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CENTRO-SYMMETRICAL MATERIAL FLOW DURING IMPACT CRATER MODIFICATION:
STRUCTURAL IMPLICATIONS. T. Kenkmann
1
and I. von Dalwigk
2
,
1
Institut für Mineralogie, Museum für
Naturkunde, Invalidenstrasse 43, D-10115 Berlin, Germany (thomas.kenkmann@rz.hu-berlin.de),
2
Department of
Geology and Geochemistry, Stockholm University, S-10691 Stockholm, Sweden.
Introduction. For complex craters, in contrast to
simple craters, the final crater shape strongly deviates
from the shape of the transient cavity. The collapse of
a transient cavity is driven by gravity forces acting on
a partly fluidized target rock [1]. This collapse induces
the mass transfer of large rock volumes. In distal parts
of a crater downward and inward sliding of huge
masses of rock occurs [1, 2]. In the centre of an im-
pact structure upward movements dominate at the
beginning of the collapse. They are followed by
downward and outward movements when the central
uplift collapses. The particle trajectory field is centro-
symmetrical with respect to the impact centre.
Working hypothesis. During the failure of a tran-
sient impact crater coherent masses (landslides) flow
into the crater cavity. Due to the centro-symmetrical
particle trajectory field individual landslides come in
close contact to each other and collide obliquely along
their sidewalls. As a consequence, steeply inclined
shear zones of a radial orientation are formed. These
radial faults can accommodate horizontal shear be-
tween adjacent landslides (Fig. 1). They also accom-
modate space incompatibilities arising from the con-
verging particle paths. Since a horizontal deposition is
not possible rock masses must be transferred to the
free surface. The transport of material to the free sur-
face takes place along steeply dipping thrust faults
which join the steep radial fault at depth (Fig. 1).
Squeezing-out of material may result in the formation
of morphological ridges of radial orientation (Fig. 1).
In terms of structural geology the resulting geometry
is referred to as a flower structure. Being an ana-
logue to transpression zones of continental strike-slip
zones we denote such ridges in impact craters as ra-
dial transpression ridges (RTR). RTRs may start to
grow where distal terraces form. They end where the
collapsing central uplift overthrusts the inward sliding
masses (Fig. 2). Volume, width and height of the
RTRs should increase towards the centre of an impact
structure.
The opposite effect is expected during the collapse
of the central uplift and the formation of a peak ring
structure when material moves downward and out-
ward. The diverging particle trajectories lead to the
formation of radially-oriented graben or trough struc-
tures which are denoted as radial transtension troughs
(RTT)(Fig. 2). They should occur in the outer area of
a central uplift or peak ring (Fig. 2). Alternatively to
the formation of localized ridges and troughs, the con-
servation of material during movement in a centro-
symmetrical vector field can be achieved by bulk
thickening of the affected material in case of con-
verging vectors or by bulk thinning in case of diverg-
ing vectors. Bulk thickening and thinning takes place
through faulting, or intergranular flow. In the follow-
ing sections we discuss only converging vector fields.
Volumetrical considerations. A simple geometri-
cal model was designed to infer the amount of bulk
thickening and transpression thickening of inward
sliding masses during crater modification. The princi-
ple assumption of this approach is the conservation of
volume of material involved in converging move-
ments. The model is designed for those craters con-
taining a central uplift or a peak ring. We assume that
the down-faulted material does not move to the centre
of the crater itself but stops moving at a certain radius
due to the uplift of the central crater floor and the fol-
lowing collision with the collapsing central uplift (Fig.
2). The geometry of the landslides and RTRs is con-
trolled by the angles  and ß, respectively. For the
RTRs we assume a triangular shape cut by the peak
ring (Fig. 2). A partitioning coefficient k is used to
