Glacially folded Outwash west of Lago Llanquihue in the southern Lake District, Chile

ABSTRACT: Folded outwash occurs in four distinct clusters in an arcuate arrangement just west of the terminal Llanquihue moraines deposited by the Lago Llanquihue piedmont ice lobe at the last glacial maximum. These clusters are physically connected along the eastern side to the Llanquihue terminal moraines. and along the western side to the Llanquihue outwash plain. Each cluster consists of three to eleven elongated ridges. The maximum height of individual ridges varies from cluster to cluster between 18 and 28 m; the maximum length of individual ridges is between 93 and 1074 m. The average orientation of the ridges ranges over a 60° sector relative to former ice-flow direction. The folded outwash sediments are cut by two distinct internal fault systems with only a faint surface expression below the Holocene top soil.

The folded outwash ridges are interpreted as a push moraine system produced by the same mechanical forces that act in a critically tapered wedge. The folded sediment is a sandy gravel with an angle of friction on the order of f = 40°. Interpretations of structural data and of mechanical comparisons point to a basal thrust plane in a sand unit with f between 24° and 30° and with a pore water pressure index of l = 0.7.

It is very unlikely that the observed and analyzed features were formed under permafrost conditions.

Introduction
Comparisons
Morphology
Structures
Mechanical interpretation
Conclusions
Acknowledgements
References

Introduction

Ice-marginal geomorphic features and sediments are widespread in formerly glaciated areas. They are the crucial data to reconstruct former glacier extensions and to decipher the physical behavior of glaciers during the ice age. Ice-marginal formations are therefore the first-order paleoclimate signal reflecting the amplitude of paleoglaciations. To extract this signal, however, requires further insights about the mechanical processes acting in these environments. In particular, little information is available on geological processes acting along a temperate piedmont ice lobe margin (see also Turbek and Lowell 1999). We report here on the analyses of an unusual feature in this context in the southern Lake District of Chile and we present possible interpretations of the results. We conclude that the folded outwash represents a variety of push moraines.

Four sets of distinct ridges occur In contact with and distal to the adjacent terminal Llanquihue moraines of the last glacial maximum (LGM) west of Lago Llanquihue (Fig. 1).These ridges lack the surface till and boulders that are common on the adjacent moraines. Rather, they are composed of gravelly outwash where the original horizontal orientation of the gavel and sand beds has been rotated by angles up to 66° from normal. Distinct fault systems cut the outwash bedding overall. the resulting elongated gravel ridges are parallel to the nearby terminal moraine ridges, except for site C (Figs. 1, 2) where they intersect. The distal side of the folded outwash borders on the undisturbed Llanquihue outwash plain of the LGM.

The age and paleoclimatic context of these features afford boundary conditions for the processes that deformed a segment of the flat Llanquihue outwash plain. The ridges of folded outwash do not display surface erosion. which suggests they formed in the most recent Llanquihue outwash, Moreover, an organic clast reworked into a folded ridge during deposition of the outwash near La Cabaña in Fig. 1 yielded an age of 29,265 +210/-205 ¹⁴C yr. BP (A 7663), indicating a late Llanquihue age for the outwash (Denton et at. 1999), because the folding must have occurred at this time or later. The paleoclimate of this time can be reconstructed from pollen analysis (Heusser et al. 1999; Moreno et al. 1999). These reconstructions indicate that the ice-free areas around the Lago Llanquihue piedmont glacier were characterized by a Subantarctic Parkland. under summer temperature about 8-9 °C lower than the present values of 1416 °C, with precipitation remaining high (Moreno et at, 1999). Therefore. the folded outwash was formed during the last glaciation in a cool and wet-temperate environment.

Index map of the study area.

Fig. 1: Index map of the study area west of Lago Llanquihue in the southern Lake District of Chile. The map shows four areas of folded outwash ridges. designated from northeast to southwest as A, B, C and D. These four areas lie just west of the outer edge of the Llanquihue terminal moraine belt of the last glacial maximum (LGM) (Andersen et al. 1999). Within each of the four areas, the folded outwash ridges are designated as push moraines that occur distal to the outermost Llanquihue terminal moraine ridges.

