Quantitative insights into the role of gravitational collapse in ... - Core

ANNALS OF GEOPHYSICS, VOL. 49, N. 6, December 2006

Quantitative insights into the role of gravitational collapse in major orogenic belts Marcello Viti, Dario Albarello and Enzo Mantovani Dipartimento di Scienze della Terra, Università degli Studi di Siena, Italy

Abstract Previous works have proposed gravitational collapse as the driving mechanism of extensional deformation of thickened continental crust. In this work we investigate the physical plausibility of this interpretation for the most important orogenic belts of the world by computing the spreading force induced by lateral variations of crustal thickness and the possible resisting forces. Two collapse mechanisms, one involving the upper crust only and the other the whole crust, have been considered. Particular attention has been devoted to constrain the uncertainty affecting such computations, mostly due to the large variability of the thermal and mechanical properties of rocks. The results obtained show that gravitational collapse is not a plausible mechanism in the four Mediterranean orogens here considered (Northern Apennines, Calabrian Arc, Hellenic Arc and Carpathians). For the other orogenic zones we have taken into account (Western US Cordillera, Central Andes, Himalayas and Central Alps) the large uncertainty that affects the estimate of spreading and resisting forces does not allow to firmly assess the feasibility of gravitational collapse.

Solarino, 1998; Gautier et al., 1999; Vanderhaeghe and Teyssier, 2001). However, in several cases the plausibility of the collapse model has been questioned and alternative driving mechanisms have been proposed (e.g., Isacks, 1988; Peltzer and Saucier, 1996; Henk, 1999; Mantovani et al., 2001, 2002, 2006; Bird, 2002). This suggests that the mere evidence of extensional deformation within an orogenic belts does not necessarily imply that gravitational spreading is the driving mechanism. In fact, most of the above-mentioned interpretations arbitrarily assume the collapse of thickened crust as feasible, independently of the related structural/tectonic context and quantitative analysis of relevant tectonic forces. However, analytical computations and numerical simulations have shown that the strength of a thickened belt may overcome the spreading force induced by lateral variations of crustal thickness (e.g., Liu, 2001) and that the strength of the foreland facing the belt may considerably contribute to maintain

Key words gravitational collapse – rheology of the crust – tectonic forces – continental crust – lithospheric stress

1. Introduction Gravitational collapse of thickened continental crust has often been invoked as a possible driving mechanism of the extensional deformation observed in orogenic domains such as the Western European Variscides, North America Cordillera, Central Andes, Tibetan Plateau, Alps, Northern Apennines and Aegean region (e.g., Dalmayer and Molnar, 1981; Dewey, 1988; Harry et al., 1993; Carmignani et al., 1994; Eva and

Mailing address: Dr. Marcello Viti, Dipartimento di Scienze della Terra, Università degli Studi di Siena, Via Laterina 8, 53100 Siena, Italy; e-mail: [email protected]

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Marcello Viti, Dario Albarello and Enzo Mantovani

mechanism of gravitational collapse which would have generated the Great Basin (southwestern U.S.A.) in early Cenozoic (e.g., Jones et al., 1999; Rey and Costa, 1999). Analogously, the assessment of the force that resists gravitational collapse is affected by many uncertainties, most of them related to the still limited knowledge of the mechanical behaviour of the crustmantle system. For instance, the strength of upper and lower crustal layers is a matter of lively debate, due to the large variability of the rheological parameters of rocks and lithospheric geotherms (Burov, 2003; Ranalli, 2003). Overlooking these sources of uncertainty compromises the reliability of the estimate of the tectonic forces responsible for gravitational collapse.

large-scale gradients of crustal thickness and topographic relief (e.g., England and Houseman, 1988; Maggi et al., 2000; Rey et al., 2001). The above considerations suggest that the feasibility of the collapse model in a given zone should be checked by properly estimating driving and resisting forces. However, this check is not an easy task due to the still incomplete knowledge of the structural, thermal and mechanical properties of the continental lithosphere. The large variability of the continental crust, in terms of its lithological composition, may cause considerable uncertainty on the depth distribution of crustal density, which in turn affects the estimate of the gravitational spreading force. An example in this sense is provided by the debate about the

Fig. 1a. Geographical location of some orogenic systems considered in this study. For each system, the mountain belt (dark grey), the underthrusting plate and the foreland zone (both light grey) are shown. Toothed lines indicate subduction or thrust boundaries. Abbreviations refer to the tectonic domains whose structural and thermal parameter are reported in table I. a) North and South America. Western United States Cordillera: CP=Colorado Plateau; GB=Great Basin; GV=Great Valley; SN=Sierra Nevada. Central Andes: AL=Altiplano Plateau; CB=Chaco Basin; NZ=Nazca Plate.

