ABSTRACT

Conventional kinematic models of fault-related folds predict fold shape from thrust ramp angle. However, these kinematic models often fail to account for certain aspects of fault-related folds, such as footwall folding. In addition, conventional model assumptions are not consistent with measured displacement distributions along some natural faults. Displacement distributions are an important part of fault-related fold interpretation, yet are frequently ignored in discussions of kinematic models. The shape of a displacement-distance graph reflects the history of fault growth. Cone-shaped (C-type) graphs generated from natural structures suggest ramp-nucleated faults. Models that assume fault growth away from a lower flat do not apply in such instances. Possible modes of fault-propagation fold evolution are described that assume ramp-nucleated faults. Low propagation rate to slip rate (PR/SR) ratios promote fold growth ahead of upper and lower fault tips, with resultant fold geometries that are consistent with natural examples and previous models, with the additional requisite of accounting for footwall folding and displacement distributions.

ACKNOWLEDGEMENTS

My sincerest gratitude to all my colleagues, friends, and family, for their combination of intellect, assistance, and moral support, during the creation of this thesis. Special thanks to:

- my advisor, Dave McConnell, for introducing me to a most interesting research problem, and for his expertise and guidance during my numerous episodes of panic.

- Tom Quick, for his assistance with developing film and prints, and for sorting out stubborn computers.

- Gautam Mitra and Kevin Stewart, for valuable comments on a manuscript comprising parts of this thesis.

- Verne Friberg and Roger Bain for their helpful suggestions towards improving the first draft.

Geometry of fold-and-thrust belts

The Appalachian Fold-and-Thrust Belt

Fault-related fold models

Fault-bend folds

Fault-propagation folds

Detachment folds

Break-thrusts

Displacement-distance diagrams

Field locations

Methodology

II. DESCRIPTION OF OUTCROP STRUCTURES

Eagle Rock, Virginia

Eagle Rock 1

Eagle Rock 2

Eagle Rock 3

Roundtop Hill, Maryland

Bergton, Virginia

III. INTERPRETATION OF STRUCTURES

Introduction

Eagle Rock 1

Eagle Rock 2a

Eagle Rock 2b

Eagle Rock 2c

Eagle Rock 3

Roundtop Hill

Bergton

IV. RE-EVALUATION OF KINEMATIC MODELS

Alternative model

Natural examples

Discussion

Fold modification via translation

Displacement-distance diagrams

V. SUMMARY

Lithologic controls on deformation

Mechanism of fault growth

Fold amplitudes

Footwall folding

Conclusions

REFERENCES

1. Simplified cross-sectional view across the central Appalachians (modified after Kulander and Dean, 1986). The lower boundary of the Massanutten-Blue Ridge sheet is the Pulaski thrust. The Waynesboro sheet is floored by a detachment in the Cambrian Rome Formation and shows a duplex geometry. The Martinsburg sheet contains numerous faults and folds (not shown) that developed over the Waynesboro sheet thrust system

2. Generalized stratigraphic column of the central Appalachian region. Undulating boundaries represent unconformities whereas sawtooth boundaries represent transgressions. Notations are Grp (Group), Fm (Formation), Ls. (Limestone), Ss. (Sandstone), and Sh. (Shale). After Kulander and Dean (1986)

3. Principal fault-fold styles in fold and thrust belts. a. fault-bend folding over a flat-ramp-flat staircase fault trajectory. b. fault-propagation folding ahead of a propagating thrust ramp tip. c. detachment folding over and ahead of a detachment thrust tip. a = fold amplitude; f = height of lowermost folded unit above the detachment. d. break-thrust in which folding precedes subsequent faulting through the fold forelimb

4. Sequential evolution of a fault-bend fold over a staircase fault trajectory, after Suppe (1983)

5. Sequential evolution of a fault-propagation fold, after Suppe (1985)

6. Sequential evolution of a fault-propagation fold developing over a pre-existing fault ramp segment (after Chester and Chester, 1990)

