Tilted block faulting, also called rotational block faulting, is a mode of structural evolution in extensional tectonic events, a result of tectonic plates stretching apart. When the upper

lithospheric A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years o ...

crust experiences extensional pressures, the brittle crust fractures, creating

detachment fault A detachment fault is a gently dipping normal fault associated with large-scale extensional tectonics. Detachment faults often have very large displacements (tens of km) and juxtapose unmetamorphosed hanging walls against medium to high-grade me ...

s. These normal faults express themselves on a regional scale; upper crust fractures into tilted fault blocks, and

ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

lower crust ascends. This results in uplift, cooling, and exhumation of ductilely deformed deeper crust. The large unit of tilted blocks and associated crust can form an integral part of metamorphic core complexes, which are found on both continental and oceanic crust.

Origin of term

The term "tilted block faulting" is a literal description of rotational extension on planar faults, which results in a uniform rotation of faults and crust. Often a "domino-style" stacking of the fault blocks occurs, creating the basis of the terminology.

Formation

Faulting, tilting, and exhumation

During extensional time periods, large, gently-dipping normal faults, called

s, can form due to relative separation of the two sides surrounding the fault. Typically, these faults can have an offset on the order of one to tens of kilometers. As the region continues to experience extensive pressures, there is an isostatic effect which moves ductile crust material underneath the fault complex. This fault system can shear the footwall, creating domal mountain ranges, which on a large scale can develop into formations known as metamorphic core complexes. If extension at the surface exceeds about 50 percent,

decompression melting Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

may permit

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural s ...

s to form; these will deform the footwall, resulting in a complex associated with intrusive and extrusive

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

s. Rocks above the detachment fault form normal faults and, at the same time, shear in a "layer-parallel" motion. This action creates a series of fault blocks, which are progressively tilted as the detachment fault progresses. The fracturing of the fault blocks can occur in a similar time frame or develop progressively.

Erosion and basin fill

As the fault blocks rotate and tip,

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...

occurs, filling the basins that are formed with associated sediment from the block into the "down-dropped corners". The basin-fill occurs concurrently with the exhumation. Calculations examining sediment infill suggest that differences in core complexes can be controlled by erosion rates and hanging wall resistance of the fault. Tilted blocks are formed under specific crustal conditions, where the lower crust is relatively warm, not hot. Hotter crust will lead to a type of formation known as a "rolling hinge" complex. The geometry of the tilted block system can be greatly affected by

subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope move ...

and isostasy.

Examples

Core complexes containing rotational fault blocks occur throughout the world. There are excellent examples in the

Southwestern United States The Southwestern United States, also known as the American Southwest or simply the Southwest, is a geographic and cultural region of the United States that generally includes Arizona, New Mexico, and adjacent portions of California, Colorado, ...

, including Arizona and

Baja California Baja California (; 'Lower California'), officially the Free and Sovereign State of Baja California ( es, Estado Libre y Soberano de Baja California), is a state in Mexico. It is the northernmost and westernmost of the 32 federal entities of Mex ...

. The more than 25 metamorphic core complexes in this region were formed during crustal extension during the mid-

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configurat ...

era. Block faulting of this nature is common in extensional settings and has been found to be an important part of physical geological models from sites around the globe, including Europe and China. Due to the availability and applicability of the systems, interest in core complexes and rotational extension systems remains high.

References