Mechanics of faulting

Transcription

1 Mechanics of faulting asfault.org Jyr-Ching Hu, Dept. Geosciences National Taiwan University

2 Strengths of active thrust-belt wedges & their basal detachments: directly determined from the covariation of surface slope a with detachment dip b, without strong assumptions about the specific strength-controlling Test: Niger delta thrust belt, the active Taiwan mountain belt, and the thrust that slipped in the M = 7.6 Chi-Chi earthquake Suppe, Geology 2007

3 Absolute fault & crustal strength from wedge tapers Basal detachments: exceedingly weak, with effective coefficients of friction ( ) that are an order of magnitude less than most laboratory friction coefficients ( ) Weak faults & strong crust: wedges are moderately strong internally, within the range of pressure-dependent strengths in deep boreholes

4 The classic thrust-fault problem Frictional resistance = b x Weight The breakup Maximum length ~20 km Courtesy of John Suppe

5 Critical-taper wedge mechanics

6 Courtesy of John Suppe

7 Critical-taper wedge taper = ab Courtesy of John Suppe

8 Critical-taper wedge mechanics Actively deforming fold-and-thrust belts & accretionary wedges: simultaneously at regional failure internally & along their base Mechanical equilibrium: between the critical taper α + β of a wedge & the strength of the wedge & its base, where α is the surface slope and β is the dip of the detachment taper = ab Davis, 1983 Courtesy of John Suppe

9 a b Wedge theory Wedge theory: infer the magnitudes of strength parameters that are consistent with observed tapers, e.g., internal & basal friction coefficients (μ = tanφ, μ b ) & depth-normalized pore-fluid pressures (λ = P f /ρgh). Mechanically homogeneous Wedge (Dahlen, 1990, equation 99): b 1 f b(1 b ) Sb gh sin 1 f 2(1 ) C gh 1 sin S b & C: non-pressure-dependent parts of the fault and wedge strength H: thickness

10 Wedge theory Equations contain a number of average regional-scale fault & crustal strength parameters, but unfortunately have little direct constraint in actively deforming regions. Equations simpified: Fault-strength terms: W 2(1 ) sin 1 sin F (1 ) S gh b b b b 1 f F a b 1 f W Wedge-strength terms C gh a b f ( F, W ) [1 (ρ f /ρ)]: ratio of the density of the overlying fluid (seawater or air) to the mean density of rock & is 1 for subaerial wedges & ~0.6 for submarine wedges

11 Wedge theory b 1 f F a b 1 f W F gh F: regional normalized basal shear traction W 1 3 gh W: normalized differential stress (see Dahlen, 1990, equations 88, 90, 91, 97)

12 Wedge theory If F & W are homogeneous then a & b are linearly related F W a 1 1 f W f W b a a b s b 0

13 Get strengths from co-variation of a & b W s 1 f 1 s s W 1 s F ba 0 W

14 Application to active wedges Dry-sand wedges on a Mylar base, Two active geologic wedges, Taiwan and the Niger delta Approximate the assumption of large-scale homogeneity 1. Approximate linear covariation of α and β 2. Rather thick (H = 5 12 km)

15 Application to active wedges Not mechanically homogeneous: thin toes (H < ~1 km) of active accretionary wedges such as the Nankai trough & Barbados show surface slopes α that decrease away from the toe, with no associated change in detachment dip β Have horizontal gradients in wedge strength, given the strong lateral variation in porosity, lithification, & hence cohesion, & probably fluid pressure.

16 Application to active wedges Basal coefficient of friction of F = μ b = 0.27 & a wedge strength W = 1.9, which corresponds to a cohesionless internal friction of μ = tanφ = Davis et al. (1983) predicts µ = 0.57 measured µ = 0.58 predicts µ b = 0.27 measured µ b = 0.3

17 Taiwan Main Detachment h=v Carena et al., geology 2002 Stepping down to deeper detachments to East Courtesy of John Suppe

18 Linear regressions of taper measurements Central Taiwan Carena et al. (2002) Deep-water compressive toe of the Niger delta Bilotti & Shaw (2005)

19 Summary W = (σ 1 σ 3 )/ρgh based on the regression slopes & obtain similar results for both wedges. Taiwan gives W = 0.6 & the Niger delta gives W = 0.7 Normalized basal shear traction F = σ /ρgh: F = 0.08 for Taiwan and F = 0.04 for the Niger delta. Observed ratio of fault strength to wedge strength F/W = σ /(σ 1 σ 3 ): 0.13 for Taiwan and 0.06 for the Niger delta. These results show that the basal detachments are exceedingly weak absolutely and relative to the wedge strengths.

20 Comparison with deep borehole data Borehole stress measurements SAFOD pilot hole: strong decrease with depth; suggesting that the measurements, which are at a depth of 1 2 km in granite, are still within the nearsurface boundary layer in which cohesion dominates Cohesive strength C = ~46 MPa: A factor of four less than the borehole-scale cohesion estimated for the SAFOD pilot hole at MPa (Hickman & Zoback, 2004).