From morphological considerations alone, the terminal moraine and the folded outwash ridges could have been confused in our reconnaissance mapping program (Andersen et al. 1999). Therefore, the present study was carried out to complement the mapping program of Llanquihue moraines (Andersen et al. 1999).

Based on the results of a number of field surveys, especially in 1995 (Gander 1996), we report here on the structural analysis and mechanical interpretations of this folded outwash. We consider the folded outwash ridges to be push moraines, following the original definition by Chamberlain (1890) (see also Kälin 1971; Gripp 1979; Gander 1996).

Comparisons

Before specific interpretations of the folded outwash ridges adjacent to the Llanquihue terminal moraines are put forward, we briefly discuss general aspects of push moraine formation. As a rule, the structures of a push moraine mirror the former stress field and, paleogeographically, the former ice margin or the former direction of ice flow, respectively. Kälin (1971) noticed two conjugated structural trends with respect to the general ice flow direction and a concentric trend parallel to the deformed ridges with its center close to the base of the glacier lobe.

The structural elements of a push moraine are part of the compressive glaciotectonic zone of an ice margin (Banham 1988; Ruszcynska-Szenajch 1988). Structures most often recorded are pressed scales with a basal thrust plane, repeated thrusting, folds and normal faults (Gander 1996). Rare structures are inverse faults, schistosity, and terraced pressed scales (due to changes in base level or uplift).

Mechanical properties that are considered to be important prerequisites for the formation of push moraines are (1) a sedimentary or topographic obstacle (Boulton 1986; Eissmann 1987), (2) high pore water pressure (van der Wateren 1985; Croot 1987; Hart 1994), (3) contrasting permeability (Thomas 1984; Owen 1988), and most controversial (4) permafrost (Gripp 1929; Berthelsen 1979; Kälin 1971). Köster (1957, 1960) concluded from model experiments that folding in the glaciotectonic zone of an ice margin is always a preliminary stage in the formation of wedges. In this view, the folded features adjacent to the Llanquihue moraine did not reach the end member conditions of the deformation process.

Morphology

The folded outwash occurs as a series of concentric elongated ridges concentrated in four distinct clusters on the distal side of the Llanquihue terminal moraines (Fig. 1 and Andersen et al. 1999). Each cluster contains three to eleven well-defined ridges. The overall dimensions for individual ridges are 93-1074 m in length and 25-93 m in width, with a maximum individual ridge-crest height some 28 m above the adjacent outwash plain. The spacing of the ridges within a cluster, or the topographic wavelength, is 51-148 m. This value is only about 50% of published values for wedge-type push moraines (Gander 1996).

The folded areas denoted as A, B, C and D in Fig. 1 are separated from each other by segments of the undisturbed Llanquihue outwash plain. It is possible that a complete rim of ridges originally extended in the form of an arcuate belt from the eastern end of area A to the western tip of area D. If so, the present gaps result from meltwater streams that cut syngenetically into the emerging ridges. The ridge complexes within areas A and B end abruptly along their western margins at a well-defined meltwater channel. Terraces occur at the edge of clusters A and B about 3 to 5 m above the undisturbed Llanquihue outwash plain. These are interpreted as up-lifted or emerged parts of the same former outwash plain that was mechanically deformed into the folded ridges.

A gross indication for the intensity of the deforming process is given by the volume of dislocated sediment. The volume of a ridge cluster is given by the following equation (Croot 1987), and the calculated values for the total visible dislocated volume is listed in Table 1:

Equation 1

The topographic amplitude or height of the outwash ridges above the adjacent outwash plain ranges up to 28 m. This amplitude decreases in all four clusters from a maximum near the Llanquihue terminal moraine ridge to the distal margin of the folds. It is impossible with the outcrops available to determine the total amplitude of folding. Our assumed value used for later calculations is based on the Interpolation of the visible surface geometry and on the results of a drill core reported at outcrop B/C.