7. Fault-propagation fold model of McNaught and Mitra (1993). Early detachment folding is followed by thrust ramping through the thickened section of the fold core, with translation and modification of the detachment fold via a fault-propagation fold mechanism

8. Measurement of parameters used to construct a displacement-distance graph (inset). Distances are measured from a reference point at the upper fault tip to beds in the hanging wall. Corresponding displacements are measured across the fault plane

9. Displacement-distance diagrams for conventional models of fault-related folding. a. Fault-bend folds (Suppe, 1983) display oblique plots through the truncated beds that were initially in the hanging wall of the fault ramp, but are horizontal everywhere else. Curve 1 shows the constant displacement maximum occurring for points originally in the hanging wall of the lower flat. Curve 2 uses displacement markers only (truncated beds in the hanging wall) and will be the type most often used. b. Fault-propagation folds (Suppe, 1985) display constant maximum displacements for points initially above the lower flat (curve 1), but curve 2 is most commonly used, as only offset markers (truncated beds) are used in its construction. c. Labeled points as used in a, showing corresponding footwall points. d. Labeled points as used in b, showing corresponding footwall points

10. Idealized cone-shaped (C-type) and mesa-shaped (M-type) normalized displacement-distance diagrams, after Muraoka and Kamata (1983)

11. Location map showing outcrop sites (solid black circles) within the Appalachian Valley and Ridge Province

12. Geologic map of the Eagle Rock Gap region, after McGuire (1970). Numbered stars refer to outcrop locations. Heavy black lines are faults. Stratigraphic units are: Dmn: Millboro and Needmore Shales; DS: Lower Devonian-Upper Silurian indifferentiated; Sk: Keefer Formation; Srh: Rose Hill Formation; Stu: Tuscarora Formation; Omb: Martinsburg Formation; Oe: Edinburg Formation (Chambersburg Group); Oln: Lincolnshire Formation and New Market Limestone; Ob: Beekmantown Formation. More recent interpretations differentiate the Wills Creek Formation from the DS (Bartholomew and others, 1982; Spencer and others, 1989) and incorporate the Oswego Formation between Omb and Stu

13. a. Representation of Eagle Rock 1, rotated to horizontal. Labeled beds and numbered faults are referred to in the text; NP= nucleation point with respect to footwall (a matching point lies in the hanging wall cut-off bed). b. (next page) Photograph of Eagle Rock 1 with 1.85 m tall person as scale

14. Displacement-distance diagrams for Eagle Rock 1 (Figure 13). a. fault 1. b. fault 2. All beds are labeled as in Figure 13, and refer to the tops of beds except where subscripted with an m (middle of bed) or a b (base of bed). Displacement maxima represent fault nucleation points (NP), and occur along the fault ramps, producing C-type profiles

15. Bed M at Eagle Rock 1 (Figure 13) showing duplex formation along a series of small-displacement faults that has imbricated the bed. Unlike normal duplexes, bed M lacks an upper and lower detachment, with displacement decreasing towards the fault tips into adjacent shales that accommodated shortening through disharmonic folding. The bed M imbricates also show both hanging wall and footwall folding, unlike normal predictions of passive footwall behavior

16. a. Simplified line drawing of Eagle Rock 2, showing faults associated with footwall folds in the Rose Hill Formation. b. (next page) Photograph of Eagle Rock 2 (hammer for scale)

17. Line drawing of outcrop Eagle Rock 2a. The outcrop has been rotated clockwise to demonstrate the footwall dominated folding associated with bed 3. Faults are numbered as in Figure 16a. Bedding thickness variations in the Tuscarora Formation sandstone are sedimentological

18. Displacement-distance diagram for Eagle Rock 2a. Letters indicate the upper contacts of beds as labeled in Figure 17

19. Line drawing of Eagle Rock 2b, showing deformation associated with fault 2 (Fig. 16). The outcrop has been rotated to illustrate the footwall folding. Beds are labeled as in Fig. 17