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Quantitative insights into the role of gravitational collapse in major orogenic belts

b

c

Fig. 1b,c. Geographical location of some orogenic systems considered in this study. For each system, the mountain belt (dark grey), the underthrusting plate and the foreland zone (both light grey) are shown. Toothed lines indicate subduction or thrust boundaries. Abbreviations refer to the tectonic domains whose structural and thermal parameter are reported in table I. b) Himalayas: GB=Gange Basin; OB=Ordos Basin; TP=Tibetan Plateau. c) Mediterranean region. Central Alps: LA=Lepontine Alps; MB=Molasse Basin; PB=Po Basin. Northern Apennines: AB=Adriatic Basin; NTB=Northern Tyrrhenian Basin; UM=Umbria-Marche Apennines. Calabrian Arc: CA=Calabria; IB=Ionian Basin; STB=Southern Tyrrhenian Basin. Hellenic Arc: CB=Cretan Basin; CR=Crete; HB=Herodotus Basin. Carpathians: MP=Moesian Platform; SEC=Southeastern Carpathians; TB=Transylvanian Basin.

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pathians (fig. 1a-c). The structural and thermal data available for each belt, along with the related uncertainties (table I), have been used to obtain realistic ranges of driving and resisting forces, which allow investigating the role of gravitational collapse in active extension.

The aim of this work is providing quantitative insights into this problem, by computing the force balance for eight major orogenic belts in the world: Western US Cordillera, Central Andes, Himalayas, Central Alps, Northern Apennines, Calabrian Arc, Hellenic Arc and Car-

Table I. Structural and thermal data concerning the 8 orogenic systems considered in this study. The first, second and third row for each zone refer to the subducting crust, the chain and the foreland, respectively. The second box (Western US Cordillera 2) only includes two rows, which are respectively referred to the chain and the foreland. See fig. 1a-c for geographical location of the various domains. e=average height of the chain above the surrounding domain; L=length of the underthrusting boundary; hW, hUC, hLC =thickness of the water layer, upper crust and lower crust, respectively; q0 =surface heat flow. Small letters close to numbers indicate the reference. For each zone of the various orogenic systems only the parameters which are relevant for the computation of spreading and resisting forces acting at that zone (F, R and T at subduction boundaries and F, R and C at nonsubduction ones) are reported. See text for explanations. Zone

e km

hW km

hUC km

hLC km

q0 (mW⋅m−2)

-

15(a,b) 20(b,d) 18(b,f)

15(a,b) 24(b,d) 12(b,f)

25-55(c) 40-57(e) 59-125(e)

Fliedner et al. (2000) Ruppert et al. (1998) (c) Wang and Munroe (1982) (d) Smith and Bruhn (1984) (e) Harry et al. (1993) (f) Satarugsa and Johnson (2000)

hW km

hUC km

hLC km

q0 (mW⋅m−2)

References

-

18(a,b) 25(a,d,e)

12(a,b) 15(a,d,e)

59-125(c) 42-94(a,d,e)

Western US Cordillera 1 Great Valley Sierra Nevada 3.5 Great Basin 1.5

Zone

e km

Western US Cordillera 2 Great Basin 1.5 Colorado Plateau 2.5

Zone Central Andes Nazca Plate Altiplano Chaco Basin

Zone Himalayas Gange Basin Tibetan Plateau

e km

L km

hW km

hUC km

hLC km

q0 (mW⋅m−2)

5. -

150(a) -

5(b) -

23(c) 28(b,e)

7(b) 45(c) 10(b,e)

70-130(d) 40-50(d)

e km

L km

hUC km

hLC km

q0 (mW⋅m−2)

5.