The topographic inclination (a) of a ridge cluster approximates a line connecting the ridge crests and can be estimated using Equation 2 following Croot (1987):

Equation 2

where

H^R_max = highest ridge (m a.s.l.)
H^R_min = lowest ridge of a cluster (m a.s.l.)
H^E = height of (undisturbed) outwash plain (m a.s.l.)
L = average length of ridges within a cluster (m)
a = topographic inclination (°)

For the fold clusters a ranges between 2.9° and 7.0° (Table 1). In the distal part of each fold cluster, the ridge morphology fades out and merges with the undisturbed Llanquihue outwash plain.

Table 1: Different parameters of the folded outwash clusters A, B, C and D in Fig. 1 with summary information on their size, including the topographic inclination (a) are listed. Volume and topographic inclination are calculated using Equations (1) and (2).

Cluster	Number of outcrops available	Height	Length	Width	Surface area	Volume	a
A	-	m	m	m	km²	10⁶ m³	°
A	0	14	300	750	0.2	1.5	4.0
B	8	15	889	1481	1.3	10.1	2.9
C	1	21	154	463	0.1	0.7	5.2
D	2	19	155	939	0.1	1.4	7.0

Height = average height of ridges
Length = distance from proximal to distal end of cluster
Width = distance from easternmost to westernmost point of a cluster
Volume = deformed sediment above surrounding outwash plain.
Number of outcrops used for structural measurements:
Cluster A = 0, cluster B = 8, cluster C = 1, cluster D = 2.

Depressions between the ridges commonly contain wetlands or small lakes, probably sealed by fine-grained slope wash or volcanic sediment. The natural drainage between the ridges has been severely modified by human activity. However, examination of outcrops indicates minimal soil erosion from the ridge tops into the depressions.

Topographic cross sections of two of the four clusters of outwash ridges and further information given in Gander (1996) show the following patterns. The ridges within areas A and B are similar to each other, but are different from those within areas C and D. The folds within areas A and B lie parallel to the LGM terminal moraine belt, which here are composed of as many as three to eleven ridges.

Area C shows a different configuration. The terminal Llanquihue moraine envelops the folded ridges along the western end of the cluster. In addition there are two distinct directions of the fold ridges at C at an angle of 26° apart (Fig. 2). These cross-cutting geometric relationships suggest that area C reflects multiple mechanical deforming events.

However, within area D there are only three larger and one smaller folds bordering a prominent LGM terminal moraine ridge. The topographic inclination with a = 7° is highest of the whole fold belt in area D. This area is special as there are only two folds but of considerable amplitude and therefore a is highest here. Area D is a small but pronounced tapered wedge.

Structures

Insights into the deformation processes come from the present orientation of the gravel beds and from the faults cutting them. All outcrops reveal medium to coarse, sandy gravel in well-preserved rhythmically bedded lenses several meter long with occasional openwork gravel lenses. However, all outcrops show massive sandy gravel beds of proximal sheet flow origin. At outcrop B/H of area B (Fig. 3), an overturned gravity microfold with a wavelength of about 0.40 m occurs within the folded outwash. Thus these sediments were deposited in a proximal to medium distal outwash position. Structural measurements come from eleven outcrops concentrated within cluster B (Fig. 3) and therefore most of the conclusions based on structural analysis are derived from there.

A major structural element is macroscale folding that conformably follows the topography of the ridges. The description in this section assumes an original horizontal bedding of the now deformed sediments, and does not take into account any faint primary depositional deviation from the horizontal. A proximal outwash plain most likely shows a primary surface dip up to 1.33° (Schreiner 1992); a negligible amount compared to the orientations reported here.

Map of cluster C.

Fig. 2: Map of cluster C in Fig. 1 showing the geometrical relationship between the bend of the Llanquihue terminal moraine around the two sets of folded outwash ridges offset by 26°. At this site the deformation causing folded outwash is at least two phased. Both axial directions of folded outwash have a corresponding direction of the terminal moraine ridge.

