Structural evolution of the Frampton growth fold system, Atwater Valley-Southern Green Canyon area, deep water Gulf of Mexico

Marine and Petroleum Geology 21 (2004) 889–910 www.elsevier.com/locate/marpetgeo

Structural evolution of the Frampton growth fold system, Atwater Valley-Southern Green Canyon area, deep water Gulf of Mexico Gianluca Grando*, Ken McClay Fault Dynamics Research Group, Geology Department, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK Received 26 June 2003; received in revised form 3 December 2003; accepted 8 December 2003

Abstract The Frampton growth anticline is part of the Atwater Valley-Southern Green Canyon frontal fold belt in the deep-water northeastern Gulf of Mexico. The anticline is located basinward of the allochthonous Sigsbee Salt sheet near the Sigsbee Escarpment. The timing and mechanisms of the formation of the frontal fold have been investigated using palinspatically restored depth sections and by the analysis of the growth stratal architecture preserved on fold limbs. The Frampton anticline is cored by autochthonous Middle Jurassic Louann salt and its western limit is bounded by the Green Knoll diapir. The fold geometry varies along strike, from a symmetric box-fold in the east, to a breached detachment fold in the west. Small-wavelength salt pillows formed during the Late Jurassic-Cretaceous in response to an early contractional deformation. These precursor structures controlled the geometry of Tertiary age folding. The Green Knoll diapir, west of the Frampton anticline, influenced deformation, leading to complex interactions between segments of the fold system and associated thrust faults. A landward-vergent thrust fault accommodated shortening adjacent to the diapir whereas folding was the main mechanism of deformation in the eastern part of Frampton anticline. Analysis of growth strata indicates that the detachment anticline developed according to a progressive limb rotation kinematic model with minor hinge migration. The fold evolution model proposed in this study could be used as an analogue for less well imaged, hydrocarbon-bearing growth folds in the deep-water province of the Gulf of Mexico, particularly those that are partially obscured by overlying allochthonous salt sheets. q 2004 Elsevier Ltd. All rights reserved. Keywords: North-eastern Gulf of Mexico; Atwater Valley fold belt; Growth fold

1. Introduction The Frampton anticline is part of the Atwater ValleySouthern Green Canyon frontal fold belt system which is a frontier petroleum exploration region located in the deep water contractional province of the north-eastern Gulf of Mexico (Fig. 1). General descriptions of the Mississippi Fan fold belt or the eastern Atwater Valley fold belt can be found in Rowan (1997), Rowan, Trudgill, and Fiduk (2000), Weimer and Buffler (1992), Worral and Snelson (1989), Wu and Bally (2000), and Wu, Bally, and Cramez (1990b). Only relatively recently however has detailed data been published on the Atwater Valley and Southern Green Canyon areas (Buddin, Williams, & Hall, 2002; Grando, McClay, & Buddin, 2002; Hall, 2002; Hall & Smith, 2001) following the recent petroleum discoveries (e.g. Mad Dog, * Corresponding author. E-mail addresses: [email protected] (G. Grando), [email protected]. ac.uk (K. McClay). 0264-8172/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2003.12.005

Atlantis and Neptune fields, Hall, 2002). The Frampton anticline is one of the frontal folds within the Atwater Valley fold belt in the northeastern Gulf of Mexico (Fig. 2). The fold belt consists predominantly of detachment folds with axial traces trending NE. Some folds are cut on one or both limbs by thrust faults. The fold belt formed in response to sedimentary loading and extension during gravity failure of the continental margin above the Middle Jurassic Louann salt (Diegel, Karlo, Schuster, Shoup, & Tauvers, 1995; Hall, 2002; Peel, Travis, & Hossack, 1995; Rowan, 1997; Rowan et al., 2000; Weimer & Buffler, 1992; Worral & Snelson, 1989; Wu et al., 1990b). Up-slope extension was accommodated by allochthonous salt sheet extrusion, extensional growth faulting and by contractional folding at the basinward limit of the autochthonous Louann salt (Peel et al., 1995; Trudgill et al., 1999). The Frampton anticline is located just basinward of the allochthonous Sigsbee Salt sheet in the proximity of the Sigsbee Escarpment (Fig. 2). The Sigsbee Escarpment is the bathymetric expression of

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Fig. 1. Regional tectonic map of the northern Gulf of Mexico showing the major structural features and the distribution of the allochthonous salt sheets. Note the broad distribution of regional and counter-regional extensional faults in the shelf areas and the downdip contractional fold belts that developed at the toe of the slope. Modified from Diegel et al. (1995).

the leading edge of a spreading allochthonous salt sheet at the base of the slope (Worrall & Snelson, 1989; Wu & Bally, 2000). The major phase of fold amplification and uplift occurred during the Late Miocene to Pliocene as recorded by the stratal terminations of syntectonic sediments deposited around the fold. The anticline is cored by Middle Jurassic Louann salt and its geometry varies along strike from a symmetric, concentric detachment fold to an asymmetric, fault-related fold. This paper discusses the structural characteristic of the Frampton anticline using a combination of 2D regional seismic profiles, 3D depth migrated seismic lines, restored crosssections and evolutionary block-diagrams. The evolution and kinematic of the fold system has been addressed using sequentially restored depth sections and through the analysis of the growth stratal geometries preserved on both fold limbs. Analysis of the syntectonic sediments, can help to discriminate between different kinematic models for fault-related folds (Hardy, Poblet, McClay, & Waltham, 1996; Poblet, McClay, Storti, & Mun˜oz, 1997; Storti & Poblet, 1997; Suppe, Chou, & Hook, 1992). Growth strata record the progressive development of the fold system (Poblet et al., 1997; Suppe et al., 1992) and well-preserved growth strata occur on both limbs of the Frampton anticline. The seismic profiles are displayed at a vertical and horizontal scale of 1:1 allowing a quantitative analysis of the geometrical stratal relationships within the growth sequences. The aim of the study is to develop a model for the evolution of these deep water fold systems, such that it

might be used to unravel fold evolution in adjacent areas that are poorly imaged on seismic data as a result of burial by overlying allochthonous salt sheets.

