an alternative interpretation for the map expression of abrupt changes in lateral stratigraphic level near transverse zones in fold thrust belts

RESEARCH PAPERAn alternative interpretation for the map expression of “abrupt” changes in lateral stratigraphic level near transverse zones in fold-thrust belts a Department of Earth Sys

RESEARCH PAPER

An alternative interpretation for the map expression of

“abrupt” changes in lateral stratigraphic level near

transverse zones in fold-thrust belts

a

Department of Earth System Sciences, Yonsei University, Seoul 120-749, Republic of Korea

b

Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA

Received 4 December 2011; received in revised form 18 January 2012; accepted 23 January 2012

Available online 14 February 2012

KEYWORDS

Lateral stratigraphic

changes;

Fold-thrust belt;

Transverse zone;

Frontal ramp;

Lateral ramp;

Displacement gradient

Abstract The map expression of “abrupt” changes in lateral stratigraphic level of a thrust fault has been traditionally interpreted to be a result of the presence of (1) a lateral (or oblique) thrust-ramp, or (2) a frontal ramp with displacement gradient, and/or (3) a combination of these geometries These geometries have been used to interpret the structures near transverse zones in fold-thrust belts (FTB) This contribution outlines an alternative explanation that can result in the same map pattern by lateral variations in stratigraphy along the strike of a low angle thrust fault We describe the natural example of the Leamington transverse zone, which marks the southern margin of the PennsylvanianePermian Oquirrh basin with genetically related lateral stratigraphic variations in the North American Sevier FTB Thus, the observed map pattern at this zone

is closely related to lateral stratigraphic variations along the strike of a horizontal fault Even though the present-day erosional level shows the map pattern that could be interpreted as a lateral ramp, the observed structures along the Leamington zone most likely share the effects of the presence of a lateral (or oblique) ramp, lateral stratigraphic variations along the fault trace, and the displacement gradient

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1 Introduction

Fold-thrust belts (FTBs) are an outstanding natural laboratory for studying architecture of rocks, deformation and tectonic evolution (e.g., Mitra, 1997; Macedo and Marshak, 1999; Paulsen and Marshak, 1999; Kwon and Mitra, 2004, 2006; Bhattacharyya and Mitra, 2009; Kwon et al., 2009) Most FTBs, most FTBs have prominent large scale arcuate map patterns (e.g., Himalayas and Alps), with thrust traces strongly convex toward the foreland; they also show a pattern of second-order salients separated by transverse zones (Lawton et al., 1994; Mitra, 1997) Adjoining salients generally exhibit significant variations in their structural

* Corresponding author Tel.: þ82 2 2123 5666; fax: þ82 2 2123 8169.

E-mail addresses: skwon@yonsei.ac.kr , earthstructure@gmail.com

(S Kwon).

1674-9871 ª 2012, China University of Geosciences (Beijing) and Peking

University Production and hosting by Elsevier B.V All rights reserved.

Peer-review under responsibility of China University of Geosciences

(Beijing).

doi: 10.1016/j.gsf.2012.01.001

Production and hosting by Elsevier

available at www.sciencedirect.com

China University of Geosciences (Beijing)

GEOSCIENCE FRONTIERS

journal homepage: www.elsevier.com/locate/gsf

In FTBs the presence of a lateral (or oblique) ramp in a fault is

commonly inferred from the relationship of the stratigraphy to the fault

in map view, even though the best evidence for it is to obtain definitive

information such as the direct measurement of the fault surface from

subsurface data Since Froidevaux (1968) and Bielenstein (1969)

pioneered the use of stratigraphic-separation diagram (SSD), these

have proven immensely useful in recognizing the geometry of a fault

surface by mapping the relative positions of hanging wall and footwall

flat- and ramp-cutoffs to deduce how a thrust fault cuts through the

stratigraphic section (e.g.,Rubey, 1973; Evans and Craddock, 1985;

Woodward, 1987; Castonguay and Price, 1995; Groshong, 1999;

