EFFECT OF LITHOSPHERIC THICKNESS ON THE FORMATION OF HIGH-ANGLE AND LOW-ANGLE NORMAL FAULTS

It is accepted that extension of a homogeneous brittle layer should produce high-angle normal faults. The rotation of the upper parts of such a fault to a low dip requires fault offset that is greater than the local lithospheric thickness. New model calculations indicate that a fault cutting lithosphere thicker than 10-20 km can build up only a few kilometres horizontal throw before a new fault replaces it. Such a small offset leaves the fault little rotated from its initial high dip angle. For thinner lithosphere, the fault offset may be much larger, leading to rotation of the upper, inactive parts of a high-angle fault to a low dip angle. In continents, this can happen where thin upper-crustal lithosphere is decoupled from mantle lithosphere by weak lower crust. The calculations suggest that normal faults begin with high dip angles and only in areas of very thin lithosphere can they be offset enough to produce low-angle fault structures. This prediction is consistent with the occurrence of low-angle faults in areas with higher than normal heat flow.