2. Geological setting of the Atwater Valley Fold Belt The Cenozoic structural activity in the northern Gulf of Mexico was mainly gravity driven, induced by sedimentary loading and progradation of the passive margin above the Middle-Jurassic Louann salt (Diegel et al., 1995; Peel & Travis, 1995; Rowan & Trudgill, 2000; Worral & Snelson, 1989). During the Miocene the influx of large volumes of sediment into the northeastern Gulf of Mexico resulted in the formation of extensional growth faults within the developing depocenters (Peel & Travis, 1995). The overall structure of the northeastern Gulf of Mexico is characterised by a combination of Middle to late Miocene gravitational linked systems of extensional faults and outboard contractional foldbelts (Fig. 1). The contractional deformation downslope was linked through the basal salt detachment back to extensional structures in the shelf area. The allochthonous Sigsbee Salt sheet dominates the middle slope evolving from the original Middle Jurassic autochthonous salt (Wu & Bally, 2000) (Fig. 1). In the downslope section beneath, and in front of the allochthonous salt sheet, lies the Atwater Valley fold and thrust belt. Only part of the Atwater Valley fold belt can be observed on seismic sections because the shallow allochthonous salt

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Fig. 2. Map showing the location of the Frampton 3D seismic survey in the Atwater Valley area and the seismic data used for this study. Green Knoll diapir and the basinward limit of the allochthonous Sigsbee salt sheet are also shown. Modified from Weimer and Buffler (1992).

sheet has over-ridden and masked the seismic expression of the landward portions of the fold belt (Hall, 2002). The 50 km wide Atwater Valley fold belt extends approximately 300 km eastward of the Green Knoll diapir in the southern Green Canyon area (Fig. 2). It consists of a regular wave-train (wavelengths approximately of 10 –15 km) of NE-trending fold axes and thrusts cored by the autochthonous Middle Jurassic Louann salt (Fig. 2). NW – SE trending anticlines also developed and are linked along strike with the NE –SW trending anticlines (Hall, 2002) (Fig. 2). Hall (2002) suggested that NE –SW anticlines may have been controlled by topographic highs in the basement and the NW –SE trends may be linked indirectly to transfer zones that developed parallel to the stretching direction of the Gulf of Mexico opening (e.g. Salvador, 1991a, b). The Atwater Valley fold belt formed at the basinward limit of the Middle Jurassic Louann salt and the original depositional limit of the salt may have been locally controlled by basement structures (Rowan et al., 2000). Rowan et al., 2000 suggested, from structural analyses and observations on the syntectonic sediment geometries in seismic sections, that the eastern Atwater Valley fold belt underwent two major phases of deformation followed by periods of relative quiescence. These are: (a) early, smallwavelength folds developed due to gravity gliding above a basinward dipping basal salt detachment during the Upper Jurassic to Cretaceous post rift thermal subsidence phase;

followed by (b) relatively quiescent phase during the Cretaceous to Middle Miocene when thermal subsidence waned and the basinward tilt of the decollement layer was reversed by flexural sedimentary loading on the upper slope; (c) renewed, large-wavelength folding caused by gravity spreading as the clastic margin prograded basinward over a landward-dipping detachment during the Middle Miocene and Pliocene (Rowan et al., 2000). 2.1. Rifted basement The main period of rifting in the Gulf of Mexico occurred during the Middle Jurassic followed by oceanic spreading continuing possibly into the Late Jurassic (Pindell, 1985; Salvador, 1987, Worral & Snelson, 1989). Thermal subsidence occurred mainly from the Late Jurassic through Early Cretaceous (Watkins, Macrae, & Simmons, 1995). Rifting of the continental margin created a set of northeast-southwest trending half-grabens and tilted fault blocks together with northwest-southeast trending transverse structural features in the Perdido fold belt (Trudgill et al., 1999) and in the Atwater Valley fold belt (Rowan et al., 2000). In the Atwater Valley fold belt there is evidence that the basement is offset by high-angle extensional faults (Hall, 2002). Basement underlying the Atwater Valley fold belt appears on 3D migrated seismic data as an irregular gently landward dipping surface. The basement tilted toward the north as a result of subsidence due to a greater sedimentary

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load in the shelf area (Hall, 2002; Peel et al., 1995; Rowan et al., 2000). Gravity/magnetic data and seismic data also suggest the presence of horsts and grabens beneath the Perdido and Atwater Valley fold belts (Rowan et al., 2000; Trudgill et al., 1999). This rifted basement architecture most likely influenced the areal extent and thickness distribution of the Jurassic salt, and in turn the shape and thickness of the autochthonous salt may have played an important role in controlling the structural styles of the overlying folded sequences. 3. Stratigraphy of the Frampton area The stratigraphic section of the Frampton fold system has been subdivided into megasequences in order to correlate the major tectonic events with the depositional units. A chronostratigraphic column showing these major megasequences and the lithologies present in the study area is shown in Fig. 3. The seismic horizons interpreted in the Frampton 3D seismic survey and their relative ages are shown separately in Fig. 4. Stratigraphic ages are based on the information provided by BP (Tim Buddin pers. comm.) and were correlated with lithological information from previous studies on the northeastern Gulf of Mexico (Rowan et al., 2000; Weimer, 1990; Weimer & Buffler, 1992; Wu, Vail, & Cramez, 1990a; Buffler, 1991). In the study area the Middle Jurassic Louann Salt is mainly concentrated in the core of the anticlines. The internal reflections within the salt unit beneath the anticline are characterised by low-amplitude discontinuous reflectors. However, there are also terminations of moderate amplitude reflectors against the base of the salt in the vicinity of the salt pinchout. The observed pinched out of the interpreted autochthonous salt unit occurs immediately basinward of the Frampton anticline (Fig. 4). The top of the salt is a low-amplitude seismic reflector, possibly due to the small acoustic impedance contrast between the salt and the overlying Challenger sequence carbonates (Fig. 4). At 12 km depth the salt does not have a distinctive seismic response and interpretation of the base-salt reflector is difficult. However, medium amplitude parallel continuous reflectors can be identified at about 11 – 12 km marking the base salt. The fairly continuous reflectors beneath the salt are probably syn-rift continental clastics deposited above the rifted basement. The Upper Jurassic to Middle Cretaceous Growth 1 megasequence II (Fig. 3), is the Challenger sequence and consists of carbonates (possibly shallow marine) that change upwards into marls and shales (Weimer & Buffler, 1992). The passage from shallow marine to deep marine deposits indicate a transgressive phase in the basin related to post rift thermal subsidence (Winker & Buffler, 1988). The seismic character of the Challenger sequence consists of parallel to sub-parallel discontinuous, moderate amplitude reflectors (Fig. 4). The upper section of the Challenger