21 Comparison with deep borehole data Borehole stress measurements KTB borehole σ 2 is vertical, whereas in compressive wedges σ 3 is vertical W* is relatively constant as a function of depth, indicating that the KTB region is dominated by pressure-dependent strength, with W* = 1.0 ± 0.2 to a depth of 8 km.

22 Constraint of a single taper a a b b 0 s F a 1 f ( a b ) W

23 Constraint of a single taper Courtesy of John Suppe

24 Wedge-strength constraints Courtesy of John Suppe

25 Courtesy of John Suppe

26 Thermal anomaly in post Chi-Chi boreholes Courtesy of John Suppe

27 Constraint of Chi-Chi thermal anomaly Tanaka et al. GRL 2006

28 Post Chi-Chi borehole stress measurements W*= Hung et al. Tectonophysics 2009

29 Constraint of borehole thermal anomaly Courtesy of John Suppe

30 Summary Upper bound on upper-crustal strength: Byerlee s law (μ = 0.85) with hydrostatic pore-fluid pressures (λ = 0.4), then W 2.2 & F 0.21, which is a weak detachment Chinshui Shale detachment: exceedingly weak, & best estimate is in the range F = σ /ρgh = Chelungpu thrust ramp is even weaker based on shear tractions σ estimated from post Chi-Chi borehole thermal anomalies and W* observed by Tanaka et al. (2006), Hung et al. (2009), and Kano et al. (2006) (F* = σ /σ n = ).

31 Summary These extreme fault weaknesses are especially striking in light of the observation that the regional pore-fluid pressures surrounding the Chinshui Shale detachment and thrust ramp are hydrostatic (λ = 0.4) (Yue, 2007). Therefore, the static ambient Hubbert & Rubey (1959) fluid-pressure hypothesis is not the cause of the weakness of the Chinshui Shale detachment or thrust ramp. Furthermore, the wedge is strong in spite of the very weak thrust ramp within it, presumably because of the internal strength of the thrust sheets in bending

32 Why are the faults so weak & the crust so strong??? F gh W 1 3 gh F W

33 Are pore-fluid pressures the solution to the weak fault problem??? F (1 ) b where P f gz need (1 ) 0.1 or 0.9 The classic Hubbert-Rubey hypothesis

34 The Chinshui shale detachment is above fluid-retention depth Z FRD therefore not classic Hubbert & Rubey fluid-pressure mechanism Courtesy of John Suppe

35 Relationship between Z FRD and Hubbert & Rubey effect. (1 ) 0.6 Z FRD Z need Z > 5Z FRD or ~10-15 km for Taiwan Courtesy of John Suppe

36 Earthquake slip is confined to geometric segments Chi-Chi earthquake Yue, Suppe & Hung, 2005

37 Coseismic folding in Chi-Chi earthquake Fault bends must be the locus of crustal strength Courtesy of John Suppe

38 Coseismic folding in Chi-Chi earthquake Courtesy of John Suppe

39 Weak faults and strong crust Continual deformation of new rock along axial surfaces. Courtesy of John Suppe

40 Coseismic folding in Chi-Chi earthquake Fault bends must be the locus of crustal strength Courtesy of John Suppe

41 Contrasting crustal strengths Courtesy of John Suppe

42 Areas of very thick deforming sediments Gulf of Mexico Niger delta Borneo Sumatra Nankai trough Cascadia Bangladesh/Myanmar Makran Gulf of Alaska New Zealand Taiwan Courtesy of John Suppe

43 Low strength of deep San Andreas fault gouge from SAFOD core David A. Lockner, Carolyn Morrow, Diane Moore & Stephen Hickman Nature, 472, 82 85, 2011

44 Weakness of the San Andreas Fault Zone Absence of a heat flow anomaly (Brune et al., 1969; Lachenbruch & Sass, 1980; Williams et al., 2004) Stress orientation across the fault (Zoback et al., 1987; Mount and Suppe, 1987),

45 Hypotheses Fault zone consists of clay gouge (Wu et al., 1975; Wu, 1978; Wang et al., 1978), especially a montmorillonite rich clay gouge that has frictional coefficients as low as ~0.1 (e.g., Wang and Mao, 1979; Chu et al., 1981; Carpenter et al., 2011; Lockner et al., 2011) Fault zone has a normal frictional coefficient but is dynamically weakened during earthquakes by shear heating & other physicochemical processes (e.g., Lachenbruch, 1980; Di Toro et al., 2011) Frictional coefficient of the fault is normal, but high porepressure in the fault zone lowers the effective normal stress on the fault and thus its frictional resistance to sliding (Rice, 1992; Byerlee, 1990)

46 SAFOD SAFOD: San Andreas Fault Observatory at Depth Study the physical & chemical processes controlling faulting & earthquake generation along an active, platebounding fault at depth

