GEOLOGY, January 2010 91

INTRODUCTIONHypervelocity impact events consist of the

following stages: contact and shock compres-sion, growth of transient cavity by ejection of crater material, and the fi nal modifi cation of transient cavity (Melosh and Ivanov, 1999). A central uplift is formed during the modifi cation stage if the transient crater is suffi ciently deep and becomes gravitationally unstable (Melosh and Ivanov, 1999; Fig. 1A). The internal struc-ture and topography of central uplifts are geo-logically important because they provide a key record of the ways in which impact melt, rock debris, and large target rock blocks from dif-ferent parts of the transient crater are emplaced and deposited to result in the fi nal form of the crater (e.g., Kenkmann and von Dalwigk, 2000; Lana et al., 2006, 2008). Current models for the emplacement of impact melt involve signifi cant centrifugal impact melt fl ow during the modi-fi cation stage, when the impact debris surges away from the rising central uplift toward the crater rim (e.g., Melosh, 1989). Subsequent resurge of the impact melt back from the crater rim toward the center of the structure is expected to occur after the collapse of the central uplift (Melosh, 1989).

In order to assess the extent of melt resurge toward the crater center on a collapsed central uplift, it is crucial to have empirical constraints on the subsurface geometry of the central uplift and the overlying impact melt and breccias. Structures of central uplifts have been stud-ied by surface geologic mapping (Lana et al., 2003; 2007; Scherler et al., 2006), and geo-physical methods have been applied to study the internal variations of central uplifts of bur-ied impact structures (e.g., Brenan et al., 1975).

However, the subsurface structural variations of well-exposed impact structures are not well defi ned. This is because previous geophysical studies on well-exposed impact structures, on which many current constraints on the internal structure of central uplifts are based, focused primarily on their relatively large-scale fea-tures with typical geophysical measurements taken at >1 km intervals (e.g., Pilkington and Grieve, 1992; Masero et al., 1997; Henkel and Reimold, 2002).

Similarly, the subsurface geometry of deca-meter-scale impact melt bodies on central

uplifts is not well constrained because previ-ous studies on impact melt have been based on mineralogical (e.g., Maier et al., 2006), borehole (e.g., Whitehead et al., 2002), surface observa-tions (e.g., Osinski, 2004), and the characteriza-tion of their physical properties (e.g., Kukkonen et al., 1992) only. Electrical resistivity tomogra-phy (ERT) is well suited for imaging local lat-eral lithologic variations (e.g., Beauvais et al., 1999), and it is also capable of providing high-resolution subsurface images of lithologic units at eroded impact structures. Although measure-ments of electrical resistivity of impact crater materials (e.g., Kukkonen et al., 1992) and mod-els from magnetotelluric studies (e.g., Masero et al., 1997; Campos-Enriquez et al., 2004) have been obtained, there have been hitherto no reports on ERT in impact cratering research.

In order to illustrate the novel application of geoelectric tomography in characterizing the subsurface geometry of central uplifts, we pre-sent fi ve ERT profi les of the shallow subsurface from the central uplift of the 245 Ma Araguainha impact structure in Brazil, the largest complex

Geology, January 2010; v. 38; no. 1; p. 91–94; doi: 10.1130/G30459.1; 4 fi gures; Data Repository item 2010016.© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

*E-mail: vincent.tong@ucl.ac.uk.

Geoelectric evidence for centripetal resurge of impact melt and breccias over central uplift of Araguainha impact structureC.H. Tong1,*, C. Lana2, Y.R. Marangoni3, and V.R. Elis3

1Department of Earth and Planetary Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, UK2Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7620, South Africa3 Instituto de Astronomia, Geofi sica e Ciencias Atmosfericas, USP Rua do Matao, 1226, Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil

ABSTRACTWe present fi ve profi les from electrical resistivity tomography (ERT), with surface con-

straints and gravity data, in the central uplift of the Araguainha impact structure in central Brazil. The central uplift, the overlying polymict breccias, and decameter-scale impact melt rocks are characterized by contrasting ranges of electrical resistivity. Our resistivity model provides empirical evidence that supports the existing model in which impact melt and brec-cias resurged toward the crater center in the fi nal stages of the cratering process. On the basis of our results from the fi rst use of ERT in impact cratering studies, we conclude that the deposition and fl ow of impact melt and breccias over the central uplift were infl uenced by the geometry of the lithologic boundaries in the central uplift.