sequence consists of a low-amplitude, homogenous package that is bound by a continuous high amplitude reflector, which corresponds to the Middle Cretaceous Sequence Boundary (MCSB), (Fig. 4). During the deposition of the Challenger sequence early salt pillow structures formed (Growth 1 megasequence II in Fig. 3) as indicated by the internal onlap geometries of the sediments within this sequence. The Middle Cretaceous MCSB regional unconformity and Mid-Late Miocene 9.1 Ma horizon define the InterGrowth megasequence III (Fig. 3). This consists of the Campeche, Lower Mexican Ridges, Middle Mexican Ridges and the early Upper Mexican Ridges units. The Campeche, Lower Mexican Ridges, Middle Mexican Ridges sequences are interpreted as deep-marine shales, chalks and marls (Feng & Buffler, 1991; Wu et al., 1990a). By the Early Miocene, (Early Upper Mexican Ridges) siliciclastic deposits entered the basin (Feng & Buffler, 1991; Weimer & Buffler, 1992) and the base of this unit is interpreted as the 15 Ma sequence boundary. The continuous high amplitude reflectors of the early Upper Mexican Ridges are interpreted to be coarse-grained turbidite sediments derived from the shelf-edge delta system (Weimer & Buffler, 1992). Above the 15 Ma sequence boundary the seismic character changes, from generally low-amplitude seismic facies with a few high amplitude reflectors, to a package of continuous, high-amplitude parallel seismic reflectors. The top of this high-amplitude package is marked by the 9.1 Ma sequence boundary, which also forms the top of the interpreted Inter-Growth megasequence III (Fig. 3). The Growth 2 megasequence IV is interpreted to be between Mid-Late Miocene to Mid-Pliocene, horizons 9.1 and 2.6 Ma, respectively and consists of the Late Upper Mexican Ridges and Cinco De Mayo sequences (Fig. 3). This Middle Miocene to Pliocene interval represents the time of maximum fold uplift and amplification as indicated by significant growth strata. The seismic character of the Growth 2 megasequence IV changes from the turbidite dominated Late Upper Mexican Ridges to the delta fan deposits of Cinco De Mayo sequence (Fig. 4). The Late Upper Mexican Ridges sequence consists of continuous parallel, low to moderate amplitude reflectors whereas the Cinco De Mayo are predominantly discontinuous low-moderate amplitude reflectors (Fig. 4). The change between the two seismic units is the 5.6 Ma sequence boundary horizon, which is a continuous high amplitude reflector (Fig. 4). The Post-Growth megasequence V, Mid-Pliocene to Present, corresponds to the Sigsbee Sequence (Fig. 3). It is characterised by chaotic reflectors within moderate amplitude discontinuous reflectors (Fig. 4). The Sigsbee sequence is part of the Mississippi Fan, which consists of primarily channel-levee deposits (Weimer, 1990) and slump related sediments. The thickness of the fan system is variable across the study area. The fan is 2.0 –3 km thick

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Fig. 3. Chronostratigraphic chart of the Frampton anticline area showing the stratigraphic section divided into megasequences. Lithologies are taken from Feng and Buffler (1991), Weimer (1990), Weimer and Buffler (1992), Winker and Buffler (1988) and Wu et al. (1990a).

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Fig. 4. Seismic characteristics of the Frampton anticline area showing the seismic character of the horizons interpreted and their relative ages.

above the Frampton anticline and increases to more than 4 km thick south of the anticline. 4. Structural styles of the Frampton fold system The Atwater Valley fold belt is defined by a series of NE trending salt-cored detachment and fault-related folds.

Two regional seismic profiles have been interpreted to show the structural geometry of the frontal fold belt system and the allochthonous salt sheet in the Atwater Valley area (Figs. 5 and 6, see Fig. 2 for line locations). The two sections through the Atwater Valley fold belt show a series of saltcored detachment folds between welded synclines (Figs. 5 and 6). The basinward fold is the Frampton anticline,

G. Grando, K. McClay / Marine and Petroleum Geology 21 (2004) 889–910 Fig. 5. Regional NW–SE seismic profile across the Atwater Valley fold belt, showing the eastern geometries of K2/Timon, Mad Dog and Frampton detachment folds. Notice the shape of the Frampton anticline on its eastern part: a box fold with well-imaged kink bands on the fold limbs. The Sigsbee allochthonous salt sheet over-rides the landward portion of the folds, partially masking their seismic expression (location of line shown in Fig. 2).

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896 G. Grando, K. McClay / Marine and Petroleum Geology 21 (2004) 889–910 Fig. 6. Regional NW– SE seismic profile across the Atwater Valley fold belt, showing the western geometries of K2/Timon, Mad Dog and Frampton detachment frontal folds close to the Green Knoll diapir. Frampton anticline to the west is a breached detachment fold characterised by a NW vergent thrust fault (location of line shown in Fig. 2).

