The Geometry and Evolution of the Jura Mountains: Fernschub mechanics

Tabea Kleineberg

03.10.2013

Paper for the excursion “Geländeseminar Alpen” led by Prof. Dr. Janos Urai, Institute of Structural Geology, Tectonics and Geomechanics and Prof. Dr. Ralf Littke, Institute of Geology and Geochemistry of Petroleum and Coal at the RWTH Aachen University.

2 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

ABSTRACT ...... 3

1. GEOLOGICAL SETTING...... 4

2. STRATIGRAPHY AND EVOLUTION OF THE JURA MOUNTAINS...... 5

2.1 BASEMENT ...... 5

2.2 THE SEDIMENT COVER ...... 5

2.2.1 The basal décollement within the Triassic evaporites ...... 7

2.3 EVOLUTION OF THE JURA MOUNTAINS...... 7

2.4 PALINSPASTIC RECONSTRUCTION ...... 8

3. STRUCTURES...... 9

3.1 EVAPORITE-RELATED FOLDS...... 9

3.2 THRUST-RELATED FOLDS ...... 9

3.3 TEAR FAULTS ...... 10

4. FERNSCHUB HYPOTHESIS...... 11

4.1 MECHANICS ...... 11

4.1.1 Kinematics ...... 12

4.1.2 Critical taper...... 13

4.2 LABORATORY EXPERIMENTS...... 14

5. CONCLUSION...... 15

6. LITERATURE...... 16

3 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

Abstract The Jura Mountains, representing the most external part of the Alpine chain at its North-western front, are divided into three parts, all featuring different structural styles, ranging from plain plateaux in the external and Tabular Jura to a well developed fold-train in the internal Jura. The Jura Mountains formed in the latest stage of the Alpine orogeny in Upper Miocene/ Lower Pliocene times and are closely linked to the Molasse Basin. Its basement is comprised of metamorphic rocks and is decoupled from the sediment cover by a basal décollement. Folding and thrusting is restricted to the sedimentary cover rocks, pointing to a thin-skinned fold-and-thrust tectonics, which require very low basal friction. When shortening in the subalpine Molasse reached the Jura, the décollement in the Triassic Evaporites sheared off into the foreland generating the thrusts and folds of the Jura Mountains. This process is called the Fernschub

hypothesis by BUXTORF (1916) and it is, at the present day, the most broadly-acknowledged theory for the formation of the Jura Mountains. Its essence and mechanics will be discussed in this paper.

4 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

1. Geological Setting The Jura Mountains are located in Central Europe, in the Northwest of Switzerland and in the East of France in front of the Western Alpine arc. Their length amounts to 370 km with a maximum width of

75 km (BECKER, 2000). Figure 1 shows the structural map of the Jura arc, with its division into the internal, external and Tabular Jura highlighted in different shades of grey. In the south, the Jura Mountains are linked with the Alpine front of the Prealps, however, in the northeast they are separated from the Alpine chain by the tertiary Molasse Basin which is up to 50 km wide close to the eastern termination of the Jura Mountains northwest of Zurich. The Molasse Basin correlates to an Oligo- Miocene foredeep, that developed in front of the Alpine orogen

(SOMMARUGA, 1998). The crucial features along the western and northern border are the Tertiary rifts

of the Bresse Depression and the Upper Rhine Graben (BECKER, 2000). The Rhine and Bresse Graben are associated with the Eocene and younger, West-European rift system. The Jura overthrusts the

Bresse Graben in the west, and the Tabular Jura in the north (SOMMARUGA, 1998). Both rifts were

active during the Eocene to Miocene, before Jura folding commenced (AFFOLTER ET AL., 2004). The remaining areas along the northern margin of the Jura folds belong to the Tabular Jura; the more or less unfolded, locally block faulted and non-decoupled sedimentary analogue of the folded Jura

cover (BECKER, 2000).

001 A[ Sommaru`a:Marine and Petroleum Geolo`y 05 "0888# 000023

Figure 1: Structural sketch of the Jura Mountains (SOMMARUGA, 1998). Fig[ 1[ Tectonic sketch of the Jura arc showing main structural units[ Legend] PHSPlateau de Haute!Saone^ ICIle Cre⇣mieu^ AMAvants! Monts^ FeFerrette^ ARAiguilles Rouges^ MBMont Blanc[ Modi_ed from Sommaruga "0884#[

Wegmann\ 0852^ Ziegler\ 0871# and\ on the other hand\ Jura and the Molasse Basin "Ziegler\ 0871^ Guellec et al[\ thin skinned thrusting and associated folding of the Jura 0889^ Gorin et al[\ 0882^ P_}ner\ 0883^ Signer + Gorin\ cover above a Triassic detachment horizon and dis! 0884^ P_}ner et al[\ 0886#[ For a more complete review placement over large horizontal distances pushed from on the evolution of the ideas on the formation of the Jura the Alps across the Molasse Basin "{Fernschub theory|# belt\ see Sommaruga "0886#[ "Boyer + Elliott\ 0871^ Buxtorf\ 0805^ Laubscher\ 0862b#[ Although balancing arguments clearly favor an allo! 1[ Geological setting chthonous interpretation\ seismic data as well as neo! tectonic arguments have recently been used to support The Jura is a small\ arcuate fold belt forming the fron! some thick!skinned basement involvement beneath the tal portion of the western Alpine arc "Fig[ 0#[ The Jura 5 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

2. Stratigraphy and Evolution of the Jura Mountains The Jura Mountains are a foreland fold-thrust belt, where the structural components involve a relatively thin sediment cover; approximately 2 km thick in the Internal Jura, deformed above a basal

décollement within the Middle and Upper Triassic evaporites (Figure 3) (AFFOLTER ET AL., 2004). The Mesozoic and Cenozoic rocks are folded at variable degrees and detached from the gently 1-5° SE

dipping pre-Triassic basement, illustrated by Figure 3 (SOMMARUGA, 1998 and BECKER, 2000). By contrast with the deformed Jura cover, the molasse fill of the foreland basin was left virtually un-

deformed by Alpine deformations (Figure 3) (AFFOLTER ET AL., 2004).

2.1 Basement The crystalline basement is composed of medium-to-high grade metamorphic and plutonic rocks, which were deformed during the Hercynian orogeny. The surface of the basement, including some Permo-Carboniferous troughs, is not strongly accentuated. Nowhere is it exposed in the Jura and

Molasse Basin (BECKER, 2000). It is characterized by two major unconformities, one below the

Carboniferous and the second below the Triassic (SOMMARUGA, 1998). Its tectonic style, the depth

and geometry and its internal deformation are still uncertain (BURKHARD, 1990). Some moderate basement elevations, however, are proven along the Vuache fault system, the eastern border zone of the Bresse Depression, the southern rim of the Permo-Carboniferous Trough of northern Switzerland and in the Oyonnax region of the southern Jura Mountains. The depth of the basement varies from more than 7 km below sea level in front of the Aar massif to more than 4 km above sea level 20 km further to the southeast (Figure 3)

(BECKER, 2000).