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Figure 1. A: Schematic snapshots (not to scale) illustrating formation of large impact craters (after Melosh, 1989; Melosh and Ivanov, 1999): growth of transient cavity, ejection of crater material (A1), and subsequent uplift of crater fl oor (A2), and fi nal modifi cation of transient cavity with collapse of central peak (A3). B: Main map showing central uplift of Araguainha impact structure in central Brazil and locations of fi ve resistivity profi les (white dotted lines; A1–A2, B1–B3) and associated gravity measurements (black diamonds). Coordinates of start and end points of resistivity profi les are listed in GSA Data Repository (see text foot-note 1). Inset shows location of Araguainha impact structure. Lithologic boundaries (gray solid lines) are based on results from previous studies (Engelhardt et al., 1992) and surface observations. All resistivity profi les begin from granite core (zero distance in Figs. 2 and 3) and end in sedimentary rocks or polymict breccias.

92 GEOLOGY, January 2010

impact crater found in South America (Fig. 1B). The 12-km-diameter central uplift of the struc-ture consists of a 4-km-wide well-exposed core of alkali Precambrian-Ordovician granites and a collar of Permian-Devonian sandstones of the Paraná Basin (Lana et al., 2006). The con-tact between the alkali granite and the Paraná sediments is partially overlain by polymict and monomict breccias (Engelhardt et al., 1992). Previous geologic mapping has shown that the nature of this contact is discordant, and sedi-mentary strata dip at high angles toward the center of the central uplift (Lana et al., 2007). The sedimentary layers record overturned sedi-mentary features, which are consistent with more than 90° rotation during development of the central uplift (Lana et al., 2007). The strata also record an early stage of large-scale thrust-ing responsible for signifi cant duplications and thickening of the strata around the granite core (Lana et al., 2006, 2008). Similar overturned features have been observed elsewhere, includ-ing those at Vredefort Dome in South Africa (e.g., Wieland et al., 2005).

METHODSWe acquired resistivity data in the study

area (Fig. 1) using the dipole-dipole confi gura-tion with 50 m electrode spacing (two poten-tial electrodes and six current electrodes). We also concurrently acquired gravity data along the fi ve profi les by using a portable LaCoste-Romberg gravimeter. Details of ERT modeling and gravity data processing can be found in the GSA Data Repository.1 Our structural inter-pretation of the profi les in this case study of ERT is based on the systematic changes in the decameter-scale resistivity features resolvable by the tomographic method.

GRANITE-SEDIMENTARY BOUNDARY IN THE CENTRAL UPLIFT

We observe a direct correspondence between the lithology identifi ed on the surface and the subsurface resistivity structures across the well-exposed boundary between the sedi-mentary strata in the collar and the granite core (Fig. 2). The higher-resistivity regions (>400 Ω m) lie immediately below the sedimentary strata as identifi ed from surface observations. In contrast, the granitic rock in the core has a typical resistivity of 150–250 Ω m. The low resistivity of the core, both relative to the resis-tivity of unfractured granite (Pilkington and Grieve, 1992) and to that of the surrounding sedimentary rocks, is consistent with the cor-

responding lower Bouguer gravity in the gran-ite region (Fig. 2). Both low resistivity and low Bouguer gravity are compatible with the results of a recent petrographic study showing that the granite core is strongly fractured (Machado et al., 2008), and our results agree with similar observations from some other impact struc-tures (Pilkington and Grieve, 1992).