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Fig. 7. (a) NW– SE trending seismic profile from the Frampton 3D survey showing the symmetric detachment box-fold in the eastern part of the area. A wellimaged, constant- width kink-band geometries developed in the pre-growth sequence. (b) Line diagram interpretation of the seismic profile showing the major megasequences. Small arrows indicate onlap and truncation geometry of stratal terminations on fold limbs.

the two landward folds, which are traps for announced oil discoveries (MadDog and K2/Timon), are overlain by the allochthonous Sigsbee salt sheet, which itself accommodated some shortening at its toe (Figs. 5 and 6). These folds formed landward of the distal pinchout of the Middle Jurassic Louann salt, which is interpreted on regional seismic lines to be just south of the Frampton anticline. The allochthonous salt sheet is a striking feature in Figs. 5 and 6. It is about 40 km long, 2 –3 km thick and characterised by

prominent high amplitude reflectors marking the base and top. The Sigsbee Escarpment is the basinward edge of the allochthonous salt sheet (Figs. 5 and 6). The emplacement and down-slope spreading of the allochthonous salt sheet is inferred, by the sub-horizontal stratal cut-off at the base of the allochthonous salt, to be Middle Miocene to PlioPleistocene in age. Above the Sigsbee salt sheet, minibasins and Tertiary salt welds (Fig. 6) formed in response to rapid Late Pliocene-Pleistocene sedimentation. Counter-regional

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Fig. 8. (a) NW– SE trending seismic profile from the Frampton 3D survey showing a rounded detachment fold. (b) Line diagram interpretation of the seismic profile showing the major megasequences. Note the presence of a thrust fault at the 96 and 30.3 Ma horizons. The fault has been interpreted as a landward vergent back-thrust. Small arrows indicate onlap and truncation geometry of stratal terminations on fold limbs.

extensional faults detached on the allochthonous salt and associated growth wedges of syntectonic sediments are clearly visible (Figs. 5 and 6). Below the allochthonous salt the deeper structures are poorly imaged. Seismic reflectors of the Challenger sequence onlap on the flanks of both of these anticlines indicating growth (Growth 1 megasequence II) during the Late Jurassic to Early Cretaceous deformation event. The Challenger sequence also thins onto the flanks of the Frampton anticline recording a similar stage of growth

of this structure. The basement is estimated at about 9 s (TWT), and appears on seismic sections as an irregular surface in places showing ‘pull-up’ effects due to the high seismic velocity of the overlying salt (Figs. 5 and 6). Below the Frampton anticline itself a high-angle NE trending extensional fault has been interpreted (Figs. 5 and 6). Basinward and at the edge of the allochthonous salt, the Frampton anticline is the most prominent structural feature in the eastern portion of the study area. Three representative

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Fig. 9. (a) NW–SE trending seismic profile from the Frampton 3D survey showing an asymmetric breached detachment fold near the Green Knoll diapir (to the west). (b) Line diagram interpretation of the seismic profile showing the major megasequences. Note the presence of a well-developed, NW vergent backthrust. Small outer-arc extensional faults are developed on the crest of the anticline. Small arrows indicate onlap and truncation geometry of stratal terminations on fold limbs.

seismic profiles from the Frampton 3D seismic survey, oriented approximately perpendicular to the fold axis, display the two-dimensional geometry of the fold system, that varies markedly from east to west along-strike. The eastern-most profile shows a symmetric detachment fold (Poblet & McClay, 1996) (Fig. 7) with well-developed kink

bands between planar axial surfaces and sharp angular hinges. To the west, the fold progressively becomes a faulted detachment fold (Figs. 8 and 9) displaying more rounded hinges. The anticline passes laterally to the west into the Green Knoll salt diapir (Fig. 2). The length of the fold hinge line before linking to the diapir is 16 km with

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Fig. 10. Structural map of the 15 Ma horizon showing the major features of the Frampton anticline and the location of the 3D seismic lines. NE– SW trending outer-arc extensional faults are found on the crest of the anticline together with smaller, oblique extensional faults. The NW vergent back-thrust is the most striking feature to the west. Minor reverse faults correspond to the kinks on the fold limbs.

maximum fold width of 10 km to the east and minimum width of 5 km to the west. A structural map and a depth slice map show the detailed features characteristic of the fold system and the along-strike variations in fold geometries (Figs. 10 and 11). A maximum fold amplitude of approximately 2 km has been measured at the 9.1 Ma horizon (Top Inter-Growth megasequence III) in the vicinity of the Green Knoll diapir. A minimum fold amplitude of 1.4 km has been measured to the east where the structure is a symmetric detachment fold (Fig. 7). In the east where the symmetric, kink-band anticline is developed, the axial surfaces on both symmetrical limbs dip at angles of 50– 608 and are approximately 4 km. The westernmost profiles (Figs. 8 and 9) show the geometries of a landward vergent asymmetric fold (Fig. 8). In proximity of the Green Knoll diapir, the anticline is a faulted detachment fold with a major northwest vergent thrust fault together with a minor, southwest vergent thrust (Fig. 9). The northwest vergent thrust dips at 40 – 458 to the SE and strikes NNE (Figs. 9 and 10). In the westernmost profile (Fig. 9) this fault cuts the entire Inter-Growth megasequence III and part of the Growth 2 megasequence IV. A maximum thrust displacement of 1.8 km has been measured at the 96 Ma horizon (Top of the Growth 1 megasequence II). In the western segment of the anticline, fault displacement is the principal mechanism that accommodates the shortening of 2.5 –3 km. In contrast to the east, where only folding

accommodates the contraction, the average shortening value is 1.6 km. The crest of the Frampton anticline shows extensional faults (Figs. 8 and 9). This extensively deformed zone is poorly imaged on the seismic sections. Poor seismic imaging is interpreted to result from numerous small-scale normal faults developed in response to outer-arc stretching during folding. The NE trending major extensional faults on the crest of the anticline can be seen both on the structural map and depth slice map (Figs. 10 and 11). A series of smaller NNE striking extensional faults has been also mapped (Figs. 10 and 11).

