The nature of the impact process has been inferred through the study of the geology of a wide variety of impact crater types and sizes. Some of the largest craters known are the multiring basins found in ancient terrains of the terrestrial planets. Of these features, those found on the Moon possess the most extensive and diverse data coverage, including morphological, geochemical, geophysical, and sample data. The study of the geology of lunar basins over the past 10 years has given us a rudimentary understanding of how these large structures have formed and evolved. The topics covered include basin morphology, basin ejecta, basin excavation, and basin ring formation

Spudis, Paul D.

English

NASA Technical Reports Server

LPI Contribution No. 790 69
References: [1] Trendall A. F. (1983) In Iron-Formations:
Facts and Problems (A. F. Trendall and R. C. Morris, eds.), 69-129.
Dsevier. Amsterdam. [2] Simonson B. M. (1992) GSA Bull.. 104.
in press. [3] Goode A. D. T. (1981) In The Precambrian Geology of
the Southern Hemisphere (D. R. Hunter, ed.), 105-203. Elsevicr,
Amsterdam. [4] Simonson B. M. and Goode A. D. T. (1989)
Geology. 17. 269-272. [5] Simonson B. M. et al (1992) Precam-
brian Res., in press. [6] Hassler S. W. (1991) Ph.D. dissertation.
University of California, Santa Barbara. [7] Hassler S. W. (1992)
Precambrian Res., in press. [8] Amdt N. T. et al (1991) Australian
J. Earth Sci.. 38. 261-281.
SAR IN SUPPORT OF GEOLOGICAL INVESTIGATIONS
OF THE SUDBURY STRUCTURE. V. Singhroy', R.
Mussakowski2, B. O. Dressier1. N. F. Trowcll', and Richard
Grieve4, 'Canad*Centrefor Remote Sensing Department of Energy,
Mines and Resources of Energy, Mines and Resources, Canada,
1Provincial Remote Sensing Office, Ontario Ministry of Natural
Resources, Canada, 'Onurio Geological Survey Ministry of Northern
Development and Mines, Canada, Oeological Survey of Canada,
Department of Energy, Mines and Resources, Canada,
Imaging radar is an important contributing source of information
for arangeof geological problems and environments. Airborne SAR
and ERS- 1 data integrated with other geoscience datasets are being
used in an attempt to characterize the crustal fracturing associated
with the Sudbury structure. This presentation highlights examples
of integrated and composite images aimed at facilitating the inter-
pretation of the Sudbury structure. This work is the result of an
ongoing cooperative multidisciplinary SAR study of the basin
carried out by the Canada Centre for Remote Sensing, Ontario's
Provincial Remote Sensing Office, the Ontario Geological Survey,
and die Geological Survey of Canada.
/&
VISCOSITY DETERMINATIONS OF SOME FRICTIONAL-
LY GENERATED SILICATE MELTS: IMPLICATIONS
FORSLIPZONERHEOLOGYDURINGIMPACT-INDUCED
FAULTING. John G. Spray, Department of Geology, University
of New Brunswick, Fredericton, New Brunswick, Canada.
Analytical scanning electron microscopy, using comb
ergy dispersive and wavelength dispersive spectrometry, has been
used to determine the major-element compositions of some natural
and artificial silicate glasses and their crystalline equivalents de-
rived by the frictional melting of acid to intermediate pro tol i ths. The
major-element compositions are used to calculate the viscosities of
their melt precursors using the model of Shaw [1] at temperatures
of 800°-1400eC. with Fe»VFe(tot) = 0.5 and for 1-3 wt% H,O.
These results are then modified to account for suspension effects
(i.e., the presence of mineral and rock clasts) in order to determine
effective viscosities.