2.2 The sediment cover The sediment cover of the Jura Mountains reaches maximum thicknesses of 1.5 km in the north, approximately 2 km in the centre and more than 3 km

in the south (BECKER, 2000). It is separated from the basement by an evaporite layer (Figure 2 and 3, compare to 2.2.1). The Jura is divided into an external and an internal part, based on different tectonic styles (Figure 1). The external Figure 2: Stratigraphy of the Jura and adjacent Jura consists of flat areas, plateaux, separated from Molasse Basin (modified after SOMMARUGA, 1998).

6 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

A[ Sommaru`a:Marine and Petroleum Geolo`y 05 "0888# 000023 002 ne*Molasse Basin region from the above large scale cross!section[

Fig[ 2[ "a# Large!scaleSommaruga "0887#^ balanced "b# cross!section Enlargement across of the the Jura Molasse Haute Cha( Basin from the external Jura to the Alps "external crystalline massifs#[ For location\ see Fig[ 1[ Modi_ed from Burkhard and Figure 3: Cross section through the Jura Mountains (SOMMARUGA, 1998).

7 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

each other by Faisceaux, narrow stripes comprising numerous small-scale imbrications and tear faults. The internal Jura consists of a well-developed fold train. At a large scale, deformation is characterized by major folds, the trend of which swings through 90° from east to south. Major tear

faults, oriented at a high angle to fold axes, cut the internal Jura at regular intervals (SOMMARUGA , 1998). The crucial rock types of the sediment cover are Triassic evaporites (compare 2.2.1) and shales, Jurassic and Cretaceous limestones (compare 2.3) and molasse sediments from Oligoene and

Miocene times (BECKER , 2000).

2.2.1 The basal décollement within the Triassic evaporites In order to shear off the sediment cover and to accomplish folding of the Jura Mountains, a basal décollement with a low basal friction is required. These properties are given by the Triassic evaporitic sequences of the Muschelkalk and the Keuper formations. The most important lithologies for generating décollement horizons are halite, anhydrite and, at depths of less than

approximately 500m, gypsum (BECKER, 2000). The distribution of halite in the Triassic (Figure 4) correlates with the location of the Jura Mountains, most obviously at their southern and eastern end. Southwards, the halite

Figure 4: Distribution of Halite of the Keuper and disappears before reaching the Alpine front, and Muschelkalk in the Jura Mountains (BECKER, 2000). anhydrite is replaced increasingly by dolomite and marl. The thickness of the Triassic decreases from more than 1000m in the southern and central Jura

Mountains to less than 50m in the North Helvetic domain, 60 km to the south (SOMMARUGA, 1998).

2.3 Evolution of the Jura Mountains After the end of the Hercynian Orogeny, the Alpine cycle started with peneplanation and a subsequent transgression in the Early Triassic. After the Jurassic, the Jura and Molasse Basin realm became part of the Alpine Tethys passive margin. During the Triassic, up to 1 km of evaporites and shales accumulated in an elliptical depocenter in the area of the future Jura arc (Figure 2 and 3). Limestones were sedimented in Dogger and Malm, as well as in the Carboniferous. During the Oligocene and Miocene, fluvial, lacustrine and marine clastic molasse sediments were deposited in the Alpine foredeep as a sedimentary wedge, the Molasse Basin. Its thickness decreases from up to 3

km in the south to a few hundred meters in the north (SOMMARUGA, 1998). In upper Miocene and lower Pliocene times, the latest stage of the Alpine orogeny, the Jura belt formed, at the front of the Alpine foredeep. Originally, the Molasse Basin extended further north-

8 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

and westwards into the region of the present Jura Mountains and is preserved in the Jura Mountains as relicts. In the Oligocene, this region experienced normal faulting synchronous with formation of the Bresse Graben. Later, the upper Miocene to early lower Pliocene the Jura deformation reactivated these extensional structures which probably played a role in the distribution of thrusts. At that time, the

frontal Jura was thrust above the eastern border of the Bresse Graben (AFFOLTER ET AL., 2004).

2.4 Palinspastic reconstruction Palinspastic reconstructions are used for a clearer understanding of the pattern of strain of a heterogeneous deformation. They are achieved by dividing the area into homogeneous domains with a subsequent restoring and best-fitting of the individual domains to realize the initial undeformed state. The structure of the Weissenstein Anticline (upper section of Figure 5) exhibits two geometrically distinguishable tectonic generations. The shortening of the wedge system decreases, which is apparent from the decreasing height of the Weissenstein

anticline. In contrast, the Ausserberg-thrust shows no Figure 5: Restored cross-section of the region "Volgelsberg" (BITTERLI, 1990). change (BITTERLI, 1990).

9 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

3. Structures Two different types of folds appear in the Jura Mountains: evaporite-related and thrust- related folds. The Molasse Basin and the external Plateaux Jura feature folds of the first mentioned type. The

second type is present in the internal Jura (SOMMARUGA, 1998). Strike-slip faults disrupt the lateral continuity of the folds, but no clear crosscutting relationships

exist between both on geological maps (AFFOLTER ET AL., 2004)

3.1 Evaporite-related folds Evaporite-related folds are broad, long-wavelength, low- amplitude folds that are cored by Triassic evaporites (Figure 6). They are difficult to recognize in the field or on geological maps, therefore an interpretation by seismic lines is required. Seismic sections prove that the folds are controlled by evaporite, salt and clay stacks within a ductile unit of the Triassic layer. Within the Plateaux Jura the folds have two long asymmetric limbs dipping with a very low angle towards the north and the south, respectively. The geometry of the folds is highlighted by a well-layered series of reflectors representing Cretaceous, Jurassic and Upper Triassic strata (Figure 6). Due to the scarcity and thinness of pure rock salt layers in the Triassic series and the lack of early extension or Figure 6: Seismic section of an Evaporite- differential sedimentary loading, no salt diapir occurs in the related anticline in the Plateau Jura (SOMMARUGA, 1998). Jura belt and the Molasse Basin (SOMMARUGA, 1998).

3.2 Thrust-related folds Thrust-related folds are characterized by high-amplitude folds. These folds formed above thrust faults stepping up from the basal Triassic décollement zone. These anticlines duplicate the entire Jurassic

sequence (Figure 7) (SOMMARUGA, 1998). The short wavelength of these folds is

thought to be due to the reduced Figure 7: Geological profile through the thrust-related folds around Grenchenberg (PFIFFNER, 2010). thickness of sediments involved in deformation in this external area. This reduced thickness is due to a period of peneplanation

10 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

(compare to 2.3), thought to have lasted from Late Cretaceous times to the onset of Jura

deformation in upper Miocene (AFFOLTER ET AL., 2004). Thrust-related anticlines are separated by broad or tight synclines. Many thrust faults are NW (NNW)-verging, for example the main thrust system (foreland-vergent thrust), and have at least kilometric dip slip. SE (SSE)-vergent thrust faults are considered as backthrusts (hinterland-vergent thrusts), and have few tens or hundreds of meters of displacements. Thrust faults include both flats and ramps. All mapped foreland-vergent thrust faults reach the surface, breaking through the

structures in the steep frontal limbs (SOMMARUGA, 1998).