5. Analysis of growth strata There are a number of different models that have been proposed for the development of growth strata in contractional fold-belts. These can be divided into two basic groups: kink-band migration models and limb-rotation models. The kink-band migration models require the fold growth by widening of fold limbs by migration of kink-band axial surfaces (Suppe, 1983, 1985; Suppe & Chou, 1992; Suppe & Medwedeff, 1990). For detachment fold systems the kink band migration model (constant limb dip and variable limb length folding of Poblet and McClay (1996)) is a self-similar mechanism where the fold grows

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change of the growth wedge on the fold limbs and the progressive evolution of angular unconformities (Fig. 12b). This limb rotation leads to a wedge-like geometry in the growth strata (Fig. 12b). 5.1. Growth strata in the Frampton anticline

Fig. 11. Depth slice image at 5340 m showing the major structural elements of the Frampton anticline. Two NE–SW outer-arc extensional faults are shown on the crest of the anticline. A series of NNE–SSW smaller extensional faults has also been found with a trend oblique to the fold axis. At this stratigraphic level the trace of the major thrust fault has not been recorded.

exclusively by axial surface migration. The fold nucleates instantaneously as a small structure with fixed limb dips and axial surface dips, and the axial surfaces migrate outwards as the layers feed into the fold to accommodate continued shortening (Fig. 12a). In contrast, the limb-rotation model (Poblet & McClay, 1996, 1997) predicts that the fold limbs rotate progressively and steepen during fold growth and amplification (Fig. 12b). The axial surface migration model leads to panels of rocks with constant dip largely parallel to the pre-growth beds thinning over the anticline crest with growth triangle geometries developed by the axial surfaces in the growth strata (Fig. 12a). In contrast limb rotation model (Poblet et al., 1997) produces a continuous dip

The growth stratal architecture preserved on both fold limbs of the Frampton anticline provides an important tool to unravel the timing and mechanisms of folding. Two major phases of growth have been recorded in Frampton anticline based on the geometry of stratal terminations preserved on fold limbs, Growth 1 megasequence II and Growth 2 megasequence IV (Fig. 3). Thickness variations within the Challenger sequence and onlap of reflectors within this stratigraphic unit are indicative of an early stage of fold growth (Growth 1 megasequence II; Figs. 3 and 7– 9). Stratal terminations have been mapped below the 96 Ma horizon at about 11 km depth (Figs. 7– 9). Seismic reflectors onlap on the core of the Frampton anticline above the top surface of the Middle Jurassic Louann salt (Figs. 7 –9). This suggests that structures early-formed during the Late Jurassic-Cretaceous deformation and have been overprinted and amplified during the Miocene – Pliocene when major deformation and fold growth occurred. Three main growth packages have been identified within the Tertiary Growth 2 megasequence IV, each separated by an unconformity and onlap surface (Figs. 7– 9). These are sequence I (9.1 –6.5 Ma), sequence II (6.5 – 4.43 Ma) and sequence III (4.43 –2.6 Ma). However, different growth pattern and thickness variations within the growth packages has been recorded on both flanks of the Frampton anticline, reflecting the asymmetry of the structure along strike. In Figs. 7 and 8 the growth strata show a symmetric pattern on both limbs of the detachment fold. Up to 2 km of synkinematic Tertiary, sediments are found at between 4.5 and 6.5 km depth (Figs. 7 and 8). The base of sequence I (top of the Inter Growth megasequence III) corresponds to the 9.1 Ma horizon (Middle Miocene). Within sequence I, growth strata dip at maximum angles of 30 – 408 on both fold limbs. Toward the crest of the anticline reflectors are truncated by the 6.5 Ma unconformity (top of sequence I) (Figs. 7 and 8). Above this unconformity, the overlying younger growth strata of sequence II decrease in dip, to angles of 10 – 158 and onlap the 6.5 Ma unconformity surface. The 4.43 Ma unconformity (top of sequence II) truncates the uppermost reflectors of sequence II in this section. The overlying sequence III growth strata are gently dipping (2 –38) and onlap the 4.43 Ma unconformity as well as overlapping the anticline crest. The almost flat-lying 2.6 Ma unconformity marks the end of this phase of anticline growth (Figs. 7 and 8). To the west where the Frampton detachment anticline is faulted, different asymmetric growth stratal architectures are found on the fold limbs (Fig. 9). On the NW back limb

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Fig. 12. Growth stratal architectures generated by different kinematic models (a) by kink band migration, (b) by limb rotation. These models assumed a constant ratio between uplift rate and syntectonic sedimentation rate and no erosional activity has been taken into account. Modified from Rafini and Mercier (2002) and Storti and Poblet (1997).

, 3 km of growth stratigraphy occur whereas , 1.8 km of growth strata are found on the SE front limb where sequence I, reflectors dip at angles of 10 – 158 and onlap onto the 9.1 Ma surface (Fig. 9). Sequence II is characterised by gently 2 –58 dipping growth strata that onlap the 6.5 Ma unconformity. The 4.43 unconformity (base of sequence III) appears as a flat surface and the overlying growth strata overlap the anticline crest (Fig. 9). On the back limb older reflectors of sequence I are steeper and highly rotated, with maximum dips of 45 –508 (Fig. 9). Up section, there is a progressive decrease in stratal dips and younger beds of sequence II, dip at angles of 30 –358. The 6.5 and 4.43 Ma unconformities mark the changes in bed dips and are characterised by truncation and onlap of overlying younger strata (Fig. 9). Sequence III growth units dip at angles of 10 –158 and the very youngest growth strata overlap the fold crest (Fig. 9). Despite the different patterns of growth strata shown in the three sections in Figs. 7 –9, the syntectonic units, generally decrease in dip up-section and thin towards the anticline crest. Younger sub-horizontal growth strata onlap the previously deposited growth units and progressively overlap the anticline. In contrast the older growth strata are generally steeper and truncated by unconformities. These growth stratal relationships indicate that during the initial phase of fold amplification uplift rate was greater than sedimentation rate. Angular unconformities developed within the Growth 2 megasequence IV as a result of variations in uplift rate and, by implication, tectonic activity. The growth strata of the Frampton anticline display fanning wedge-like geometries

on the fold limbs, indicating progressive limb rotation during fold evolution. Lack of growth triangles and panels of growth strata parallel to underlying units in the Growth 2 megasequence IV on both limbs of the Frampton anticline indicates that kink-band migration was not the main mechanism of folding. A 3D block diagram (Fig. 13) shows the along strike structural variations of the Frampton fold system and the differences in growth stratal geometries between the box fold to the east and the fault-related fold to the west. The eastern profiles show similar growth stratigraphies on both limbs (Figs. 7, 8 and 13) reflecting the symmetry of the structure whereas, to the west, substantial differences exist between the growth architectures on both limbs (Figs. 9 and 13). However in all the profiles (Figs. 7– 9 and 13) the growth strata display fanning wedge geometries thinning over the fold crest indicative of progressive limb rotation.