20. Footwall-dominated folding associated with fault 1 (Fig. 16) at Eagle Rock 2c. Beds are labeled as in Figs. 17 and 19. Beds C and I are thinly interbedded sandstone and shale. All other beds are quartz sandstones

21. Displacement-distance plot for Eagle Rock 2c showing a C-type profile. Labeled points refer to basal contacts of beds as labeled in Fig. 20. Bed It is the top bed in unit I (Fig. 20)

22. Eagle Rock 3, rotated to horizontal to emphasize the fault-propagation fold geometry. The fault cuts upsection through sandstones and dies out into a fold developed in interbedded sandstones and shale of the Rose Hill Formation

23. Line-drawing of a fault-propagation fold in interbedded limestone and shale of the Wills Creek Formation, Roundtop Hill, near Hancock, Maryland. Shaded beds are shale; plain beds are limestone

24. Displacement-distance diagram for the structure at Roundtop Hill, Maryland. There is a gradual decrease in displacement upsection towards the point where the fault becomes layer-parallel within inclined beds (Fig. 23) and slip supercedes folding as the dominant shortening mechanism. Labeled points refer to the top contacts of beds as labeled in Fig. 23

25. Line-drawing of fault-propagation folding that accommodated shortening in the zone between en echelon fault segments, in Devonian (Chemung Group?) sandstone, siltstone and shale, near Bergton, Virginia. Dark shaded beds represent thinly interbedded units

26. Displacement-distance diagrams for the upper and lower en echelon fault segments at Bergton, Virginia. Labels refer to the tops of labeled beds in Fig. 25, except those subscripted to indicate the middle, or base of a bed

27. Restored cross-section for Eagle Rock 1 using area-balancing techniques. See text for discussion

28. a. Kink-type folding representation of the structure at Eagle Rock 2a, as seen in Fig. 17. Note both footwall and hanging wall deformation, with differing fold morphology. b. Inverted Suppe (1985)-type fault-propagation fold, showing predicted kink-fold morphology. The footwall fold at Eagle Rock 2a contains an additional kink band not predicted by the Suppe (1985) model

29. Reconstruction of the sequence of events leading to the evolution of Eagle Rock 2a. a. Incipient thrusting along fault 4 (Fig. 16). b. Formation of a single kink-band footwall fault-related fold beneath fault 4. c. A layer-parallel fault (fault 3, Fig. 16) in the existing fold becomes a thrust ramp as it propagates downsection. Coeval fault-propagation folding modifies the existing geometry of the fault 4 fold. d. As the footwall fold of fault 3 develops, kink bands in the fold crest that reflect the initial fault 4 footwall fold geometry become progressively smaller. Forelimb beds are steepened and thinned

30. a. Eagle Rock 2b simplified as a kink-style fold. The additional backlimb kink-band is not predicted by conventional fault-related fold models. Layer thickness variation in bed C was facilitated by cataclastic flow of shale. b. Bed A prior to the development of fault 2 (Fig. 16). Circles are reference points. c. Bed A after faulting and associated folding. The reference points indicate that displacement decreases towards the lower end of the fault ramp

31. Conceptual model demonstrating the evolution of Eagle Rock 2b. Fault-propagation folding developed ahead of an existing thrust ramp segment, as in the Chester and Chester (1990) model. In this case, the fold became pinned along the forward syncline hinge, and the fault ceased to propagate. Slip accommodation involved the eradication of the backlimb hanging wall ramp segment, allowing the steep backlimb segment to increase in size

32. Kink-type folding representation of the evolution of Eagle Rock 2c. a. incipient faulting nucleates in the region of bed I (Fig. 20). b. fault-propagation folding ahead of both fault tips involves only a single kink band. c. translation of the footwall structure beneath a lower flat results in modification of the initial fault-propagation fold forelimb

33. Representation of Eagle Rock 3 hanging wall as a kink-style fault-propagation fold. The overall geometry resembles the Chester and Chester (1990) model. Labeled kink segments are referred to in text

