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The origin of oriented lakes in the Andean foreland, Parque Nacional Torres del Paine (Chilean Patagonia)
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Geomorphology 97 (2008) 502 – 515 www.elsevier.com/locate/geomorph
The origin of oriented lakes in the Andean foreland, Parque Nacional Torres del Paine (Chilean Patagonia) Joseph Gonzales ⁎, Atilla Aydin Stanford University, Geological and Environmental Sciences, 450 Serra Mall, Braun Hall, Bldg 320, Stanford, CA 94305, USA Received 19 January 2007; received in revised form 2 July 2007; accepted 4 September 2007 Available online 11 September 2007
Abstract The Parque Nacional Torres Del Paine and surrounding area in the Magallanes foreland basin in Chilean Patagonia is the site for numerous lakes fed by glaciers and rivers in the Andean highlands to the west. The lakes are elongate and have conspicuously systematic orientations. We hypothesize that the origin of the oriented lakes lies in the fault system, composed of a right-lateral strike-slip fault set oriented 58° from north, a left-lateral strike-slip set oriented 87°, and a thrust fault set oriented 167°, that exists within the underlying rocks. To test this hypothesis quantitatively, we determined the shape and orientation of the lakes by fitting each lake with an ellipse of appropriate aspect ratio, and later with multiple ellipses consistent with the composite geometry of some lakes. We then examined the faults in the area in terms of their kinematics, orientation and distribution. The distribution of lake orientations showed three distinct groups which appear to correspond to the three main fault groups. For lakes fitted with multiple ellipses, the difference in means between the right-lateral, left-lateral, and thrust faults and their corresponding groups of lakes are 3.05°, 1.57°, and 5.17°. Using a Kolmogorov–Smirnov (K–S) statistical test to compare the orientations of faults with respect to the lakes suggests that there is not a strongly significant difference between the fault orientations and the corresponding lake groups. These results indicate that the faults have a profound control on the orientation, shape, and distribution of the lakes. We attribute this to faults and their damage zones being weaker and therefore prone to a faster rate of erosion, and to stress perturbations associated with discontinuous faults resulting in localized high density fracturing and surface subsidence. These results have implications for lake and drainage system morphologies in other foreland basins along the Andes and other similar settings. © 2007 Elsevier B.V. All rights reserved. Keywords: Lake morphology; Faults; Damage zone; Chile
1. Introduction The Andean foreland, and particularly the Parque Nacional Torres Del Paine are the sites for numerous
⁎ Corresponding author. Tel.: +1 650 725 8708; fax: +1 650 725 0979. E-mail address: [email protected] (J. Gonzales). 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.09.001
oriented lakes, ranging from 0.03 km2 to 40 km2 in size. The origins of these lakes and the reason for their alignment are not well known. The only relevant effort in this regard is that of Plafker (1964) who analyzed the orientation, shape, and distribution of the lakes in continental sediments in the Bolivian foreland, and suggested that aerial subsidence or sags in the surface of the sediments overlying fracture-bounded basement blocks may have been the cause for lake alignment.
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In this paper we begin with the premise that the structural and tectonic settings play a profound role in geomorphology through deformation, erosion, and weathering. We test this premise first at the small scale in the Parque Nacional Torres Del Paine by analyzing the geometry and distribution of the lakes, characterizing the structural elements of the area, and examining the interplay between structural elements, particularly faults and their damage zones. We use mechanical principles for stress distributions and damage zones associated with discontinuous faults to support our hypothesis for the oriented lakes in the detailed study area. We then examine a larger area surrounding the national park to see if the results from the detailed study are applicable to the greater area. Finally, we compare the conclusions from this study to the Bolivian foreland and propose that our results may be applicable to other regions with similar structural settings. 2. Geographic and geological settings The study area is located in the Parque Nacional Torres Del Paine, centered about 51°S and 73°W in the eastern part of the N–S-trending Andean orogenic belt in Chilean Patagonia (Fig. 1A). Most of the field data in this study was collected from an area that includes a major syncline, the Silla Syncline, and bounding thrust cored anticlines. Also present are many lakes, including
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Lago Grey, Lago Nordenskjold, Lago Pehoe, Lago Sarmiento, and Lago Toro (Fig. 1B). The Magallanes Basin has been characterized as a retro-arc foreland basin (Wilson, 1991), related to the Andean Orogeny (Fig. 1C). Prior to the orogeny, in the Late Jurassic to the Early Cretaceous, the area had undergone extension related to the breakup of Gondwana, which resulted in the development of a back arc basin (Bruhn et al., 1978, Gust et al., 1985, Fildani and Hessler, 2005). The change from extensional to compressive environments and the creation of the Magallanes Basin is believed to be the result of the eastward progression of Andean orogenic activity. The Magallanes foreland basin was filled in the Late Cretaceous to early Tertiary, with approximately 7 km of sediments (Wilson 1991; Biddle et al., 1986; Crane, 2004). The sediments in the Silla Syncline are part of the Cerro Toro Formation, which is present throughout much of the Magallanes Basin (Fig. 2). The thickness of the Cerro Toro Formation is estimated at 2000–2500 m (Katz, 1963; Wilson, 1991). The age of the formation, based on fossil evidence, is middle to upper Senonian (Katz, 1963), but might be younger based on recent zircon analysis of the underlying Punta Barrosa Formation (Fildani et al., 2003). The lithology of the Cerro Toro Formation is dominantly mudstone and thin bedded fine sandstone turbidites containing several thick (at times greater than 200 m) sequences of conglomerate
Fig. 1. A) Satellite image showing the location of major lakes in the study area. White box is the location of the detailed study area shown in Figs. 2 and 6. B) W–E section across the Andean subduction zone and the Magallanes foreland basin, adapted from Wilson (1991).
