The development of Archean crust remains as one of the significant problems in earth science, and a major unknown concerning Archean terrains is the nature of the deep crust. The character of crust beneath granulite terrains is especially fascinating because granulites are generally interpreted to represent a deep crustal section. Magnetic data from this area can be best modeled with a magnetized wedge of older Archean rocks (granulitic gneisses) underlying the younger Archean greenstone terrain. The dip of the boundary based on magnetic modeling is the same as the dip of the postulated thrust-fault reflection. Thus several lines of evidence indicate that the younger Archean greenstone belt terrain is thrust above the ancient Minnesota Valley gneiss terrain, presumably as the greenstone belt was accreted to the gneiss terrain, so that the dipping reflection represents a suture zone. Seismic data from underneath the granulite-facies Minnesota gneiss terrain shows abundant reflections between 3 and 6 s, or about 9 to 20 km. These are arcuate or dipping multicyclic events indicative of layering

Johnson, R. A.

Pierson, W. R.

Smithson, S. B.

English

NASA Technical Reports Server

1R6-28556
SOME EXAMPLES OF DEEP STRUCTURE OF THE ARCHEAN FROM GEOPHYSICS;
S.B. Smithson, R.A. Johnson and W.R. Pierson, Department of Geology and
Geophysics, Program for Crustal Studies, University of Wyoming, Laramie,
Wyoming 82071
/The development of Archean crust remains as one of the significant
problems in earth science, and a major unknown concerning Archean terrains
is the nature of the deep crust. The character of crust beneath granulite
terrains is especially fascinating because granulites are generally
interpreted to represent a deep crustal sectionTj
Some of the oldest Archean crust in North America is found in the
Precambrian of Minnesota, which has dates of 3.6-3.8 b.y. (1). Here the
ancient Minnesota Valley gneiss terrain is bounded on the north by the Great
Lakes tectonic zone (2) and a series of late Archean greenstone belts. The
Great Lakes tectonic zone was presumed to be a steeply dipping Archean
border but COCORP crustal reflection data (3,4) shows a gently dipping
reflection beneath the zone. Because some of the best crustal reflectors
may be fault zones along which mylonites have formed within the ductile
regime (5,6), the reflection could be caused by a mylonitic thrust fault.
It could equally well be interpreted as a mylonitic listric normal fault,
especially since younger rocks lie on older across the fault. However, the
indication of a major recumbent fold above the fault reflection (4) suggests
that this event is coming from a thrust fault; mylonitization along the
fault zone is the postulated cause of the strong, multicyclic-reflection
(6). jJMagnetic data from this area can be best modeled with a magnetized
wedge of older Archean rocks (granulitic gneisses) underlying the younger
Archean greenstone terrain (-Fig. 1). The dip of the boundary based on
magnetic modeling is the same as the dip of the postulated thrust-fault
reflection. Thus several lines of evidence indicate that the younger
Archean greenstone belt terrain is thrust above the ancient Minnesota Valley
gneiss terrain, presumably as the greenstone belt was accreted to the gneiss
terrain, so that the dipping reflection represents a suture zoneT) This
dipping reflection, however, with a thickness of about 1-2 km, seems
remarkably simple to represent an Archean suture. Its apparent simplicity
may just be a function of resolution in the seismic survey and the postu-
lated suture may be more geologically extensive and broader.
Other dipping events beneath the greenstone terrain may also be reflec-
tions from thrusts. Thus by at least late Archean time, a horizontal tec-
tonic regime was dominant. This may reflect the operation of
plate-tectonics mechanisms, and if greenstone belts are formed around island
arcs, then their presence is more evidence for early plate tectonics.
[[Seismic data from underneath the granulite-facies Minnesota gneiss
terrain shows abundant reflections between 3 and 6 s, or about 9 to 20 km.
These are arcuate or dipping multicyclic events indicative of layeringT]
Flat events continue to 10 s or more than 30 km. The gneisses consist of
granodioritic to tonalitic gneisses with mafic layers or schlieren and gar-
nets (7). Foliation in the gneiss generally dips moderately. Although the
layered sequences underlying the gneiss could be mylonites their arcuate
geometry and general geologic setting suggest that these events are caused
