Rift Zones and
Extensional Boundaries
Normal Faults
Horsts and Grabens
►
Conjugate pairs of
normal faults produce
ridges called HORSTS
and down-dropped
valleys called GRABENS
Listric Normal Faults and
Detachement Faults
►
►
Small rift valleys can be produced by regular normal faults
Regional extension involves normal faults that curve into a horizontal
orientation at depth (LISTRIC NORMAL FAULTS)
Old versus Current View
The Basin and Range
►
The Basin and Range region (Nevada) is underlain by a series of listric
normal faults that merge into a basal detachment fault
Half Graben
►
An asymmetric basin
that develops when a
single listric normal fault
develops
►
Hanging wall slides
down the fault wall and
rotates
Mesozoic Rift Basins
(e.g., Portland and Newark Basins)
►
The Mesozoic basins that are the source of brownstone
are half graben structures that formed during the rifting
of Pangea
Rollover Anticline
Monoclines
Rift Development
► Heat
build-up in
mantle
► Thermal
expansion
and up-warp of
crust
► Development
of
normal faults that
result in crustal
thinning
► Eruption
lavas
of basaltic
Rift in
Iceland
Rift Zones
“Triple Junction”
Aulocogens
An aulacogen is a
failed arm of a triple
junction of a rift system
Mid-Ocean Ridges:
Extensional Plate Boundaries
Dike Swarms
Rifting of
Pangea