Variations in the geometry, distribution and thickness of Cambrian igneous and sedimentary units within southwest Oklahoma are related to a late Proterozoic - early Paleozoic rifting event which formed the Southern Oklahoma aulacogen. These rock units are exposed in the Wichita Mountains, southwest Olkahoma, located on the northern margin of a Proterozoic basin, identified in the subsurface by COCORP reflection data. Overprinting of the Cambrian extensional event by Pennyslvanian tectonism obsured the influence of pre-existing basement structures and contrasting basement lithologies upon the initial development of the aulacogen

Gilbert, M. C.

Mcconnell, D. A.

English

NASA Technical Reports Server

RELATIONS BETWEEN EXTENSIONAL TECTONICS AND MAGMATISM WITHIN 
THE SOUTHERN OKLAHOMA AULACOGEN 
D.A. McConnell, Center for Tectonophysics and Department of Geology, and 
M.C. Gilbert, Department of Geology, Texas A&M University, College Station, 
Texas 77843. 
Variations in the geometry, distribution and thickness of Cambrian 
igneous and sedimentary units within southwest Oklahoma are related to 
a late Proterozoic - early Paleozoic rifting event which formed the Southern 
Oklahoma aulacogen. These rock units are exposed in the Wichita Mountains, 
southwest Oklahoma, located on 'the northern margin of a Proterozoic basin, 
identified in the subsurface by COCORP reflection data [I]. Overprinting 
of the Cambrian extensional event by Pennsylvanian tectonism obscured 
the influence of pre-existing basement structures and contrasting basement 
lithologies upon the initial development of the aulacogen. 
Gilbert [2] identified the beginning of the rifting stage as the 
rise of a basaltic liquid which formed the intrusive Glen Mountain layered 
complex (GMLC), with an extrusive equivalent, the Navajoe Mountain 
basalt-spilite group (NMBS) (~ig. lb). This was probably accompanied 
by brittle faulting which accomodated extension in the upper crust (not 
shown in Fig. 1). The greater than 4 km thick GMLC [3] was intruded at 
shallow depths and must have resulted in extensive updoming of the overlying 
basement rocks of the Proterozoic basin margin. Thermal expansion must 
have added to the doming component, perhaps exposing the gabbros to erosion. 
The GMLC consists dominantly of heteradcumulate/adcumulate anorthositic 
layers. Abundant strongly laminated plagioclase layers indicate a high 
degree of compaction which may reflect seismicity (faulting) contemporaneous 
with consolidation. The associated NMBS formed uniformly thick (300 m) 
flows across the area [4]. 
Faulting of the area (now in the general vicinity of the Pennsylva- 
nian Frontal Fault zone) on arcuate, concave-upward, normal faults, resulted 
in rotation of the gabbro layering to a north dip as rifting initiated 
within the aulacogen (Fig. lc). Dips estimated from mineralogical layering 
within compositional zones in the GMLC, and from the attitude of the zones 
themselves, are presently about 10 degrees to the north. Cambrian tilting 
would have been at higher angles but would have been rotated back during 
Pennsylvanian compression. Igneous activity was confined to the aulacogen 
and resulted in a relatively higher heat flow than on the surrounding 
craton. This suggests that extension occurred along low-angle normal 
faults analogous to those described for Cenozoic crustal thinning in the 
Basin and Range province [5]. Post-extensional isostatic re-equilibration 
resulted in further erosion of the gabbro body and overlying country rock 
(~ig. Id). The latter may have provided the source for the Meers 
metaquartzite which was then deposited locally in fault block sub-basins. 
A layering discordance is observed between the GMLC and later Roosevelt 
gabbros [2] which show a less well defined mineralogic layering, also 
dipping to the northeast. Therefore, block faulting must have rotated 
the host GMLC before intrusion of the Roosevelt Gabbros. A Cambrian age 
is inferred for faulting which truncated the Proterozoic basin layering 
to the south [I]. In addition, faulting is also documented to have taken ' 
place along strike, to the southeast, at this time [4]. Faulting continued 
during the unroofing erosional event of the two gabbro units because both 
show layering discordance with overlying granitelrhyolite. 
