Basement fault blocks and fault block patterns

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Using magnetics in petroleum exploration
Author	Edward A. Beaumont, S. Parker Gay
Link	Web page
Store	AAPG Store

The basement fault block pattern in sedimentary basins was formed in multiple tectonic and metamorphic episodes during the Archean and Proterozoic eras. Basement tends to control most of the local structure and much of the stratigraphy within the overlying, younger sedimentary section. It is along the shear zones, or block boundaries of the basement, that we generally find the faults or other structures in the overlying sedimentary section. These zones of weakness are periodically reactivated by tectonic stresses or gravitational loading. Consequently, they have influenced depositional patterns and locations of structures throughout geologic time.

Study of shield areas

Figure 1 Landsat and SLAR images of exposed Precambrian crystalline crust. Copyright: Gay courtesy International Basement Tectonics Assoc.

The Landsat and SLAR images of exposed Precambrian crystalline crust (Figure 1 show highly lineated terrains and demonstrate that the linears fall into multiple parallel or subparallel sets of varying strike directions. These overlapping fracture sets cut the basement into blocks of varying shapes and sizes. This collection of basement blocks is the basement fault block pattern.

Figure 2 . Copyright: LaBerge, 1976; courtesy International Basement Tectonics Assoc.

Precambrian surface topography

The intensity of fracturing and mylonitization of the rocks in shear zones explains why these zones generally erode low and why they tend to control the topography of the Pre-Cambrian surface. This surface, in turn, controls much of the structure in the lower part of the sedimentary section through gravitational compaction of the sedimentary rocks.

Canadian Shield example

Figure 2 is a geologic map of an area of crystalline basement in central Wisconsin on the southern edge of the Canadian Shield. Here, outcrops and rock exposures in shallow excavations, roadcuts, etc., abound. It is possible to map the basement geology in considerable detail. Five things stand out:

• A series of parallel to subparallel shear zones has been mapped.
• There is obvious periodicity to the shear zones, the spacing between them varying from about 4–8 km (2.5–5 mi).
• There are rock type changes across these zones.
• The width of the shear zones varies from about length::1 km (LaBerge, personal communication) up to length::2.5 km or more.
• The shear zones and geology truncate abruptly and change style across line A–A′.

See also

• Interpreting magnetic data
• Magnetic field: local variations
• Magnetics: interpreting residual maps
• Magnetics: petroleum exploration applications

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