3.3 Tear faults Tear faults are small strike-slip faults which affect the whole Mesozoic and Cenozoic cover but do not

show any offset of the basement top within the seismic resolution. (TWISS ET AL., 2007 and

SOMMARUGA, 1998). These faults are defined here as belonging to an allochthonous cover with a transcurrent movement, and terminating into a décollement zone that can be well recognized on geological maps (Figure 8 right). They are sinistral and occur mainly in the Internal Jura; oriented NW-SE in the southern Jura, NNW-SSE to N-S in the central Jura and NNE-SSW in the eastern Jura (Figure 8 right). In Figure 8 the left part shows different types of tear faults. A mixture of A and B would represent the most common type in the Jura Mountains, where shortening is accommodated by trust- related folds on both sides of

Figure 8: Left: thrust sheets segmented by tear faults (Twiss et al., 2007). the tear fault. Right: location of tear faults on the Jura anticline map after Heim 1916 (SOMMARUGA, 1998).

11 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

4. Fernschub hypothesis The Fernschub hypothesis was conceived by Buxtorf in 1907. It states that the process begins with the subduction of the European plate accompanied by the rise of the Aar massif (Figure 3). The resulting push affected the sedimentary wedge of the Molasse Basin and the linked Jura Mountains, resulting in deformation of the sediment cover and shear of décollement nappe. The shear was enhanced by the basal detachment, consisting of a Triassic salt layer with low friction. Beneath this

detachment, the rocks remain mostly undeformed (Figure 3) (DAHLEN, 1990).

4.1 Mechanics Mechanically, a fold-and-thrust belt is similar to a wedge of snow in front of a moving snow plough

(DAHLEN, 1990). The snow deforms until it develops a constant critical taper, after which it slides stea-

dily, continuing to grow at constant taper as additional material is accreted at the toe (DAVIS, 1983). The mechanical possibilities of thrusting an extensive thin sheet of sediments are controlled by the

amount of friction at its base (LAUBSCHER, 1961), therefore an increase in the sliding resistance increases the critical taper. In contrast, an increase in the wedge strength decreases the critical taper, since a stronger wedge can be thinner, not deforming while sliding constantly over a rough

base (DAHLEN, 1990). A computed specific friction value is 30 kg/cm2 for the base of the Plateaux Jura, and three times this value for the Molasse Basin. Those low values are due to plastic yielding of salt within the Triassic

evaporite series (LAUBSCHER, 1961).

DAVIS (1983) emphasized that fold-and-thrust belts features two further characteristics besides the above-mentioned basal décollement. First, large horizontal compression in the strata that overlie the décollement (see chapter 2.2 and 3) and second, a distinctive wedge shape of the deformed strata tapering toward the margin of the mountain belt (compare to Figure 3). The Jura Mountains exhibit all three characteristics.

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(ˆˆiˆ   (ˆ+ˆ   0T?ˆ EWd4^ˆ4ggjf€Vb4uXfdˆVqˆ}4^X=ˆCfjˆ4ˆ~?=P?ˆfCˆ d4kkf~ˆu4g?k ˆ)ˆˆ  (3) (DAHLEN, 1990).  0T?ˆ EWd4^ˆ4ggjf€Vb4uXfdˆVqˆ}4^X=ˆCfjˆ4ˆ~?=P?ˆfCˆ d4kkf~ˆu4g?k ˆ)ˆˆ  ~T@n@ˆ  4d=ˆ ,ˆ 4j@ˆ b@4q|j@=ˆ Xdˆ j4=X4dqˆ "?:4|q@ˆ uT@ˆ :jVuX:4^ˆ s4g@jˆ Xqˆ Equation (3) gives a final approximation for a narrow tapered wedge, α+β << 1, if α and β are Pf}@kd?=ˆ~T@n@ˆ  4d=ˆfd^„ˆ,ˆ6„ˆ4j@ˆsT?ˆb@4q|j@=ˆ|d}4k„VdPˆ'!'+ Xdˆqsk@dPsTˆj4=X4dqˆfCˆsT?ˆ"?:4|q@ˆq4d=ˆ4d=ˆuT@ˆsT?ˆ:jVuX:4^ˆ64q4^ˆCkX9sXfd ˆs4g@jˆ Xqˆ 6fsTˆPf}@kd?=ˆuT?ˆ ~X=uTˆfd^„ˆ6„ˆ4d=ˆsT?ˆsT?ˆ|d}4k„VdPˆT@XPTuˆqsk@dPsTˆfCˆ 4ˆ6|^^=f ?jˆfCˆsT?ˆq4d=ˆ~?=P?ˆ4d=ˆPjf~ˆsT?ˆ64q4^ˆ^X]?ˆ CkX9sXfd ˆ/T?ˆ 1/2 measured in radians. Due to its critical taper, the wedges’ width and the height grow at a rate of‡ ! t . Pkf~sTˆ6fsTˆ uT?ˆVqˆ~X=uTˆq?^DqYbV^4kˆ4d=ˆsT?ˆVdˆT@XPTuˆsT?ˆ q?dq?ˆfCˆ 4ˆuT4sˆ6|^^=f ?jˆsU?ˆ ~?=P?ˆ~?=P?ˆ4sˆPjf~ˆsVb?ˆ^X]?ˆtˆ Xqˆ Vd=Xr†/T?ˆ ‡ ! uVdP|XqT46^?ˆPkf~sTˆThe wedges Vqˆ q?^DCkfbˆqYbV^4kˆ at uU?ˆ time ~?=P?ˆVdˆ t sT?ˆ and 4uˆq?dq?ˆ time sXb?ˆt ˆuT4sˆ 2t b4PdVL?=ˆ are sU?ˆ proportionally ~?=P?ˆsVb?qˆ4sˆ sVb?ˆ identical, tˆ Xqˆ Vd=Xr† with time 2t magnified 21/2 times by WIB6264 - Technische Hochschule Aachen on 09/24/13. For personal use only. uVdP|XqT46^?ˆ!dˆ@jf=VdPˆ~@=P@ˆCkfbˆuU?ˆ~X^^ˆ~?=P?ˆ4ss5Xdˆ4uˆsXb?ˆ4ˆ=„d4bV9ˆt ˆb4PdVL?=ˆqs@4=„ˆqs4s@ˆsVb?qˆ~T@dˆsT@ˆ 4;;j@‡

