Syntectonic deposits and punctuated limb rotation in an Albian submarine transpressional fold (Mutriku village, Basque-Cantabrian basin, northern Spain) L.M. Agirrezabala* Estratigra®a eta Paleontologia Saila, Euskal Herriko Unibertsitatea, 644 postakutxa, 48080 Bilbao, Spain H.G. Owen Natural History Museum, Cromwell Road, London SW7 5BD, UK J. GarcõÂa-MondeÂjar Estratigra®a eta Paleontologia Saila, Euskal Herriko Unibertsitatea, 644 postakutxa, 48080 Bilbao, Spain ABSTRACT strata and angular unconformity geome- Cenozoic foreland basins. Fewer cases have tries, temporal variation of deformation been reported from strike-slip basins. Field Deep-water syntectonic deposits and an- rates, and bedding-parallel faulting indi- and modeling studies support two contrasting, gular unconformities record the denudation cate folding by progressive limb rotation geometrically based kinematic models: instan- and deformation history of the middle Cre- and associated ¯exural slip. Local northwest- taneous limb rotation and progressive limb ro- taceous Aitzeta structure, interpreted here southeast compression is deduced in for- tation. The former model is based upon kink- as a monoclinal syncline associated with the mation of the Aitzeta syncline. The pres- band migration and displays constant dips on high-angle reverse Mutriku fault (Basque- ence of bedding-parallel oblique faulting the fold limbs (Suppe, 1983; Suppe et al., Cantabrian basin, northern Spain). Sedi- and minor drag folds suggests a component 1997). The second model produces limb ro- mentological and structural analyses, com- of right-oblique movement along the Mu- tation and increasing limb dips during growth bined with a precise chronostratigraphy triku fault. (Riba, 1976; Holl and Anastasio, 1993; Hardy based on ammonites, permit us to docu- and Poblet, 1994; VergeÂs et al., 1996; Schnei- ment in detail the history of this syncline Keywords: Albian, Basque, folding, rates, during ;0.53 m.y. (Late Albian, Callihopli- sedimentation, transpression. der et al., 1996). In the case of the Cretaceous tes auritus Subzone). Tectonism occurred Aitzeta syncline, the geometry of its preserved during two major and two minor short- INTRODUCTION growth strata suggests an origin of progressive lived pulses. The sedimentary signatures of limb rotation. However, this geometry was these pulses include unconformities and re- Precisely dated deposits that correlate with produced by repeated pulses of nearly instan- sedimented deposits derived from the up- the growth of tectonic structures provide a rec- taneous limb rotation that are recorded in an- lifted rocks, showing inverted clast stratig- ord of deformation histories. In the peri- gular unconformities in syntectonic sedimen- raphy. In contrast, intervening phases of Atlantic Mesozoic Basque-Cantabrian basin, tary deposits. tectonic calm were characterized by gentle northern Spain (Boillot and Malod, 1988), the This study is based on detailed sedimento- hemipelagic sedimentation. The major tec- deposition of deep-water strata and the devel- logical and structural mapping at 1:5000 scale, tonic pulses were characterized by strong opment of angular unconformities ¯anking the measurements of stratigraphic sections, bio- rotation and uplift of the syncline limb. The Albian Aitzeta structure (Figs. 1 and 2) permit stratigraphic dating of sedimentary units, fa- two major pulses coincided with the lower us to establish its deformation history. This cies analysis, and construction of structural local structure appears on a geologic map as and upper limits of the auritus Subzone, re- cross sections. The major tectonic phases rec- a fossilized homoclinal overturned succession spectively, and resulted in the following ognized within the Aitzeta area occurred dur- limb-rotation and uplift values: 428 and 250 and is interpreted here as the overturned limb ing a period of ;0.53 m.y. (Callihoplites au- m in the ®rst pulse and 668 and 100±140 m of a fault-related monoclinal syncline with a ritus ammonite Subzone of the European in the second pulse. A maximum duration buried hinge. Biostratigraphical data, based on province zonation of the late Albian). The es- of 0.12 m.y. for each major pulse has been ammonites and foraminifera, provide a high- calculated, from which limb-rotation and resolution temporal framework, which is used timated local compressive deformation took uplift rates of 0.358/k.y. and 2.08 m/k.y. to both reconstruct the depositional and de- place in a general strike-slip regime (Chouk- (®rst pulse) and 0.558/k.y. and 0.83±1.16 m/ formation history and to determine deforma- roune and Mattauer, 1978). Thus, this example k.y. (second pulse) are estimated. Growth tion rates in the Aitzeta structure. will be useful for comparison