160(a) -

15(b) 24(c)

25(b) 46(c)

58-106(c,d)

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References

(a)

(b)

(a)

Ruppert et al. (1998) Satarugsa and Johnson (2000) (c) Harry et al. (1993) (d) Hinojosa and Mickus (2002) (e) Lowry and Smith (1995) (b)

References

(a)

Giese et al. (1999) Flueh et al. (1998) (c) Schmitz et al. (1999) (d) Springer and Förster (1998) (e) Lucassen et al. (2001)

(b)

References

(a) (b)

Kayal (2001) Cattin et al. (2001)


Table I (continued). Zone Himalayas Ordos Basin

e km

L km

hUC km

hLC km

q0 (mW⋅m−2)

1.

-

19(c)

24(c)

48-72(c,d)

References

(c) (d)

Zone Central Alps Po Basin Lepontine Alps Molasse Basin Zone

e km

L km

hUC km

hLC km

q0 (mW⋅m−2)

2.5 0.5

125(a,b) -

20(a) 30(a,b,c) 20(a,b)

10(a) 30(a,b,c) 10(a,b)

60-80(d) 68-110(c)

e km

L km

hUC km

hLC km

q0 (mW⋅m−2)

100(a) -

20(b) 15(a,d,e) 12(e)

10(b,c) 20(a,d,e) 10(e)

40-60(f, g) 67-147(f)

Northern Apennines Adriatic Basin Umbria-Marche Belt 1. N. Tyrrhenian Basin -

Zone

e km

L km

hW km

hUC km

hLC km

q0 (mW⋅m−2)

120(a) -

4(a,b) 4(a,d)

7(a,b) 20(a) -

8(a,b) 6(a) 10(a,d)

40-60(c) 75-143(e)

e km

L km

hW km

hUC km

hLC km

q0 (mW⋅m−2)

1.5 -

150(a) -

3(a,b) 2(a)

10(a,b) 13(a) 6(a)

11(a,b) 20(a) 8(a)

50-60(c) 60-70(c)

Calabrian Arc Ionian Sea Calabria 1. S. Tyrrhenian Basin -

Zone Hellenic Arc Herodotus Basin Crete Cretan Basin Zone

e km

Carpathians Moesian Platform Southeastern 1 Carpathians Transylvanian 0.5 Basin

L km

hUC km

hLC km

q0 (mW⋅m−2)

180(a) -

14(b) 20(b)

16(b) 30(b)

50-70(b)

-

12(b,c)

18(b,c)

40-60(b,c,d)

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Wang (2001) Hu et al. (2000) References

(a)

Okaya et al. (1996) Marchant and Stampfli (1997) Cermak and Bodri (1996) (d) Della Vedova et al. (1995) (b)

(c)

References

(a)

Ponziani et al. (1995) Okaya et al. (1996) (c) Finetti et al. (2001) (d) Giudici and Gualteri (1998) (e) Gualteri et al. (1998) (f) Della Vedova et al. (1995) (g) Pasquale et al. (1997) (b)

References

(a)

Van Dijk et al. (2000) De Voogd et al. (1992) (c) Cataldi et al. (1995) (d) Duschenes et al. (1986) (e) Della Vedova et al. (1995)

(b)

References

(a)

Bohnhoff et al. (2001) De Voogd et al. (1992) (c) Cermak (1993) (b)

References

(a)

Linzer et al. (1998) Fan et al. (1998) (c) Andreescu et al. (2002) (d) Serban et al. (2001) (b)


Fig. 2. Driving (solid arrows) and resisting (empty arrows) forces presumably involved in the gravitational collapse of a thickened crust. F=gravitational spreading force; R=tensional strength of the chain; C=compressional strength of the foreland; T=frictional resistance at the underthrusting boundary (thick line), whose length is L in the text.

F − (R + C) $ 0

2. Dynamics of gravitational collapse

(2.1a)