Annu. Rev. Earth Planet. Sci. 1990.18:55-99. Downloaded from www.annualreviews.org   uVfd4k„ˆ(Figure 9!dˆ@jf=VdPˆ~@=P@ˆVdM|‚ˆ). The growth is denoted k4u?ˆfDˆDk~X^^ˆ?qTˆ4ss5Xdˆb4u?kV4^ˆ4ˆ=„d4bV9ˆVdvfˆas selfuT?ˆ-wf?ˆsiqs@4=„ˆmilar (Vqˆ64^4d9?=ˆqs4s@ˆ DAHLEN~T@dˆ6„ˆ, 1990).sT?ˆsT@ˆ?kfqV}?ˆ4;;j@‡ ?J|€ˆuVfd4k„ˆ%XP|j?ˆVdM|‚ˆk4u?ˆ /T?ˆfDˆDkqu?4=„qs4u?ˆ?qTˆb4u?kV4^ˆ~V=uTˆVdvfˆuT?ˆfCˆ4ˆwf?ˆ|dVCfjb^„ˆVqˆ64^4d9?=ˆ?jf=XdPˆ6„ˆsT?ˆ~?=P?ˆ?kfqV}?ˆXqˆ PX}?dˆ?J|€ˆ%XP|j?ˆ6„ˆuT@ˆG`|€ˆ 64^4d:?ˆ/T?ˆqu?4=„qs4u?ˆ:fd=VsXfdˆ~V=uTˆfCˆ4ˆ|dVCfjb^„ˆ( " (+ ?jf=XdPˆ~?=P?ˆXqˆ PX}?dˆ6„ˆuT@ˆG`|€ˆ64^4d:?ˆ:fd=VsXfdˆ A2q?:a iˆ A2ˆ
S 9C A2q?:a iˆ A2ˆ
S ~T?o?ˆ Vqˆ sT?ˆ k4s?ˆ fCˆ9C?kfqVfdˆ !ˆ qs@4=„qs4s@ˆ ~@=P@ˆ b|qsˆ 9fdsVd|4^^„ˆ ~T?o?ˆ ^ Vqˆ sT?ˆ k4s?ˆ fCˆ ?kfqVfdˆ !ˆ qs@4=„qs4s@ˆ ~@=P@ˆ b|qsˆ 9fdsVd|4^^„ˆ =@Cfjbˆ^7fxTˆufˆ4::fbbf=4x?ˆxT?ˆZdN|€ˆfFˆCj?qTˆb4x?jZ4^ˆZdufˆZyqˆyf?ˆ4d=ˆ sfˆ=@Cfjbˆb4Xds4Xdˆ7fxTˆXuqˆufˆ:jXsX94^ˆ4::fbbf=4x?ˆs4g?jˆ4P4XdqsˆxT?ˆZdN|€ˆ?jfqXfdˆfFˆCj?qTˆb4x?jZ4^ˆZdufˆZyqˆyf?ˆ4d=ˆ sfˆb4Xds4XdˆXuqˆ:jXsX94^ˆs4g?jˆ4P4Xdqsˆ?jfqXfdˆ $ÿHƒġ ¢
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by WIB6264 - Technische Hochschule Aachen on 09/24/13. For personal use only. OP by WIB6264 - Technische Hochschule Aachen on 09/24/13. For personal use only. Annu. Rev. Earth Planet. Sci. 1990.18:55-99. Downloaded from www.annualreviews.org Annu. Rev. Earth Planet. Sci. 1990.18:55-99. Downloaded from www.annualreviews.org

@]+EC)9@2]W+)2+] #OO#9@J] #] )Y@#?:'] JO+#)YJO#O+] W9)O5]39U+A] 6Y] 9qiŠ`# _L:C lK @]+EC)9@2]W+)2+] #OO#9@J] #] )Y@#?:'] JO+#)YJO#O+] W9)O5]39U+A] 6Y] 9qiŠ`# _L:C lK 13 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

4.1.2 Critical taper 1158 DAVIS ET AL.' MECHANICSOF FOLD-AND-THRUSTBELTS The critical taper for the compressive wedge in Figure A 9 is governed by the balance of four forces in moving direction. First, a gravitational body force; second (in a submarine regime), the pressure of water overburden; third the frictional resistance of sliding over a basal décollement; and fourth, the compressive push of the normal tractions acting on the two side walls of the

wedge (DAVIS, 1983). !% S $ !S S =7¬

.=¬ (4) !% S $ !S S =7¬ N{—jž——{Š‡¬Equation (4) shows that the increase of the critical •b .=¬ /=¬ ;] dM Ȓtaper correlates with the increase of the coefficient of  Ȓ Ȓ
Ȓ Ȓ Ȓ ĹȒ Ȓ

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Ȓ äšř 0 C •* N{—jž——{Š‡¬ȒȒ
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 Ȓ@Ȓ Ȓ

Ȓ the basal friction μb, as it was already 'ο mentioned in ȒȒ> Ȓ/=¬ š>Ȓ 4ȒبȒţ>ȒØȒ ;] Ȓ@ ȒQʀF\Ћ ' Ȓš>Ȓ >@Ȓ  TȒ ôƨ ¬­ƨ ©uåùřf dM ȒȒ4.1,  Ȓ while š Ȓ it  Ȓ decreases Ȓ Ȓ
Ȓ Ȓ by 4'Ȓ Ȓ an 2@Ȓ increase  Ȓ of ĹȒ the  Ȓ

Ȓ;] internal ~ ȒýȒ@Ȓ Ȓ

Ȓ 4Ȓ äšř Ȓ 'ȒȒ
ƤȒ ȒȒ
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Ȓ % 
Ȓ

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 Ȓ%Ȓ Ȓ ÅȒ@Ȓ Ȓ

Ȓ
4
Ȓ Ȓ friction angle ϕ. For the typical value for dry sand of ²ƨ 'ο Ȓ ȒȒȒ> Ȓ 4Ȓ%Ȓš>Ȓ
Ȓ-ŁȒ 4ȒبȒţ>ȒȒØȒȒ% Ȓ
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@Ȓ Ȓ Ȓ õ4'Ȓ<Ȓ2@Ȓ ;] ~ ȒýȒFig. 6. Mohrdiagram 4Ȓillustrating the state of stress(a) at somepoint within the wedge and (b) at thebase of the ϕ=30°, the critical surface slope is given by α≈⅓(μb-2βwedge.) TheFigure 10: Mohr diagram depicting the state of quantities 4•and 4•, are the angles of internal and basal friction, and ½ and %, are the angles between o-• and  'ȒdM Ȓ ȒȒ
Ȓ Ȓ   Ȓ Ȓ7$«¬Ȓ
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Ȓ 
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4
Ȓ @Ȓthe wedge Čand  Ȓat thebase of thewedge. The basal shear traction r/, is given by the intersection of the ²ƨ frictionalstress (A) at some point within the wedge and failure law Irl = •,rr,* withthe Mohr stress circle corresponding to the basal depth H. AHLEN Ȓ X Ȓ(DȒ 4Ȓ³4Ȓ, 1990).%Ȓ
Ȓ
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Ȓ<Ȓ  
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Ȓ  Ȓ Ȓ
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:Ȓ 
ȒȒ 
Ȓ Ȓ
 @Ȓ(2 ¬Š