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Fig. 2. Geologic map of the Silla Syncline and surrounding area, slightly modified from Wilson (1991). Note location of Silla Syncline within the detailed study area (center of figure).
and coarse sandstone deposits (Crane, 2004). In addition to turbidites, there is evidence for debris and slurry flows within the formation (Crane, 2004). 3. Lake characteristics To investigate a possible relationship between the oriented lakes and faults, we compared the orientations and fabrics of the fault sets in the study area to the orientations, shapes, and distributions of the lakes. To obtain a quantitative measure of lake orientation, we used a MATLAB-based program to fit an ellipse to each lake in the study area (Fig. 3). We took the long axis of the ellipse as the overall lake orientation which we later compared to the fault orientations. The short axis of each ellipse corresponds to the width of a lake and together
with long axis, defines the approximate 2-D shape of each lake in map view. This technique of ellipse fitting is similar to that for determining strain in deformed grains (Mulchrone and Choudhury, 2004). The distribution of lake orientations (Fig. 4A) defines three main groups. The first group, with a mean value of 54.22°, is the most common, making up 45% of the lake orientations. The second group, with a mean value of 88.86°, is the least common at 15%. The third group, with a mean value of 172.69° is intermediate at 40%. Note that in this paper orientations are listed in degrees from north in a clockwise direction (see Table 1). While the ellipse fitting provides information on the length and width of each lake, the depths of most of the lakes are unknown. The only lakes for which the depths are known are for two of the larger lakes in the area,
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Fig. 3. Single ellipses fit to each lake in the study area, with the long axis of the ellipse representing the overall lake orientation. A histogram of the orientation data is presented in Fig. 4A.
namely Lago Grey with a depth of 450 m, and Lago Sarmiento with a depth of 312 m (Corporación Nacional Forestal (CONAF), personal communication, June 10, 2007). 4. Structural characteristics We mapped the major structural features of the Silla Syncline and surrounding area using enlarged aerial photographs as base maps. The resulting map (Fig. 5) shows major north-northwest to south-southeast-trending folds (hereafter referred to as N–S-trending folds) and sub-parallel thrust faults, and NE–SW and E–W-trending strike-slip faults, which are each briefly described below. 4.1. Folds and thrust faults There are a series of folds with north–south axial trends paralleling the Andean orogenic grain in the region,
originally mapped by Wilson (1991). Fold amplitude and wavelengths appear to be interdependent with the changing lithology of the formation, with large wavelength folds (up to 8 km) involving in the coarse-grained units, and smaller ones in the fine-grained units. The Silla Syncline, plunging to the north, is the largest of the mapped folds, with a width of up to 4 km and contains three coarse-grained sequences of the Cerro Toro Formation (Crane, 2004) that form the syncline's core. East and west of the Silla Syncline, the lithology is dominantly composed of mudstone and fine-grained sandstone, interpreted as turbidites (Crane, 2004). These units are cut pervasively by thrust faults, and also display folds with smaller wavelengths than that of the Silla Syncline. In some cases, these folds are associated with a zone of thrust faults at their cores or limbs. The eastern edge of the syncline is bound by a zone of numerous approximately N–S-trending thrust faults and associated folds, oriented sub-parallel to the syncline
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Fig. 4. Histograms for A) lake orientations from single ellipse fits. Groups 1 through 3 have mean orientations of 54.22°, 88.86°, and 172.69° respectively. B) Right- and left-lateral strike-slip and thrust fault data, with mean orientations of 58.06°, 86.79°, and 167.50° respectively. C) Partitioned lake segment orientations from multiple ellipse fits. Groups 1 through 3 have mean orientations of 55.01°, 88.36°, and 172.67°, respectively. D) Large scale lake and river orientations from ellipse fitting. Groups 1 and 3 have mean orientations of 62.38° and 163.26°, respectively.
axis. In this zone the individual thrust faults, with offsets ranging from several cm to about 30 m, are visible and generally accommodate east-directed transport and E–W shortening. In addition to thrust faults, bedparallel slip occurs along layer boundaries and within finer grained sediments. Slickenside orientations and offset layers show that slip occurred in a top to the west sense in the western limb of the Silla Syncline and top to the east sense in the eastern limb, consistent with flexural slip folding. The thrust faults do not occur in the coarse units of the Silla Syncline. Typically, thrust faults zones have calcite or other precipitated minerals, closely spaced cleavages, and many slip surfaces and breccia zones. All these features, together with mudstone host rock, result in weak fault zones making them susceptible for high rates of erosion and relatively lower elevations (Fig. 6A).
striking right- and left-lateral sets (Fig. 5, Table 1). These faults have spectacular geomorphic expressions in the form of incised valleys (Fig. 6B). Although the fault valleys are typically heavily vegetated, covering the fault core, fractures forming the fault damage zone are preserved in some outcrops flanking the valleys. Fig. 7A shows the fracture density within an approximately 6 m-thick conglomerate layer on both sides of a fault valley at the northern end of the Silla Syncline. The figure illustrates that the presence of the damage zone, Table 1 Mean lake and fault orientation data with comparison between undivided, divided, and large scale lake segment orientations Group Mean fault orientation (angle from N)
Divided lake segment orientation (angle from N)
Large scale lake and river orientation (angle from N)
Mean (difference from fault in parentheses)
4.2. Strike-slip faults The study area is cut by many strike-slip faults in an apparent conjugate pattern, with NE–SW- and E–W-
Undivided lake segment orientation (angle from N)
1) 2) 3)
58.06 86.79 167.50
54.22(− 3.84) 88.86(+2.07) 172.69(+5.19)
55.01(− 3.05) 88.36(+1.57) 172.67(+5.17)
62.38(+4.32) none present 159.26(− 8.24)
J. Gonzales, A. Aydin / Geomorphology 97 (2008) 502–515 Fig. 5. Structural map of the study area showing major folds, thrust faults, and strike-slip faults. Structures are dashed where inferred. Note the location of the Silla Syncline within coarse-grained rock units (coarse-grained rock locations from Crane, 2004), and N–S-trending thrust faults within fine-grained rock units. A histogram of fault orientation data is presented in Fig. 4B. 507
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Fig. 6. A) View to the north of thrust faults in fine-grained lithologies and topographic lows. Inset is a map of the lake in (A) showing the surrounding strike-slip and thrust faults. B) View to the east-southeast along a strike-slip fault valley with an axial lake.
