Kattenhorn, S.A. (1998)

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A 3D mechanical analysis of normal fault evolution and joint development in perturbed stress fields around normal faults


Normal faults are frequently segmented at the Earthıs surface, forming discontinuous fault traces that attest to a complex evolutionary history. However, field observations provide only limited information on the 3D characteristics and mechanics of fault systems. Inferences about 3D fault geometries and fault evolution may be difficult to accurately establish from 2D outcrop data. It is necessary to capture the three dimensionality of normal fault systems in order to develop conceptual and mechanical models for normal fault evolution in the Earth's crust. This thesis investigates the role of three dimensionality in the evolution of normal fault systems and joint development in perturbed stress fields induced by slipping normal faults. This is accomplished using numerical models that examine the effect of 3D fault geometry and configuration on fault growth tendency and stress field perturbations. Field observations provide information on fault and joint characteristics and timing relationships that are crucial to the development of a plausible hypothesis of the genetic relationship between faulting and jointing. Finally, 3D seismic data interpretations are used to characterize the 3D geometry of a normal fault system and to corroborate numerical results of fault evolution and mechanical interaction between mutually adjacent fault segments.

Normal faults in nature have a tendency towards being longer than they are tall. I used 3D numerical models of elliptical normal faults in an elastic body in order to investigate the growth tendency of faults in the Earth's crust. The models incorporated the contribution of an increasing lithostatic load with increasing depth on fault slip behavior. The increasing overburden results in a greater frictional resistance to slip with increasing depth along the fault surfaces. As a result, normal faults subject to a constant tectonic tension are more prone to slip near the top than the bottom, and slip distributions are skewed towards the upper fault tip. A corresponding maximum in energy release rate at the upper tip suggests that fault growth will occur preferentially in an up-dip direction, irrespective of fault shape. Such a growth tendency for isolated faults would result in tall faults, unlike what is generally observed in nature.

However, when laterally segmented fault geometries are considered, mechanical interaction effects between the fault segments are shown to overwhelm the effect of the lithostatic load on fault growth tendency. Mechanically interacting fault segments preferentially propagate towards each other and may eventually link together. For the case of multiple fault segments, as is frequently observed in normal fault patterns at the Earth's surface, lateral propagation and linkage would result in composite fault surfaces that are longer than they are tall, in agreement with documented natural examples.

As fault segments slip and grow, they perturb the stress field near to the faults. The associated stress magnitudes may be high enough to cause rock to break, forming joints along the fault. At a field example in Arches National Park, Utah, joints formed almost perpendicular to fault strike, contrary to an expected fault-parallel orientation had the joints formed in the same stress field as the faults. Field relationships confirm the genetic relationship between normal faulting and joint development, and suggest that joints grew in the perturbed stress field around the faults. Numerical analyses indicate that the relative orientation of faults and joints is dictated by the location along the fault tipline in 3D. At the upper tip of an elliptical fault, joints will form with strikes parallel to the strikes of the normal faults. However, at the lateral tips, joints may form perpendicular to fault strike. These end-member cases indicate that a range of joint orientations are possible depending on where along the tipline the stresses are high enough for joints to develop.

The characteristics of the remote stresses are also important in determining the orientations of joints forming near to faults. In numerical models where faults undergo small slip increments (< 30 cm), the perturbed stress field is overwhelmed by the remote field within a short distance of 1 km long faults. If the maximum tensile stress is perpendicular to the faults, joints forming perpendicular to fault strike will not propagate beyond a few meters of the faults. However, in Arches National Park, joints propagated several hundred meters away from faults no more than 4 km long. I therefore investigated the role of the horizontal fault-parallel remote stress on the orientation of joints. As the ratio of fault-parallel to fault-perpendicular stress increases, so too does the distance away from the fault affected by the perturbed field. This is particularly true for relay zones between overlapping fault segments. Joints forming at high angles to fault strike are most likely to continue propagating away from the fault in a relay zone if the fault-parallel stress approaches, or slightly exceeds, the fault-perpendicular remote stress during fault slip. The relative magnitudes of these stresses at any time during the faulting history is probably highly dependent on the tectonic stress state, the lithostatic stresses (which may or may not be isotropic), and the effect of stress relaxation induced by fault slip.

In order to accurately characterize the 3D geometry of a normal fault system, I interpreted faults using a 3D seismic dataset from an oilfield in southern England. My interpretations indicate that fault segmentation is prevalent in 3D, however, lithology can exert a strong control on fault evolution where the stratigraphic section is variable and characterized by thick shale sequences. Fault nucleation was highly controlled by rock type, particularly sandstones and limestones. This led to a predominance of lateral segmentation over vertical segmentation. Multiple slip maxima at the same stratigraphic level on apparently continuous fault surfaces attest to initial segmentation. Mechanical interaction and lateral propagation led to subsequent linkage and the formation of composite faults that are longer than they are tall, in agreement with numerical results. Some degree of vertical segmentation is apparent where faults nucleated at different levels of the stratigraphy, however, linkage of segments across vertical steps was inhibited by the presence of thick shales, which behaved in a ductile manner around fault tips. Lateral propagation in the fault system in southern England was thus enhanced by stratigraphic influences. Mechanical interaction between laterally overlapping fault segments resulted in steep tiplines bounding relay zones, as is predicted from numerical model results of propagation tendencies within relays. Horizontal tiplines are common where vertically stepping segments mechanically interact across shale units.

The integration of seismically interpreted 3D fault geometries, 3D numerical models based upon the principles of linear elasticity, and field observations thus provide a means of accurately characterizing normal fault systems as well as providing insights into the mechanical evolution of segmented normal faults and associated joint development in the Earth's crust. These insights allow 2D outcrop data to be visualized and interpreted in the context of a complex 3D evolutionary process of fault and joint growth.


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Citations:


This thesis has been cited in the following 4 works:


Tselentis, G.-A., Gkika, F., 2005. Boundary element slope instability modeling of Corinth Canal, Greece due to nearby fault activation. In: Brebbia, C.A., Da Conceicao Cunha, M., eds, COASTAL ENGINEERING VII, THE BUILT ENVIRONMENT 78: 368 pp.

Kattenhorn, S.A., Aydin, A., Pollard, D.D., 2000. Joints at high angles to normal fault strike: an explanation using 3-D numerical models of fault-perturbed stress fields. JOURNAL OF STRUCTURAL GEOLOGY 22 (1): 1-23.

Maerten, L., Pollard, D.D., Karpuz, R., 2000. How to Constrain 3-D Fault Continuity and Linkage Using Reflection Seismic Data: A Geomechanical Approach. AAPG BULLETIN 84 (9): 1311-1324.

Kattenhorn, S.A., Pollard, D.D., 1999. Is lithostatic loading important for the slip behavior and evolution of normal faults in the Earth's crust? JOURNAL OF GEOPHYSICAL RESEARCH 104: 28,879-28,898.


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