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SOME EXAMPLES OF DEEP STRUCTURE OF THE ARCHEAN FROM GEOPHYSICS 99
Smithson, S.B. et al.
by layered gneisses that could represent a supracrustal sequence (although
with transposed layering). This indicates that no mafic residuum from ana-
texis closely underlies the gneiss unless it is highly heterogeneous. The
significance of layering may rather be to indicate a large-scale migmatite
terrain. This reflection data furnishes important information concerning
what underlies a granulite terrain. The rocks appear to be a heterogeneous,
layered, deformed sequence of rocks with moderately dipping foliation.
Seismic data is available from two areas within the Archean Wyoming
province, the Wind River Mountains which are comprised of some of the oldest
rocks of the Wyoming province, and the Laramie Range. The COCORP Wind River
lines that imaged the Laramide Wind River thrust fault crossed the flank of
a greenstone belt at South Pass (8). Except for one strong event at 2.5 s,
the seismic section does not show many reflections from the upper crust.
This is somewhat surprising because of the different velocity contrasts
expected from within the greenstone belt rocks; however, the explanation for
this might be the steep dips that are present near the surface.
The crust beneath the Wind River Mountains produces discontinuous but
strong reflections all through the crust and complex arcuate events at 8 s.
on line 1A (10). Interpretation of these events on the basis of fold struc-
tures exposed up-plunge in the hanging-wall block of the Wind River uplift
suggests that the deep crust is not significantly different from the folded
high grade metamorphic rocks intruded by granites exposed in the core of the
range. Furthermore, Sraithson et al. (1980) suggest that a crustal thickness
of 35-40 km was attained by 3 b.y. based on the interpretation of complex
fold structure. Constraints on deep crustal development may best be deter-
mined from the reflection data.
A Proterozoic suture zone in southeastern Wyoming marks the border bet-
ween the Archean Wyoming province (which includes the basement rocks of the
Wind River Mountains) and 1.7 b.y. crust to the south (10). The suture zone
consists of a steeply dipping mylonite zone from 1 to 7 km wide (10). This
area has been the site of extensive geophysical studies including acquisi-
tion of seismic reflection and gravity data by the University of Wyoming
(11,12) and COCORP seismic lines (13). These data have provided constraints
on the interpretation of surficial geological features as well as deep
crustal structure. Most of the seismc data comes from the Proterozoic
terrain south of the suture zone. Gravity interpretation indicated that
charnockitic syenite associated with the Laramie anorthosite complex was
underlain at several kilometers depth by mafic rock (14), and later seismic
work showed refractions coming from a high-velocity (mafic) zone. Both
seismic reflection interpretation (.11,13) and gravity interpretation suggest
that the Laramie anorthosite itself is about 6 km thick. Reflections from
depths of 4 to 18 km are found beneath the vast 1.4 b.y. Sherman granite and
indicate the presence of heterogeneities that could represent features such
as xenoliths or the base (or far below the base) of the batholith (11).
100 SOME EXAMPLES OF DEEP STRUCTURE OF THE ARCHEAN FROM GEOPHYSICSSmithson, S.B. et al.
iLMany ambiguities still exist in the interpretation of deep crustal
reflections! Complexities are illustrated by a zone of reflections
resemblingan unconformity on COCORP line 5. When this data is migrated to
move dipping events into proper geometric position, the reflections then
form the shape of a dipping saucer. This could come from a layered mafic
intrusion (15), and modeling, which is generally the only check on a deep
crustal reflection interpretation, indicates that this is a plausible
conclusion (16, Fig. H, p. 96).
The shear zone marking the Proterozoic suture may have been detected by
the CQ63RP reflection survey which suggests that it dips about 50° southeast)
(13). Although Moho depths were picked from the COCORP data, the seismic
data are highly ambiguous and unreliable on this question. Gravity data,
however, indicates a change in crustal structure across the
Archean-Proterozoic suture (12). The gravity field decreases to the
southeast at the suture zone (Fig. 2) and the interpretation of this obser-
vation is that the crust thickens to the south and/or crustal density
decreases to the south (12). Because of Laramide effects, some long-
wavelength thickening of the crust occurs to the south, but an abrupt change
