构造地质学双语6faultsets

构造地质学双语6faultsets

Chapter 6 Fault sets

1.Extensional structure

Extensional structure is usually composed of series normal faults due to the streching of the lithosphere. Consequently they are the chief structural components of many sedimentary rift basins where they have major significance for hydrocarbon exploration. They can also be found in deltas, at the rear edges of huge gravitation slumps and slides. The extensional structure can occur in different scale , at different depth and under different geological condition. The main structure patterns will be shown in following:

(1)listic fault

In some situations the faults can become gently dipping at depth so that they have a spoon (or listric) shape. Listric faults can be defined as curved normal faults in which the fault surface is in concave upwards; its dip decreases with depth. These faults also occur in extension zones where there is a main detachment fracture following a curved path rather than a planar path. Hanging wall blocks may either rotate and slide along the fault plane (eg slumps), or they may pull away from the main fault, slipping instead only along the low dipping part of the fault. Roll-over anticlines will often form between bedding planes and the main fault plane as a result of the

(2)grabens and horsts.

The structures are the product of several normal faults forming in close proximity and dipping

in opposite directions. This leads to the subsidence of a hanging wall (graben) between two foot walls (horst). A horst is a raised elongated block of the earth's crust lying between two normal faults, and a graben is an elongated block of earths crust lying between two normal faults. When a horst and graben are beside each other, they are considered to be a horst and graben structural

(3) Domino faults

Normal faults may not always dip in opposite directions when they form. Instead, they may actually dip in similar directions. This will lead to a 'domino' structure in which adjacent faults will slip down relative to the fault next to it. These structures are also known as half-grabens and are accompanied by a tiling of the fault blocks. Sedimentary layers may also bend towards the fault plane.

(4) Detachments and metamorphic core complexes

Although most active normal faults can be shown to dip at angles steeper than 50 degrees, there are examples of very low-angle normal faults. These are often termed "detachments" - although this is a pretty vague term! Detachment faulting is associated with

large-scale extensional

tectonics.

Detachments show gentle dips and often expose high grade metamorphic rocks in their footwalls. These footwalls can be termed metamorphic core complexes. 2.

Thrust structure

Thrusts are reverse faults and commonly dominate the structure of collision mountain belts. Some thrusts have moved a long way - many mountain belts have thrusts that have moved many

tens of kilometers. Thrust faults often move older rocks on top of younger rock and influence the positions of ore deposits and hydrocarbons.