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Geology

Unfolding: An inverse approach to 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

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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 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 , 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 ϳ37.2 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 40Њ to 70Њ.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 ϳ0.5 km (Figs. 1B and 3). tectonic strata across the anticline-growth are associated with numerous folds (Riba, Given the typical structural styles of the 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).

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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 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 ϳ0.16 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- ϳ1.3Њ/104 yr. In contrast to long-term rates, though sedimentation rates were slow prior ated to ϳ0.16 mm/yr. However, because this short-term rates (at the scale of single mag- to 35.37 Ma (Fig. 4), they remained faster rate is less than the vertical uplift rate (0.22 netic chrons) provide more insight on the than the rate of forelimb uplift and resulted mm/yr), growth strata show offlap geome- interplay among variables that control the in onlap during early fold growth. Between tries and gentle thinning toward the crest of

176 GEOLOGY, February 1996 Downloaded from geology.gsapubs.org on November 6, 2014 the extent to which layer-parallel slip con- tributes to the total shortening is difficult to quantify from field relations, some such slip is implied by our restorations, specifically in the lower stratal units (Fig. 4B).

DISCUSSION Because both kinematics and structural geometry should be used to assess the va- lidity of a cross section (Geiser, 1988), un- folding provides a useful constraint on the geometrical evolution of a fold and a check on the viability of an interpretation. Unfold- ing of dated growth strata reveals a detailed history of fold kinematics and changes in var- iables (rates of uplift, horizontal displace- ment, and limb rotation), which, along with rates of sediment accumulation and periods of erosion, affect the internal organization of the growth strata. The resultant geomet- ric restorations can be compared with pre- dicted geometries of published folding mod- els. Our restorations at Can Juncas yield three critical fold-evolution observations (Fig. 4). First, the syncline axial surface changes its orientation through time, result- ing in a broad hinge region. Axial surfaces of the present-day section are localized in this hinge region. Second, the forelimb rotates to a steeper dip toward the syncline with time. Third, the fold exhibits variable rates of crestal uplift and limb rotation during growth. Delineation of the axial surfaces at each step helps define the growth and amplifica- tion of the fold. The fold grows and the fore- limb rotates because the horizontal separa- tion of the axial surfaces decreases with time, the axial surfaces concurrently rotate from a steeper to a shallower inclination, and the length of the forelimb does not change during folding (Fig. 4). Through time, the resultant deformational path of material points in the forelimb is forward and upward with respect to the syncline, un- til the forelimb is vertical. Subsequently, the path of these material points is forward and slightly downward (overturned limb). Figure 4. A: Detailed cross section of growth syncline at Can Juncas showing position of Although the Can Juncas fold shows a su- magnetostratigraphy (Burbank and Verge´s, 1994). Prior to 36.4 Ma, growth strata filled paleo- perposition of axial surfaces within the valley, and are not involved in unfolding. B: Unfolded geometry with strata younger than 34.96 hinge regions of the fold, other detachment Ma removed. For this and subsequent stages, total forelimb rotation and incremental move- folds in the Ebro region show pregrowth and ment of material points are shown. Positions of both synclinal and anticlinal axial surfaces are depicted. C: Unfolded geometry at 35.37 Ma. D: Mean chron-length rates of horizontal and growth strata passing forward through the vertical translation, limb rotation, and sediment accumulation during fold growth. Instanta- anticlinal axial surface, such that crestal neous rate changes at chron boundaries are not geologically meaningful. Ratios of vertical segments become incorporated into a uplift to accumulation define intervals of onlap and offlap. lengthening forelimb (Milla´net al., 1994). Whether axial surfaces have been fixed or the anticline. Coevally, the forelimb steep- During this stage, the uplift rate of the ma- active is often not easily discerned from a ened at a rate of ϳ0.8Њ/104 yr, and the fold terial points in growth strata at 34.96 Ma preserved fold geometry. Past changes in the achieved ϳ85% of its maximum height exceeded both the accumulation rate and orientation and position of such surfaces, (Fig. 4B). Subsequently, from 34.96 to 34.67 the uplift rate of the material point in pre- however, can be defined through sequential Ma, accumulation rates dropped to ϳ0.13 growth strata. When combined with a rapid unfolding of growth strata. mm/yr, whereas forelimb rotation increased rate of horizontal shortening (ϳ0.46 mm/ Given the time and geometric control at almost 2.5 fold to ϳ2.0Њ/104 yr (Fig. 4B). yr), pronounced offlap developed. Although the Can Juncas anticline, rates of limb ro-