where fracture density is greater (one or more fractures every 2 m) than what would be expected if fracture spacing were proportional to layer thickness (1 fracture every 6 m). A photograph supporting the notion of fracture localization associated with strike-slip faults from another location is shown in Fig. 7B. From these examples, we infer that the width of a strike-slip fault valley is an expression of the fault's damage zone such that prominent valleys in the study area are most often the locations of relatively larger (N 5 m slip) strike-slip faults. Both sets of strike-slip faults are found in the coarsegrained units, and to a lesser degree in the fine-grained units (Fig. 5). In terms of orientations, some variation in the left-lateral set from generally E–W to NW–SE occurs in the western part of the study area. Besides their orientations, between the two sets of strike-slip faults there are several differences in terms of length, offset and frequency. In general fault segment length and slip are larger for the right-lateral set, with average values of approximately 450 m and 9 m, respectively. For the leftlateral set, the average segment length and slip are approximately 330 m and 5 m, respectively. Furthermore, the maximum observed slip is 40 m for the right-
lateral set and 8 m for the left-lateral set. The frequency of right-lateral fault segments (Fig. 4B) is greater than that of the left-lateral set, with a ratio of about 3:1. 5. Comparison of the orientations and distributions of lakes and faults Table 1 shows the three main groups of lake orientations from Fig. 4A and the distribution of the fault orientations from Fig. 4B. A comparison of these distributions shows strong similarities between groups 1, 2, and 3 of lake orientations and right-lateral, leftlateral, and thrust fault orientations, respectively. These data sets also show similarity in their relative frequencies: Group 1 lake orientations are three times more common than Group 2 lake orientations, as is the case for the corresponding right-lateral and left-lateral sets. 6. Refined lake segment geometry A cursory survey of lake shapes reveals that most lakes, especially large ones have irregular boundaries, which can be characterized as dog-leg or staircase geometry. Based on the extension of the shoreline boundaries of these lakes
J. Gonzales, A. Aydin / Geomorphology 97 (2008) 502–515 Fig. 7. A) Fracture density from a scanline across a right-lateral fault, indicating the presence of the damage zone. B) An outcrop example of the damage zone, with an increasing number of fractures towards the inner fault zone. Photograph is looking to the west, showing south side of a strike-slip fault valley. Note that the outcrop in (B) is not the location for the data in (A), due to variable topography preventing photographing of the outcrop in (A).
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Fig. 8. Ellipses fit to lake segments with partitioning based primarily on their boundaries and the structures in the surrounding land. A histogram of the orientation data is presented in Fig. 4C.
into the surrounding land, and their spatial correspondence to the faults mapped in Fig. 5, it is apparent that these lakes are compound features and are associated with multiple faults or fault segments. It is therefore sensible to partition these compound lakes into individual component segments and compare their orientations with the corresponding fault orientations, in order to obtain a refined representation of lake geometry. Fig. 8 shows multiple ellipses fit to compound lakes by dividing the lakes into individual components, based on lake boundaries and the fault traces on land in the neighborhood of the lakes. These new segmented data (Table 1, Fig. 4C) provide an improvement in the comparison of mean values for the lake segments and the corresponding faults. This comparison shows that the lake orientation distributions determined by partitioning of compound lakes into their components improves the correlation between the distributions of lake orientations and fault orientations.
7. Statistical analysis Examining the means of the unrefined and refined lake orientations provides a good indication that fault and lake orientations are similar. More importantly, an inspection of the maps provides evidence for the spatial correlation between the faults and the lakes. In addition to the visual spatial correlation and comparison of the means of the orientations, statistical analyses can further illustrate this possible relationship quantitatively. First, to test if each population of faults and lakes are distinct, we used a Kolmogorov–Smirnov (K–S) statistical test between each of the different faults types, as well as each of the three groups of lakes. P values, representing a measure of statistical difference, were 0.0 in all of the comparisons between populations, indicating that there is a significant difference between each of them. Next, using the K–S test we examined if there is a significant difference in orientation among the groups of faults and
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Fig. 9. K–S test comparison percentile plots for fault and lake data. The dashed line in each plot corresponds to lake data and the solid line corresponds to fault data. Plots A through C use unrefined lake data, and plots D through F use refined lake data.
the groups of lakes. Fig. 9A through C show plots comparing the orientations of each fault set to the corresponding groups of unrefined lakes. Fig. 9D through
F show plots comparing the orientations of each fault set with the corresponding groups of refined lakes. Overall, the analysis indicates that while there may be some
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Fig. 10. Ellipses fit to large scale rivers and lakes. The lakes from the previous analysis of the detailed study area (within the white box) are not included here. To compare with large scale structural features, this figure is the same scale as Fig. 2. A histogram of the orientation data is presented in Fig. 4D.
statistical difference between the orientations of lakes and faults, it is not a strong statistical difference. Note that for each strike-slip fault set and their related groups of lakes, the P value increases from the unrefined lake orientations to the refined lake orientations. Specifically the increase is from 0.013 to 0.050 for the right-lateral/ group 1 comparison and from 0.036 to 0.134 for the left-
lateral/group 2 comparison. This indicates that the difference between the fault sets and refined lakes is less significant than that between the faults and unrefined lakes, supporting our discussion in the previous section. Possible reasons that the P value decreased for the thrust fault/group 3 lakes comparison could be related to the small sample size for the thrust faults and a poorly
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Fig. 11. A) Areas of mean stress reduction around a single frictionless fault segment, and between frictionless echelon fault segments, modified from Segall and Pollard (1980). C) Area of reduced mean stress between highly overlapped faults with a low coefficient of friction (0.0001), modified from Ohlmacher and Aydin (1997). D) A single fault, and (E) echelon fault segments showing delineated fault and lake relationships which are consistent with the model results.
constrained relationship between the thrust faults and lake boundaries. 8. Large scale lake and river geometry To determine if there are similarities between fault orientations and other major hydrologic bodies at a larger scale, we used the ellipse fitting technique on the major lakes and rivers in an area of approximately 4100 km2 surrounding the original detailed study area (Fig. 1B), but excluding those lakes and faults already covered in our earlier analyses of the detailed area. Fig. 10 shows the satellite image of the larger area with the ellipses representing the orientation and overall shapes of the major lakes and rivers. The resulting distribution of lake and river orientations (Fig. 4D, Table 1) shows similarities to previous distributions from the detailed study of the small-scale area: Groups 1 and 3 of the lake orientations are well represented and are close to the corresponding fault orientations. However, Group 2 does not appear to be present in the histogram, the possible reason for which will be discussed later. 9. Explanation for dependence of lake geometry on fault geometry Based on orientation, shape, and distribution of the lakes, and those of the fault sets, we suggest that the faults control lake formation and geometry. First, fault zones are almost always associated with damage zones, the sizes of which are proportional to fault slip and related stress perturbations (Scholz, 2002; Knott, 1996). With increasing slip, a fault's damage zone becomes longer and wider with the development of new