The critical factors in controlling the viscosities of the silicate
melts are SiOa and H2O contents and temperature, as has been well
established for silicate melts of magmatic origin. Additionally, for
fault-generated melts, the effects of shear thinning can reduce the
viscosity to a significant degree. At 1200°C, the viscosities range
from 7 p for the more basic melt sample (40 wt% SiO2) to 1 x 105 p
for the more acid melt sample (64 wr% SiO2). These viscosities arc
low and indicate that at least the friction melt derived from the more
basic protolith would have been highly fluid within the slip zone
during displacement The effects of shear thinning at very high
strain rates would reduce these viscosities even further.
Where friction melts are generated during coseismic slip (to
form pseudotachylites) this implies mat the melt may help to
lubricate the sliding interface and dissipate stored strain energy.
These results are contrary to the views of earlier workers, who
suggested that any melts generated by frictional heating would
possess high viscosities and so inhibit slip. Part of this inference was
based on the erroneous assumption thai parudot achy lite generation
involved the bulk fusion of wallrocks. Although a pseudotachylite
matrix plus clasts has a very similar chemistry to the wallrock
lithology, the matrix typically possesses a more basic chemistry and
hence, due to its lower SiOa content, a significantly lower viscosity
than that of its protolith. On the other hand, smaller entrained clasts
(<1 cm diameter) are typically felsic and dominated by quartz.
These results have implications for the generation of
pseudotachylitic breccias as seen in the basement lithologies of the
Sudbury and Vredefort structures and possibly certain dimict lunar
breccias. Many of these breccias show similarities with the more
commonly developed pseudotachylite fault and injection veins seen
in endogenic fault zones that typically occur in thicknesses of a few
centimeters or less. The main difference is one of scale: Impact-
induced pseudotachylite breccias can attain several meters in thick-
ness. This would suggest that they were genera ted underexception ally
high slip rates and hence high strain rates and that the friction melts
generated possessed extremely low viscosities.
Reference: [1] Shaw H. R. (1972) Am. J. Sci.. 272.870-893.
THE LARGE IMPACT PROCESS INFERRED FROM THE
GEOLOGY OF LUNAR MULT1RING BASINS. Paul D.
Spudis, Lunar and Planetary Institute, Houston TX 77058, USA.
The nature of the impact process has been inferred through the
study of the geology of a wide variety of impact crater types and
sizes. Some of the largest craters known are the multiring basins
found in ancient terrains of the terrestrial planets [e.g., 1]. Of these
features, those found on the Moon possess the most extensive and
diverse data coverage, including morphological, geochemical, geo-
physical, and sample data. The study of the geology of lunar basins
over the past 10 years [2-4] has given us a rudimentary understand-
ing of how these large structures have formed and evolved.
Basin Morphology: Basins on the Moon begin to form at
diameters of about 300km, the 320-km-diameter Schrodinger being
an example [5,6]. At these diameter ranges, only two distinct rings
are apparent; the transition diameter at which multiple rings appear
is uncertain, but appears to be between 400 and 500 km in diameter
[6]. Above these diameters, basins possess multiple rings, as few as
three and as many as seven [ 1 3,6] . In every basin, one ring appears
to be very prominent and is believed to correspond structurally to the
topographic rim of complex craters. This ring has various names
(basin rim of [5], Ring IV of [6], MOR of [7]), but corresponds to
the Cordillera ring of the Orientale Basin. Rings inside and outside
this ring are recognized, each having distinct morphology. Basin
inner rings tend to be clusters or aligned segments of massifs,
arranged into a crudely concentric pattern; scarp like elements may
or may not be present Basin outer rings tend to be much more scarp-
like and massifs are rare to absent Within a certain subset of basins
on the Moon (e.g., Crisium [8], Humorum [9]). the main topo-
https://ntrs.nasa.gov/search.jsp?R=19930000996 2020-03-17T10:03:32+00:00Z
70 1992 Sudbury Conference
graphic rim is not evident. These basins appear to have undergone
a different style of postimpact modification, possibly related to
rapidly changing thermal conditions within the Moon 3.9 Ga ago
[8.9].