GEOLOGY, February 1996 177 Downloaded from geology.gsapubs.org on November 6, 2014 tation, horizontal and vertical displace- that the axes of folds are commonly posi- Fischer, M. P., Woodward, N. B., and Mitchell, ment, and sediment accumulation can be tioned by differential thicknesses of pre- M. M., 1992, The kinematics of break-thrust decoupled and independently compared growth strata, because heterogeneities may folds: Journal of , v. 14, p. 451–460. (Fig. 4). Throughout growth, rates of limb localize strain. Geiser, P. A., 1988, The role of kinematics in the rotation increase due to changes in the construction and analysis of geological cross ratio of horizontal to vertical displace- CONCLUSIONS sections in deformed , in Mitra, G., ment. Whereas the rate of uplift is greater Geometric models for fold development and Wojtal, S., eds., Geometries and mech- anisms of thrusting with special reference to than horizontal shortening during the initial (Suppe, 1983; Suppe and Medwedeff, 1990) the Appalachians: Geological Society of phases of folding (until the forelimb has at- have served to focus attention on and to pro- America Special Paper 222, p. 47–76. tained 50Њ dip), horizontal displacement vide testable hypotheses for the kinematics Hardy, S., and Poblet, J., 1994, Geometric and rates equal and subsequently exceed uplift and sequential growth of folds. Similarly, ge- numerical model of progressive limb rota- after 34.96 Ma, resulting in shearing of the ometries of growth strata are predicted to tion in detachment folds: Geology, v. 22, p. 371–374. forelimb of the anticline. The growth-strata record sensitively the evolving shape of the Jamison, W. R., 1987, Geometric analysis of fold geometry is not completely defined, how- forelimb and backlimb of a fold. Given this development in overthrust terranes: Journal ever, simply by comparison of rates of uplift prediction, interpretations of seismically of Structural Geology, v. 9, p. 207–219. and accumulation. Increased rates of limb imaged growth strata have been used to il- Medwedeff, D. A., 1992, Geometry and kinemat- ics of an active, laterally propagating wedge rotation, driven by an acceleration of hori- lustrate some folding models (Suppe et al., thrust, Wheeler Ridge, California, in Mitra, zontal displacement and coupled with offlap, 1992). Neither the steep forelimb dips at S., and Fisher, G. W., eds., Structural geology result in a strong angular discordance within Can Juncas anticline nor a fold of its scale, of fold and thrust belts: Baltimore, Maryland, the youngest growth strata (Fig. 4, A and B). however, would be clearly imaged by typical Johns Hopkins University Press, p. 3–28. Although the limb rotation rates we have seismic data. Yet, despite its small scale, the Milla´n,H., Aurell, M., and Melendez, A., 1994, Synchronous detachment folds and coeval documented are chron-length averages, pro- high-quality exposure and precise dating of sedimentation in the Prepyrenean External nounced tapering of several individual beds the growth strata at Can Juncas provide Sierras (Spain): A case study for a tectonic suggests that limb rotation rates vary on powerful tools for examination of fold de- origin of sequences and system tracts: Sedi- shorter time scales. velopment through time. Whereas forward mentology, v. 41, p. 1001–1024. predict Ramsay, J. G., 1974, Development of The evolving shape and related deforma- models fold evolution, unfolding of folds: Geological Society of America Bulle- tion rates of the Can Juncas fold can be growth strata reconstructs incremental fold tin, v. 85, p. 1741–1754. compared with simple geometrical folding evolution. Furthermore, unfolding permits Riba, O., 1976, Syntectonic unconformities of the models which allow for increasing limb dips calculation of rates of deformation which, Alto Cardener, Spanish Pyrenees: A genetic during growth. Models of symmetric chev- when combined with rates of deposition, interpretation: Sedimentary Geology, v. 15, p. 213–233. ron (Ramsay, 1974), sinusoidal (Rockwell et emphasize the interactions among variable