fractures (Myers and Aydin, 2004). This increased fracture density weakens the fault zone and makes it susceptible to a faster rate of physical erosion and chemical weathering relative to its surroundings. Second, mechanical models show that faulting related deformation is heterogeneously distributed about the faults due to stress perturbations associated with slip (Segall and Pollard, 1980). The perturbed stress fields result in both uplift and subsidence at certain areas about the faults (Bertoluzza and Perotti, 1997). Third, stress concentrations can also result in local high density opening mode fractures with high connectivity, thereby enhancing the erosion (Cruikshank and Aydin, 1994). A summary of stress perturbations around single and en echelon strikeslip faults with varying degrees of friction is shown in Fig. 11A through C. Specific faults and their overlaps in relation to lake occurrence are shown in Fig. 11D and E, where the fault and lake geometries are similar to the model results discussed above. The locations of these examples are also shown in Fig. 5. 10. Discussion Our field data of the orientations, shapes, and distributions of lakes and faults in the Andean foreland in Chilean Patagonia show strong similarities between the geomorphology of the lakes and the underlying fault structures. The distributions and orientations of the three major lake groups match quite well with the three fault sets (Fig. 4). The thrust faults commonly occur in muddominated base rocks to the east and west of the Silla Syncline, so do the lakes identified as Group 3. The right-lateral strike-slip faults in the detailed study are the most prominent and pervasive, so are the lakes in Group
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1. The left-lateral faults are poorly developed in the detailed study area and similarly the corresponding lakes in Group 2 occur less frequently there (Fig. 4A, B, and C). In fact, this group of poorly developed faults cannot be detected at a larger scale (Figs. 4D and 10). The correspondence between the fault orientations and the lake orientations in the larger area surrounding the detailed study area also holds well, even without the refinement of the image segmentation due to the lack of structural data at that scale. The results highlighted from the modeling of stress perturbations around strike-slip faults are used to rationalize the role of faults on lake morphology. The stress perturbations in combination with the damage zone distribution result in areas of higher fracture density at fault tips and in between overlapping or parallel fault segments, which may make these domains more susceptible to weathering and erosion. In addition, these domains are likely to subside as they are pulled apart (Bertoluzza and Perotti, 1997), resulting in sags, usually called pull-apart basins (Aydin and Nur, 1982). If suitable hydrogeologic conditions were met, such as high rates of rainfall, glacial melt, high water table, and low evaporation, a lake may form. Current glacier bodies and glacial polish on certain outcrops indicate that the glaciers also play a role as a physical agent of the erosion (Harbor et al., 1988) responsible for the lake formation. However, the initial glacier valley locations were likely guided by previously eroded fault valleys or zones of high fracture density. Larger lakes in the study area appear to have more complicated geometries. Many of these complexities are likely due to the interplay between thrust faults and various strike-slip faults (Fig. 6A). Based on field relationships, it is likely that strike-slip faulting and thrust faulting were active simultaneously, although thrust faulting and folding appear to have initiated first (Gonzales, 2006). Having both types of faults active at the same time is consistent with the predominant E–W Andean orogenic compression that is the mutual driving force. The formation of lakes related to thrust faults is not addressed here because the downthrown block of a thrust fault is depressed by definition and therefore the downthrown block of thrust fault is capable of collecting water and the upthrown block is capable of damming the water body. In addition, thrust faults occur in muddominated fine-grained sequences and are therefore weaker than the surrounding stiffer rocks. While this study focuses on one area, the results could provide insights to other oriented lakes observed in Andean foreland and craton which are covered by
young sediments and therefore do not offer unambiguous clues about the basement structures. The best known case for this is the oriented lakes in the Beni Basin in Bolivia, identified by Plafker (1964), where oriented lakes define two prominent orientations; orogen-parallel (NW–SE) and orogen-perpendicular (NE–SW). Plafker noted surface lineaments sharing the same alignment with lake boundaries, and supposed that lineament and lake formation was caused by downdropped fracture-bounded basement fault blocks, or sediment compaction over the basement blocks. In terms of our results in this paper, the two dominant orientations of the lakes in the Beni Basin may be attributed to orogen-parallel thrust faults and one welldeveloped strike-slip fault set nearly perpendicular to the local orientation of the orogenic belt to the west. In contrast to our study, Plafker (1964) did not consider the effect of a damage zone and stress perturbations and the related mechanical weakening and subsidence around the block bounding lineaments. Nevertheless, even though the mechanisms for lake formation are different in our study and in Plafker's, our results corroborate Plafker's suggestion that lake geometries and lineaments in surface deposits can be controlled by structural heterogeneities in underlying basement rocks, and that surface features can be used to infer the structural behavior of the area under the surface deposits. 11. Conclusions Our field data shows a striking correlation between lake orientation, shape, and distribution and the faults. This is supported by visual evidence as well as statistical analyses. It is therefore compelling to conclude that the formation of the lakes in the study area is fault controlled. The nature of the faults, with its implications for erosional processes in the study area, along with models of fault-induced deformation and related surface subsidence, lend support for this interpretation. The proposed correspondence between the lakes and the underlying faults offered in this paper may be helpful in understanding similar phenomena in analogous tectonic settings, for example the Beni Basin in Bolivia, especially if the area is covered by surface deposits obscuring the interrelationship between basement structures and lake geometry. Acknowledgements This work was supported by the Stanford Rock Fracture Project. We would like to thank Kingshuk Choudhury for suggesting the ellipse fitting technique
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and George Hilley for reviewing an early version of the manuscript. We would also like to thank the Corporación Nacional Forestal (CONAF) for their permission to undertake the study and for providing the depth information for Lago Gray and Lago Sarmiento. References Aydin, A., Nur, A., 1982. Evolution of pull-apart basins and their scale independence. Tectonics, 1 (1), 91–105. Bertoluzza, L., Perotti, C.R., 1997. A finite-element model of the stress field in strike-slip basins: implications for the Permian tectonics of the Southern Alps (Italy). Tectonophysics, 280, 185–197. Biddle, K.T., Uliana, M.A., Mitchum Jr., R.M., Fitzgerald, M.G., Wright, R.C., 1986. The stratigraphic and structural evolution of the central and eastern Magallanes Basin, southern South America. In: Allen, P.A., Homewood, P. (Eds.), Foreland Basins: International Association of Sedimentologists Special Publication, vol. 8, pp. 41–63. Bruhn, R.L., Stern, C.R., de Wit, M.J., 1978. Field and geochemical data bearing on the development of a Mesozoic volcano-tectonic rift zone and back-arc basin in southernmost South America. Earth and Planetary Science Letters, 41, 32–46. Crane, W.H., 2004, Depositional history of the Upper Cretaceous Cerro Toro Formation, Silla Syncline, Magallanes Basin, Chile [Ph.D. thesis], Stanford University, Stanford, California. Cruikshank, K.M., Aydin, A., 1994. Role of fracture localization in arch formation, Arches National Park, Utah. Geological Society of America Bulletin 106, 879–891. Fildani, A., Cope, T.D., Graham, S.A., Wooden, J.L., 2003. Initiation of the Magallanes foreland basin: timing of the southernmost Patagonian Andes orogeny revised by detrital zircon provenance analysis. Geology, 31 (12), 1081–1084. Fildani, A., Hessler, A.M., 2005. Stratigraphic record across a retroarc basin inversion: Rocas Verdes-Magallanes Basin, Patagonian Andes, Chile. Geological Society of America Bulletin, 117 (11/12), 1596–1614.