The deformations observed are rare microscale folding, macroscale folding, and faulting. The present orientation of the deformed bedding planes in cluster B shows the magnitude of the folds. Stereoplots of these beds reveal a plastic fold around an axis. However, the spread of the measurements on the orientation of deformed bedding planes depends on the size of the outcrop in a ridge. The more completely exposed a folded ridge, the more accentuated is the spread of values. In the small outcrop B/D (Fig. 3), the scattering of the values for the poles of the deformed bedding planes is only 13°. In outcrop B/A, which opens up an almost complete cross section of the ridge, the measured deviation is 65°. The axis of the folded outwash corresponds to the strike of the individual ridges. The poles for the deformed bedding planes (Fig. 3) mirror the bending of the folds as a whole in this area a counterclockwise evolution from east to west of the bending of the deformed bedding planes over an angle of 70° corresponds to the concentric bending of the whole belt of folded ridges as well.

There are two sets of faults cutting the folded outwash: high-angle faults with dips of >60° and low-angle faults. Figure 4 illustrates typical examples for high-angle faults (Nos. 1, 2, 4, 7 and 70) and low-angle faults (Nos. 3, 5, 6, 44 and 71). The low-angle faults are preferably associated with the distal parts of the proximal ridge, as well as with the distal ridges (northern half of outcrop B/B in Fig. 4). The low-angle faults are responsible for the visible fault asymmetry exposed in outcrops (Fig. 4). Fault planes with more than 0.50 m displacement are largely part of the low-angle fault system and are likewise asymmetrically oriented and distributed in the ridges (Figs. 4, 5). The amount of displacement of low-angle faults is generally on the order of one meter. In addition, many low-angle faults are reversed (No. 60 in outcrop B/B, Fig. 4) and low-angle fault planes dip toward the direction of the former Llanquihue glacier front. Both fault systems strike parallel to the orientation of the long axes of the ridges. Within an individual ridge (Figs. 4, 5) the high-angle fault planes are more symmetrically oriented and distributed preferentially around vertical rather than low-angle faults. The low-angle faults reflect the direction of deformation, whereas the high-angle faults reflect both syn-deformation extensive normal faulting typical for antiformal structures and the post-deformation settlement of the sediment beds.

The complex of folded outwash ridges that forms push moraines within area B.

Fig. 3: The complex of folded outwash ridges that forms push moraines within area B of Fig. 1. The symbols for push and terminal moraine ridge best fits for the bedding planes from 340° at outcrop B/A to 270° at outcrop B/L (Figs. 4, 5). This shift corresponds not only to the degree of bending of the ridges in area B but also to a concentric bending of all the folded ridges. When combining both small- and large-scale bending we see a concentric pattern with its center about 8 to 10 km to the south-east.

The deformation appears multi-phased, as some additional observations attest. In cluster B, the average orientation of the deformed bedding planes and the average orientation of all measured faults differ by an angle of 18°. The deformation of the bedding has a mean azimuth of 050°, whereas the cross-cutting faults display a more easterly azimuth of 068°. The observation that the low-angle faults verge to the distal side of the fold clusters with an orientation of azimuth of 340° points to the fact that the faulting also has a genetic connection with the folding. This systematic difference of 18° in strike orientation between the bedding and the fault planes can be considered as a strong indication of a two-phase event. During a first phase the primary «plastic» folding was formed. The second phase then caused a 18° shift of the main stress axis to the northeast. This new orientation would then have pushed up the ridges to the present height by «brittle» folding producing the fault planes. This order of events is also supported by the surface expression of the faults on the ridges (Fig. 4), where a faint morphological impact is visible.

Cross sections of the push moraine complex.

Fig. 4: Cross sections of the push moraine complex at station B/B and at station B/C. See Fig. 3 for location. The shaded area is topsoil. The numbers refer to the measured structural elements. This outcrop (B/B-B/C) was the best available exposure of the internal structure of the folded outwash ridges.

Decreasing dip for the push moraine complex.