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; +, ,; ~ATBNS I~INAL TECTONICS /MAGMATISM- so OKLA AULACOGEN 
72 McConnell, D.A. and Gilbert, M.C. 
Rhyolite was extruded onto the eroded gabbro unconformity during 
middle Cam5rian. At the southeast end of the aulacogen the rhyolite is 
1500 m thick to the northeast and absent to the southwest, indicating 
that it was laid down in a half graben [4]. North of the Wichita Mountains 
the Earlton rhyolite is known to be more than 1300 m thick, ranging up 
to 4000 m in places, but it is unknown to the south of the area. 
Consequently, a similar half graben structure is postulated for the 
Wichitas, rather than a later tilting and erosional event such as that 
suggested earlier [4]. 
Eruption of the rhyolite may also have been strongly fault controlled. 
Mapping of ash-flow eruptions elsewhere has shown them to be circular 
caldera structures overlying pendant-shaped silicic plutons. No caldera 
structures have been identified in the exposed Wichita Mountains. The 
lack of distinct gravity lows in the otherwise prominent positive Bouguer 
gravity confirm that subcaldera structures are missing. Therefore, the 
rhyolites must have been erupted along linear fissures, perhaps the normal 
faults themselves. 
Within the Wichita Mountains, rhyolite is present overlying GMLC 
but it has not been shown to be as thick as to the northeast. This may 
be a consequence of erosion associated with Pennsylvanian uplift, however, 
the conglomerates derived solely from this erosion have a granitic source 
E63. Alternatively, the intrusion of the Roosevelt gabbros may have 
stabilized the central Wichita 'block' sufficiently to lessen its subsequent 
rotation and subsidence. The largest volume of rhyolites was, therefore, 
laid down in rapidly subsiding fault basins north of the central block 
(~ig. le). COCORP evidence suggests that the rhyolites spread beyond 
the bounding faults of the aulacogen onto the cratonic platform [I, p. 
$111. In the Arbuckle Mountains, to the southeast, no major basic 
intrusives are observed and the rhyolites are contained within the 
boundaries of the faulted region. 
The rhyolite-gabbro contact on the stable central block was favored 
for the intrusion of the Wichita Granite Group as relatively thin (500 
m), but laterally extensive, sills. The granites crystallized from a 
dry, high temperature (900°-10000~+) magma which rose near its liquidus. 
Bulk compositions plot on or near the 4 kb dry boundary curve in the system 
Qtz-Ab-Or suggesting derivation from a source about 10-15 km deep. This 
implies rather large mafic plutons at intermediate depths in the crustal 
column as well as feldspathic-rich "continental" layers available for 
partial melting. The thermal pulse related to the granitelrhyolite 
formation would have been strong enough to have potentially generated 
geothermal gradients of 60-80°c/km within the aulacogen proper. The Meers 
metaquartzite shows variable metamorphism from low-grade involving 
chloritelgreen-white micas through rare andalusite-bearing rocks to 
prevalent sillimanite-cordieritespinel grade. These had previously been 
thought due to contact metamorphism but may additionally reflect a more 
regional event of the higher postulated heat flows. 
Because the granite/rhyolite source is thought to have been at about 
10-15 km, and because the rhyolites may have erupted from the faults, 
it is possible that the listric basal zone for the extensional faults 
was the zone of partial melting itself. This zone would have directly 
overlain the gabbroic mass responsible for the melting and which later 
acted{ as a source for diabase dikes. 
The last igneous activity was a set of late diabase dikes which show 
EXTENSIONAL TECTONICS/MAGMATISM-SO OKLA AULACOGEN 
McConnell, D.A. and Gilbert, M.C. 7 3 
trace element patterns of two interrelated types: 1) a monotonic pattern 
characteristic of primary continental tholeiites and 2) a modified monotonic 
pattern with small positive Eu anomalies. The latter set must have tapped 
crystallizing basaltic magma chambers in the crust. Presumably these 
are the ones providing the heat necessary to produce the silicic melts 
at 10-15 km depths. 
Igneous activity ceased before the onset of deposition in the late 
Cambrian, as diabase dikes do not cut the Reagan sandstone at the base 
of the sedimentary section. This is consistent with a high level crustal 