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Geomorphology 97 (2008) 502 – 515 www.elsevier.com/locate/geomorph
The origin of oriented lakes in the Andean foreland, Parque Nacional Torres del Paine (Chilean Patagonia) Joseph Gonzales ⁎, Atilla Aydin Stanford University, Geological and Environmental Sciences, 450 Serra Mall, Braun Hall, Bldg 320, Stanford, CA 94305, USA Received 19 January 2007; received in revised form 2 July 2007; accepted 4 September 2007 Available online 11 September 2007
Abstract The Parque Nacional Torres Del Paine and surrounding area in the Magallanes foreland basin in Chilean Patagonia is the site for numerous lakes fed by glaciers and rivers in the Andean highlands to the west. The lakes are elongate and have conspicuously systematic orientations. We hypothesize that the origin of the oriented lakes lies in the fault system, composed of a right-lateral strike-slip fault set oriented 58° from north, a left-lateral strike-slip set oriented 87°, and a thrust fault set oriented 167°, that exists within the underlying rocks. To test this hypothesis quantitatively, we determined the shape and orientation of the lakes by fitting each lake with an ellipse of appropriate aspect ratio, and later with multiple ellipses consistent with the composite geometry of some lakes. We then examined the faults in the area in terms of their kinematics, orientation and distribution. The distribution of lake orientations showed three distinct groups which appear to correspond to the three main fault groups. For lakes fitted with multiple ellipses, the difference in means between the right-lateral, left-lateral, and thrust faults and their corresponding groups of lakes are 3.05°, 1.57°, and 5.17°. Using a Kolmogorov–Smirnov (K–S) statistical test to compare the orientations of faults with respect to the lakes suggests that there is not a strongly significant difference between the fault orientations and the corresponding lake groups. These results indicate that the faults have a profound control on the orientation, shape, and distribution of the lakes. We attribute this to faults and their damage zones being weaker and therefore prone to a faster rate of erosion, and to stress perturbations associated with discontinuous faults resulting in localized high density fracturing and surface subsidence. These results have implications for lake and drainage system morphologies in other foreland basins along the Andes and other similar settings. © 2007 Elsevier B.V. All rights reserved. Keywords: Lake morphology; Faults; Damage zone; Chile
1. Introduction The Andean foreland, and particularly the Parque Nacional Torres Del Paine are the sites for numerous
⁎ Corresponding author. Tel.: +1 650 725 8708; fax: +1 650 725 0979. E-mail address: [email protected] (J. Gonzales). 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.09.001
oriented lakes, ranging from 0.03 km2 to 40 km2 in size. The origins of these lakes and the reason for their alignment are not well known. The only relevant effort in this regard is that of Plafker (1964) who analyzed the orientation, shape, and distribution of the lakes in continental sediments in the Bolivian foreland, and suggested that aerial subsidence or sags in the surface of the sediments overlying fracture-bounded basement blocks may have been the cause for lake alignment.
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In this paper we begin with the premise that the structural and tectonic settings play a profound role in geomorphology through deformation, erosion, and weathering. We test this premise first at the small scale in the Parque Nacional Torres Del Paine by analyzing the geometry and distribution of the lakes, characterizing the structural elements of the area, and examining the interplay between structural elements, particularly faults and their damage zones. We use mechanical principles for stress distributions and damage zones associated with discontinuous faults to support our hypothesis for the oriented lakes in the detailed study area. We then examine a larger area surrounding the national park to see if the results from the detailed study are applicable to the greater area. Finally, we compare the conclusions from this study to the Bolivian foreland and propose that our results may be applicable to other regions with similar structural settings. 2. Geographic and geological settings The study area is located in the Parque Nacional Torres Del Paine, centered about 51°S and 73°W in the eastern part of the N–S-trending Andean orogenic belt in Chilean Patagonia (Fig. 1A). Most of the field data in this study was collected from an area that includes a major syncline, the Silla Syncline, and bounding thrust cored anticlines. Also present are many lakes, including
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Lago Grey, Lago Nordenskjold, Lago Pehoe, Lago Sarmiento, and Lago Toro (Fig. 1B). The Magallanes Basin has been characterized as a retro-arc foreland basin (Wilson, 1991), related to the Andean Orogeny (Fig. 1C). Prior to the orogeny, in the Late Jurassic to the Early Cretaceous, the area had undergone extension related to the breakup of Gondwana, which resulted in the development of a back arc basin (Bruhn et al., 1978, Gust et al., 1985, Fildani and Hessler, 2005). The change from extensional to compressive environments and the creation of the Magallanes Basin is believed to be the result of the eastward progression of Andean orogenic activity. The Magallanes foreland basin was filled in the Late Cretaceous to early Tertiary, with approximately 7 km of sediments (Wilson 1991; Biddle et al., 1986; Crane, 2004). The sediments in the Silla Syncline are part of the Cerro Toro Formation, which is present throughout much of the Magallanes Basin (Fig. 2). The thickness of the Cerro Toro Formation is estimated at 2000–2500 m (Katz, 1963; Wilson, 1991). The age of the formation, based on fossil evidence, is middle to upper Senonian (Katz, 1963), but might be younger based on recent zircon analysis of the underlying Punta Barrosa Formation (Fildani et al., 2003). The lithology of the Cerro Toro Formation is dominantly mudstone and thin bedded fine sandstone turbidites containing several thick (at times greater than 200 m) sequences of conglomerate
Fig. 1. A) Satellite image showing the location of major lakes in the study area. White box is the location of the detailed study area shown in Figs. 2 and 6. B) W–E section across the Andean subduction zone and the Magallanes foreland basin, adapted from Wilson (1991).