control whether the space problem is compensated by
bulk thickening of the inward sliding material or by
the formation of RTRs. The model allows to determine
k and the amount of bulk thickening if (1) the hori-
zontal displacement and the average thickness of the
landslide, (2) the tectonic uplift of the RTR, and (3) 
and ß are known. All these parameters can be inferred
from structural investigations.
The Siljan Impact structure: A case study.
Structural investigations at the Siljan Impact Struc-
ture, Sweden, (61°2N, 14°52E) have revealed that
the formation of transpression ridges plays a crucial
role during modification of the transient impact crater.
The centre of the structure consists mainly of shocked
and brecciated Proterozoic granites. They form a to-
pographical high of approximately 30 km in diameter,
which coincides with the former peak-ring [3]. The
centre is surrounded by a ring-shaped depression
which contains deformed sedimentary rocks of Palaeo-
zoic age. The apparent diameter of the impact struc-
ture was assumed to be 52 km [3]. Recent investiga-
tions have shown that fractures and pseudotachylites
related to the impact event can be observed in a zone
of 65 km diameter (Fig. 3)[4]. We regard this diame-
ter as coincident with the final crater diameter. The
diameter of the transient cavity is then ~42 km. We
assume that the Palaeozoic sediments at their present
position had been located at the rim of the transient
Lunar and Planetary Science XXXI
1041.pdf
CENTRO-SYMMETRICAL MATERIAL FLOW DURING CRATER MODIFICATION: T. Kenkmann and I. von Dalwigk
cavity and slumped towards the crater centre when the
transient cavity collapsed. Inward movements of the
deformed Palaeozoic strata are in the order of 6 to 7
km.
The Palaeozoic ring becomes broad at some places
and is restricted to narrow zones at other places (Fig.
3). At those localities characterized by a restricted
width of Palaeozoic exposure, Proterozoic footwall
rocks form narrow bridges which are bordered by ra-
dial faults. The tectonic uplift inferred from stratigra-
phy is 500 m on average. We suggest that these
structural highs were uplifted by oblique compression
during inward sliding and hence represent the relics of
RTRs. At least six RTRs were discovered, dominantly
in the western and northern part of the ring structure.
Using the diameter estimate of 65 km for the final
crater and assuming a thickness of the inward moving
slumps of 500 m (the main detachment horizon is at
the base of the Palaeozoic stratum) we derive a bulk
thickening factor in the order of 1.8-2.0 with respect
to the initial slump thickness. 10-20 % of the addi-
tional volume increment is involved into the formation
of the RTRs and 80-90 % contribute to a bulk thick-
ening of the down-faulted masses.
RTRs in other craters?
To assess the signifi-
cance of RTRs in modified impact structures one has
to identify them in other craters. On Earth, their iden-
tification may dominantly be controlled by the degree
of erosion of the impact crater. Their discovery is most
likely possible if erosion has removed the impact melt
sheet. The
Decaturville Impact structure
, Missouri (6
km

), the
Carswell impact structure
, Canada, (39
km

) and the
Araguainha Dome
, Brazil, (40 km

)
show structural features which are consistent with the
concept of RTRs.
The detection of RTRs on the terrestrial planets is
only possible if they are expressed morphologically.
Most of the complex impact craters display a flat floor
between the terraced wall rim and the central uplift
which is caused by an impact melt sheet covering the
crater floor along the ring depression. A chance to de-
tect RTRs is only given if they break through the im-
pact melt sheet. Small impact craters, lacking an co-
herent impact melt sheet but still showing a complex
shape, are potenially good candidats to detect RTRs.
References
. [1] Melosh, H. J. and Ivanov, B. A.
(1999).
Ann. Rev. Earth Planet. Sci
. 27, 385-415. [2]
Kenkmann, T., et al. In
ESF-Proceeding Volume,
Cambridge
, in press. [3] Grieve, R. A. F. (1988).
Deep drilling in crystalline bedrock
; 1., 328-348. [4]
von Dalwigk, I and Kenkmann, T. (2000).
Nordic
Geol. Meeting, Trondhejm
.
Lunar and Planetary Science XXXI
1041.pdf