47 San Andreas Fault

48

49 Fault Contact at 10,063 ft Highly Deformed Siltsone Clay Gouge 2.5 cm Granite Cobble Conglomerate

50 San Andreas Fault Observatory at Depth (SAFOD) A 15-year effort of Mark Zoback, Steve Hickman, and Bill Ellsworth North American Plate Steve Direct measurement of the processes that control earthquakes

51 Location of SAFOD site SAFOD site: located at the NW end of the rupture zone of the 1966 and 2004 M 6 Parkfield earthquakes, in the transition between the creeping and locked sections of the SAF At surface, the fault is creeping at a rate of 1.8 cm/yr. Numerous earthquakes occur directly on the SAF at 3-12 km

52 Geophysical logs & generalized lithology from phase 2 of the SAFOD project SAF is a broad zone of anomalously low P- and S-wave velocity and resistivity

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54 Why Parkfield? Transition between the locked portion of the fault to the SE & the segment of the fault to the NE where slip dominantly occurs by aseismic creep

55 Geologic cross-section parallel to the trajectory of the SAFOD borehole San Andreas Fault damage zone: SDZ, CDZ, & NBF

56 2007 => creeping strands: southwest deforming zone (SDZ) central deforming zone(cdz) Depth:2.7 km, damage zone:200 m wide 56

57 Methods XRD 1.6 m and 2.6 m fault gouge of 31 m of core Powder(<150 μm ) sheared in gouge layer(1- or 2-mm-thick, 25.4-mm-diameter) DI water & brine X-ray diffraction to determine mineral composition. Strength Triaxial apparatus σ n = 40, 120, 200 MPa p = 1 MPa V = 1.15, 0.115, μm /s In situ & steady state

58 X-Ray Diffraction SDZ & CDZ: porphyroclasts of serpentinite and sedimentary rock dispersed in a matrix of Mgrich clays

59 SW side NE side X-Ray Diffraction CDZ:Sap (>60%) SDZ:Sap, Cor, Q, F The two gouge zones: product of shearing-enhanced metasomatic reactions between serpentinite & adjoining sedimentary rocks. Q = quartz Cc = calcite K = K feldspar Pl = plagioclase (albite?) Chl = chlorite Km = K - micas Srp = serpentine Sap = saponite Cor = corrensite

60 SW side NE side X-Ray Diffraction CDZ:Sap (>60%) SDZ:Sap, Cor, Q, F The two gouge zones: product of shearing-enhanced metasomatic reactions between serpentinite & adjoining sedimentary rocks. Q = quartz Cc = calcite K = K feldspar Pl = plagioclase (albite?) Chl = chlorite Km = K - micas Srp = serpentine Sap = saponite Cor = corrensite

61 Sample strength: in situ σ n = 122 MPa V = 1.15 μm/s μ:coefficient of friction τ :shear stress σ n :effective normal stress p:pore pressure Compositional change SW => NE SDZ & CDZ: 0.13 μ 0.21 Saponite (μ ~0.05)

62 Strength & sliding rate Dieterich-Ruina law (a-b) < 0 => unstable slip (a-b) > o => stable creep μ ss = steady-state friction coefficient Slip rate (μm/s) : V = 1.15 (F), (M), (S) Serpentinite porphyroclast from the SDZ: a b = ± All other core samples have positive rate sensitivity: 1. Outside the foliated gouge zone: < a b < CDZ:a b = ± SDZ:a b = ±

63 Argument Low frictional strength (µ 0.15) of foliated gouge: (1) lack of an observed heat flow anomaly (2) maximum compressive stress oriented at a high angle to the fault trace (3) no evidence of pore pressure elevated in fault zone The positive dependence of strength on slip rate of the fault gouge material is consistent with deformation by creep rather than by earthquakes. Stable creep and low strength Boness & Zoback, 2006

64 Stress state Depth : 2.7km τ = 17 MPa, σ n = 122 MPa p = Tembe et al., 2009

65 Summary The laboratory strength measurements of the SAFOD fault core materials at in situ conditions, demonstrating that at this locality & this depth the San Andreas fault is profoundly weak (μ = 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault. Deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms

66 References 11. Zoback, M. D., et al. (1987), New evidence on the state of stress of the San Andreas Fault, Science, 238, Scholz, C. H. (2000), Evidence for a strong San Andreas fault, Geology, 28, Zoback, M. D. (2000) Strength of the San Andreas. Nature 405, Carpenter, B. M., C. Marone, and D. M. Saffer (2011) Weakness of the San Andreas Fault revealed by samples from the active fault zone, Nature Geoscience, doi: /ngeo Wang, C-Y (2011) High pore pressure, or its absence, in the San Andreas Fault, Geology, 39, , doi: /G Zoback, M., Hickman, S. Ellsworth, W. and the SAFOD Science Team (2011) Scientific Drilling Into the San Andreas Fault Zone An Overview of SAFOD s First Five Years, doi: /iodp.sd Collettini, C., Niemeijer, A., Viti C. & Marone, C. (2009) Fault zone fabric and fault weakness, Nature 462, 36,