Basin Ejecta: Basins display textured eject* deposits, extend-
ing roughly an apparent crater radius beyond the main topographic
rim. Ej ecu may display various morphologies, ranging from wormy
to hummocky deposits (e.g., Hevelius Formation of Orientale, Fra
Mauro Formation of Imbrium [5 ]) to knobby textured material (e.g.,
Alpes Formation of Imbrium [5]). The cause of these variations in
ejecta morphology are not known. At Orientale. knobby material is
largely confined within the Cordillera scarp while hummocky
materials appear to be mostly restricted beyond this boundary
[5,10]. However, at Imbrium, both units are restricted beyond the
topographic rim (Apennine ring) and display a curious "bilateral"
double symmetry [1.3]; this relation remains unexplained. Outside
the limits of the basin textured ejecta are found both fields of
satellitic craters (secondaries [5,11]) and light plains deposits
(Cay ley Formation [5]). These materials contain both primary basin
ejecta and local materials, the local materials being predominant
[12].
Impact melt sheets are observed on the floors of relatively
unflooded basins, such as Orientale [1.5,10]. A class of impact melts
in the Apollo sample collections possess basaltic major-element
chemistry, have a KREEP trace-element pattern of varying concen-
tration, and all have ages of about 3.8-3.9 Ga [1,13]. These rocks,
collectively called "lo w- K Fra Mauro" basalts, arc probably related
to basin impact melts [13-15]. Although the exact number is
contentious, at least three major compositional subdivisions of the
LKFM melt group can be recognized; each may correspond to a
different multiring basin, the Imbrium [13,15], Serentitatis [15],
and Nectaris Basins [16]. A curious fact about lunar LKFM melts is
that they cannot be produced through the fusion of known lunar
pristine rock types [13.14], suggesting the occurrence of unknown
crustal lithologies on the Moon. The LKFM melts were probably
generated at middle to lower crustal levels [13,15].
Basin Excavation: The preservation of preexisting topogra-
phy within the main topographic rim provides some constraints on
the size of the excavation cavity of multiring basins. At Orientale,
pre-existing craters and basins can be mapped [2,17,18] within the
Cordillera scarp (950 km diameter) and some structures [17] may
extend inside the outer Rook ring (620 km diameter). These obser-
vations suggest that the excavation cavity for Orientale must have
been less than about 600 km in diameter [2]. The minimum size is
difficult to constrain; the innermost ring (400 km diameter) may
provide a lower limit to cavity size [2]. These constraints observed
at the Orientale Basin are paralleled by similar relations of prebasin
topography preserved within the Imbrium Basin (1160-km main rim
diameter), where the prominence of the Apennine Bench indicates
that the excavation cavity for the Imbrium Basin must be less than
about 800 km in diameter [3]. These data indicate that the excava-
tion cavity of multiring basins is between about 0.4 and 0.6 times the
diameter of the apparent crater diameter [2-4,17].
Basin depths of excavation can be inferred from the composition
of basin ejecta. At Orientale. basin ejecta are very feldspathic,
having normative composition of noritic anorthosiie, and mafic
(basaltic) components cannot be present in quantities greater than a
few percent [2]. Because evidence from other basins [16,20] and
impact melts from Imbrium and Serentitatis [15] suggest a more
mafic crustal composition at depth, this basin ejecta composition
strongly suggests that basin excavation was limited to upper crustal
levels [2]. At Orientale. the crust may be as thick as 100 ± 10 km
[21]; thus, the maximum depth of excavation is on the order of about
50 km, suggesting an effective depth of excavation of about 0.1 ±
0.02 times the diameter of the excavation cavity [2-4]. Data from
well-studied complex craters on the Earth suggest that the excava-
tion cavity of complex craters is on the order of 0.5 to 0.65 times the
diameter of the apparent crater [19]; the maximum depths of
excavation are on the order of 0.09-0.12 times the excavation cavity
diameter [19]. These numbers compare favorably with the admit-
tedly poorly resolved lunar values [ 1-3], a conclusion substantiated
by certain analytical methods [22]. The relatively shallow effective
depths of excavation predicted by these various models account for
the relative paucity of very deep crustal or mantle materials within
the returned Apollo lunar samples [5,23].
