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Geology.Gsapubs.Org on November 6, 2014 Downloaded from geology.gsapubs.org on November 6, 2014 Geology Unfolding: An inverse approach to fold kinematics Jaume Vergés, Douglas W. Burbank and Andrew Meigs Geology 1996;24;175-178 doi: 10.1130/0091-7613(1996)024<0175:UAIATF>2.3.CO;2 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geology Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes Geological Society of America Downloaded from geology.gsapubs.org on November 6, 2014 Unfolding: An inverse approach to fold kinematics Jaume Verge´s Departament de Geologia Dina`mica, Geofı´sica i Paleontologia, Universidad de Barcelona, 08071 Barcelona, Spain Douglas W. Burbank Department of Geological Sciences, University of Southern California, Los Angeles, California 90089 Andrew Meigs ABSTRACT in the fold’s forelimb. The oldest growth Preserved fold shapes usually reveal little about their kinematic evolution. Syntectonic strata prominently onlap paleotopography strata preserved in growth synclines in contact with a fold, however, can permit ‘‘unfold- that existed prior to fold growth. Clear onlap ing’’: a sequential reconstruction of fold growth backward through time from a geometry of the forelimb persisted as folding initiated. observed at present to an initial undeformed state. Such reconstructions can define the From the synclinal axial surface to the crest kinematics of fold growth. Growth strata associated with anticlinal forelimbs in the Ebro of the anticline, growth strata show a gen- basin exhibit depositional tapering of beds on fold flanks and progressive limb rotation. eral thinning defined by marked changes in Unfolding a well-dated detachment fold defines its kinematic evolution and coevally varying dip of both individual beds and stratal units rates of shortening, forelimb uplift, and forelimb rotation. Interplay of these rates with (Fig. 4A). These strata comprise 170 m of sedimentation rates controls onlap and offlap relations. fluvial deposits characterized by siltstone and sandstone interbedded with conglomer- INTRODUCTION Spanish examples display four salient char- ate beds up to 25 m thick. Paleocurrent in- Folds are a fundamental structure of con- acteristics when seen in profile (Fig. 1): (1) dicators define flow parallel to the fold crest tractional orogens, and yet deciphering their the full succession of growth strata displays and synclinal axis. kinematic history continues to be controver- a wedge-shaped geometry; (2) single beds or Few successions of growth strata have sial. Given a present-day cross section of a groups of beds within the wedge-shaped do- been dated with precision sufficient to define fold, we are usually unable to determine un- main thicken across the forelimb from the short-term variability in rates of sedimenta- ambiguously the history of fold growth. Fold anticlinal crest to a maximum thickness near tion, uplift, shortening, or forelimb rotation. geometries from some thrust belts are con- the synclinal axis; (3) angular unconformi- Paleomagnetic dates on the growth strata of sistent with geometrically based kinematic ties may occur between groups of beds in the the Can Juncas anticline (Burbank and models, often with self-similar growth pat- wedge-shaped domain; and (4) growth Verge´s,1994), however, provide ages for terns (Medwedeff, 1992; Shaw and Suppe, strata above the forelimb show an increase three horizons from which rates of sediment 1994). Other studies suggest that preserved in dip from stratigraphically higher to lower accumulation and fold growth can be fold shapes represent only the end product strata. Whereas the relations described be- calculated. of a geometrically variable growth history low apply equally well to backlimbs and (e.g., Ramsay, 1974). The crux of the prob- forelimbs, our discussion is restricted to UNFOLDING: REVERSE STEP-BY-STEP lem rests in the motion of rocks with respect forelimbs, where angular stratal geometries FOLD RESTORATION to fold hinges and the angles of fold limbs tend to be more pronounced. The methodology relies on a step-by-step with time. Two different data sets can po- A well-exposed example of the interrela- unfolding of stratal units backward through tentially resolve fold evolution: distribution tions between syntectonic deposition and time. For each time step, younger growth of strain throughout the fold (Fischer et al., evolving structures occurs at Oliana along strata are removed, the uppermost deposi- 1992) and growth