The granite-sediment boundary, which coin-cides with a region of high lateral resistivity gradient, is subvertical, dipping slightly out-ward from the core in both resistivity profi les (Fig. 2). This tomographic result provides the fi rst subsurface evidence that the granite-sedi-ment boundary, which is partially covered by breccias (Engelhardt et al., 1992), is a well-defi ned structural interface. This observation suggests that both the granite and sedimentary materials can result in the fi nal core-collar geometry without kilometer-scale brecciation or megablock rotation (see also Lana et al., 2003; Wieland et al., 2005). There is a pro-nounced vertical narrow low-resistivity region within the sedimentary rocks (Fig. 2). Given its vertical geometry and resistivity contrast, this feature is interpreted as a signifi cant frac-ture in the sedimentary strata fi lled with other lithologic materials, which may correspond to cataclastic fault zones or the injected breccia commonly found in and around central uplifts (Dressler and Reimold, 2004). This feature is likely to represent concentric faults that dis-placed sediments outward as observed in other crater structures (Spray et al., 2004). Although it is not possible to determine the width of the interpreted faults owing to resolution limita-tions, the consistent geometry of this structure at all depths suggests that this low-resistivity anomaly is required by the data.

BRECCIAS AND IMPACT MELT BODIES: SUBSURFACE GEOMETRY

Figure 3 shows the granite region, which is characterized by a similar range of resistivity (100–250 Ω m) and Bouguer gravity values (~–67 mGal) as shown in Figure 2. Bouguer gravity values increase consistently away from the granitic areas (~1–3 mGal/km along the pro-fi les), although there are some variations in the local gradient and in the length of the profi les (Figs. 2 and 3). However, we observe three dis-tinct resistivity anomalies in the polymict brec-cias: (1) low-resistivity zones, characterized by anomalously low resistivity of 30–80 Ω m dip-ping toward the granite core (profi les B1 and B3); (2) high-resistivity blocks, characterized by resistivity higher than 300 Ω m (profi les B1–B3); and (3) regions of anomalously high resis-tivity relative to other lithologic units in the pro-fi les (1000–2500 Ω m) in the lower parts of the model (XHRs in profi le B1). Unlike the granite-sediment boundary, the subsurface resistivity structure shown in the granite-breccia profi les does not show clear delineations separating the polymict breccias from the granite, despite the consistent increase in Bouguer gravity from the core toward the breccias (Fig. 3) similar to the pattern observed in Figure 2.

Previous fi eld observations and petrographic studies (Engelhardt et al., 1992) have shown that the polymict breccias at Araguainha con-sist of a mixture of (1) shocked and unshocked sedimentary and granitic rock fragments, and (2) blocks of impact melt rocks. These are the two distinct lithologic components that can best explain the observed contrast in resistivity in terms of porosity and that are also compatible with the geometry of the low-resistivity zones and high-resistivity blocks. The low-resistivity

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1GSA Data Repository item 2010016, methods, experimental geometry and photograph of impact melt rocks, is available online at www.geosociety.org/pubs/ft2010.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

Figure 2. Profi les from electrical resistivity tomography (ERT) and total Bouguer gravity in vicinity of granite-sediment boundary (A1–A2). Solid white lines indicate interpreted core-collar boundary (granite to sedimentary strata) as identifi ed from resistivity models with surface constraints. Dotted white line indicates interpreted fracture with infi ll in sedimentary strata. HRB—high-resistivity block. Lithologic groups are based on surface observations and previous study (Engelhardt et al., 1992). Note that resistivities match at crossing point and that gravity data have an estimated error of 0.15 mGal arising predominantly from uncer-tainties in altitude measurement.