Basin Ring Formation: A wide variety of mechanisms has
been proposed to account for the formation of basin rings (see
review hi [ 1,5,7]). In my opinion, none of them are entirely plausible
and the formation of rings constitutes the last great unsolved puzzle
of multiring basin formation. Ring-forming mechanisms can be
divided into two broad groups (see [ 1,6]): (1) forcible uplift due to
fluidizationof the target [5,24]; (2) concentric, brittle fracturing and
failure of the target on regional (megaterraces [25]) to global scales
(lithospheric fracturing [27]). Geological evidence supports por-
tions of all of these models, but none of them completely or
unequivocally. One constraint that has emerged from the examina-
tion of a variety of basins on a number of terrestrial planets is that
basin rings are spaced at a constant factor, namely the famous V2
relation observed between adjacent rings [ 1,6,27]. Originally pro-
posed only for the Orientale Basin [27,28], it has been found to be
valid for all of the terrestrial planets and some icy satellites [1.6].
Because geological evidence supports divergent ring-forming mod-
els, it may be that the ring-locating mechanism is different from the
ring-forming mechanism [6]. Thus, large-scale crustal foundering
(megatemcing) could occur along concentric zones of weakness
created by some type of resonant wave mechanism (fhudization and
uplift); such immediate crustal adjustment could then be followed
by long-term adjustment due to lithospheric fracturing. If the
conundrum of ring genesis can be resolved, we will possess a good
understanding of all of the principal phases of formation of multi-
ring basins.
References: [1] Spudis P.O., in press. The Geology of Multi-
ring Impact Basins. Cambridge Univ. [2] Spudis P. D. et al. (1984)
Proc. LPSC 15th. in JGR. 89. C197. [3] Spudis P. D. et al. (1988)
Proc. LPSC 18th. 155. [4] Spudis P. D. et al. (1989) Proc. LPSC
19th. 51. [5] Wilhelms D. E. (1987) US. Geol. Sitrv. Prof. Pap.
1348. [6] Pike R. J. and Spudis P. D. (\W1) Earth Moon Planets, 39,
\29.[l]CioftS.K..(m\)lnMulti-Ring Basins. Proc.LPS12A(f.
Schultz and R. B. Merrill, eds.). 207. 227, Pergamon. New York.
[8] Spudis P. D. et al. (19*9) LPSC XX. 1042. [9] Spudis P. D. et al.
(1992) LPSC XXIII, 1345. [10] McCauley J. F. (1977) PEP1.15.
220. [11] Wilhelms D. E. (1976) Proc. LSC 7th. 2883. [12] Ober-
beck V. R. (1975) Rev. Geophys. Space Phys.. 13.337 [13] Spudis
P. D. et al. (1991) Proc. LPS. Vol. 21.151. [14] McCormick K. et
al. (1989) Proc. LPSC 19th. 691. [15] Ryder G. and Wood J. A.
(1977) Proc. LSC 8th. 655. [16] Spudis P. D. (1984) Proc. LPSC
15th. in JGR. 89. C95. [17] Schultz P. H. and Spudis P. D. (1978)
LPSC IX. 1033. [18] King J. S. and Scott D. H. (1978) NASA 77V-
79729,153. [19] Grieve R. A. F. et al. (1981) InMutii-Ring Basins.
Proc. LPSC 12A (P. Schultz and R. Merrill, eds). 37, Pergamon,
New York. [20] Spudis P. D. and Davis P. A. (1986) Proc. LPSC
17th. in JGR. 92. E188. [21] Bills B. and Ferrari A. J. (1976) Proc.