@ȒȒ õ
 
Ȓ<ȒȒÅ Ȓ 
'Ȓbe written 
Ȓas ȒȒ weakness, either because of a lower intrinsic strengthor GƘƙƨ Ƨƨ g because of elevated fluid pressures. For a wedge with In a critically tapered wedge, the AʽήĿο horizontal •'0 = /XbO'z*= ,U,b(1- Ko)pgH (8) `> ȒdM Ȓ Ȓ`Ȓ`Ȓ`Ȓ   Ȓ `—`²Ȓǻ>2> :ȒȒM O  ƺȒ2  Ȓ%ȒæȒ% Ȓ—Ȓ²>`²—@ȒȒ @Ȓ`—> ȒČ uniforminternal properties /x and X, we must necessarily E¦¦ο E¨¨ο where/x0= tan (b0is thebasal coefficient of friction and )to is have(1 - •.b)ld,b • (1 - X)/xfor the baseof the wedgeto be a the generalizedHubbert-Rubey ratio (6) on the base. In throughgoingdecollement. X Ȓ 4TȒ³4Ȓ Ȓ 
Ȓ
4ȒȒ  Ȓ@ȒȒȒ ȒkŠ2Ȓ% Ȓ% Ȓ<Ȓ ŐȒ8Ȓ X
Ȓ Ȓgg}%Ȓ Ȓ compressive stress σxx is related with the lithostaticintroducing overburden the basalÊЋvalues go and )to stress we allowexplicitly σzzfor≈ -ρgz To determine by the the remaining Coulomb unknown quantity tr; in the

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 @ȒȒ
 
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ȒȒ Ȓ/?¬ 11ř h Y # $. 4 Ȓ Ȓ Ȓ Ȓ 4Ȓš  Ȓ  Ȓ  ȒÊЋ  Ȓ ȒȒO Ȓ „ łȒ ƍƎƨ Ŗŗ ĺīôř ŘЋ
Ȓ yȒcȒXȒ-  qȒ 4Ȓ Ȓ 
Ȓ Ȓ failure ǭȒTherefore, a thinner (subcritical) wedge with a greater σ
ȒtÃȒ Ȓ2Ȓ-  
Ȓ qȒȒ
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Ȓ until  Ȓ it Ȓ gets 
Ȓ Ȓ critical. <Ȓ~Ȓ Ȓ 4Ȓ

In contrast, 4Ȓš  Ȓ  ȒȒa thicker  Ȓ  Ȓ 
:Ȓ(supercritical) ÊЋ  Ȓ
Ȓ Ȓ 'ȒȒ '
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4
Ȓ 4Ȓ Ȓ  
RȒ Ȓ ƍƎƨ  Ȓ
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 Ȓ ȒŖŗ ĺīôř ǭȒ 4Ȓwould not deform if no fresh material tÃȒ Ȓ  2:Ȓ2Ȓ 
ȒȒȒ  ȒÃȒttȒȒȒtćÃȒ Ȓ<Ȓc@Ȓwould be encountered at the toe (
% t4Ȓ 4Ȓt%ȒȒ% Ȓ Ȓ Ȓ % ćt Ȓ  Ȓ DAHLEN, 1990).  Ȓ@Ȓ

Ȓ›  4Ȓ ȒO  Ȓ
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Ȓ  Ȓ In Figure 10F ,` aY Mohr diagraśĔä m is shown depicting the state of stress in such a wedge, where (A) Ȓ 'Ȓ  @Ȓ|ł1ÒÓřŠĜȒȑ 
ȒƮ Ȓ Ȓ Ȓ  ȒȒ  Ȓ
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 Ȓ Ȓ  yȒ @Ȓ 4Ȓillustrates the stre›  4Ȓ  2:Ȓ (Ȓ  Ȓ (
Ȓ  Ȓss at an arbitrary point and (B) at the base of the wedge.  (4 'ȒȒȒ( % 
Ȓ (Ȓ<Ȓc@Ȓ"%ȒȒ  Ȓ( 4Ȓ "(Ȓ Ȓ ȒȒ%  Ȓ  Ȓ ψ and ψb are the angles F `O śĔä by WIB6264 - Technische Hochschule Aachen on 09/24/13. For personal use only. Y  'Ȓbetween σ|ł1ÒÓř ȑ1 and the x axis within the wedge and base of the wedgeƮ Ȓ ȒȒ  Ȓ>  Ȓ%ȒȒ Ȓ Ȓ  yȒ, while ϕ and ϕb represent the Annu. Rev. Earth Planet. Sci. 1990.18:55-99. Downloaded from www.annualreviews.org 8ȒȒ (ƚ Ȓ (  ( 4Ȓ%Ȓ(X Ȓ( "ȒȒȒ(  "( Ȓ Ȓ 4¶¹ @Ȓ › Ȓangles of the internal and basal friction. The intersection of the frictional failure law |2@ȒȒ> Ȓ%Ȓ >Ȓ  Ȓ`>Ȓ>  Ȓ>Ȓ˜ž ¨Ȓ8Ȓ τ| = μbσn* with by WIB6264 - Technische Hochschule Aachen on 09/24/13. For personal use only. the Mohr stress circle corresponding to the basal depth H gives the basal shear traction τ > Ȓ>Ȓ @ Ȓ2@Ȓ>O˜ž Ȓ>
>
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³Ȓƚ ȒȒȒ
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 ȒȒȒȒ Ȓ%Ȓ Ȓª
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C
and minimu >Ȓ
@Ȓ  Ȓ Ȓ`>Ȓm effective compressive stresses, respectively (  Ȓ>  ȒȒȒ>Ȓ 4Ȓ˜ž ¨Ȓ'Ȓ8Ȓ DAVIS, 1983).  @  Ȓ> Ȓ <Ȓ>Ȓ¬Ȓ @ Ȓ Ȓ2@Ȓ Ȓ>O˜ž Ȓ
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:Ȓ  ȒȒ  ȒȒȒ ŠȒ  Ȓ<Ȓ Absolute fault and crustal strength from wedge tapers