6. Depth to detachment calculations In order to determine the possible models for the fold evolution (Poblet & McClay, 1996) for the Frampton anticline it is necessary to calculate the depth to detachment. Three main geometric models have been proposed for the evolution of detachment folds involving a competent unit displaced over a ductile horizon (Poblet & McClay, 1996). These are: (1) constant limb dip where the detachment fold grows by limb lengthening (Mitchell & Woodward, 1988);

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Fig. 13. Block diagram showing the along-strike variations of structural styles of the Frampton anticline (from Figs. 7 –9).

(2) constant limb length where the detachment fold grows by limb rotation (De Sitter, 1956); (3) variable limb length and variable limb dip where the detachment fold grows by both limb rotation and limb lengthening (Dahlstrom, 1990). Models 1 and 2 are area balanced for both the competent and the ductile units, assuming depth to detachment variations. In contrast model 3 is always area balanced and the depth to detachment does not change during fold evolution (Poblet & McClay, 1996). Classically the depth to detachment in fold systems has been estimated by using the Chamberlin’s law (1910). According to this law which is based on the conservation of area in a 2D cross-section, the area of the fold core divided by the shortening (comparison between bed length before and after folding, obtained from the fold profile) gives the depth to detachment. Poblet and McClay (1996) models 1 and 2 do not satisfy the conditions under which Chamberlin’s law may be applied. In fact for these models the detachment depth varies during fold amplification in order for the ductile unit to be locally area balanced. Thus in a fold formed according to model 1 or 2, the uplifted area occupied by the ductile unit in the fold core divided by the shortening may not give the real depth to detachment. In contrast in model 3 (limb rotation and

limb lengthening), the original regional elevation is not shifted vertically due to thinning by migration of the ductile unit toward the core of the anticlines therefore the area of the ductile unit is assumed to be constant. In areas where the real depth to detachment is known, i.e. from seismic data as in our study, the comparison between the real depth to detachment and the depth to detachment calculated using Chamberlin’s law may allow us to evaluate particular fold models. If the estimated depth to detachment and the real depth to detachment coincide, any of the models may be valid (Poblet & McClay, 1996). Alternatively, if the measurements differ by a significant amount, model 3 is probably not applicable. An analysis to calculate the depth to detachment for the Frampton anticline has been carried out on 6 seismic depth cross-sections along the eastern portion of the structure. In these cases Chamberlin’s law has been applied to the fold area occupied by the ductile unit (Louann salt) and divided by the shortening measured in the competent unit immediately above the ductile unit (Table 1). Variation of the parameters used in the calculation has been observed for the different sections (Table 1). Seismic interpretation suggests that the detachment layer in the study area is actually about 1.2 km below the MCSB

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Table 1 Shortening and depth to detachment calculations, for the Frampton Anticline Section

Ll (km)

L0 (Km)

S (km)

A (km2)

D (km)

A B C Line 1 D E Line 2 F

11.5 12.4 13 12.7 12 12.8

13 14 14.6 14.5 13.5 14.2

1.5 1.6 1.6 1.8 1.5 1.4

6.5 7 6.24 8.6 5.6 6.8

4.3 4.3 3.9 4.7 3.7 4.8

Sections A –F are located from E–W at about 2 km spacing and include lines 1 and 2 (Fig. 10). L1 is the Present Day horizontal distance between the synclinal hinges on either sides of the Frampton anticline, L0 the bed length after folding measured at MCSB (96 Ma) horizon, S is the shortening ðS ¼ L0 – LlÞ; A is the area on the fold core occupied by the salt unit and D the depth to detachment.

(96 Ma horizon), similar to the value of 1 km proposed by Rowan et al. (2000) for the adjacent structures of the eastern Atwater Valley fold belt. The difference between the real and estimated depths to detachment (the estimated depth to detachment is almost three times larger then the real depth, see Table 1) indicate that the Frampton anticline probably did not form according to model 3 whereby both limb rotation and limb lengthening accommodate shortening. The growth stratal patterns indicate that the anticline grew via progressive rotation of the fold limbs. Growth stratal geometries also indicate that the Frampton fold did not form according to model 1 of constant limb dip and variable limb length-kink band migration model. Based on this analysis it is likely that the Frampton anticline developed above a thick detachment layer (Louann salt) in which the fold grew by limb rotation with constant limb length. According to this kinematic model, salt first flowed from beneath synclines into the cores of the anticline and then flowed out again as the fold tighteed leading to a variation of the detachment depth (Homza & Wallace, 1995; Poblet & McClay, 1996). The depth to detachment calculations applied in this paper may however produce errors in the results—e.g. due to lateral movement of salt beneath the fold in response to nearby diapirism. It was not possible to take this potential lateral movement into account when calculating the depths to detachment described above.

7. 2D cross-section restorations In order to understand the geological evolution of the study area, a northeast-southwest trending regional depth converted at a 1:1 scale seismic profile (Fig. 14a) was sequentially restored to key stratigraphic levels (Fig. 14a). This depth converted section was digitised and retrodeformed using the 2DMove program by MidlandValley. Structural restorations were made for different stratigraphic horizons (i.e. time periods), showing the development of the