heating which could then decay relatively rapidly. Normal gradients must 
have been obtained, and thus relative cooling, so that basin subsidence 
could begin. 
The aulacogen trend influenced deposition in southern Oklahoma and 
northern Texas during the lower Paleozoic as the isopach map of the Arbuckle 
group (upper Cambrian-Lower Ordovician) shows thinning from the aulacogen, 
to the southwest (Denison, 1984, personal communication). This thinning 
can not be explained by erosion along Pennsylvanian structures aligned 
east-west  atad ad or Arch, Red River Arch). It,is suggested that the thinning 
occurs across a peripheral bulge which formed a positive feature adjacent 
to the aulacogen during the Lower Paleozoic. This is consistent with 
thermal models of basin evolution [7]. Within southern Oklahoma the axis 
of maximum deposition migrated northward, parallel to the aulacogen trend, 
during the Paleozoic [8, 91. 
Pennsylvanian tectonism uplifted the igneous rocks to the surface 
along 30-40 degree reverse faults [I]. Prior to uplift these rocks would 
have been in the brittle field of deformation. As these rock types would 
have high ultimate strengths in compression it is suggested that they 
failed preferentially along suitably oriented pre-existing lines of 
weakness. Consequently, the faults delineated by COCORP [I] may represent 
reactivated Cambrian normal faults. Rotation along these faults would 
have been in the opposite sense during the Pennsylvanian, than in the 
Cambrian, causing shallowing of the original dips and rotating the granite 
sill until it dipped gently to the southwest in some places. 
REFERENCES 
[I] Brewer, J.A., Good, R., Oliver, J.E., Brown, L.D. and Kaufman, S., 
1983, COCORP profiling across the Southern Oklahoma aulacogen: 
overthursting of the Wichita Mountains and compression within 
the Anadarko Basin; Geology, v. 11, p. 109-114. 
[2] Gilbert, M.C., 1983, Timing and chemistry of the igneous events 
associated with the Southe-rn Oklahoma aulacogen; Tectonophysics, 
v. 94, p. 439-455. 
[3] Powell, B.N. and Phelps, D.W., 1977, Igneous cumu"1ates of the Wichita 
province and their tectonic implications; Geology, v. 5, p. 
52-56. \ 
[4] Ham, W.E., Denison, R.E. and Merritt, C.A., 1964, Basement rocks and 
structural evolution of southern Oklahoma; Okla. Geol. Surv. 
Bull. 95, 302 pp. 
EXTENSIONAL TECTONICS/MAGMATISM-SO %LA AULACOGEN 
McConnell, D.A. and Gilbert, M.C. 
151 Spencer, J.E., 1984, Role of tectonic denudation of warping and uplift 
of low-angle normal faults; Geology, v. 12, p. 95-98. 
[6j Edwards, A.R., 1959, Facies changes in Pennsylvanian rocks along the 
flank of Wichita Mountains; in, Petroleum geology of southern 
Oklahoma - a symposium, v. 2, AAPG, (~ds.1 Mayes, J.W., 
Westheimer, J., Tomlinson, C.W. and Putman, D.M., p. 142-156. 
171 Oxbureh, E.R., ,1982, Heterogeneous lithospheric stretching in early 
history of orogenic belts; in, Mountain building processes, 
(~d.) Hsu, K.J., p. 85-93. 
181 Amsden, T.W., 1975, Hunton Group (Late Ordovician, Silurian and Early 
~evonian) in the Anadarko Basin of Oklahoma; Okla. Geol. Surv. 
Bull. 121, 214 pp. 
[ 9 ]  Hill, G.W., 1984, The Anadarko Basin: a model for regional petroleum 
accumulations; p. 1-23 in, Tech. Proc. 1981 AAPG Mid-Continent 
regional meeting, Oklahoma City Geological Society (Ed.) Borger, 
J.G., 221 pp. 
FIGURE CAPTION 
Figure 1. Development of the Southern Oklahoma aulacogen. 
a. Postulated pre-aulacogen basement configuration; 
b. initial extension accompanying intrusion of GMLC and extrusion 
of NMBS is masked by physical and thermal doming; 
c. continued extension results in faulting and rotation of 
igneous units; 
d. isostatic re-equilibration, intrusion and rotation of 
Roosevelt gabbros; 
e. continued extension results in extrusive rhyolites 
accumulation northeast of central, stabilized Wichita block; 
granite sill intrudes along rhyolitelgabbro contact. 
EXTENSIONAL TECTONICSIMAGMATISM-SO OKLA AULACOGEN 
McConnell, D.A. and Gilbert, M.C. 
PROTEROZOIC BASIN 
SW 
AULACOGEN CRATON 
NE 
Wichita Granite Gp. Glen Mtn.Lqyered Complex 
a Roosevelt Gabbro B Navaloe Mtn Bosalt-Sp~l~te Gp 
U Carlton Rhyolite Gp. 


Relations between extensional tectonics and magmatism within the Southern Oklahoma aulacogen

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