Fig. 5: The poles of fault planes, with decreasing dip for the push moraine complex of area B in Fig. 1. The base line is the best-fit equators, * is the best-fit equator pole.

In addition, area C (Figs. 2, 3) displays two strike orientations of the fold ridges. Each direction parallels a different segment of the adjacent moraine ridge. It is unlikely that both of these cross-cutting sets can result from the same deforming event. It is more likely that the two distinctly defined stress fields acted during two phases of deformation. The cross cutting relationships of the ridge morphologies point to an early easterly oriented stress field, overprinted later by a strong northerly deformation. This is in apparent contradiction to the relationships at the southwestern end of cluster B, where a shift of 70° is noticed in the orientation of all measured structural features yield an average orientation of 328°. This corresponds to the overall regional stress parallel to the flow direction of the Lago Llanquihue piedmont glacier as it advanced to the terminal moraines at the proximal edge of the folded outwash ridges (Fig. 1) (Andersen et al. 1999). The discussion about a two or one phased event may also be related to very local differences in the deforming forces when the Llanquihue glacier lobe split in a series of locally active sublobes, each one to two kilometers wide, Such a splitting of the main lobe and the existence of a more complex local glacier front would help to explain the crosscutting configuration at cluster C Figs. 6, 7.

Major s<sub>1</sub> and minor s<sub>3</sub> stress components, before, during, after the advance of the glacier.

Fig. 6: Major s₁ and minor s₃ stress components, before, during, and after the advance of the Lago Llanquihue piedmont glacier to the terminal moraine ridges shown in Fig. 1 and in Andersen et al. (1999). The undisturbed situation with potential fault planes is shown in (a). With additional stress, the axis of stress and the potential fault planes rotate by w as shown in (b). The range of most probable orientations of fault planes after the completion of (a) and (b) is shown as shaded wedges in (c).

Distribution of fault plane poles.

Fig. 7: Distribution of fault plane poles at exposures B/B and B/C in area B in Fig. 1 and 3.

Mechanical interpretation

We consider the folded outwash proximal to the Lago Llanquihue terminal moraines to represent a push moraine system that is very similar to a critically tapered wedge, with the horizontal stress being applied by the advancing Lago Llanquihue piedmont glacier. Although these descriptive features show many similarities to push moraines, this classification does little to explain the mechanical processes under which the features formed. For this we adopt the critically tapered wedge model of Davis et al. (1983) and test it by the application of two analyses (Fig. 8). The model explains the mechanics of snow plowing as a lateral stress forms an accretionary wedge. The form of the wedge is defined by the angle that develops between the lower detachment surface and the height of the pushed material when compressional forces and the internal friction of the snow or sediment or rock are in equilibrium.

The shape of the wedge produced by this mechanism is controlled by horizontal stress components, by the internal friction of the deformed material, by the friction along the basal thrust plane and by pore water pressure (Fig. 8). The sum of all the acting stresses in the wedge must be zero, because the deformed wedge is not «accelerated». This means that the wedge does not collapse and that the deformation stops when the horizontal stress approaches zero.

In this context the equation to balance the forces becomes:

Equation 3

where

F_g^- = horizontal component of gravity
F_f^- = component of friction along basal thrust plane
F_x⁺ = horizontal component of compression

Stress components in a sub-aerial tapered wedge.

Fig. 8: Stress components in a sub-aerial tapered wedge after Davis et al. (1983). F_x⁺ is the horizontal component of compression, F_g^- is the horizontal component of gravity, F_f^- is the component of friction along the basal thrust plane, (a) is the topographic angle of slope, and (b) is the angle of inclination of the basal thrust plane.

We have measured a to be 2.9° (Table 1) but must take probable ranges of l from the literature and compute the ranges of b that balance the equation (Fig. 8) (full computations are in Gander 1996). In bedrock folds and thrust belts or in accretionary wedges these features are expressed as follows.

