectory and location of the subducted extinct mid-oceanic ridge (MOR). The aseismic behavior is probably caused by the subducted ridge, which is still hot and is deforming plastically. This is supported by the heatflow data, which shows high values along the extinct MOR. The subducted part of the MOR may serve as the weakest zone where this tear could be localized. The tear may also explain the cause of the abrupt termination of the eastern chain of volcanoes south of 18°N lat. The above model, which is a refinement of the model introduced by Yang et al. [Tectonophysics 258 (1996) 85], is consistent with the observed seismicity and deformation pattern, observed bathymetric features, spatial distribution and geochemical character of volcanism in northern Luzon.

Structural cross sections based on a gravity survey of parts of the Quetico and Wawa subprovinces near Thunder Bay, Ontario

In this study, gravity data from 350 new gravity stations are combined with those from 50 previously surveyed stations in a detailed Bouguer anomaly map of a portion of the Quetico and Wawa subprovinces north and west of Thunder Bay, Ontario.In general, high gravity values characterize the southern and southwestern part of the area where metavolcanic rocks of the Wawa subprovince dominate. Much of the Quetico subprovince forms a broad gravitational low, reflecting extensive exposures of gneisses, schists, and migmatites. Well-defined gravity lows are associated with several granitic intrusive bodies.Three- and [Formula: see text]-dimensional gravity models of subsurface configuration of the density contrasts, representative of major rock units, indicate a trough-like structure for the metavolcanic rocks of the Wawa subprovince. This trough-like structure is flanked by a domical feature in the granitoid rocks to the south. North of the metavolcanic rocks, a succession of low-grade greywackes and slates occupies a basinal structure. These structures form the principal subsurface elements of the Wawa subprovince in this area.The gneisses, schists, and migmatites of the Quetico subprovince form a thick, southward-dipping, wedge-shaped structure that may extend under the structures of the Wawa subprovince. This wedge-shaped structure is underlain by a model unit of greater density representative of mafic gneisses and amphibolites. The denser substratum is modelled with local abrupt changes in dip corresponding in position with the Quetico and Hawkeye Lake faults.

Potential for Significant Oil and Gas Fracture Reservoirs in Cretaceous Rocks of Raton Basin, New Mexico

The Raton basin in New Mexico contains up to 4,700 ft (1,430 m) of marine shale and subordinate sandstone, limestone, coal, and terrestrial shale of Cretaceous age. Clastic reservoir rocks are sparse in the Cretaceous section below the Trinidad Sandstone, and drilling them has, to date, led to disappointing results. However, brittle carbonate- and silica-rich interbeds in the shale intervals are capable, under the proper conditions, of providing highly productive fracture reservoirs containing commercial deposits of oil and/or gas. Other Rocky Mountain basins have fracture reservoirs that have proven to be productive of hydrocarbons where there is maximum curvature of brittle interbeds in shale sequences at fairly shallow depths. Fracture reservoirs can occur along anticlines, synclines, monoclines, and in areas where there is an abrupt change in dip and/or strike. Numerous flexures in the Raton basin could have excellent fracture systems. Carbonate-rich beds of the Greenhorn Limestone, the Codell Sandstone Member, the Fort Hays Limestone Member, and the overall Niobrara Formation appear to be the most widespread and thickest intervals that might develop fracture reservoirs. Silica-rich siltstone and orthoquartzite interbeds in the Graneros, Carlile, and Pierre Shales may provide additional zones capable of fracturing. Minor gas was produced from fracture reservoirs in the Niobrara, Carlile, and Greenhorn formations at the Garcia field in the Raton basin in southern Colorado. In addition, there have been at least 35 shows of oil and/or gas from Cretaceous rocks of the New Mexico part of the Raton basin, with 23 of the shows from the Graneros, Greenhorn, Carlile, Niobrara, and Pierre formations--units mostly lacking indi enous porosity. Cored Pierre Shale in 1 well has vertical fractures with oil stain, and it seems likely that, in most of the other wells, the shales and the limestones, which lack indigenous porosity, have fractures that may contain hydrocarbons. Development of natural gas from fracture reservoirs in the Niobrara Formation is currently taking place in the Denver-Julesburg basin immediately northeast of the Apishapa arch, which forms the northeastern flank of the Raton basin. It should be noted that the Codell Sandstone Member, primarily on the western steeply dipping limb of the Denver-Julesburg basin, has become a new target as a result of several recent commercial oil discoveries in a fractured reservoir environment. Favorable areas for development of fracture reservoirs within the Cretaceous rocks of the Raton basin are along the synclinal bend at the foot of the western limb of the basin, along the hinges of smaller flexures within the basin, and where there are abrupt changes in dip and/or strike. Detailed structure-contour and lithofacies maps are needed to refine the broad targets outlined here.

A model for lower continental crust

This proposed model is based on geological, geophysical and geochemical data. Previous models suggested for the lower continental crust consisted of basalt, gabbro, or charnockitic rocks; however, experimental and field petrological data indicate that the bulk of crustal rocks are metamorphic. A lower crust of heterogeneous metamorphic rocks also agrees with seismic reflection results which show numerous reflections from “layering”. Geothermal conditions favor a “dry” charnockitic or gabbroic lower crust rather than an amphibolitic lower crust because heat production data imply that wet amphibolitic rocks would have a higher heat production than their dry metamorphic equivalents. Relatively high velocities from field and laboratory measurements in such low-density rocks as granite, syenite, anorthosite and granulitic rocks in general imply that the composition of the lower crust is more felsic than gabbro. Variation in seismic velocity and depths from crustal refraction studies and numerous seismic reflections all indicate a highly heterogeneous lower crust. The lower crust, which has traditionally been described as gabbroic or mafic, may consist of such diverse rocks as granite gneiss, syenite gneiss, anorthosite, pyroxene granulite, and amphibolite, interlayered on a small scale, deformed, and intruded by granite and gabbro. Interlayering of these rocks explains the presence and character of seismic reflections. Abrupt changes in dip, tight folding, disruption of layers, intrusion, and changes in layer thickness explain the characteristic discontinuity of deep reflections. Igneous intrusions may be floored by metamorphic rocks. The lower crust consists of a complex series of igneous and metamorphic rock of approximate intermediate composition.

Note sur la geologie des environs d'Arcy-sur-Cure et de Saint-More (Yonne)

Abstract The abrupt change in dip of Rauracian (Jurassic) deposits in the vicinity of Arcy-sur-Cure and Saint-More in Yonne, France, is not due to tectonic dislocation but to differences in the dip of the terminal zone of a Rauracian reef and that of the surrounding marls.