GEOLOGY, January 2010 93

zones are interpreted to represent materials that make up the matrix of the polymict breccias, along with rock fragments in which the high-resistivity blocks are found. The anomalously low resistivity can be explained by the hetero-geneous composition of the polymict matrix, which is characterized by higher porosity linked to the presence of signifi cant microfractures (Engelhardt et al., 1992). The low resistivity is also consistent with values observed in breccias found in other impact structures (Kukkonen et al., 1992). As for the impact melt rocks, they have been shown to have higher resistivity than breccias because of the lower porosity of the crystalline or glassy impact melt matrices (Kuk-konen et al., 1992). From outcrop observations, meter- to decameter-scale blocks of impact melt rock are common features of polymict breccias in several parts of the Araguainha impact struc-ture (Engelhardt et al., 1992; Fig. DR1 in the Data Repository), and these blocks have also been observed in other impact structures (e.g., Osinski et al., 2005).

The resistivity of the high-resistivity blocks is consistently one order of magnitude higher than that of the surrounding low-resistivity zones in all three profi les (profi les B1–B3). Our results indicate that the high-resistivity blocks are unlikely to represent the highly fractured granite in the core because of its signifi cantly lower resistivity (Figs. 2 and 3). A high-resis-tivity block of similar size and resistivity is also observed near the granite-sediment boundary (profi le A1), and for the same reasons, the fea-ture is likely to represent a block of impact melt.

For this interpretation, each high-resistivity block may represent a single decameter block or multiple blocks of impact melt with an equiva-lent effective resistivity.

Anomalously high-resistivity regions at the bottom of profi le B1 are likely to represent the sedimentary target rocks because they are the only rock type found around the granite core that simultaneously satisfi es the observed resis-tivity, size, and geometry. First, the anomalously high-resistivity regions are highly unlikely to represent the heavily fractured granite core given their high resistivity. Second, although the anomalously high-resistivity regions have resis-tivity values similar to some interpreted impact melt rocks, their physical dimensions (spanning hundreds of meters laterally) are not compatible with those of the impact melt rocks. Third, the anomalously high-resistivity regions are char-acterized by the presence of vertical fractures similar to those found in profi le A2. This con-fi guration is consistent with the emplacement geometry of sedimentary strata onto the granite core. The higher resistivity of this interpreted sedimentary strata compared to those in profi les A1–A2 can be explained by the reduced fl uid-fi lled fractures at greater depths.

EMPLACEMENT OF BRECCIAS AND IMPACT MELT BODIES OVER CENTRAL UPLIFT

We observe that the low-resistivity zones and high-resistivity blocks are arranged as subho-rizontal structures, gently dipping toward the granite core (profi les B1–B3). In profi le B3, we

note that adjacent high-resistivity blocks show a similar dipping geometry. The gently dipping geometry of the high-resistivity blocks and low-resistivity zones indicates that these rocks were deposited during the fi nal stages of structural uplift or immediately after the target rocks ceased to move. Given recent stratigraphic and structural evidence for structural uplift of the granite core and its subsequent gravitational collapse to form a peak-ring structure (Lana et al., 2007, 2008), the inclined orientation of both the high-resistivity blocks and low-resistivity zones toward the core in different locations from the polymict breccias (profi les B1–B3) refl ects the topography of the crater fl oor, where signifi cant amount of rock fragments and melt rocks accumulated (Engel-hardt et al., 1992). The observed sharp change in resistivity from the anomalously high-resistivity regions to the rock units above (profi le B1) there-fore defi nes the morphology of the crater fl oor immediately above the sedimentary collar of the central uplift; this is consistent with the interpre-tation that anomalously high-resistivity regions represent the sedimentary strata.