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Fig. 2. Geologic map of the Silla Syncline and surrounding area, slightly modified from Wilson (1991). Note location of Silla Syncline within the detailed study area (center of figure).
and coarse sandstone deposits (Crane, 2004). In addition to turbidites, there is evidence for debris and slurry flows within the formation (Crane, 2004). 3. Lake characteristics To investigate a possible relationship between the oriented lakes and faults, we compared the orientations and fabrics of the fault sets in the study area to the orientations, shapes, and distributions of the lakes. To obtain a quantitative measure of lake orientation, we used a MATLAB-based program to fit an ellipse to each lake in the study area (Fig. 3). We took the long axis of the ellipse as the overall lake orientation which we later compared to the fault orientations. The short axis of each ellipse corresponds to the width of a lake and together
with long axis, defines the approximate 2-D shape of each lake in map view. This technique of ellipse fitting is similar to that for determining strain in deformed grains (Mulchrone and Choudhury, 2004). The distribution of lake orientations (Fig. 4A) defines three main groups. The first group, with a mean value of 54.22°, is the most common, making up 45% of the lake orientations. The second group, with a mean value of 88.86°, is the least common at 15%. The third group, with a mean value of 172.69° is intermediate at 40%. Note that in this paper orientations are listed in degrees from north in a clockwise direction (see Table 1). While the ellipse fitting provides information on the length and width of each lake, the depths of most of the lakes are unknown. The only lakes for which the depths are known are for two of the larger lakes in the area,
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Fig. 3. Single ellipses fit to each lake in the study area, with the long axis of the ellipse representing the overall lake orientation. A histogram of the orientation data is presented in Fig. 4A.
namely Lago Grey with a depth of 450 m, and Lago Sarmiento with a depth of 312 m (Corporación Nacional Forestal (CONAF), personal communication, June 10, 2007). 4. Structural characteristics We mapped the major structural features of the Silla Syncline and surrounding area using enlarged aerial photographs as base maps. The resulting map (Fig. 5) shows major north-northwest to south-southeast-trending folds (hereafter referred to as N–S-trending folds) and sub-parallel thrust faults, and NE–SW and E–W-trending strike-slip faults, which are each briefly described below. 4.1. Folds and thrust faults There are a series of folds with north–south axial trends paralleling the Andean orogenic grain in the region,
originally mapped by Wilson (1991). Fold amplitude and wavelengths appear to be interdependent with the changing lithology of the formation, with large wavelength folds (up to 8 km) involving in the coarse-grained units, and smaller ones in the fine-grained units. The Silla Syncline, plunging to the north, is the largest of the mapped folds, with a width of up to 4 km and contains three coarse-grained sequences of the Cerro Toro Formation (Crane, 2004) that form the syncline's core. East and west of the Silla Syncline, the lithology is dominantly composed of mudstone and fine-grained sandstone, interpreted as turbidites (Crane, 2004). These units are cut pervasively by thrust faults, and also display folds with smaller wavelengths than that of the Silla Syncline. In some cases, these folds are associated with a zone of thrust faults at their cores or limbs. The eastern edge of the syncline is bound by a zone of numerous approximately N–S-trending thrust faults and associated folds, oriented sub-parallel to the syncline
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Fig. 4. Histograms for A) lake orientations from single ellipse fits. Groups 1 through 3 have mean orientations of 54.22°, 88.86°, and 172.69° respectively. B) Right- and left-lateral strike-slip and thrust fault data, with mean orientations of 58.06°, 86.79°, and 167.50° respectively. C) Partitioned lake segment orientations from multiple ellipse fits. Groups 1 through 3 have mean orientations of 55.01°, 88.36°, and 172.67°, respectively. D) Large scale lake and river orientations from ellipse fitting. Groups 1 and 3 have mean orientations of 62.38° and 163.26°, respectively.
axis. In this zone the individual thrust faults, with offsets ranging from several cm to about 30 m, are visible and generally accommodate east-directed transport and E–W shortening. In addition to thrust faults, bedparallel slip occurs along layer boundaries and within finer grained sediments. Slickenside orientations and offset layers show that slip occurred in a top to the west sense in the western limb of the Silla Syncline and top to the east sense in the eastern limb, consistent with flexural slip folding. The thrust faults do not occur in the coarse units of the Silla Syncline. Typically, thrust faults zones have calcite or other precipitated minerals, closely spaced cleavages, and many slip surfaces and breccia zones. All these features, together with mudstone host rock, result in weak fault zones making them susceptible for high rates of erosion and relatively lower elevations (Fig. 6A).
striking right- and left-lateral sets (Fig. 5, Table 1). These faults have spectacular geomorphic expressions in the form of incised valleys (Fig. 6B). Although the fault valleys are typically heavily vegetated, covering the fault core, fractures forming the fault damage zone are preserved in some outcrops flanking the valleys. Fig. 7A shows the fracture density within an approximately 6 m-thick conglomerate layer on both sides of a fault valley at the northern end of the Silla Syncline. The figure illustrates that the presence of the damage zone, Table 1 Mean lake and fault orientation data with comparison between undivided, divided, and large scale lake segment orientations Group Mean fault orientation (angle from N)
Divided lake segment orientation (angle from N)
Large scale lake and river orientation (angle from N)
Mean (difference from fault in parentheses)
4.2. Strike-slip faults The study area is cut by many strike-slip faults in an apparent conjugate pattern, with NE–SW- and E–W-
Undivided lake segment orientation (angle from N)
1) 2) 3)
58.06 86.79 167.50
54.22(− 3.84) 88.86(+2.07) 172.69(+5.19)
55.01(− 3.05) 88.36(+1.57) 172.67(+5.17)
62.38(+4.32) none present 159.26(− 8.24)
J. Gonzales, A. Aydin / Geomorphology 97 (2008) 502–515 Fig. 5. Structural map of the study area showing major folds, thrust faults, and strike-slip faults. Structures are dashed where inferred. Note the location of the Silla Syncline within coarse-grained rock units (coarse-grained rock locations from Crane, 2004), and N–S-trending thrust faults within fine-grained rock units. A histogram of fault orientation data is presented in Fig. 4B. 507
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Fig. 6. A) View to the north of thrust faults in fine-grained lithologies and topographic lows. Inset is a map of the lake in (A) showing the surrounding strike-slip and thrust faults. B) View to the east-southeast along a strike-slip fault valley with an axial lake.
where fracture density is greater (one or more fractures every 2 m) than what would be expected if fracture spacing were proportional to layer thickness (1 fracture every 6 m). A photograph supporting the notion of fracture localization associated with strike-slip faults from another location is shown in Fig. 7B. From these examples, we infer that the width of a strike-slip fault valley is an expression of the fault's damage zone such that prominent valleys in the study area are most often the locations of relatively larger (N 5 m slip) strike-slip faults. Both sets of strike-slip faults are found in the coarsegrained units, and to a lesser degree in the fine-grained units (Fig. 5). In terms of orientations, some variation in the left-lateral set from generally E–W to NW–SE occurs in the western part of the study area. Besides their orientations, between the two sets of strike-slip faults there are several differences in terms of length, offset and frequency. In general fault segment length and slip are larger for the right-lateral set, with average values of approximately 450 m and 9 m, respectively. For the leftlateral set, the average segment length and slip are approximately 330 m and 5 m, respectively. Furthermore, the maximum observed slip is 40 m for the right-
lateral set and 8 m for the left-lateral set. The frequency of right-lateral fault segments (Fig. 4B) is greater than that of the left-lateral set, with a ratio of about 3:1. 5. Comparison of the orientations and distributions of lakes and faults Table 1 shows the three main groups of lake orientations from Fig. 4A and the distribution of the fault orientations from Fig. 4B. A comparison of these distributions shows strong similarities between groups 1, 2, and 3 of lake orientations and right-lateral, leftlateral, and thrust fault orientations, respectively. These data sets also show similarity in their relative frequencies: Group 1 lake orientations are three times more common than Group 2 lake orientations, as is the case for the corresponding right-lateral and left-lateral sets. 6. Refined lake segment geometry A cursory survey of lake shapes reveals that most lakes, especially large ones have irregular boundaries, which can be characterized as dog-leg or staircase geometry. Based on the extension of the shoreline boundaries of these lakes
J. Gonzales, A. Aydin / Geomorphology 97 (2008) 502–515 Fig. 7. A) Fracture density from a scanline across a right-lateral fault, indicating the presence of the damage zone. B) An outcrop example of the damage zone, with an increasing number of fractures towards the inner fault zone. Photograph is looking to the west, showing south side of a strike-slip fault valley. Note that the outcrop in (B) is not the location for the data in (A), due to variable topography preventing photographing of the outcrop in (A).
