The Role of Mechanical Stratigraphy in Fault-Propagation Fold Evolution: A Case StudyDaniel Hoin, Brett Mays, and Benjamin Surpless; Trinity University, Department of Geosciences, San Antonio, TX 78212

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Figure 2B. Stratigraphy of the Still-well anticline area. Based on St. John (1965).

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Previous studies have demonstrated the influence of mechanical stratigraphy upon rock deformation, suggesting that interval thickness and competence contrast between intervals can be linked to the formation and evolution of contractional structures (e.g., Biot, 1961; Shackleton et al., 2005) as well as the formation and termination of fractures within those systems (e.g., Narr and Suppe, 1991; Shackleton et al., 2005). The Stillwell anticline is a Laramide-age, asymmetric structure to the east of Big Bend National Park in west Texas. The anticline is an east-vergent fault-propagation fold (Surpless and Quiroz, 2010), with subhorizontal fault propagation likely controlled by relatively weak strata (Mays et al., 2012). Field observations and fold geometry suggest that the tipline of the fault system has not breached the surface but likely has affected a significant portion of the Santa Elena limestone stratigraphy that best defines the fold system. To evaluate the effects of rheology on deforma-tion, we approached our investigation of the Stillwell anticline with several research questions in mind:

Research Questions: • Are there significant variations in unit rigidity within strata of the Santa Elena limestone? • How is strain distributed within and between strata of different rigidities? • How do variations in lithology affect the initiation and evolution of contractional features?

To address these questions, we mapped cross-sectional exposures of the Santa Elena limestone at the outcrop scale, performed detailed field-based testing of unconfined compressive strength of more resistant limestone beds (using a Schmidt hammer), and performed slope analysis to place qualitative constraints on weaker, shaley intervals within the Santa Elena stratigraphy.

Figure 2A. Shaded relief map with major Laramide-age faults and folds of the Big Bend region (in red) and the location of Marathon uplift. Sierra Del Carmen (SDC) and Santiago Mountains (SM) are shown in yellow. Inset shows distribution of deformation associated with the Laramide orogeny and the location of the Texas Lineament (TL). Modfied from Muehlberder and Dickerson (1989), Miller et al. (1992), and Surpless and Quiroz (2010).

Conclusions

References

Acknowledgements Special thanks to the Black Gap Wildlife Management Area staff for supporting our field research over the past several years; Mike Pittman, Travis Smith, and Dewey Stockbridge have generously provided housing and logistical support. Thanks also to the Trinity University Geoscience Depart-ment and the Ed Roy Field Research Fund for providing funding and logistical support.

Pre-Laramide Tectonics

The Stillwell anticline is part of the Texas Lineament, a northwest-trending, 80km-wide zone of fundamental crustal weakness (Fig. 2A; Albritton and Smith, 1957; Sears and Price, 1978; Muehlberger, 1980). Rifting between South and North America beginning in the Late Triassic likely resulted in shal-low marine deposition during the Lower to Middle Cretaceous (e.g., Muehl-berger, 1980; Page et al., 2008), including limestone, claystone, and siltstone (Fig. 2B). Marine sedimentation in the region ended with the initiation of Laramide deformation (e.g., Lehman and Busbey, 2007; Page et al., 2008).

The Laramide Orogeny

The Late Cretaceous Laramide orogeny (80-35 Ma) formed contractional structures from Montana south into northern Mexico (Fig. 2A; inset). Laramide aged structures are most commonly localized along reactivated fault zones (e.g., Tweto, 1975; Allmendinger, 1992). Anticlines associated with the Laramide orog-eny are characterized by steep narrow fore-limbs and shallow dipping backlimbs (e.g., Miller et al., 1992). In the Big Bend region (Fig. 2A), signficant faults and folds are associated with the Santiago and Sierra del Carmen Mountains (SM and SDC, respectively).

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The Stillwell anticline is a relatively small-scale (8km length, 550m width, with 250m structural relief ) asymmetric, fault-propagation fold (Fig. 3A), likely formed by fault reactivation during the Laramide orogeny (Surpless and Quiroz, 2010), similar to other Laramide-age structures in the western U.S. The fold is best defined by the resistant Santa Elena limestone. The Stillwell anticline displays three left-stepping bends along the fold’s axial trace, divid-ing the anticline into two simple segments to the north and south (North Segment and South Segment in Fig. 3A) with a complex central segment likely controlled by an underlying fault system (Surpless and Quiroz, 2010).