Wilkerson et al., 2002).Woodward (1987) further suggested that

SSDs can be drawn in along-strike (longitudinal) and transport-parallel

(transverse) directions, even though transverse SSDs are rare and often

very short because of lack of adequate exposures parallel to the

regional transport direction Therefore, longitudinal SSDs are

more commonly used for recognizing “abrupt” changes in lateral

stratigraphic level along a fault as evidence for the presence of a lateral

ramp with stair-step geometry (i.e., flat-ramp-flat) (e.g.,Woodward,

1985; Castonguay and Price, 1995) While the stair-step lateral ramp

geometry may be common in nature as suggested by subsurface

information (e.g.,Boyer and Elliott, 1982;Fermor, 1999; Rowan and

Linares, 2000), the same geometrical relationship, in terms of

rela-tive position of fault trace with respect to the stratigraphic section in

map view, can also be obtained in other ways (e.g., frontal ramp with

displacement gradient,Wilkerson et al., 2002) These existing models

(lateral ramp vs displacement gradient) or some combination of these

end-member cases have been used for interpreting the structures near

transverse zones in FTBs (Bayona et al., 2003) We have outlined an

alternative explanation to the previous end-member cases for the origin

of “abrupt” changes in lateral stratigraphic level of a thrust fault using

a natural example, namely the Leamington transverse zone of the

North American Sevier FTB (Fig 1)

2 Alternative model

2.1 The model

The model proposed here applies to situations where there is an

abrupt lateral (along strike) change in stratigraphic thickness of the

original sedimentary basin from which the FTB evolves Such

a situation may exist where a younger basin margin cuts across an

ancient crustal boundary (e.g., an old fault zone) at high angle If the

ancient zone is reactivated during any portion of the basin history, it

will result in the basin being significantly deeper on one side of the

boundary than the other during at least part of its history As a direct

consequence of this lateral variation in basin shape, parts of the initial

sedimentary package will be thicker on one side of the boundary than

the other In addition, parts of the stratigraphic package will have an initial tilt toward the deeper portion of the basin in the zone of tran-sition between the shallow and deep portions of the basin (Fig 2) During subsequent deformation to form an FTB the basal detachment typically forms at the basementecover contact, and successive thrusts cut up-section from this level A basal thrust formed at the base of the thinner section could propagate laterally across the transverse boundary at a constant depth, thereby cutting

“up-section” through a series of tilted units (Fig 2a) A thrust formed

at the base of the thicker section could propagate laterally but would tend to climb section as it approached the lateral edge of the deeper basin; across the transverse zone it would tend to flatten out at the base of the thinner section, thereby cutting “down-section” through tilted beds (Fig 2b) In either case, a flat portion of the thrust would

be cutting up through tilted beds in the undeformed state After movement of the thrust, both the hanging wall cutoffs and the foot-wall cutoffs would appear as “ramp cutoffs” because of the angular relationship between the fault and bedding On an SSD the faultebedding relationship would suggest that the fault had cut

“up-section”, thus suggesting the presence of a “lateral ramp” The lateral variations are different between the models

2.2 Natural example of the Leamington transverse zone

An example of this alternative interpretation can be seen in the Leamington transverse zone area of south-central Utah (Fig 1) The Leamington zone is located along the boundary between two prominent salients (viz., Provo and central Utah segments) of the Sevier FTB (Fig 1), across which there are dramatic changes in stratigraphic thickness The zone follows an old crustal boundary

at the southern end of the deep, Upper Paleozoic Oquirrh basin in south-central Utah (Peterson, 1977; Hintze, 1988; Royse, 1993;

Figure 1 Map of Provo salient and central Utah segment of the Sevier fold-thrust belt showing transverse zones with the major thrust faults Box indicates location of Leamington area shown in more detail inFig 3a SRe Sheeprock CR e Canyon Range

Lawton et al., 1994; Paulsen and Marshak, 1999) In this area, the

Canyon Range and Leamington Canyon thrusts are essentially the

same fault as shown on a simplified geologic map in Fig 3a

(Kwon and Mitra, 2006)