and towards the underthrusting boundary when

It has been demonstrated that a lateral variation of crustal thickness produces a spreading force, whereas thickening of the underlying lithospheric mantle generates an opposite effect (Rey et al., 2001). An upper bound for the spreading force can therefore be obtained by considering the difference in crustal thickness between the chain and the related foreland (e.g., Liu and Shen, 1998; Liu, 2001; Liu and Yang, 2003). Assuming that the spreading force only acts in the crust, the collapse of a thickened belt requires that the crust is decoupled from the lithospheric mantle by a zone of low mechanical strength, as revealed by rheological profiles of continental lithosphere (e.g., Bird, 1991; Viti et al., 1997; Meissner and Mooney, 1998). In our computations, we have adopted a structural scheme involving a thickened crustal domain confined by a thinner foreland, on one side, and by a subduction or collision boundary, on the other (fig. 2). The spreading force (F) generated in the thickened crust is resisted by the tensional strength of the chain (R), the compressional strength of the foreland (C) and the frictional resistance at the underthrusting boundary (T). Gravitational collapse can only occur when the spreading force overcomes the strength of the chain (e.g., Liu, 2001): F−R≥0. However, this condition is not sufficient, due to the contribution of the other boundary resistances. In particular, lateral collapse towards the foreland occurs when

F − (R + T) $ 0 .

(2.1b)

In the next we describe the formulation adopted for estimating the above driving and resisting forces. 2.1. Spreading force The gravitational spreading force is induced by the horizontal gradient of potential energy stored in the crust of the orogenic and foreland domains, which depends on the lateral variations of crustal thickness and density (e.g., Molnar and Lyon-Caen, 1988). Various mechanisms of gravitational collapse, involving different portions of the crust, have been discussed (e.g., Rey et al., 2001). In this work, we have investigated both the possibilities that the collapse is confined to the upper crust and that it involves the whole crust. In these cases, lateral spreading would be allowed by low-strength layers respectively at the base of the upper and lower crust, as predicted by strength envelopes (e.g., Meissner and Mooney, 1998). For the simplified structural model shown in fig. 3 the average spreading force (F) per unit length of the orogenic belt (Nm−1) is given by (e.g., Coblentz and Sandiford, 1994; Liu, 2001) 1294


ing collapse and depends on the collapse mechanism considered (upper or whole crust; hFU≤ ≤ zF≤hFU +hFL from fig. 3). C represents a lower bound for the strength of the foreland, since it does not take into account the contribution of the mantle facing the crustal root of the chain. To evaluate R and C, constitutive equations, describing the inelastic deformation of rocks, have to be adopted (Kohlstedt et al., 1995). In computing classical strength envelopes (Goetze and Evans, 1979), two basic deformation mechanisms, i.e. frictional slip on optimally oriented faults (in the brittle field) and dislocation creep (in the crystal plastic field) are taken into account. However, a large amount of experimental results,

zc

# ∆p (z) d z

F=

(2.2a)

−e

z

∆p (z) = g

# 6 ρ (ζ) − ρ (ζ)@ d ζ C

F

(2.2b)

−e

where ∆p is the difference of lithostatic pressure in the chain and foreland, ρC and ρF are the related depth-distributions of rock density and g is the gravity acceleration (9.81 ms−2). e is the topographic height of the chain above the foreland, z is the depth measured downward from the top of the foreland and zC is the the depth of the bottom of the collapsing crustal layer. It follows from fig. 3 that zC =hCU for collapse of the upper crust, and zC =hCU +hCL for collapse of the whole crust. The above computation of F applies to both cases of gravitational collapse shown in fig. 1a-c, i.e. towards the foreland or towards the subducting plate. The structural data about foreland, chain and subducting plate, required to estimate F by eq. (2.2a), are reported in table I, for each of the orogenic systems considered. 2.2. Strength of the chain and the foreland The integrated tensional strength of the thickened crust of the chain (Nm−1) is defined by (e.g., Liu and Shen, 1998; Liu, 2001) zC

# ∆σ (z) d z

R=

T

(2.3a)

−e

where ∆σ T (z) is the depth-distribution of the tensional strength of the orogenic belt and zC is defined as in eq. (2.2a). Analogously, the compressional strength of the foreland is defined by Fig. 3. Schematic structural model used in computations of spreading force and resistances. Light, medium and dark gray depicts upper crust, lower crust and upper mantle, respectively. ρUM =density of the upper mantle; ρCU, ρCL (ρFU, ρFL)=density of the upper and lower crust in the chain (foreland); hCU, hCL (hFU, hFL)=thickness of the upper and lower crust in the chain (foreland); e=topographic height of the chain above the foreland; z=depth.

zF

# ∆σ (z) d z

C=

C

(2.3b)

0

where ∆σC(z) is the depth-distribution of the compressional strength. The depth zF corresponds to the bottom of the crust of the foreland oppos1295