A large portion of the allogenic materi-als accumulated in the vicinity of the granite-sediment boundary, which is a clear depression related to the rotation and outward collapse of the sedimentary target (Lana et al., 2008). Our surface observation shows that impact melt rocks found in the polymict breccia region have fl ow textures (GSA Data Repository Fig. DR1). Given that the impact structure is well preserved (Lana et al., 2006), the observed dipping geom-etry of the polymict breccias and impact melt implies that the molten material and breccias fl owed toward the inner part of the peak ring and accumulated near the granite-sediment boundary (Fig. 4). This fl ow is compatible with the observation that parts of the high-resistivity blocks overlie the granite-sediment boundary and cover parts of the granite core or are found in the granitic region (Figs. 1 and 2). Note that the inward-dipping orientation of the struc-tures with fl ow texture in the topographic low at the granite-sedimentary boundary provides evidence for the centripetal resurge. The sup-port for the centripetal fl ow direction does not depend on whether the breccias or impact melts were completely removed from the central uplift or from the topographic low.

Since the polymict breccia region in the topographic low above the crater fl oor is con-sistently characterized by inward-dipping struc-tures (Figs. 1 and 3), the centripetal deposition and the inferred centripetal fl ow of breccias and impact melt (Fig. 4) are representative of the central uplift of the Araguainha impact structure. This model implies that signifi cant amounts of impact debris surged back toward the crater center following the gravitational collapse of the central uplift. This centripetal

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Figure 3. Profi les from electrical resistivity tomography (ERT) and total Bouguer gravity in vicinity of granite-polymict-breccia boundary (B1–B3). Key features identifi ed include high-resistivity blocks (HRBs), low-resistivity zones (LRZs), and anomalously high-resistivity re-gions (XHRs). Dotted white line (B1) indicates signifi cant fractures between XHRs. Lithologic groups are based on surface observations and previous study (Engelhardt et al., 1992). Note that resistivities match at crossing point (profi les B2 and B3) and that gravity data have an es-timated error of 0.15 mGal arising predominantly from uncertainties in altitude measurements.

94 GEOLOGY, January 2010

resurge of impact breccias and melt in the fi nal stages of the impact cratering process suggests that the direction of fl ow and deposition in the fi nal stages of the cratering process was deter-mined by the crater fl oor topography, which is likely to be complex, as revealed in this case study at Araguainha. This is particularly true at the lithologic boundaries for craters with mixed target rocks. The level of topographic complex-ity of crater fl oors is directly related to the ways in which the target rock blocks were emplaced, and the pre-impact rheology of the target rocks is a key factor that determines their geometry of emplacement (Melosh and Ivanov, 1999; Gulick et al., 2008). In other words, the infl uence of the fl ow dynamics of breccias and impact melt on surface-crater morphology varies with the pre-impact rheology. We therefore anticipate differ-ent planetary bodies in the solar system where layered target rocks are common (Senft and Stewart, 2007; Collins et al., 2008) to display distinct patterns of surface morphology in the vicinity of their central uplifts, refl ecting their near-surface pre-impact rheological structures.

ACKNOWLEDGMENTSWe gratefully acknowledge funding received from

Birkbeck, University of London (Faculty of Science Grant); Claude Leon Foundation, South Africa; and FAPESP (São Paulo, Brazil project 05/51530). We thank the Editor Andrew Barth, Joanna Morgan, and an anonymous reviewer for their constructive comments.

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Manuscript received 15 June 2009Revised manuscript received 6 August 2009Manuscript accepted 12 August 2009

Printed in USA

Central uplift

Decameter-scale impact melt bodies

Crater center

Crater rim

Granitecore

10

0 m

Erosion level

Sedimentary collar

Deposition

Impact breccias

Figure 4. Schematic diagram (not to scale) showing centripetal deposition of impact melt and impact breccias in central uplift. This model is based on decameter-scale structures (interpreted impact melt bodies), all of which dip gently toward crater center in topographic low (Fig. 3). Our tomographic model shows that lithologic boundaries within central uplift, which refl ect pre-impact rheological structure, are likely to be related to centripetal deposition. Present erosion level (Lana et al., 2006) is included for com-parison. Note that peak rings and impact melt have been differentially eroded, result-ing in previously observed internal rings exposed on crater fl oor (Lana et al., 2007). Granite-sedimentary contact is discordant (Lana et al., 2006, 2007, 2008).