with deforma- Fault-related growth folds and linked tion rates in similar regimes, both modern and *E-mail: [email protected]. growth strata have been widely documented in ancient. GSA Bulletin; March 2002; v. 114; no. 3; p. 281±297; 12 ®gures; 3 tables. For permission to copy, contact [email protected] q 2002 Geological Society of America 281 AGIRREZABALA et al. Figure 1. (A) Simpli®ed geologic map of the Basque-Cantabrian region with location of B. (B) Simpli®ed geologic map of the Mutriku area. The square indicates the area of study (Fig. 2). GEOLOGIC SETTING cluded the development of short-lived com- another to the southeast (Fig. 1B). A cover of pressional tectonic structures like Aitzeta. syntectonic deposits overlaps the eastern mar- The Aitzeta structure is located close to the In map view, the Aitzeta structure appears gin of the structure. These deposits record the coastal Mutriku village in the northeastern as a fossilized overturned homocline, inter- punctuated growth of the Aitzeta syncline dur- part of the Basque-Cantabrian region (Fig. 1). preted here as the rotated limb of a monoclinal ing the late Albian auritus Subzone, a time During Cretaceous time the area was located syncline (Figs. 1 and 2). It reaches 2 km (min- interval of ;0.53 m.y. on the southwestern European plate margin, imum) in strike length and 0.5 km (maximum) Tertiary deformation was relatively weak in which was created by oblique opening of the in dip-direction width and strikes northeast. the study area (Figs. 1 and 2). It consisted of Bay of Biscay rift. This rift was affected by The high-angle Mutriku and Garate faults sep- (1) reactivation of the Cretaceous Mutriku major sinistral strike-slip deformation (e.g., arate the Aitzeta structure from two little- fault and diapirism of Triassic evaporites, (2) Choukroune and Mattauer, 1978), which in- deformed domains, one to the northwest and reactivation of the monoclinal syncline and 282 Geological Society of America Bulletin, March 2002 SYNTECTONIC DEPOSITS AND LIMB ROTATION IN A SUBMARINE TRANSPRESSIONAL FOLD Figure 2. (A) Geologic map of the study area, showing locations of Figures 2B, 4, 5, and 7. (B) Noninterpretive shallow A±A9 cross section. The submarine northwesternmost part has been projected from coastal data. Bold arrows in B indicate tops of beds. tilting of its hinge line (average plunge: 238 Aitzeta and southeast domains. Five lithostrat- Ondarroa unitÐsiliciclastic conglomerates, toward 0738), and (3) appearance of northeast- igraphic units of Middle Albian±Late Albian sandstones, and lutites; Mutriku unitÐbiotur- trending minor faults and gentle folds of meter age (Berriatu, Ondarroa, Mutriku, Alkolea, bated muddy sandstones; Alkolea unitÐme- to decameter scale. This paper focuses on the and Deba, respectively) are recognized (Table gabreccias, marls, and limestones (Figs. 4 and late Albian deformation. 1). Complex stratigraphy and geometric rela- 6, A±C, and Table 2); and Deba unitÐalter- tionships among the units are observed in the nating sandstones and lutites (Figs. 6D and 7). STRATIGRAPHY key outcrop of Alkolea point (Figs. 2, 4, and The units are grouped into three distinct se- 5). The units are composed of the following quences: An early middle Albian (Lyelliceras Figure 3 shows two representative strati- main lithofacies: Berriatu unitÐmarls, lime- lyelli Subzone) to early late Albian (Hystero- graphic sections of the study area in both the stone megabreccias, lutites, and sandstones; ceras varicosum Zone) pretectonic sequence, Geological Society of America Bulletin, March 2002 283 AGIRREZABALA et al. Figure 3. Schematic stratigraphic sections of the Aitzeta structure and southeast domain, with indication of pretectonic, syntectonic, and posttectonic sequences; ammonite biozonation; and angular unconformities. Sbz.Ðsubzone, SQ.Ðsequence, syntect. a middle late Albian (Callihoplites auritus Aitzeta structure, and that sequence is de- auritus Subzone (5 Mortoniceras (Mortoni- Subzone) syntectonic sequence, and a late scribed in detail in this paper. Accurate ceras) in¯atum Zone sensu stricto of the late Albian (Mortoniceras rostratum Sub- chronostratigraphy of the sedimentary units Tethyan zonation). The exception is used to zone) posttectonic sequence. We utilize the is based on ammonites and foraminifera fos- facilitate discussion of the chronometry and pre®xes pre-, syn-, and post- to refer to the sils and is given in the faunal Appendix Table the tectonic phases in relation to the dating phases of tectonic development of the Aitzeta A1. The ammonite zonation for the late Al- work of Gradstein et al. (1994). Stratigraphy structure and the succession of sedimentary bian used here follows that given in LoÂpez- of the area has been established in