A longitudinal SSD constructed from the western limb of the

folded Canyon Range thrust to the folded Leamington Canyon

thrust, before the emplacement of later Tintic Valley thrust, shows

an increase in the stratigraphic-separation from south to north

(Sussman, 1995; Lawton et al., 1997; Kwon and Mitra, 2001, 2006;

Kwon, 2004) (Fig 3b) The stratigraphic-separation diagram

further shows the possible stratigraphic position of a southward

dipping large lateral (or oblique) ramp (w10 km long), where the

fault climbs up-sectionw2.5 km laterally from Lower Paleozoic

sedimentary rocks in the footwall of the Canyon Range thrust to

Middle Paleozoic sedimentary rocks in the footwall of the

Leamington Canyon thrust (Sussman, 1995; Kwon and Mitra, 2001,

2006; Kwon, 2004) (Fig 3b) Within the Leamington zone the

first-order folded Leamington Canyon thrust and the second-first-order

asymmetric smaller folds have hinges parallel to the regional

ENE-WSW oblique zone trend with moderate plunges; these

structures further indicate the presence of a lateral (or oblique)

ramp However, considering that the Leamington zone is the

southern margin of the deep, PennsylvanianePermian Oquirrh

basin, with related lateral variations in stratigraphy, the lateral ramp

should dip northward toward the deeper part of the basin, rather than

to the south as suggested by the SSD (Fig 3b) In addition, farther to

the east (in the Nebo area) the oblique ramp dips toward the NW as

clearly seen on seismic sections (Constenius, 1998)

The restoration of an admissible cross-section in the

Lea-mington zone area suggests that the Proterozoic hanging wall

rocks of the Leamington Canyon thrust were displaced

south-eastward a minimum of about 27 km (Kwon and Mitra, 2001,

2006; Kwon, 2004); the rocks were also folded presumably as

they were carried over a ramp Thus, in order to understand the

stratigraphic position of the Canyon Range/Leamington Canyon

thrust with respect to predeformational lateral stratigraphic

vari-ations, we need to address the problem in the context of a more

regional stratigraphic section A regional stratigraphic section

(Fig 4) using available stratigraphic columns (Hintze, 1988) and

restored sedimentary prisms (Mitra, 1997) extends across the

Leamington zone from the Canyon Range of the central Utah

segment to the Sheeprock Mountains of the Provo salient (Fig 1)

Approximate line of section (SRe CR) is shown onFig 1 This

section is used to illustrate the inferred stratigraphic position of the Leamington Canyon/Canyon Range thrust (before thrusting)

on a predeformational lateral stratigraphic section (Fig 4)

It clearly shows that the “abrupt” changes in lateral stratigraphic level along the Canyon Rangee Leamington Canyon thrusts, that are observed in the longitudinal SSD, are a magnified represen-tation of the lateral variations in stratigraphy related to the predeformational basin along a fault

Therefore, the observed stratigraphic cut-off patterns along the Leamington Canyon thrust were not formed by a south-dipping lateral thrust-ramp More likely, the map patterns represent vari-ations in stratigraphic cutoffs along fault-strike that formed where the flat-lying Leamington Canyon/Canyon Range thrust transected

a northward-dipping stratigraphic-section from lower Paleozoic sedimentary rocks to Middle Paleozoic sedimentary rocks (Figs.2a and4) A northward-dipping lateral (or oblique) ramp (Fig 2b) that is suggested by other evidence (e.g., first-order folded Leamington Canyon thrust and second-order asymmetric folds) most likely lies farther to the north in the subsurface (Kwon,

2004), and its position is closely related to the old crustal boundary of the predeformational basin shape of the Oquirrh basin (Fig 2b)