34. Suggested evolution of the fault-propagation fold at Roundtop Hill, near Hancock, Maryland (Fig. 23). The structure is interpreted to be superimposed on the backlimb of a pre-existing detachment fold (a). The fault propagated upsection, losing displacement into a layer-parallel flat having the same inclination as the fault ramp (b)

35. Kink-fold representation of folding between adjacent, overlapping en echelon fault segments near Bergton (Fig. 25). Duplex formation ahead of the upper tip of the lower fault resulted in additional shortening ahead of the lower tip of the upper fault. This resulted in tightening of the syncline between the fault segments, and caused folding of the underlying fault plane

36. Proposed fault geometry for a homogeneously-strained forelimb, single-kink fault-propagation fold. a. initial section with an incipient 30° ramp-nucleated fault that propagates upsection and downsection. b. basic fold geometry showing the excess bed area (shaded) which must be accommodated by internal deformation in order for the shortening profile to be balanced. c. a possible balanced configuration with excess area accommodated by heterogeneous bed thickening throughout the section

37. Possible area-balanced end-member fault-propagation fold styles produced by a ramp-nucleated fault with a constant dip of 30° that propagated both upsection and downsection. a. undeformed configuration. b. migrating-hinge, double-kink type. c. heterogeneously-strained forelimb, fixed-hinge, single-kink type

38. Fault-propagation fold in sandstone and siltstone of the Pennsylvanian Gizzard Group, near Dunlap, Tennessee (after Mitra, 1990)

39. Translated fault-related fold within limestone of the Ordovician Edinburg Formation, Frazier Quarry, Harrisonburg, Virginia. An additional kink-band was produced by translation of the fold from a ramp onto an upper flat

40. Effects of translation on fault-related folds, shown for a 22° ramp and no forelimb thinning. a. translation of a migrating-hinge, double-kink fault-propagation fold. The shaded kink bands are those that will not occur for a fault-bend fold. Thinning is produced in the kink-band that results from translation of beds from the ramp onto the upper flat. b. conventional fault-bend fold, showing no additional forelimb kink segments. c. translation of a single-kink fault-propagation fold via Mode II forelimb modification. The shaded kink band will not occur in a fault-bend fold. d. expected geometry for translation of a single-kink fault-propagation fold with Mode I forelimb modification

41. Displacement-distance diagrams for various types of fault-related folding. a. Ramp-nucleation fault-propagation folds have cone-shaped profiles with maximum displacement occurring in the bed in which the fault nucleated. b. Curve 1: fault-bend folds produce mesa-shaped profiles if both hanging wall and footwall fault-bend folding occurs. The maximum displacement occurs for offset points in the hanging wall and footwall that are still along the thrust ramp. The curve thus has a flat top. Curve 2: translated ramp-nucleation fault-propagation folds have cone-shaped profiles

42. a. translated hanging wall and footwall single-kink, ramp-nucleation fault-propagation folds. b. translated hanging wall and footwall fault-bend folds

43. Fault-propagation fold classification scheme using displacement-distance diagram profiles. A panoply of hybrid fault-fold types are possible. Ramp-nucleation type fault-propagation folds have C-type profiles. Conventional fault-propagation folds propagate upsection from a lower flat with decreasing displacement towards the fault tip. Break-thrusts involve buckle folding that pre-dates folding. If the PR/SR ratio is high during fault growth, with later fold translation, a horizontal profile is produced with a displacement decrease around the upper fault tip. If no fault tip is visible, the profile is a straight line at displacement = 1. For finite PR/SR ratios, break-thrusts will undergo fault-propagation fold modification, with resulting profiles resembling either conventional fault-propagation folds, or if the fault nucleates in the forelimb, ramp-nucleation type (ramp-nucleation type break-thrusts)

44. Illustration of diminishing fold amplitude upsection as a result of parallel-like folding in unfaulted layers