LSC 7th, frontispiece. [22] Croft S. K. (1985) Proc LPSC 15th. in
JGR. 90. C828. [23] Taylor S. R. (1982) Planetary Science. LPI,
LPI Contribution No. 790 71
Houston. 481 pp. [24] Baldwin R. B. (1981) In Multi-Ring Basins.
Proc. LPSC12A (P. Schultz and R. Merrill, eds.). 275. Pergamon.
New York. [25] Head J. W. (1974) Moon, 11.327. [26] Melosh H.
J. (1988) Impact Cratering, Oxford. [27] Hartmann W. and Kuiper
G. (1962) Comm. Lunar Planet. Lab.. 1,51. [28] Fielder G. (1963)
Nature. 198.1256. ~
Tj\ ^/'2'^J^ O/'/Uj'^
SUDBURY PROJECT (UNIVERSITY OF MONSTER-
ONTARIO GEOLOGICAL SURVEY): (3) PETROLOGY,
CHEMISTRY, AND ORIGIN OF BRECCIA FORMATIONS.
D. Stoffler1, A. Dcutsch1, M. Avermann1-2, P. Brockmeyer1-2, R.
Lakomy1-2, and V. Muller-Mohr1-2, 'Institut fur Planetologie and
KJeologisch-Palaontologisches Institut, Universitat Munster,
Wilhelm-Klemm-Str. 10 and Correnstr. 24, W-4400 Munster.
Within the Sudbury Project of the University of Munster and the
Ontario Geological Survey [1] special emphasis has been put on the
breccia formations exposed at the Sudbury structure (SS) because of
their crucial role for the impact hypothesis [2]. They were mapped
and sampled in selected areas of the North, East, and South Ranges
of the SS ([3] and Fig. 1 of [2]). The relative strati graphic positions
of these units are summarized in Fig. 1. Selected samples were
analyzed by optical microscopy, SEM.microprobc.XRF and IN A A,
Rb-Sr and Sm-Nd-isotope geochemistry [4], and carbon isotope
analysis.
This abstract summarizes the results of petrographic and chemi-
cal analyses for those stratigraphic units that were considered the
main structural elements of a large impact basin (see [1]).
Basement and Related Breccias (Fig. 1): The crystalline
rocks underlying the Sudbury Igneous Complex (SIC), collectively
called foot wall rocks [5], display three types of impact-induced
effects: (1) An 8-10-km-wide zone with planar deformation fea-
tures in quartz immediately below the SIC indicating peak shock
pressure up to about 20 GPa [6]. (2) An irregular, mostly lens-
shaped, discontinuous heterolithic breccia zone along the contact of
the SIC (Footwall Breccia=FB) that occasionally occurs in dikelike
"intrusions" in the footwall rocks. The breccia matrix is crystalline
with a dioritic composition and intersertal texture in an upper zone
near to the SIC and a tonali tic- to-granitic composition and poikili tic
to granular texture in a lower zone. The matrix texture is caused by
thermal annealing and partial melting due to the overlying melt
complex [7-9]. The clast lithologies in this breccia and its chemical
THICKNESS
800m
600m
700-900 m
100-200 m
5-70 m
300-500 m
0-3001
LITHOLOGY
_—_—_"Onw»Un Formmtion _—_-•_
o o o o o o o o o o o o o
ooo Upper Black Member ° »
O.-O— Q_.Q_ Q _O P .^O.
o o Lower Black Member o
2300m
0-150 m
INTERPRETATION
post-impact
sedimentary fill
reworked breccia material
redeposited suevitic breccias
final fall-back
suevitic breccias
inclusions of melt-breccia
clast rich melt-breccia
inclusions of suevitic breccias
clast free
impact melt sheet
clast rich melt rock
thermally metamorphosed
clastic breccia
dike breccias
brecciated crater floor
Fig. 1. Stratigraphic sequence at the
Sudbury structure with the present genetic
interpretation of the lithological units
(modified from [12]).


The large impact process inferred from the geology of lunar multiring basins

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