Lineations in the brittle regime

Lineations in the Brittle Regime

Some lineations occur only on fracture surfaces. They are not fabric-forming elements and are more characteristic of the brittle regime in the upper crust. These lineations form by mineral growth in extension fractures, as striations carved on the walls of shear fractures and faults, by intersections between fractures and by fracture curvatures that form early during fracturing. 

Mineral lineations in the brittle regime tend to be restricted to fiber lineations, where minerals have grown in a preferred direction on fractures. The growth of minerals on fractures usually requires that the fractures open to some extent, either as true extension fractures or as shear fractures with a component of extension. Furthermore, the minerals must grow in a preferred direction for a lineation to be defined. Minerals such as quartz, antigorite, actinolite, gypsum and anhydrite may appear fibrous on fractures. 

Two perpendicular sets of mineral lineations on a fault surface in serpentinite. The two directions indicate movements under two different stress fields. Leka Ophiolite, Scandinavian Caledonides.

Mineral fibers are found in many extensional or Mode I fractures. The orientation of the fibers is commonly taken to represent the extension direction. Curved fibers are sometimes seen, implying that the extension direction has changed during the course of deformation, or that shear has occurred during or after the formation of the fibers. 

Fiber lineation (talk) in extension fracture where the fibers have grown perpendicular to the fracture walls as the fracture opened.

Even though extension is involved in the formation of fibrous mineral lineations, this does not imply that such lineations are restricted to extension fractures. Because of the irregular shapes of most shear fractures, a component of extension may occur in extensional stepovers, and minerals may grow as walls separate. The mineral fibers then grow on the lee side of steps or other irregularities, precipitated from fluids circulating on the fracture network. Thus,the senseof slipis detectablefrom the relation between fiber growth and fracture geometry.

Formation of fiber lineation in irregular shear fractures. (a) Early stage. (b) Final stage. In (c) the upper wall is removed for inspection. Groove lineations (striations) are found on surfaces that have not opened during faulting.

Non-planar shear fractures can contain extensional (pull-apart) segments where fibers can grow and form a local lineation.

Striations or slickenlines are lineations found on shear fractures and form by physical abrasion of hanging-wall objects into the footwall or vice versa. The smooth and striated slip surface itself is called a slickenside. 

Slickensides with slickenlines, formed by cataclastic shearing of epidote on a post-Caledonian normal fault in the Precambrian of West Norway.

Slickensides tend to be shiny, polished surfaces coated by a 1 mm thick layer of crushed, cohesive fault rock. Hard objects or asperities can carve out linear tracks or grooves known as fault grooves. The term groove lineation can be used for this type of slickenlines. 

Fault grooves on striated fault surface separating limestone from sandstone. Moab Fault, Utah.

Such mechanically formed slickensides may show similarities with glacially striated surfaces. Close examination of many slickensided slip surfaces reveals that they are formed on mineral fill or that they actually are fiber lineations.

There are two principal types of slickenlines: those that form by mechanical abrasion (striations) and those formed by fibrous growth (slip fiber lineations).

Minerals may grow during the movement history of a fault, and it is common to find a combination of fiber lineations and striations. Such lineations may be of the same or different ages, and sometimes two or more different sets of lineations of different minerals exist that may or may not be striated due to mechanical abrasion.

Geometric striae relate to the irregular or corrugated shape of a slip surface. Such irregularities may have a preferred orientation or axis in the slip direction and appear as lineations on an exposed wall. A special type of geometric striae is the cigar-shape seen on the walls of deformation band cluster zones. Geometric striae and physical striae or slickenlines commonly coexist. 

Lineations in a zone of deformation bands. The lineation points down dip and is an expression of the cigarshaped geometry of undeformed
volumes of rock between the deformation bands. Compass for scale. Entrada Sandstone, San Rafael Desert, Utah.

Intersection lineations are found on fractures where the main slip plane is intersected by secondary fractures such as Riedel fractures or tensile fractures. The lines of intersection typically (but not necessarily) form a high angle to the slip direction, in marked contrast to striae and mineral lineations that tend to parallel the slip direction. Finally, we mention a type of lineation that is typical for some deformed limestones. It forms perpendicular to well-developed pressure solution seams where shortening occurs across the fracture surface and is composed of tubular structures known as slickolites. These structures tend to point in the direction of contraction and slickolites are thus a kind of lineation that is kinematically different from the other lineations.
Credits: Haakon Fossen (Structural Geology)

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