John Suppe* Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA

ABSTRACT (Davis et al., 1983). This assumption of failure allows wedge theory to form The strengths of mountain belts and major faults have been notori- relationships, based on mechanical equilibrium, between the critical taper ously diffi cult to constrain and there is ongoing debate over the control- α + β of a wedge and the strength of the wedge and its base, where α is the ling mechanisms and stress magnitudes. Here we show that the strengths surface slope and β is the dip of the detachment (Fig. 1). These strengths of active thrust-belt wedges and their basal detachments can be directly potentially may be controlled by many different mechanisms operating at determined from the covariation of surface slope α with detachment dip various scales, even with much of the rock remaining undeformed. Further- β, without strong assumptions about the specifi c strength-controlling more, the strength-controlling mechanisms may be quasi-static or dynamic. mechanisms. Even a single taper measurement (α, β) can strongly For example, the strength of some basal detachments is the far-fi eld stress constrain the set of possible wedge and detachment strengths. This necessary for ruptures to propagate in brief great earthquakes that are sepa- theory is tested with dry sand wedges and then applied to the Niger rated in time by centuries, such as the 2004 Sumatra M = 9.0 earthquake, delta thrust belt, the active Taiwan mountain belt, and the thrust that whereas other detachments have strengths controlled by static or quasi-static slipped in the M = 7.6 Chi-Chi earthquake. Their basal detachments processes, such as the continuous creep of detachments in salt. are shown to be exceedingly weak, with effective coeffi cients of friction Wedge theory traditionally has been used to infer the magnitudes of (0.04–0.1) that are an order of magnitude less than most laboratory strength parameters that are consistent with observed tapers, for example friction coeffi cients (0.6–0.85). In contrast, these wedges are moderately internal and basal friction coeffi cients (µ = tanφ, µb) and depth-normalized strong internally, within the range of pressure-dependent strengths in pore-fl uid pressures (λ = Pf/ρgH). An example of such a relationship deep boreholes. These results confi rm the existence of exceedingly weak between wedge taper (α, β), thickness H, and strength parameters is the faults and strong crust, which raises important causal questions. elegant general weak-base theory of Dahlen (1990), which we take as our starting point, using the special case of a mechanically homogeneous Keywords: fault strength, wedge mechanics, fold-and-thrust belt, Niger14 wedgeThe Geometry and Evolution of the Jura Mountains: Fernschub mechanics (Dahlen, 1990, equation 99): Tabea Kleineberg 319001 delta, Taiwan, Chi-Chi earthquake.   βρρ11− ( fbbb) + µ ( − λρ) + S gH INTRODUCTION αβ+=   , (1a)  sin φ  The question of the strength of the crust and the faults within it has 1  221 C gH  − (ρρf ) + ( − λ)  + ρ been long outstanding. In his 1855 paper on isostasy, George Airy recog- 4.2 Laboratory experiments 1− sin φ  nized that the strength of mountain belts must be less than the strength of The theory mentioned in 4.1 states that the covariation of surface slope α with décollement dip β is intact rock, but the extent and mechanisms of weakening remain diffi cult to where Sb and C are the non-pressure-dependent parts of the fault and wedge constrain. We have limited knowledge of in situ conditions, and theories con- used strength.to Such determine equations contain the strengths a number of of average active thrust regional-scale-belt fault wed ges and their basal décollement. It was tain regional-scale strength parameters that are diffi cult to observe. Available and crustal strength parameters about which we would like to know much stress measurements from deep boreholes are commonly a signifi cant fraction tested with more, but unfortunatelya sandbox deformational model have little direct constraint in actively in the laboratory deforming by SUPPE (2007), among others. The of the small-sample laboratory frictional strength (Byerlee, 1978), suggesting regions. Therefore we simplify Equation 1a to something more manageable that the upper crust can be relatively strong (Brudy et al., 1997; Townend and experiments were conducted to recast criticalby collecting the fault-strength terms as F = µb(1 − λ b) + Sb-/ρtaper gH and wedge the mechanics into a very simple form, to Zoback, 2000). In contrast, the existence of thin intact thrust sheets, some of wedge-strength terms as W = 2(1 − λ)[sinφ/(1 − sinφ)] + C/ρgH, obtaining which exceed 50–100 km in length, implies very weak detachments relative determine absolute regional strength 1  F sion and probably fl uid pressure. Mechanically heterogeneous wedges to internal strength, which is the classic thrust-fault problem addressed by βρρ− ( f ) + αβ+=  . (1b)require more observational constraints and are beyond the scope of this Hubbert and Rubey (1959). Low-taper accretionary wedges and fold-and- +1α  W from observed geometries in  − (ρρf 1 ) +  F Dry-sandshort papersion wedge and(see probablyFletcher, 1989; fl uid pressure.Dahlen, 1990, Mechanically equations heterogeneous98 and 103). wedges thrust belts also require weak detachments relative to crustal strength within βρρ − ( f ) + αβ+= . α (1b)Carenarequire etmore al. (2002)observational presentedappropriate constraints a set of andtaper geologic are measurements beyond conditions.the scope across of The this these wedges (Davis et al., 1983). Nevertheless, the existence, cause, and 1− ρρ + W Mylar This simplifi ed equation 8° raises a hope( f that) we might constrain the Taiwanshort (Fig. paper 2A) (see that Fletcher,shows a 1989;quasi-linear Dahlen, relationship 1990, equations of negative 98 and slope, 103). magnitude of such apparent fault weakness remain controversial. Much of base fault and wedge strengths (F, W) from appropriate observationsβ of wedge as predictedCarena for homogeneous et al. (2002) wedges.mechanical presented Similarly, a set model of Bilotti taper and measurements in Shaw the (2005) laboratory across the controversy has focused on apparently weak wrench faults such as the shape (α, βThis). This simplifi seems ed plausible equation because raises athe hope only that remaining we might term constrain in the thepresented Taiwan regional (Fig. 2A(~100) that km) shows measurements a quasi-linear of relationshiptaper in the of deep-water negative slope, San Andreas and Sumatran faults (Brune et al., 1969; Lachenbruch and Sass, Take-up spool equation,fault [1 and − ( ρwedge/ρ)], containsstrengths the (F ratio, W) offrom the appropriatedensity of the observations overlying fl ofuid wedge thrust as belt predicted of the toefor ofhomogeneous the Nigerconsists of delta wedges. that showSimilarly,a bottomless box containing a quasi-linear Bilotti and relation- Shaw (2005) 1980; Mount and Suppe, 1987, 1992; Zoback et al., 1987; Scholz andf Hanks, (seawatershape or ( αair), β ).to This the seemsmean densityplausible of because rock and the is only thus remaining 1 for subaerial term in theship withpresented a negative regional slope (~100 (Fig. km)2B). measurements We compute the of normalizedtaper in the wedge deep-water 2004) and on low-angle normal faults (Xiao et al., 1991; Axen, 2004). sand with transparent side walls, which wedgesequation, and ~0.6 [1 for− ( ρsubmarine/ρ)], contains wedges. the ratio Furthermore, of the– densitys it can of be the shown overlying that fl uidstrength thrust W belt= (σ of −the σ toe)/ρgH of thebased Niger on deltathe regression that show slopesa quasi-linear and obtain relation- The purpose of this paper is to show that we can recast critical-taperf 1 3 F is the(seawater regional or normalized air) to the basalmean4° shear density traction of rock F = and σ /ρisgH thus, determined 1 for subaerial similar ship results with for a negative both wedges. slope Taiwan(Fig. 2B). gives We W compute = 0.6 and the the normalized Niger delta wedge wedge mechanics (Davis et al., 1983; Dahlen, 1990) into a very simple form τ sits upon a sheet of Mylar. The base by thewedges failure onand the ~0.6 detachment, for submarine and Wwedges. is the normalized Furthermore, differential it can be stressshown thatgives strengthW = 0.7, W which = (σ indicate− σ )/ρ gHmoderately based on strong the regression wedges, asslopes discussed and obtain that allows us to directly determine or strongly constrain absolute regional- 1 3 W = (Fσ is− σthe)/ ρregionalgH at failure normalized (see Dahlen, basal shear1990, tractionequations F 88,= 90,/ 91,gH ,97). determined In in thesimilar next section.results for The both normalized wedges. Taiwan basal sheargives tractionW = 0.6 andF = the σ /NigerρgH, delta scale strength, simply from the observed geometries of1 active2 critical-taper στ ρ β upon which it lies is flat τ and rigid this paper we show two ways to apply this simplifi ed Equation 1b. which canα= be0° considered an effective coeffi cient of friction, is F = 0.08 wedges, given appropriate geologic circumstances. Weby theapply failure this on theory, the detachment, and W is the normalized differential stress gives W = 0.7, which indicate moderately strong wedges, as discussed TheW = critical ( surface)/ gH at slope failure α (seeof a Dahlen,mechanically 1990, equationshomogeneous 88, 90, wedge 91, 97). forIn Taiwanin the and next F section.= 0.04 for The the (Figure normalized Niger delta. 