Rockwell, T. K., Keller, E. A., and Dembroff, al., 1988), and detachment folds (Hardy and rates that determine the geometry of an G. R., 1988, Quaternary rate of folding of the Poblet, 1994) predict that with a constant evolving fold and related growth strata. Ap- Ventura Avenue anticline, western Trans- rate of horizontal shortening, high initial plication of this methodology to other folds verse Ranges, southern California: Geologi- cal Society of America Bulletin, v. 100, rates of uplift decay rapidly. In contrast, the with associated growth strata should result p. 850–858. rate of uplift at Can Juncas does not de- in a more complete description and under- Shaw, J. H., and Suppe, J., 1994, Active faulting crease with time because the rate of hori- standing of natural folding paths. and growth folding in the eastern Santa Bar- zontal shortening accelerates. Similar to the bara Channel, California: Geological Society ACKNOWLEDGMENTS model predictions, however, constant rates of America Bulletin, v. 106, p. 607–626. Supported by the Subprograma Perfecciona- Suppe, J., 1983, Geometry and kinematics of of uplift at Can Juncas can only be sustained miento de Doctores and Direccio´n General de bend folding: American Journal of Science, by increasing rates of shortening through Investigacio´n Cientı´ficayTe´cnica (grants PB91- v. 283, p. 648–721. time. In the face of steady accumulation, 0252 and PB91-0805 to Verge´s),the National Suppe, J., and Medwedeff, D. A., 1990, Geom- these fold models each predict stratigraphic Science Foundation (grants EAR-9018951 and etry and kinematics of fault-propagation EAR-9304863 to Burbank), the Petroleum Re- folding: Eclogae Geologicae Helvetiae, offlap, followed by onlap. Onlap geometries search Fund of the American Chemical Society v. 83, p. 409–454. dominate the early folding at Can Juncas, (grants PRF-17625 and PRF-23881 to Burbank), Suppe, J. S., Chou, G. T., and Hook, S. C., 1992, and thus we interpret lower rates of vertical and the American Association of Petroleum Ge- Rates of folding and faulting determined uplift for this time interval. ologists, the Geological Society of America, and from growth strata, in McClay, K. R., ed., Sigma Xi (grants to Meigs). We thank M. Lo´pez- Rates of forelimb rotation are dependent Thrust : London, Chapman and Blanco for field assistance, and R. Allmendinger, Hall, p. 105–122. on the geometry of folding, rates of short- J. Suppe, G. Axen, and two anonymous reviewers Verge´s,J., 1993, Estudi geolo`gic del vessant sud ening, and the wavelength of the growing for constructive reviews. del Pirineu oriental i central. Evolucio´cin- fold. Fold wavelength is largely a function of ema`tica en 3D [Ph.D. thesis]: Barcelona, the thickness of pregrowth strata. Compa- REFERENCES CITED University of Barcelona, 203 p. Burbank, D. W., and Verge´s,J., 1994, Recon- Verge´s,J., and Mun˜oz, J. A., 1990, Thrust se- rable limb rotation rates would be expected struction of topography and related depo- quences in the southern central Pyrenees: only if the rates of shortening were scaled to sitional systems during active thrusting: Socie´te´ge´ologique de France, Bulletin, v. 8, variations in the pregrowth strata thickness Journal of Geophysical Research, v. 99, p. 399–405. and fold wavelength. p. 20,281–20,297. Burbank, D. W., Verge´s,J., Mun˜oz, J. A., and The hinge of the detachment fold at Can Manuscript received April 21, 1995 Bentham, P. A., 1992, Coeval hindward- and Revised manuscript received October 12, 1995 Juncas separates pregrowth strata of normal forward-imbricating thrusting in the central Manuscript accepted October 23, 1995 thickness from contiguous strata that had southern Pyrenees: Timing and rates of been thinned by erosion prior to the folding shortening and deposition: Geological Soci- described here (Fig. 4). It might thus appear ety of America Bulletin, v. 104, p. 1–18. that this is not a particularly representative example of detachment folding, due to atyp- ical initial conditions. It is likely, however,

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