A basal thrust plane separates mechanically severely deformed rock units above from little deformed units below. The dip of the thrust plane separating these units is towards the deforming force. Horizontal compression (shortening) occurs in the rock units above the thrust plane. The deformed rocks above the basal thrust plane form a wedge-shaped morphology tapered toward the distal side of the wedge. Because we see a nearly one to one correspondence with similar features of the Lago Llanquihue folded outwash we try to balance the force equation (Equation 3).

The following assumptions are made for this test. The Coulomb-Navier Criterion can describe the observed brittle faulting in the folded outwash. Thus normal stress and shear stress are linearly depending. In addition, they depend on the shearing properties of the sediment (cohesion and angle of friction, Lang and Huder 1985) and basal friction must be smaller than internal friction. By definition, the deformed sandy gravel is cohesionless. For this analysis we take only one deforming event as expressed in cluster B.

To reconstruct pore water pressure values, we assume a hydraulic connection between the outwash and the glacier system. Thus maximum pore water pressure at the base of the ice is assumed to be the same as in the push moraine. Hence the pore water pressure within a glacier can be measured in boreholes or in moulins. It depends strongly on precipitation, mainly rainfall, and on the deforming velocity of the ice.

Published values tor the pore water pressure index (l is the ratio of water pressure over lithostatic pressure = ice in the case at a glacier and is a number without units) averages between 0.48 and 0.77 (Mathews 1964; Iken 1972; Boulton and Vivian 1973; Iken and Bindschadler 1986; Jansson 1995). The average published value that we use for our comparisons is l = 0.60 ± 0.1, which corresponds to a water table in the glacier ice at about 55 ± 10% of its thickness. Regional precipitation values when the Llanquihue piedmont glacier lobe was extended exceeded 2000 mm (Moreno et al. 1999). This high precipitation would assure high water levels throughout the linked glacier-outwash system.

A basal thrust plane could not be observed. However, a clay horizon has been reported by the farm owner on the occasion of a core drilling for aggregate prospecting at about 14 m depth below the base of the outcrops B/B and B/C. If this horizon was taken to be the thrust plane, however, an unrealistic angle (b, Figs. 8, 9) for the basal slip plane of -0.3° dipping away from the primary stress would result. Therefore, we conclude that the basal thrust plane is probably deeper than the clay horizon at 14 m depth.

Published values for the magnitude of b are rare; Kälin (1971) assumed in his examples in Axel Heiberg Island a dip of less than 10°. Köster (1960) concluded from physical experiments that the penetration depth of a push moraine corresponds approximately to the average width of the pushed ridges. By transferring Köster's results to the case of area B, this penetration depth of deformation is about 90 m for our example. This results in b = 5.7°.

With a = 2.9°, with an assumed value of b = 5.7° ± 1° in clay or silty sand and with an angle of friction for the gravel of 40°, for the clay of 24° and for the sand of 30° (Lang and Huder 1985), we calculate an index for pore water pressure to be l = 0.69 -0.28/+0.10 which compares favorably with average published values cited above. Fig. 9 shows other combinations of l and b which allow Equation 3 to balance.

The relationship between the shape of a push moraine and pore water pressure.

Fig. 9: The relationship between the shape of a push moraine and pore water pressure. Shown is the topographic angle of slope (a) and the inclination of the basal thrust plane (b) for pore water pressure indices (l_pm) of a push moraine made up of gravel (with f = 40) as well as the slip plane in clay (f_b = 24°) or silty sand (f_b = 30°). B marks the position of values for the push moraine complex in area B of Fig. 1.

Based on estimated physical properties of the sediments involved and on measured deformation geometries, our calculations produce comparable values for pore water pressures as measured on glaciers. These agreements require a basal shear plane, most likely at 90 m depth in the sand unit in the proximal part of cluster B. Overall, we conclude that the folded outwash ridges with the observed surface geometries and deformations can be produced with the tapered wedge mechanism.

