Jp. Hogan et al., MAGMA TRAPS AND DRIVING PRESSURE - CONSEQUENCES FOR PLUTON SHAPE AND EMPLACEMENT IN AN EXTENSIONAL REGIME, Journal of structural geology, 20(9-10), 1998, pp. 1155-1168
The level of emplacement and final form of felsic and mafic igneous ro
cks of the Wichita Mountains Igneous Province, southwestern Oklahoma,
U.S.A. are discussed in light of magma driving pressure, lithostatic l
oad, and crustal magma traps. Deposition of voluminous A type rhyolite
s upon an eroded gabbroic substrate formed a subhorizontal strength an
isotropy that acted as a crustal magma trap for subsequent rising fels
ic and mafic magma. Intruded along this crustal magma trap are the A-t
ype sheet granites (length/thickness 100:1) of the Wichita Granite Gro
up, of which the Mount Scott Granite sheet is typical, and smaller plu
tons of biotite bearing Roosevelt Gabbro. In marked contrast to the su
bhorizontal granite sheets, the gabbro plutons form more equant stocks
with flat roofs and steep side wails. Late Diabase dikes cross-cut al
l other units, but accompanying basaltic flows are extremely rare in t
he volcanic pile. Based on magmastatic calculations, we draw the follo
wing conclusions concerning the level of emplacement and the shape of
these intrusions. (1) Magma can rise to a depth at which the magma dri
ving pressure becomes negligible. Magma that maintains a positive driv
ing pressure at the surface has the potential to erupt. (2) Magma asce
nt may be arrested at a deeper level in the crust by a subhorizontal s
trength anisotropy (i.e. crustal magma trap) if the magma driving pres
sure is greater than or equal to the lithostatic load at the depth of
the subhorizontal strength anisotropy. (3) Subhorizontal sheet-intrusi
ons form along crustal magma traps when the magma driving pressure gre
atly exceeds the lithostatic load. Under such conditions, the magma dr
iving pressure is sufficent to lift the overburden to create the neces
sary space for the intrusion. (4) Thicker steep-sided stocks or bathol
iths, with flat roofs, form at crustal magma traps when the magma driv
ing pressure approximates that of the lithostatic load. Under these co
nditions, the necessary space for the intrusion must be created by oth
er mechanisms (e.g. stoping). (5) Subvertical sheets (i.e. dikes) form
when the magma driving pressure is less than the lithostatic load at
the level of emplacement. (C) 1998 Elsevier Science Ltd. All rights re
served.