anticlines and the emplacement of the over-riding allochthonous salt sheet. After each stage of structural restoration, the thickness of the underlying section was expanded to correct for the sedimentary compaction caused by the layer previously removed during the restoration process. Decompaction was carried out using porosity values from the northern Gulf of Mexico provided by BP (Tim Buddin pers. comm.). Salt does not undergo normal sedimentary compaction and is treated as an uncompactable material for the purpose of the restoration presented in this paper. The section was pinned at the right-hand eastern end in the undeformed sequence and a loose line was positioned on the left-hand side of the section. The applied restoration algorithm varied: vertical simple shear was used for the units above the allochthonous salt canopy and flexural slip unfolding was used to restore the underlying detachment anticlines and the growth strata which onlap on fold limbs. The sediments above the allochthonous salt subsided as a result of vertical salt withdrawal due to differential sedimentary loading. The reconstructions for these sequences therefore used vertical simple shear to reverse the effect of the differential salt withdrawal. In contrast, restoration of the salt cored anticline was carried out using the flexural slip unfolding algorithm because this models bedding plane slip which was probably the principal deformation mechanism between the folded units (cf. Poblet & McClay, 1996). The results of six stages of restoration are shown in Fig. 14. Although the restorations have been carried out going back in time, the results are most easily presented by discussing the forward evolution of the area. The restoration shows an average total shortening between 5 – 7% (2 – 3 km) depending on the reference horizon used. The main phase of shortening and uplift occurred during the Late Miocene to Early Pliocene (Fig. 14). The Top 96 Ma restoration (Fig. 14i) possibly indicates that early salt-related structures formed in the Late Jurassic to Cretaceous. These structures are probably formed in response to differential sedimentary loading and/or an early contractional phase. This deformation event is supported by the presence of growth strata within the Challenger sequence (Figs. 7– 9). Salt flow formed small salt pillows, swells and ridges separated by minibasins. The 18.1 Ma restoration (Fig. 14h) shows that detachment folds developed superimposed on the earlier salt structures. Deformation of the MCSB and the presence of subtle thickness variations, observed from seismic sections (Figs. 7– 9), between the Late Cretaceous (96 Ma) and Oligocene (30.3 Ma) suggested that salt movement may have occurred during this interval probably due to a pulse of contractional deformation. The geometry of the Frampton anticline was influenced in a large part by the location and shape of these earlier small pre-Tertiary salt pillows and ridges. From 18.1 to 15 Ma (Fig. 14g) the early formed salt-related structures were relatively stable indicating a quiescent phase. The 9.1 Ma restoration (Fig. 14f) indicates

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Fig. 14. Balanced cross-section of a depth converted regional northeast-southwest trending seismic profile across the Atwater Valley fold belt. The sequential restoration was carried out using the 2DMove program. Length of the depth converted section is 35 km, total amount of shortening 2.8 km (7%).

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Fig. 14 (continued )

a minor deformation event suggesting reactivation and amplification of the fold system. The 6.5 Ma horizon (Fig. 14e) represents the deposition of the first significant Tertiary growth strata. The restoration shows that

a palaeoerosion surface existed prior to the formation of the 6.5 Ma unconformity. Deposition of Late Miocene and Early Pliocene strata corresponded with the major phase of fold amplification and

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uplift of the Frampton anticline. This deformation was synchronous with the main phase of salt withdrawal and synclinal welding. From 5.6 Ma the basinward emplacement of the allochthonous salt sheet above the Atwater Valley fold belt began (Fig. 14d). Sub-horizontal stratal cut-offs at the base of the allochthonous salt indicate the timing of salt emplacement. From restorations at 5.6 Ma to Present Day (Figs. 14b– d), during the emplacement and spreading of the overlying allochthonous salt sheet, it appears that the buried folds (e.g. Mad Dog) ceased to grow. Shortening was accommodated mainly by downslope spreading of the allochthonous salt sheet rather than folding. In contrast the major phase of uplift and amplification of the Frampton anticline occurred at this time (Late Miocene to Pliocene). The Frampton anticline stopped growing during the Late Pliocene and was buried by later Mississippi fan sediments (Fig. 14b). From these structural restorations it can be inferred that the emplacement and downslope spreading of the overlying allochthonous salt dramatically affected the evolution of the Atwater Valley fold belt. Sub-salt folds such as the Mad Dog (Fig. 14b) show less crestal structural relief than Frampton anticline suggesting that this eastern fold ceased growing earlier than the Frampton structure.

8. Discussion The differences in structural styles observed along strike of the Frampton anticline are interpreted to be related to the shape and thickness of the original salt layer and to topographic highs developed in the basement (Hall, 2002). The Frampton anticline is located close to the distal pinchout of the Middle Jurassic Louann salt and in front of the Sisgbee Escarpment. The basinward salt pinchout may have controlled the development of the contractional structure by increasing the frictional resistance at the base of the sedimentary pile preventing further basinward translation of the overburden (Letouzey, Colletta, Vially, & Chermette, 1995). This could also explain the development of the NW landward vergent thrust and associated fold found in this study area. In addition, the basement extensional fault system may have also acted as a backstop, influencing the evolution of the overlying Frampton anticline. An earlier phase of salt movement possibly triggered by contraction and/or differential loading created a complex pattern of low-relief salt structures prior to the onset of the Tertiary shortening. This has been interpreted from the analysis of the growth strata within the Challenger sequence (Figs. 7 – 9) and also from the results of the structural restoration (Fig. 14i). The deformation was probably caused by gravity gliding on the Louann salt above a basinward tilted regional detachment in response to Late Jurassic –Cretaceous differential thermal subsidence (Rowan et al., 2000). This may indicate that the early-formed