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Fig. 8. Ellipses fit to lake segments with partitioning based primarily on their boundaries and the structures in the surrounding land. A histogram of the orientation data is presented in Fig. 4C.
into the surrounding land, and their spatial correspondence to the faults mapped in Fig. 5, it is apparent that these lakes are compound features and are associated with multiple faults or fault segments. It is therefore sensible to partition these compound lakes into individual component segments and compare their orientations with the corresponding fault orientations, in order to obtain a refined representation of lake geometry. Fig. 8 shows multiple ellipses fit to compound lakes by dividing the lakes into individual components, based on lake boundaries and the fault traces on land in the neighborhood of the lakes. These new segmented data (Table 1, Fig. 4C) provide an improvement in the comparison of mean values for the lake segments and the corresponding faults. This comparison shows that the lake orientation distributions determined by partitioning of compound lakes into their components improves the correlation between the distributions of lake orientations and fault orientations.
7. Statistical analysis Examining the means of the unrefined and refined lake orientations provides a good indication that fault and lake orientations are similar. More importantly, an inspection of the maps provides evidence for the spatial correlation between the faults and the lakes. In addition to the visual spatial correlation and comparison of the means of the orientations, statistical analyses can further illustrate this possible relationship quantitatively. First, to test if each population of faults and lakes are distinct, we used a Kolmogorov–Smirnov (K–S) statistical test between each of the different faults types, as well as each of the three groups of lakes. P values, representing a measure of statistical difference, were 0.0 in all of the comparisons between populations, indicating that there is a significant difference between each of them. Next, using the K–S test we examined if there is a significant difference in orientation among the groups of faults and
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Fig. 9. K–S test comparison percentile plots for fault and lake data. The dashed line in each plot corresponds to lake data and the solid line corresponds to fault data. Plots A through C use unrefined lake data, and plots D through F use refined lake data.
the groups of lakes. Fig. 9A through C show plots comparing the orientations of each fault set to the corresponding groups of unrefined lakes. Fig. 9D through
F show plots comparing the orientations of each fault set with the corresponding groups of refined lakes. Overall, the analysis indicates that while there may be some
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Fig. 10. Ellipses fit to large scale rivers and lakes. The lakes from the previous analysis of the detailed study area (within the white box) are not included here. To compare with large scale structural features, this figure is the same scale as Fig. 2. A histogram of the orientation data is presented in Fig. 4D.
statistical difference between the orientations of lakes and faults, it is not a strong statistical difference. Note that for each strike-slip fault set and their related groups of lakes, the P value increases from the unrefined lake orientations to the refined lake orientations. Specifically the increase is from 0.013 to 0.050 for the right-lateral/ group 1 comparison and from 0.036 to 0.134 for the left-
lateral/group 2 comparison. This indicates that the difference between the fault sets and refined lakes is less significant than that between the faults and unrefined lakes, supporting our discussion in the previous section. Possible reasons that the P value decreased for the thrust fault/group 3 lakes comparison could be related to the small sample size for the thrust faults and a poorly
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Fig. 11. A) Areas of mean stress reduction around a single frictionless fault segment, and between frictionless echelon fault segments, modified from Segall and Pollard (1980). C) Area of reduced mean stress between highly overlapped faults with a low coefficient of friction (0.0001), modified from Ohlmacher and Aydin (1997). D) A single fault, and (E) echelon fault segments showing delineated fault and lake relationships which are consistent with the model results.
constrained relationship between the thrust faults and lake boundaries. 8. Large scale lake and river geometry To determine if there are similarities between fault orientations and other major hydrologic bodies at a larger scale, we used the ellipse fitting technique on the major lakes and rivers in an area of approximately 4100 km2 surrounding the original detailed study area (Fig. 1B), but excluding those lakes and faults already covered in our earlier analyses of the detailed area. Fig. 10 shows the satellite image of the larger area with the ellipses representing the orientation and overall shapes of the major lakes and rivers. The resulting distribution of lake and river orientations (Fig. 4D, Table 1) shows similarities to previous distributions from the detailed study of the small-scale area: Groups 1 and 3 of the lake orientations are well represented and are close to the corresponding fault orientations. However, Group 2 does not appear to be present in the histogram, the possible reason for which will be discussed later. 9. Explanation for dependence of lake geometry on fault geometry Based on orientation, shape, and distribution of the lakes, and those of the fault sets, we suggest that the faults control lake formation and geometry. First, fault zones are almost always associated with damage zones, the sizes of which are proportional to fault slip and related stress perturbations (Scholz, 2002; Knott, 1996). With increasing slip, a fault's damage zone becomes longer and wider with the development of new
fractures (Myers and Aydin, 2004). This increased fracture density weakens the fault zone and makes it susceptible to a faster rate of physical erosion and chemical weathering relative to its surroundings. Second, mechanical models show that faulting related deformation is heterogeneously distributed about the faults due to stress perturbations associated with slip (Segall and Pollard, 1980). The perturbed stress fields result in both uplift and subsidence at certain areas about the faults (Bertoluzza and Perotti, 1997). Third, stress concentrations can also result in local high density opening mode fractures with high connectivity, thereby enhancing the erosion (Cruikshank and Aydin, 1994). A summary of stress perturbations around single and en echelon strikeslip faults with varying degrees of friction is shown in Fig. 11A through C. Specific faults and their overlaps in relation to lake occurrence are shown in Fig. 11D and E, where the fault and lake geometries are similar to the model results discussed above. The locations of these examples are also shown in Fig. 5. 10. Discussion Our field data of the orientations, shapes, and distributions of lakes and faults in the Andean foreland in Chilean Patagonia show strong similarities between the geomorphology of the lakes and the underlying fault structures. The distributions and orientations of the three major lake groups match quite well with the three fault sets (Fig. 4). The thrust faults commonly occur in muddominated base rocks to the east and west of the Silla Syncline, so do the lakes identified as Group 3. The right-lateral strike-slip faults in the detailed study are the most prominent and pervasive, so are the lakes in Group