Figure 3B (above) shows the fold geometries of the Stillwell anticline defined by the Middle Santa Elena Forma-tion (see Section 4). The North and South segments of the fold display asymmetric box fold geometries with shal-low southeast dipping backlimbs (<300), steeply northeast dipping forelimbs (>500), and relatively long, sub-horizontal middle limbs. We discuss deformation within the hinge zone in Section 5 (photograph 5A is boxed in the figure above). The photograph above shows the strong, thick-bedded limestone layers with interbedded weak, shalely limestone.

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The hinge zone of the asymmetric Stillwell anticline is the area of greatest layer curvature, and field data reveal that outcrop-scale strain is focused within and between beds of the Santa Elena limestone at this lo-cation and in the forelimb of the fold (Fig. 5A). Ramp-flat geometries are common, suggesting the impor-tance of interlayer slip between competent beds, and in the hinge zone, we have documented macro-scale faults that terminate in the weaker, relatively shale-rich beds. In addition, we documented thickening of weaker units in areas of higher average strain.

Figure 5A. Contractional deformation in the hinge zone of the Stillwell Anticline. Figure 5C. Fracturing adjacent to a fault in the forelimb of the Stillwell anticline.

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Abrahams, A. D. and Parsons. A. J. 1987, ‘Identification of Strength Equilibrium Rock Slopes: Further Statistical Considerations’, Earth Surface Processes and Landforms, Vol. 12, 631-635.Abritton, D., Jr., and Smith, J., Jr., 1957, The Texas lineament: Proceedings, Twentieth International Geological Congress, Mexico City, sec. 5, p. 501 – 518.Allmendinger, R., 1992, Fold and thrust tectonics of the western United States, exclusive of the accreted terranes: In Burchfiel, B., Lipman, P., and Zoback, M. eds., The Cordilleran Orogen: conterminous U.S., Geological Society of America, v. G-3, p. 583 – 608.Henry, C.D., Price, J.G., and McDowell, R., 1983, Presence of the Rio Grande rift in West Texas and Chihuahua: El Paso Geological Society Guidebook, no. 15, p. 108 – 118.Henry, C.D., and Price, J.G., 1986, Early Basin and Range development in Trans-Pecos Texas and adjacent Chihuahua – Magmatism and orientation, timing, and style of extension: Journal of Geophysical Research, v. 91, no. B6, p. 6213 – 6224.Maler, M., 1990, Dead horse graben: a west Texas accommodation zone: Tectonics, v. 9, p. 1357 – 1368.Moon, J. S. and Selby, M. J. 1983, ‘Rock mass strength and scarp forms in southern Africa’, Geografiska Annaler, 65A, 135-145.Moustafa, A.R., 1983, Analysis of Laramide and younger deformation of a segment of the Big Bend region, Texas: Ph.D. Thesis, University of Texas at Austin, Austin, Texas, 278 p.Moustafa, A.R., 1988, Structural geology of the Sierra del Carmen, Trans-Pecos, Texas: Geological Quadrangle Map GQ-0054, University of Texas at Austin, Bureau of Economic Geology, 28 p.Muehlberger, W.R., 1980, Texas Lineament Revisited: New Mexico Geological Society Guidebook, 31st Field Conference, Trans-Pecos Region, p. 113 – 121.Sears, J., and Price, R., 1978, The Siberian connection: a case for Precambrian separation of the North American and Siberian cratons: Geology, v. 6, p. 267 – 270.Shackleton, J. R. Cooke, M. L. and Sussman, A. J. 2005, ‘Evidence for temporally changing mechanical stratigraphy and effects on joint-network archi tecture’, Geology; February 2005; Vol. 33; No. 2; p. 101-104.St. John, B.E., 1965, Structural geology of Black Gap area, Brewster County, Texas: Ph.D. Thesis, University of Texas at Austin, Austin, Texas, 200 p.St. John, 1966, Geology of Black Gap area, Brewster County, Texas: Geological Quadrangle Map GQ-0030, University of Texas at Austin, Bureau of Economic Geology, 18 p.Tweto, O., 1975, Laramide (Late Cretaceous – early Tertiary) orogeny in the southern Rocky Mountains, in Curtis, B, ed., Cenozoic history of the southern Rocky Mountains: Geological Society of America Memoir 144, p. 1 – 44.Vidal-Royo, O. Hardy, S. and Munoz, J. A. 2011, ‘The roles of complex mechanical stratigraphy and syn-kinematic sedimentation in fold development: insights from discrete-element modelling and application to the Pico del Aguila anticline (External Sierras, Southern Pyreness)’, Geological Society, London, Special Publications, 349, 45-60.