11, basal tTheop shearobserved right) traction (ratioDAVIS Fof , =fault 1983). / gH By , confi rming the existence of very weak detachments and strongσ1 − wedges. σ2 ρ στ ρ is linearlythis paper related we to show the dip two of ways the detachmentto apply this β simplifi, as shown ed Equation by rearranging 1b. strengthwhich to wedgecan be strengthconsidered F/W an = effective στ /(σ1 − coeffi σ3) iscient 0.13 of for friction, Taiwan is andF = 0.08 Equation 1b: 0° 0.06 for the Niger delta. These pulling results showthe that Mylar the basal she detachmentset, the sand are will ABSOLUTE STRENGTH FROM WEDGE TAPERSThe critical surface slope α0° of a mechanically4° homogeneous wedge 8° for Taiwan and F = 0.04 for the Niger delta. The observed ratio of fault is linearly related to the dip of the detachment β, as shown by rearrangingexceedingly strength weak toβ wedge absolutely strength and relativeF/W = toσ /(theσ wedge − σ ) strengths.is 0.13 for Taiwan and A central premise of critical-taper wedge mechanics is that actively F W get pressed τ 1 against 3 the back wall, deforming fold-and-thrust belts and accretionary wedgesEquation are α1b: simultane-= − Dry-sand βtapers, (2a)0.06 for the Niger delta. These results show that the basal detachments are 11−αW   W  − (ρρff) +  − ( ρρ) + COMPARISONexceedingly WITHweak absolutely DEEP BOREHOLE and relative to DATA the wedge strengths. ously at regional failure internally and along their base, which is plausible Figure 11: Schematic model of critical taper laboratory measurements. F W miming the process of plate subduction. because these wedges have reached their present shape by deformationα = Figure 1. Critical-taper− measurementsβ, of dry-sand(2a)We wedges compare on the Mylar wedge strengths W from Taiwan and the Niger delta Linear detachment regression (Davis gives et al., a 1983).slope to Linear compute regression the fault gives and a slope wedge which is the equation of a 11line− ρρofff negative + W slope − ρρ + W with stressCOMPARISON measurements WITH from twoDEEP scientifi BOREHOLE c boreholes DATA (Fig. 3). In the Ger-  (strengths (s = 0.66) ± S0.14UPPE , 2007).and( an )intercept = 5.6° 0.2°, which we use to To minimize inhomogeneities, the sand βα=0° man± KTB borehole σ is vertical (Brudy et al., 1997), whereas in compres- *Current address: Department of Geosciences, National Taiwan University, compute the fault and wedge strengths (F, W ) usingWe Equations compare2 the 3 wedge strengths W from Taiwan and the Niger delta s , (2b)sive wedges is vertical; therefore we represent the KTB stress data as 1 Roosevelt Road, sec. 4, Taipei 106, Taiwan. which is the equation of ααneeds to be packed evenly and side wall friction ianda line= 4 β of=(see0 −negative βtext). slope with stressσ3 s reduced by a graphite coating ( measurements from two scientifi c boreholesDAVIS (Fig., 1983). 3). In the Ger- W* = (σ1 − σ3)/σ3, which is directly comparable to W. W* is relatively con- man KTB borehole σ2 is vertical (Brudy et al., 1997), whereas in compres- where αβ = 0 and s are the slopeIn order to and intercept compute the fault and wedge strengthsobtained by linear regression stant as a function of ,depth, two equations are necessary to draw a linear indicating that the KTB region is dominated by αα= β=0 − s β, (2b)sive wedges σ3 is vertical; therefore we represent the KTB stress data as © 2007 The Geological Society of America. For permissionof suitable to copy,data contact(α, β) fromCopyright an active Permissions, mechanically GSA, or homogeneous [email protected] wedge . pressure-dependentW* = (σ − σ )/strength,σ , which with is directly W* = 1.0 comparable ± 0.2 to a todepth W. W* of 8is km. relatively con- GEOLOGY,Geology, December December 2007; 2007 v. 35; no. 12; p. 1127–1130;(Fig. doi: 1). 10.1130/G24053A.1;From Equation 2 we 5 fifiregression (Figure 11 bottom left). gures.nd that wedge strength W is a very simple Both resultIn contrast, from1 W3 in the11273 the r Californiaearranging SAFOD of equation (99) pilot hole (Hickman from andD AHLEN where αβ = 0 and s are the slope and intercept obtained by linear regression stant as a function of depth, indicating that the KTB region is dominated by functionof suitableof the slope data of(α the, β) regression from an active mechanically homogeneous wedgeZoback, pressure-dependent 2004) shows a strong strength, decrease with withW* =depth 1.0 ± (Fig. 0.2 to3), a suggestingdepth of 8 km.that (1990). The wedge strength W is a simple function of the slopthe measurements, which are at a depthe of the reg of 1–2 kmression ( in granite, areequation (4) still within ) and (Fig. 1). From Equation 2 swe fi nd that wedge strength W is a very simple In contrast, W in the California SAFOD pilot hole (Hickman and W = 1− ρρ, (3)the near-surface boundary layer in which cohesion dominates (cf. Dahlen function of the slope of theequation (5) shows that the fault strength F is the regression intercept β1− regressions  ( f ) Zoback, 2004) shows a strong decrease α=0 withtimes wedge strength. depth (Fig. 3), suggesting that et al., the1984). measurements, The cohesive which strength are atC a= depth~46 MPa of 1–2 given km byin granite,linear regression are still within and fault strength F is simply the regressions intercept β (Fig. 1) times of the data is a factor of four less than the borehole-scale cohesion estimated W = 1− ρρ, α=0 (3)the near-surface boundary layer in which cohesion dominates (cf. Dahlen wedge strength  ( f ) for the SAFOD pilot hole at 197–212 MPa (Hickman and Zoback, 2004). 1− s (4) et al., 1984). The cohesive strength C = ~46 MPa given by linear regression Knowing C, we obtain the pressure-dependent component of the stress of and fault strength F is simply the regression intercept β (Fig. 1) times of the data is a factor of four less than the borehole-scale cohesion estimated FW= β . α=0 (4) wedge strength α=0 (5) for the SAFOD pilot hole at 197–212 MPa (Hickman and Zoback, 2004). Knowing C, we obtain the pressure-dependent component of the stress of Therefore, we should be ablewhere s is the slope to determine the wedge. and detachment FW= β . (4)4° A) strengths (W, F) simply from the linear α covariation=0 of surface slope α Taiwan tapers with detachment dip β in mechanicallyBased on this linear regression, it is possible to determine the wedge and homogeneous wedges, based on décollement strength. Therefore, we should be able to determine the wedge and detachment linear regression. 4° A) strengths (W, F) simply from the linear covariation of surface slope α 2° Taiwan tapers with detachment dip in mechanically homogeneous wedges, based on β = 7.7° APPLICATION TO ACTIVEβ WEDGES α α = 0° linear regression. 2° We fi rst consider laboratory experiments with dry-sand wedges on a 0° 5° 10° 15° 20° β Mylar base, looking at the response of critical surface slope α to changes β = 7.7° APPLICATION TO ACTIVE WEDGES α α = 0° in detachment dip β (Fig. 1). Linear regression of the data of Davis et al. –s = –0.37 We fi rst consider laboratory experiments with dry-sand wedges on a-2° 0° β (1983) yields a basal coeffi cient of friction of F = µb = 0.27 and a wedge 5° 10° 15° 20° strengthMylar W = base, 1.9, whichlooking corresponds at the response to a cohesionlessof critical surface internal slope friction α to changesof = tanin detachment = 0.57. These dip βvalues (Fig. are1). Linearsimilar regression to the basal of theand datainternal of Davis fric- et al. –s = –0.37 µ φ -4° -2° (1983) yields a basal coeffi cient of friction of F = µb = 0.27 and a wedge tions (µb = 0.3, µ = 0.58) measured independently by Davis et al. (1983), suggestingstrength the W viability = 1.9, which of estimating corresponds wedge to anda cohesionless fault strengths internal from friction the of2° B) linearµ covariation = tanφ = 0.57.of andThese , ifvalues we can are fi similarnd suitable to the geological basal and examples internal fric- Niger delta tapers α β -4° –s = –0.55 with variabletions (µ bdetachment = 0.3, µ = 0.58)dip β measuredand plausible independently mechanical by homogeneity. Davis et al. (1983),α Wesuggesting consider the two viability active geologic of estimating wedges, wedge Taiwan and and fault the strengths Niger delta, from the 2° B) linear covariation of and , if we can fi nd suitable geological examples1° Niger delta tapers that may approximate the assumptionα β of large-scale homogeneity because = –0.55 with variable detachment dip β and plausible mechanical homogeneity. –s they show approximate linear covariation of α and β, and because they are α βα = 0° = 3.35° rather thickWe (H consider = 5–12 twokm); active therefore geologic their wedges, strengths Taiwan are less and likely the Niger to be delta, 1° rapidlythat changing may approximate laterally. In the contrast, assumption the thin of large-scaletoes (H < ~1homogeneity km) of active because 0° β accretionarythey show wedges approximate such as the linear Nankai covariation trough andof α Barbados and β, and show because surface they are 0° 1° 2° 3°βα = 0° = 3.35° 4° slopesrather α that thick decrease (H = 5–12away km);from thereforethe toe, withtheir nostrengths associated are lesschange likely in to be rapidly changing laterally. In contrast, the thin toes (H < ~1 km) of activeFigure 2. Linear regressions of taper measurements across detachment dip β, indicating that they are not mechanically homogeneous. central 0° Taiwan and the deep-water compressive toe of the Niger β Such accretionarywedges are expected wedges suchto have as the horizontal Nankai troughgradients and in Barbados wedge strength, show surface delta (Carena0° et al., 2002; 1° Bilotti and 2°Shaw, 2005), which 3° gives the 4° given slopesthe strong α that lateral decrease variation away in fromporosity, the lithifitoe, withcation, no andassociated hence cohe- change faultin and wedge strengths using Equations 3 and 4 (see text). Figure 2. Linear regressions of taper measurements across detachment dip β, indicating that they are not mechanically homogeneous. central Taiwan and the deep-water compressive toe of the Niger Such wedges are expected to have horizontal gradients in wedge strength, delta (Carena et al., 2002; Bilotti and Shaw, 2005), which gives the given the strong lateral variation in porosity, lithifi cation, and hence cohe- fault and wedge strengths using Equations 3 and 4 (see text). 1128 GEOLOGY, December 2007