In the following we take a second, independent approach to test the critically tapered wedge hypothesis. We estimate the stress situation required to fold the outwash and to produce the observed fault pattern. According to «Anderson's Theory of Faulting» (Davis et al. 1983; Davis 1984) fault planes form at an angle of 45° - ½ f to the plane given by stress axes s₁ and s₂. For an undisturbed outwash unit the main stress is s₁ = g. Therefore the main stress in an undisturbed situation acts vertically, and the resulting fault planes should be steep (approximately 65° for clean, well-graded gravel with f = 40° (Fig. 6).

The horizontal shear stress (s₂) that produced the compressional deformations in the form of the folded ridges is provided by advance of the Lago Llanquihue piedmont glacier. The orientation of the resulting fault planes is a function of an angle (w) between the vertical and the bisector of the angle between the two fault planes to be expected (Fig. 6).

Following «Anderson's Theory of Faulting» (Davis et al. 1983) the angle of internal friction (f) for the slip plane where rupture occurs can also be estimate. It corresponds to the angle of the vertical and the angle between the two centers of the clusters (which mark the most likely fault planes; Figs. 6, 7).

In our case w is between 7° and 20° and f is between 29° and 34° and thus are average values for sand to gravelly sand (Lang and Huder 1985).

The ridges of cluster B with an estimated height of h_pm = 45 meter exert a vertical stress s_z of 0.88 MPa (s_z = r_pm × g × h_pm ). By measuring the angle of rotation w of fault planes, a horizontal glacier shear stress t_b of 0.11 to 0.32 MPa can be deduced (t_b = ss_z × tan w = 0.88 MPa × tan 7°<=>20°; Figs. 6, 7).

In the published literature, the values for t_b are mainly 0.15 ± 0.05 MPa (Nye 1969; Paterson 1981; Boulton 1981 pers. communication in van der Wateren 1985). If this analysis is valid, it implies that the glacier shear stress (t_b) can be estimated to an acceptable degree of accuracy through a geometrical analysis of fault planes in a push moraine system, under the reasonable assumption that the observed faults are due to brittle deformation.

Conclusions

Landform geometry, fold axis, and fault orientations on the outwash ridges near Lago Llanquihue allow the following conclusions to be drawn.

a) The average of all measured structures (deformed bedding planes and faults) is between 049° and 229° orientation. The values converge at 318°, which is the direction of the ice flow during initial deformation or the axis of the main horizontal stress.

b) Deformation of the bedding planes and the orientation of the fault planes are offset at an angle of 18°. A two-step deformation may explain this offset especially in cluster C where two distinct directions of the fold ridges lie 26° apart.

c) The theory of a critically tapered wedge, with reasonable assumptions, allows an approximation for the index of pore water pressure on the order of l = 0.7. Under these conditions a reported clay horizon is unlikely to be the basal thrust plane. Rather a basal thrust plane some 90 m deep in a sandy bed with a high pore water pressure is more likely. The critical influence of the pore water pressure in such a deforming system makes it very unlikely that the outwash area was frozen during deformation. This agrees with pollen records from the surrounding region (Heusser et at. 1999; Moreno et al. 1999).

d) The orientation of the fault planes to the vertical is asymmetric for the folded outwash within area B. This allows a model calculation of the glacier shear stress of about 0.2 MPa.

e) These conclusions are consistent with a model of the push moraines of folded outwash behaving as a critically tapered wedge of sediment. A basal thrust plane developed from a horizontal main shear stress associated with the advance of the Lago Llanquihue piedmont glacier to its maximum LGM position. This conclusion explains an area of proglacial push moraines in folded outwash at the limit of the Lago Llanquihue moraine belt (Andersen et al. 1999).

Acknowledgements

Very cordial thanks are transmitted to the Puerto Varas research team and to Baggy Gander. Most formal thanks go to a reviewer for his patient shake-up of an earlier version of the manuscript. We thank the United States National Science Foundation, the Unites States National Oceanographic and Atmospheric Administration, and the Swiss Nationalfonds Grant No. 21-043469.95/1 for financial support.

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Glacially Folded Outwash

near Lago Llanquihue, southern Lake District, Chile

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