3 Discussion and conclusions

3.1 Origin of “abrupt” changes in lateral stratigraphic level near transverse zones within FTBs

As pointed out byWilkerson et al (2002), the origin of “abrupt” changes in lateral stratigraphic level, which is commonly inter-preted in the literature as the presence of lateral (or oblique) ramps from map patterns and derivative relationships, is not necessarily unique Based on kinematic and geometric models, they evaluated the possible origins of “abrupt” changes in stratigraphy and the pitfalls of the methods that have been used for identifying lateral/ oblique ramps in FTB, and suggest three possible mechanisms: (1)

a lateral decrease in magnitude of slip on the underlying fault (and, therefore, frontal ramp with displacement gradient), (2) presence of lateral (or oblique) thrust-ramp where the fault cuts laterally up-section along strike from a deeper to a shallower level detachment, or (3) a combination of a frontal ramp with displacement gradient and the presence of a lateral ramp

Figure 2 Schematic diagrams showing models of “abrupt” changes in lateral (along-strike) stratigraphic level, related to the original sedimentary basin shape, near transverse zones in FTBs a: Model showing a case of “a thrust formed at the base of the thinner section” b: Model showing a case of “a thrust formed at the base of the thicker section” Cross symbols represent lateral variations in original basin shape Gray indicates footwall of a thrust fault Red arrow in each figure indicates fault propagation direction T.D.e transport direction

The difference in fault-surface geometry for the above

interpre-tations is the presence of a lateral (or oblique) ramp versus a frontal

ramp The occurrence of a lateral/oblique ramp serves as a cross-link

between offset segments of a frontal ramp In contrast, the other

interpretations have only a frontal ramp with a displacement gradient

along it; such gradients can result from differences in rheology, strain

rate, overburden, pore-fluid pressure, or a combination of these

factors (Wilkerson, 1992), and are commonly referred to as

a mechanism for explaining paleomagnetically determined

vertical-axis rotations (e.g.,Allerton, 1998; Bayona et al., 2003)

3.2 Alternative interpretation for the origin of “abrupt”

changes in lateral stratigraphic level near transverse zones

The alternative interpretation presented in this contribution offers an

additional explanation to the previous end-member cases for the

origin of “abrupt” changes in lateral stratigraphic level near

transverse zones We have shown that the same map pattern can be obtained as a result of lateral stratigraphic variations along a frontal-ramp without vertical-axis rotations, as seen along the Leamington zone at the south end of the Provo salient in the Sevier FTB Within the Provo salient, most of the shortening is taken up by fault propagation folding, so that the actual translation on indi-vidual thrusts is relatively small (Mitra, 1997) In addition, the Leamington transverse zone experiences only small amounts of superimposed vertical-axis rotations by block rotations during later folding and subsequent fold-tightening of the Leamington Canyon thrust (Kwon and Mitra, 2004) Taking these two obser-vations into consideration, the effect of displacement gradient, if it exists, for the observed “abrupt” changes in lateral stratigraphic level, would be fairly small

Even though the present-day map expression of the Leamington zone area shows lateral variations in stratigraphy along the strike of the thrust fault on map view and in lateral cross-sectional view, the observed structures in the Leamington zone might be controlled by

Figure 3 a: Generalized geologic map of the Leamington transverse zone (LZ) area showing the major structures b: The longitudinal SSD drawn from the northernmost Canyon Range thrust (CRT) to the Leamington Canyon thrust (LCT) that is essentially the same fault (Kwon and Mitra, 2006) The stratigraphic-separation increases continuously from lower Paleozoic sedimentary rocks of the CRT footwall to the LCT footwall, suggesting possible presence of footwall lateral (or oblique) ramp

one prominent mechanism (viz., lateral variations in stratigraphy

along a fault) while reflecting contributions from other mechanisms

As shown in the example of the Leamington transverse zone area,

“abrupt” changes in lateral stratigraphic level near the transverse zone

in a FTB most likely share the effects of the presence of lateral (or

oblique) ramp, lateral variations in stratigraphy along a fault, and

displacement gradients on a frontal ramp

Acknowledgments

The original idea of this paper is from S Kwon’s Ph.D research

performed at the University of Rochester This research was

partially supported by MLTM of Korean Government Program

20052004 to S Kwon

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