in crustal thickness or change in crustal density or both occurs at the
boundary and has apparently persisted since middle Proterozoic time (12).
The southern Proterozoic province is believed to consist of deeply eroded
roots of a migrating chain of island arcs and a continental margin that
evolved from an Early Proterozoic Atlantic-type passive margin to a con-
vergent margin in the Middle Proterozoic with the accretion of island arc
terrains (10). This is somewhat similar to the early Paleozoic history of
the Appalachians.
GREENSTONE
TERRAIN
N 0
GREAT LAKES
TECTONIC ZONE
GNEISS
TERRAIN
km 10
20J
Figure 1. Interpretation of seismic sections showing structural
relationships between different terrains in Minnesota Archean
separated by a mylonitic thrust zone.
SOME EXAMPLES OF DEEP STRUCTURE OP THE ARCHEAN FROM GEOPHYSICS
Smithson, S.B. et al. 101
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DISTANCE FROM CHEYENNE BELT (Km)
Figure 2. Gravity profiles (A) over Proterozoic suture (Cheyenne
belt) in southeastern Wyoming. Profiles are from three different
areas and show a general decrease in gravity across the Cheyenne
belt. Gravity model B explains the decrease in gravity solely on
the basis of a change in crustal thickness and gravity model C com-
bines a change in crustal density with a small change in crustal
thickness as an alternative interpretation (After Johnson et al.,
1984).
References
1. Goldich, S.S. & Hedge, C.E. 1974. '3,800 myr granitic gneiss in south
western Minnesota1. Nature, 252, p. 467-468.
2. Sims, P.K., Card, K.D., Morry, G.B., & Peterman, Z.E. 1980. 'The Great
Lakes tectonic zone - A major crustal feature in North America'.
Geol. Soc. Amer. Bull. 91, £^  690-698.
3. Gibb, A.K., Payne, B., Setzer, T., Brown, L.D., Oliver, J.E. & Kaufman, S.
1984. 'Seismic-reflection study of the Precambrian crust of central
Minnesota1. Geol. Soc. Am. Bull. 95, p. 280-294.
4. Pierson, W.R. 1984. A geophysical study of the contact between the
greenstone-granite terrain and the gneiss terrain in central Minnesota.
Unpubl. M.sT Thesis, University of Wyoming, Laramie, 84 p.
5. Smithson, S.B., Brewer, J.A., Kaufman, S., Oliver, J., & Hurich, C. 1979.
'Structure of the Laramide Wind River uplift, Wyoming, from COCORP deep
reflection data and from gravity data'. J. Geophys. Res. 84Z p.
5955-5972.
6. Fountain, D.M., Hurich, C.A & Smithson, S.B. 1984. 'Seismic reflectivity
of mylonite zones in the crust'. Geology^ 12, p^ 195-198.
102 SOME EXAMPLES OF DEEP STRUCTURE OF THE ARCHEAN FROM GEOPHYSICS
Smithson, S.B. et al.
7. Grant, J.A. 1972. 'Minnesota River Valley, southwestern Minnesota1, in
Sims, P.K. and Morey G.B., eds. Geology of Minnesota-A centenial
volume. Minnesota Geological Survey, p. 177-198.
8. Condie, K.C. 1972. 'A plate tectonics evolutionary model of the South
Pass Archean greenstone belt, southwestern Wyoming'. 24th Int'l.
Geol. Congr. Sect. _!, p. 10U-112.
9. Smithson, S.B., Brewer, J.A., Kaufman, S., Oliver, J.E. & Zawislak, R.L.
1980. 'Complex Archean lower crustal structure revealed by COCORP
crustal reflection profiling in the Wind River Range, Wyoming1.
Earth Planet. Sci. Lett. JI6, £._ 295-305.
10. Karlstrom, K.E. & Houston, R.S. 1984. 'The Cheyenne belt: Analysis of
a Proterozoic suture in southern Wyoming'. Precambrian Research,
25, £._ 415-446.
11. Smithson, S.B., Shive, P.N., & Brown, S.K. 1977. 'Seismic reflections
from Precambrian crust'. Earth Planet. Sci. Lett. 35, p. 134-144.
12. Johnson, R.A., Karlstrom, K.E., Smithson, S.B., & Houston, R.S. 1984.
'Gravity profiles across the Cheyenne Belt, a Precambrian crustal
suture in southeastern Wyoming'. J. Geodynamics, 1, in press.
13. Allmendinger, R.W., Brewer, J.A., Brown, L.D., Kaufman, S., Oliver,
J.E., & Houston, R.S. 1982. 'COCORP profiling across the Rocky
Mountain Front in southern Wyoming, part 2: Precambrian basement
structure and its influence on Laramide deformation'. Geol.
Soc. Am. Bull. 93, £._ 1253-1263.
14. Hodge, D.S., Owen, L.B. & Smithson, S.B. 1973. 'Gravity interpretation
of Laramie anorthosite complex, Wyoming'. Geol. Soc. Amer. Bull.
84, p. 1451-1464. .
15. Johnson, R.A., Pierson, W. & Smithson, S.B. 'Reprocessing of crustal
reflection data'. Int'l Symp. on Deep Structure of the
Continental-Crust, submitted to American Geophysical Union,
Geodynamics Series.
16. Wong, Y.K., Smithson, S.B. & Zawislak, R.L. 1982. 'The role of
seismic modeling in deep crustal reflection interpretation,
Part I'. University of Wyoming Contributions to Geology, 20,
£._ 91-109.


Some examples of deep structure of the Archean from geophysics

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