David Peacock

Geometry and Development of Relay Ramps in Normal Fault Systems

Relay ramps occur between normal fault segments that overstep in map view. The geometry and evolution of exposure-scale relay ramps are described from the Somerset coast, England, and are compared with larger scale ramps from elsewhere. Relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments; these groups are interpreted as being evolutionary stages. In stage 1, the segments do not interact. Stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp. In stage 3, connecting fractures start to break the relay ramp. Stage 4 is when the relay ramp is destroyed to produce a single fault that has an along-strike bend. These evolutionary stages can develop through ti e, but they can also be seen spatially. A branch line between normal faults or an along-strike bend may represent a stage 4 relay, with progressively earlier stages occurring updip or downdip. Characteristic variability in displacement-distance profiles for fault segments and linked faults accompanies the interaction and linkage processes. Displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps. Relay ramps often contribute to a minimum in total fault displacement at a linkage point.

Mesoscale strike-slip faults and damage zones at Marsalforn, Gozo Island, Malta

Well-exposed strike-slip faults in limestones in the north-western part of Gozo show damage zones that can be grouped into three categories based on their location along faults; tip damage, linking damage and distributed damage. The predominant fracture types within damage zones include extension fractures and secondary faults. Tip damage zones usually show wedge-shaped patterns formed by antithetic faults and extension fractures, commonly accompanied by block rotation. Several fractures are combined at linking damage zones, typically with the concentration of a high intensity of fractures. Structures in distributed damage zones are typically similar to the classical Riedel shear pattern. Evolutionary and 3D models are proposed in terms of the geometries of damage zones for small displacement strike-slip fault zones. Different evolutionary routes depend on fault tip modes and locations.

Modeling Tip Zones to Predict the Throw and Length Characteristics of Faults

A map of faults in a 60 km2 area of the southern North Sea has been produced from three-dimensional seismic data. The faults shown on the map obey power-law cumulative-frequency distributions for throw (power-law exponent, D, ~ 2.7) and length (D ~ 1.1). Simulations have been carried out to correct for sampling biases in the data and to make predictions of the throw and length scaling characteristics of the faults. The most important bias is caused by poor resolution of the small displacement tip zones of faults. The raw data show considerable scatter in their length:throw ratios, but they more closely fit a linear relationship if a length of 500 m is added to each fault, thereby making up for the zones near the fault tips with throws (~ 15 m) below seismic resolution. Further variability in the data may be caused by such geological factors as fault interaction. Tip lengths have been extended to simulate the actual fault pattern in the study area. Maps produced by this procedure can be used to estimate the true connectivity of the fault network. Extending the faults results in greater connectivity than shown by the raw data, which may cause greater compartmentalization of the rock mass. This greater compartmentalization has implications for hydrocarbon exploitation if the faults ©Copyright 1996. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received October 2, 1995; revised manuscript received February 15, 1996; final acceptance July 15, 1996. 2Geomechanics Research Group, Department of Geology, University of Southampton, Southampton Oceanographic Centre, Empress Dock, European Way, Southampton, SO14 3ZH, United Kingdom. 3Present address: British Gas, Gas Research Centre, Ashby Road, Loughborough, Leicestershire, LE11 3QU, United Kingdom. 4Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, United Kingdom. 5Present address: Rock Deformation Research Group, Department of Earth Sciences, University of Leeds, Leeds, LS2 9JT, United Kingdom. Mobil North Sea Ltd. is thanked for funding G. Pickering and for providing seismic data. Funding for field work in Somerset was given by the University of Plymouth. Kevin Biddle, Nancye Dawers, James Handschy, and William Shea are thanked for their careful reviews.

A fractal model and energy dissipation for en echelon fractures

Abstract Observations from the field and the laboratory show that en echelon fractures within fracture zones have a Riedel within Riedel structure. The tensile failure mechanism of formation of en echelon fractures can be described by the pile-ups of shear crack-dislocations. A fractal model can be used to simulate the Riedel within Riedel geometry, allowing the direct measurement of the fractal dimensions of en echelon fracture systems. The energy dissipation of the en echelon fracture system can be deduced using a fractal damage evolution model which explains the evolution process of the system. The fractal nature of the fractures can be used to derive an accurate estimate of total energy dissipation.

Dressel 2-4 Amphorae and Roman Trade with India: the Evidence from Nevasa

This paper examines in detail an assemblage of Roman amphorae sherds recovered from the important Indian site of Nevasa, from the point of view of origin, chronology and implications of trade between the Mediterranean and India. Roman amphorae are crucial, for they provide direct evidence for the movement of agricultural produce of great economic significance, principaly wine, olive oil and various fish products. A detailed study of amphorae can therefore provide vital evidence of the export and import of important foodstuffs not readily available from other sources. It seems likely that all but one of the 63 amphora sherds found at Nevasa belong to the late Republican/early Imperial Dressel 2-4 form, which normally carried wine. Moreover petrological analysis suggests that these vessels are almost certainly Italian in origin and can most likely be dated to between the period 20/25 BC and AD79. The trade in Italian wine to India is mentioned in the Periplus Maris Erythraei, a first century AD shipping guide to the Red Sea, Persian Gulf and Indian Ocean. The amphora finds from Nevasa, and other Indian sites, provides archaeological evidence of this trade and, in addition, clearly indicates that the main thrust of the Mediterranean wine trade to India was in Italian wines. Finally, the Mediterranean chronology of the amphorae are related to the stratigraphical sequence of Nevasa and the possibility is discussed of local industries being connected with the exchange commodities of the Indo-Mediterranean trade.