strata (Suppe et al., 1992). the eastern oblique margin of the South tional surface or bounding unconformity is Growth strata deposited above the limb of a Central Unit (Fig. 1, A and B). Here, be- restored to its original prefolding orienta- deforming fold can place clear geometric tween ;37.2 and 34.0 Ma, an imbricate sys- tion, and underlying growth and pregrowth constraints on fold growth through time. tem of southeast-directed thrusts developed strata are concurrently back rotated. In Herein we describe a methodology for a on the backlimb of the Oliana anticline co- these fluvial strata, where paleoflow is par- step-by-step restoration of growth strata evally with deposition of growth strata allel to the anticlinal trend, a horizontal dep- backward through time (termed ‘‘unfold- (Verge´sand Mun˜oz, 1990; Burbank et al., ositional surface in cross section can be as- ing’’) to determine fold kinematics. Al- 1992). The fold and thrust system is de- sumed. Transverse rivers or alluvial fans though our examples formed above detach- tached above a decollement in Upper Trias- may have slopes of a few degrees. If known, ment folds, the methodology is independent sic evaporites (Verge´s,1993). Folds in both such slopes can be integrated into the of fold history and should be applicable to pregrowth and growth strata show kink- restoration. other folding styles. shaped geometries with rounded hinges and A deformed-state cross section, parallel interlimb angles ranging from 408 to 708.We to the tectonic transport, at Can Juncas (Oli- DETACHMENT FOLDS AND RELATED focus on strata preserved in a growth syn- ana) has been built using dip data, map dis- GROWTH STRATA cline attached to the Can Juncas anticline, tribution, measured stratigraphic sections, Along the margins of the Ebro foreland which has an overturned forelimb and a and lengths of tapered and untapered syn- basin (Fig. 1), well-preserved growth strata wavelength of ;0.5 km (Figs. 1B and 3). tectonic strata across the anticline-growth are associated with numerous folds (Riba, Given the typical structural styles of the syncline pair (Fig. 4A). Restorations for two 1976), some of which can be defined as de- folds and faults above evaporites in the stages (35.37 and 34.96 Ma) have been made tachment folds (Fig. 2). In our usage, a de- southern Pyrenees and the absence of sur- by pinning the unthinned growth strata in tachment fold is one that forms by buckling face faults at Can Juncas, we interpret this the distal part of the growth syncline well above a detachment which is subparallel to fold as a detachment anticline. beyond the folded domain, preserving original layering and in which the majority A pronounced unconformity marks the stratal areas, and minimizing changes in of shortening is accommodated by folding, contact between the base of the growth taper in order to minimize bed-length rather than faulting (Jamison, 1987). The strata and the overturned pregrowth strata changes during folding (Fig. 4, B and C). Geology; February 1996; v. 24; no. 2; p. 175–178; 4 figures. 175 Downloaded from geology.gsapubs.org on November 6, 2014 Figure 1. A: Geologic map of Ebro basin with locations of field examples. B: Growth fold at Can Juncas showing prominent thinning toward anticlinal crest in pregrowth strata (top right). C: Internal geometry of evaporite-cored La Garriga fold is dominated by clear onlap of overturned fold forelimb (arrows). D: Growth syncline near Horta de Sant Joan (Barranco dels Estrets) in Catalan Coastal Ranges showing overturned basal conglomerates (at right) deposited on deformed carbonate and siltstone. Note prominent internal unconformity that younger growth strata progressively onlaps (arrows). Figure 2. Detachment fold geometry and thickness variations in growth syncline as re- lated to its forelimb and backlimb. The deformation path of the forelimb has been broken into horizontal and vertical components by tracking the motion of ma- Figure 3. Regional cross section showing imbricate system of thrusts and folds above backlimb terial points near the top of the forelimb of Oliana anticline. Growth strata are linked to Can Juncas detachment anticline. Shaded units (Fig. 4). in pregrowth stratigraphy are competent layers. Overall, during 0.7 m.y., growth strata ac- cumulated at a mean rate of ;0.16 mm/yr, as the forelimb rotated at a mean rate of fold shape and patterns of growth strata. Al- 35.37 and 34.96 Ma, accumulation acceler- ;1.38/104 yr.
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