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1. The left-lateral faults are poorly developed in the detailed study area and similarly the corresponding lakes in Group 2 occur less frequently there (Fig. 4A, B, and C). In fact, this group of poorly developed faults cannot be detected at a larger scale (Figs. 4D and 10). The correspondence between the fault orientations and the lake orientations in the larger area surrounding the detailed study area also holds well, even without the refinement of the image segmentation due to the lack of structural data at that scale. The results highlighted from the modeling of stress perturbations around strike-slip faults are used to rationalize the role of faults on lake morphology. The stress perturbations in combination with the damage zone distribution result in areas of higher fracture density at fault tips and in between overlapping or parallel fault segments, which may make these domains more susceptible to weathering and erosion. In addition, these domains are likely to subside as they are pulled apart (Bertoluzza and Perotti, 1997), resulting in sags, usually called pull-apart basins (Aydin and Nur, 1982). If suitable hydrogeologic conditions were met, such as high rates of rainfall, glacial melt, high water table, and low evaporation, a lake may form. Current glacier bodies and glacial polish on certain outcrops indicate that the glaciers also play a role as a physical agent of the erosion (Harbor et al., 1988) responsible for the lake formation. However, the initial glacier valley locations were likely guided by previously eroded fault valleys or zones of high fracture density. Larger lakes in the study area appear to have more complicated geometries. Many of these complexities are likely due to the interplay between thrust faults and various strike-slip faults (Fig. 6A). Based on field relationships, it is likely that strike-slip faulting and thrust faulting were active simultaneously, although thrust faulting and folding appear to have initiated first (Gonzales, 2006). Having both types of faults active at the same time is consistent with the predominant E–W Andean orogenic compression that is the mutual driving force. The formation of lakes related to thrust faults is not addressed here because the downthrown block of a thrust fault is depressed by definition and therefore the downthrown block of thrust fault is capable of collecting water and the upthrown block is capable of damming the water body. In addition, thrust faults occur in muddominated fine-grained sequences and are therefore weaker than the surrounding stiffer rocks. While this study focuses on one area, the results could provide insights to other oriented lakes observed in Andean foreland and craton which are covered by
young sediments and therefore do not offer unambiguous clues about the basement structures. The best known case for this is the oriented lakes in the Beni Basin in Bolivia, identified by Plafker (1964), where oriented lakes define two prominent orientations; orogen-parallel (NW–SE) and orogen-perpendicular (NE–SW). Plafker noted surface lineaments sharing the same alignment with lake boundaries, and supposed that lineament and lake formation was caused by downdropped fracture-bounded basement fault blocks, or sediment compaction over the basement blocks. In terms of our results in this paper, the two dominant orientations of the lakes in the Beni Basin may be attributed to orogen-parallel thrust faults and one welldeveloped strike-slip fault set nearly perpendicular to the local orientation of the orogenic belt to the west. In contrast to our study, Plafker (1964) did not consider the effect of a damage zone and stress perturbations and the related mechanical weakening and subsidence around the block bounding lineaments. Nevertheless, even though the mechanisms for lake formation are different in our study and in Plafker's, our results corroborate Plafker's suggestion that lake geometries and lineaments in surface deposits can be controlled by structural heterogeneities in underlying basement rocks, and that surface features can be used to infer the structural behavior of the area under the surface deposits. 11. Conclusions Our field data shows a striking correlation between lake orientation, shape, and distribution and the faults. This is supported by visual evidence as well as statistical analyses. It is therefore compelling to conclude that the formation of the lakes in the study area is fault controlled. The nature of the faults, with its implications for erosional processes in the study area, along with models of fault-induced deformation and related surface subsidence, lend support for this interpretation. The proposed correspondence between the lakes and the underlying faults offered in this paper may be helpful in understanding similar phenomena in analogous tectonic settings, for example the Beni Basin in Bolivia, especially if the area is covered by surface deposits obscuring the interrelationship between basement structures and lake geometry. Acknowledgements This work was supported by the Stanford Rock Fracture Project. We would like to thank Kingshuk Choudhury for suggesting the ellipse fitting technique
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and George Hilley for reviewing an early version of the manuscript. We would also like to thank the Corporación Nacional Forestal (CONAF) for their permission to undertake the study and for providing the depth information for Lago Gray and Lago Sarmiento. References Aydin, A., Nur, A., 1982. Evolution of pull-apart basins and their scale independence. Tectonics, 1 (1), 91–105. Bertoluzza, L., Perotti, C.R., 1997. A finite-element model of the stress field in strike-slip basins: implications for the Permian tectonics of the Southern Alps (Italy). Tectonophysics, 280, 185–197. Biddle, K.T., Uliana, M.A., Mitchum Jr., R.M., Fitzgerald, M.G., Wright, R.C., 1986. The stratigraphic and structural evolution of the central and eastern Magallanes Basin, southern South America. In: Allen, P.A., Homewood, P. (Eds.), Foreland Basins: International Association of Sedimentologists Special Publication, vol. 8, pp. 41–63. Bruhn, R.L., Stern, C.R., de Wit, M.J., 1978. Field and geochemical data bearing on the development of a Mesozoic volcano-tectonic rift zone and back-arc basin in southernmost South America. Earth and Planetary Science Letters, 41, 32–46. Crane, W.H., 2004, Depositional history of the Upper Cretaceous Cerro Toro Formation, Silla Syncline, Magallanes Basin, Chile [Ph.D. thesis], Stanford University, Stanford, California. Cruikshank, K.M., Aydin, A., 1994. Role of fracture localization in arch formation, Arches National Park, Utah. Geological Society of America Bulletin 106, 879–891. Fildani, A., Cope, T.D., Graham, S.A., Wooden, J.L., 2003. Initiation of the Magallanes foreland basin: timing of the southernmost Patagonian Andes orogeny revised by detrital zircon provenance analysis. Geology, 31 (12), 1081–1084. Fildani, A., Hessler, A.M., 2005. Stratigraphic record across a retroarc basin inversion: Rocas Verdes-Magallanes Basin, Patagonian Andes, Chile. Geological Society of America Bulletin, 117 (11/12), 1596–1614.
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