Dear Dr. Subrahmanyam
Please I would like to ask about
How do vertical (90 degree dip) dip-slip faults; neither strike-slip nor diagonal slip; form?
What are conditions that produce low-angle normal faults at the upper crustal level???
When does lithosphere stop stretching even so extensional stresses are active?
Dear Ahmed, Goodmorning and Fraternal Greetings!
I am happy to answer your question to the best of my knowledge.
the answers are marked 1,2,3 according to your questions as given below. though these questions are thought provoking and bit difficult to answer, I have tried my level best as lot of research is still going on on the subjects what you have asked me.
Dip-slip faults can occur either as "reverse" or as "normal" faults. A normal fault occurs when the crust is extended. Alternatively such a fault can be called an extensional fault. The hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is called a graben. An upthrown block between two normal faults dipping away from each other is called a horst. Low-angle normal faults with regional tectonic significance may be designated detachment faults.
Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. On the other hand, if the displacement is parallel to the dip and at right angles to the strike, then the fault is a dip‐slip fault. However, sometimes the displacement is neither parallel to the strike nor to the dip, and, in such cases the fault is classified as an oblique‐slip fault
Low-angle normal faults (LANFs or detachment faults; dip <30°) are a major class of faults that became widely accepted by earth scientists only in the late twentieth century.
“S” reflecting horizon, at the base of the tilted block layer, progressively cuts into higher levels of the upper continental crust oceanward. This S horizon is best interpreted as a low-angle normal fault within the upper crust with an original dip toward the continent.
Spencer (1984) recognized an isostatic mechanism that tilts LANFs: as the upper-plate load is withdrawn laterally, the footwall progressively rebounds and arches into a dome, rotating the LANF to gentler, even opposite, dips. This “rolling hinge” mechanism was later invoked to rotate steep normal faults to gentle dips, but compelling continental examples are unknown.
Formation of brittle normal faults with low initial dips is an unsolved mechanical problem. It requires (1) rotation of the stress field as the LANF is approached, and/or (2) other special and unusual(?) mechanical, boundary, and initial conditions, or suggests that we do not understand how to apply laboratory results on the fracture of intact rocks to natural time and space scales.
Crust can extend on large, planar, non rotational normal faults that dip at a very low angle (0-20 degrees) through the entire upper seismogenic crust. Very low angle normal faults move aseismically (i.e. by creep), that they are spatially separated from high normal faults (which would otherwise cut them) or that the sample period used is not long enough. These faults are mostly inactive now.
Where the lithosphere undergoes horizontal extension at a normal fault or rifting center, the crust will stretch until faulting occurs, either by a system of normal faults (which creates horsts and grabens) or by a system of listric faults. These fault systems allow the region to stretch, while also decreasing its thickness. A thinner crust subsides relative to thicker, undeformed crust.
I hope these explanations meet your requirement,
Dr. D S Subrahmanyam