Kinematic forward computer modelling of the coupled fault-fold system beneath the Stillwell anticline closely approximates the documented cross-sectional geometry of the fold (figure above; Mays et al., 2012). Finite strain ellipses (in red and blue, above) record changes in shape associated with layers above the propagating fault system and strongly support field evidence which suggests that greatest strain is focused in the hinge zone and forelimb of the anticline (see dark red strain ellipses). The pre-cise shape of the field-documented hinge zone and forelimb regions could not be attained with mod-eling, perhaps due to the range of mechanical behaviors observed in those strata.

Modeling6

5 Hinge Zone Deformation and Mechanical Behavior

4 Mechanical Stratigraphy of the Santa Elena Limestone

The Stillwell Anticline3

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Introduction1

Fault and folds in the Big Bend region are primarily northwest trending, indicating that compressional stresses were northeast-oriented during the Laramide (Moustafa, 1983; 1988; Maler, 1990). The Stillwell anticline (SA in Fig. 2A) is associated with this event; by the Middle Eocene, the Big Bend area transitioned from contractional to ex-tensional deformation, causing normal faulting throughout the region (Henry et al., 1983; Henry and Price 1986).

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Figure 5B. Ramp-flat geometries and duplex structures within a 1-m thick limestone bed.

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Figure 4A. Local Santa Elena stratigra-phy, modified from St. John (1965).

To evaluate mechanical stratigraphy within the Santa Elena anticline, we performed field-based analysis of layers exposed within the Santa Elena anticline (highlighted in the stratigraphic section at left), where these same layers were gently dipping and relatively undeformed (see star on Fig. 2A for location). This analysis included qualitative description of changes in slope as well as direct measurement of compressive strength (in MPa) of more resistant units using a Schmidt hammer. The photos shown here display the stratigraphic units measured as well as Q value statis-tics.

The Q value is based on incoming and recoil velocities within the handheld Schmidt Hammer. The Q Value can be converted to MPa thereby determining the rock’s approximate compressive strength (Fig. 4B).

Section 4analysis location

To evaluate qualitative changes in rock strength, we also used the concept of the strength equilibrium enve-lope, where changes in rock slope can be loosely corre-lated with changes in rock strength (e.g., Abrahams and Parsons, 1986). In addition to field observations, we uti-lized topographic profiles constructed perpendicular to slopes not affected by a significant drainage system, using USGS DEM data within ArcMap. The slope profile shown below displays both the derived profile as well as the positions of the more resistant units for which we have compressive strength data.

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ResultsSchmidt hammer testing reveals variation in rock strength among more resistant limestone layers, with values loosely constrained between 30 MPa and 100 MPa (Fig. 4B), suggesting that 30 MPa may be a maximum value for the more shale-rich units in the Santa Elena stratigraphy. The angles of individual beds appear to suggest that thick, strong beds crop out with high slope values but may not signifi-cantly change the average slope in their vicinity if surrounded by weaker units (units I. and II., left); conversely, thick packages of strong beds dis-play a higher average slope (see units III. - VII., above). Thus, it appears that slope gradient can provide a loose constraint on the mechanical stratigra-

phy in an area, permitting division of mechanical packages based on changes in average slope.

We also documented ramp-flat geometries and duplex structures within the stronger limestone beds (Fig. 5B; see 5A for location). These structures suggest that a portion of the strain is accommodated within the stronger beds, and intense fracturing adjacent to faults (Fig. 5C; see 5A for location) also supports intra-bed deformation within the mechanically strong units. Thus, while there is evidence of ductile behavior in the weaker beds in the Santa Elena, brittle deformation is distributed both between and within mechanically strong beds, suggesting a range of deformation mechanisms have accomodated strain in the hinge zone.

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• Schmidt hammer analyses, field observations, and topographic slope analysis suggest that average slope is loosely correlated with rock strength. Stratigraphic intervals with a greater percentage of resistant/strong limestone layers exhibit greater average slope values, while in- tervals with few or no limestone layers (i.e., shalier intervals) display lower average slope values. • Based on the above finding, average slope can inform construction of mechanical stratigraphy, with significant changes in slope revealing signficant changes in mechanical behavior.

• The greatest bed-scale contractional deformation is focused within the hinge and forelimb zone, coincident with the maximum strain positions predicted by kinematic modeling.

• Field data suggest that a range of deformation mechanisms (e.g., fracturing, ductile thickening, intra-bed faulting, interlayer slip) take up strain during the folding of layers within the Santa Elena limestone.

• The above-mentioned bed-scale deformation mechanisms likely reduce stress at the fault tip, so that greater fault slip must have occurred to produce the observed fold geometries. These mechanisms might explain the difference between kinematic model results and observed cross-sectional geometries.

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