1128 GEOLOGY, December 2007 15 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

5. Conclusion The Jura Mountains and the Molasse Basin are closely linked and share a mutual evolution. Figure 12 depicts a simplified sketch of their evolution and the accompanied formation into an foreland fold- and thrust belt. In the buckling stage, folds evolve to low-amplitude buckle folds in response to compression induced by the rise of the Aar-massif. The weak Triassic evaporites infill the space generated by rising anticlines at the space of the sedimentary cover. After thickening of the basal

zone, fault ramps nucleate and prograde upwards, doubling the sediment cover (SOMMARUGA, 1998). This evolutionary stage resulted in the present day structural units of the Jura Mountains. Evaporite- and thrust-related folds, as well as tear faults are its major tectonic features. In contrast, the Molasse Basin stayed relatively undeformed. The paper by Eva Görke elaborates in more detail on the sedimentary wedge, as well as further features of the Molasse Basin and the paper by Simon Freitag concentrates on the Hydrocarbon- system within the Jura-arc-Molasse-Basin-system. The underlying mechanics of fold-and-thrust belts can be described by the wedge forming in front of a snow plough/bulldozer. Various papers (both older and more recent) give accurate descriptions of these processes, so that fold-and-thrust belts and accretionary wedges are one of the best

understood deformational features of the Earth’s upper crust (DAHLEN, 1990).

Figure 12: Conceptual evolutionary stages of the Jura foreland between 20-15 Ma. This sketch is without scale (SOMMARUGA, 1998).

16 The Geometry and Evolution of the Jura Mountains: Fernschub mechanics Tabea Kleineberg 319001

6. Literature

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AFFOLTER, T., GRATIER, J.-P. (2004): Map view retrodeformation of an arcuate fold-and-thrust belt: The Jura case; J. Geophys. Res. 109, B03404

BECKER, A. (2000): The Jura Mountains - an active foreland fold-and-thrust belt; Tectonophysics 321, 381-406

SOMMARUGA, A. (1999): Décollement tectonics in the Jura foreland fold-and-thrust belt; Marine and Petroleum Geochemistry 16, 111-134

BITTERLI, T (1990): The kinematic evolution of a classical Jura fold : a reinterpretation based on 3- dimensional balancing techniques (Weissenstein Anticline, Jura Mountains, Switzerland) ; Eclog. Geol. Helvet., 83, 493-511

PFIFFNER, O.A. (2010): Geologie der Alpen; Haupt: Bern, Stuttgart, Wien.

TWISS, R. J., MOORES, E. M. (2007): Structural Geology; Freeman and Company: New York.

BURKHARD (1990): Aspects of the large-scale Miocene deformation in the most external part of the Swiss Alps (Subalpine Molasse to Jura fold belt). Eclog. Geol. Helvet., 83, 559-583.

DAHLEN, F.A. (1990): Critical taper model of fold-and-thrust belts and accretionary wedges: Annual Review of Earth and Planetary Sciences, v. 18, p. 55–99.

DAVIS, D., SUPPE, J., AND DAHLEN, F.A., 1983, Mechanics of fold-and-thrust belts and accretionary wedges: Journal of Geophysical Research, v. 88, p. 1153–1172.

SUPPE, J. (2007): Absolute fault and crustal strength from wedge tapers. The Geological Society of America, v. 35, no. 12, p. 1127-1130.