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structures are more likely to have been the result of down-dip contraction. However, the relief on top salt surface in Fig. 14i might possibly be also due to errors accumulated during the progressive restoration. Correct interpretation of the seismic sections is crucial because errors arising from the misinterpretation of the Present Day salt geometry will be carried through the restoration procedure and produce artefacts in the final restored sections. Estimation of thickness variations of the autochthonous Louann salt through time from 2D restorations may also be problematic because the salt is not area balanced and most likely has undergone complex three-dimensional flow. In the restorations presented in this paper (Fig. 14) it has not been possible to take lateral salt movement into account. 3D restorations potentially permit better estimations of autochthonous salt thicknesses and will be utilised in our future research. The Frampton structure shows a close relationship between fold thrusting and diapirism. The Green Knoll diapir is located to the west along strike of the fold (Fig. 2). The diapir probably localised the contractional strain and has been squeezed and rejuvenated whereas thrusting and folding accommodated the shortening in the Frampton fold to the east of the diapir. The presence of thrusted diapirs in the northern Gulf of Mexico has previously been reported and models for their development have been proposed (Hall, 2001; Rowan et al., 2000; Vendeville & Nilsen, 1995; Vendeville & Rowan 2002). In these models, an earlier phase of salt movement triggered mainly by differential loading created salt ridges and minibasins prior to the onset of lateral shortening. Subsequently the ridges grew and eventually evolved into passive diapirs (Vendeville & Nilsen, 1995). During contraction the diapirs were rejuvenated and the shortening drove further diapirism and lateral salt extrusion. The diapirs localised the contraction (by acting as a weak zone) and during shortening were squeezed and linked along strike with folds and thrust faults (Rowan et al., 2000). A significant amount of shortening could also have pinched off the diapir stem, forming secondary salt weld serving as a thrust fault (Vendeville & Rowan 2002). A summary synoptic model is proposed for the structural evolution of the Frampton anticline (Fig. 15). This has been constructed based on structural analysis from the seismic sections, observations from the growth stratigraphy and the results of cross-section restorations. The model implies four major phases of fold evolution followed by the deposition of the Mississippi Fan sequence. These are: (a) NE trending extensional faulting in the rifted basement influenced the early stages of salt movement producing a salt swell or ridge. Localised differential sedimentation may have also triggered this early formation of minibasins above the autochthonous salt; (b) these early salt structures acted as nucleation sites for small-wavelength salt pillows that developed during the Late Jurassic-Cretaceous contractional deformation; (c) In the west the Green Knoll diapir started to grow passively, possibly driven by

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Fig. 15. Synoptic block diagram showing the structural evolution of the Frampton anticline through time: (a) pre-existing structures formed above original salt layer in places controlled by a basement extensional fault; (b) small wavelength salt pillows developed above the earlier salt structures during the Late JurassicCretaceous deformation; (c) period of relatively tectonic quiescence, and passive growth of the Green Knoll diapir; (d) major fold amplification during Late Miocene– Early Pliocene, with fold segments linked along strike to form the complete Frampton anticline. The Green Knoll diapir was squeezed and rejuvenated during this major contractional event; (e) deposition of the Mississippi fan delta sediment, draping over the fold. The Green Knoll diapir, shows seabed relief indicative of continuing salt movement to the Present Day.

differential sedimentary loading and/or subtle lateral contraction; (d) the main detachment fold amplification occurred during the Late Miocene-Early Pliocene contractional event, synchronous with the main phase of synclinal welding. Distinct, early-formed fold segments linked along-strike and formed the complete Frampton detachment fold system. In the western part the Green Knoll diapir was squeezed and pinched-out from the original salt source. A landward reverse fault developed possibly forming an inclined secondary salt weld. Thrusting

accommodated the shortening adjacent to the diapir whereas folding developed along strike to the east; (e) subsequent to stage d the Frampton anticline was buried by Late Pliocene-Pleistocene sediments of the Mississippi Fan deltaic system and growth (fold amplification) ceased. 9. Conclusions † The Atwater Valley fold belt is the basinward response to updip loading and gravity-driven extensional collapse of

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†

†

†

†

†

†

a post-rift passive margin section underlain by salt. Much of the downslope translation was accommodated by folding above the autochthonous salt as well as by the extrusion of allochthonous salt sheets. The Frampton anticline is the southernmost frontal fold of the Atwater Valley fold belt. It is a salt cored faulted detachment fold and formed as a result of shortening above the autochthonous Middle Jurassic Louann salt layer. The fold trends NE and is located basinward of the Sigsbee Escarpment in close proximity to the distal pinchout of the autochthonous Louann salt. Fold profiles vary from that of a symmetric detachment anticline in the east to an asymmetric, faulted anticline in the west. The symmetric box-fold in the east accommodates shortening of 1.4 –1.8 km and minimum fold amplitude of 1.4 km has been measured. In contrast, in the west, the symmetric box fold is replaced by a faulted landward vergent detachment anticline. Thrusting mainly accommodates shortening of 2.5– 3 km and maximum fold amplitude of approximately 2 km has been measured. Two main phases of growth of the Frampton anticline have been identified by the analysis of growth stratal geometries. A smaller early phase of growth is interpreted to have occurred during Late Jurassic-Early Cretaceous deformation possibly in response to gravity gliding above a basinward dipping basalt salt detachment during post-rift thermal subsidence. This was followed by a Late Miocene-Early Pliocene major contractional event caused by gravity spreading as the passive margin prograded basinward. Significant growth and amplification of the Frampton anticline occurred during this stage. The basement rift architecture below the Frampton anticline and the thickness and distribution of the autochthonous salt may have also influenced the subsequent fold geometries. An underlying NE trending extensional fault has been mapped in the basement and this most likely controlled the location and orientation of the Frampton anticline. The Green Knoll diapir was probably triggered by a salt ridge or pillow formed above this extensional fault. During subsequent shortening the diapir was squeezed and the salt re-mobilised. Pre-existing salt structures in the Middle Jurassic Louann salt situated above a rifted basement are interpreted to have controlled the position and initial geometry of the Frampton anticline. These early-formed structures acted as nucleation sites for the subsequent Late Jurassic-Early Cretaceous deformation that formed small wavelength salt pillows. These precursor salt structures were reactivated during the main Miocene folding. The Frampton anticline displays variable geometries along strike that might possibly indicate the progressive evolution and linkage of individual fold segments formed above such pre-existing salt pillows. Progressive angular unconformities developed within the Tertiary growth megasequence result from variations in

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the anticline uplift rate and fold shortening. The growth strata on limbs of the Frampton anticline display wedgelike geometries, indicating progressive rotation of both limbs during fold evolution. A lack of growth triangles and flat-lying stratal panels parallel to the underlying pregrowth units of the Frampton anticline indicate that kinkband migration was probably not the main fold mechanism.

Acknowledgements This study was conducted by the Fault Dynamics Research Group, Royal Holloway University of London, as part of the Fault-Related Growth Fold Project sponsored by BP Exploration and Production. Seismic data was kindly provided by WesternGeco. We are grateful to Midland Valley Ltd. for providing the 2D and 3DMove computer programs for balancing cross sections. We thank Tim Buddin for his helpful suggestions and discussion on the structural restoration techniques to balance the regional profiles presented in this paper. We also thank Frank Bilotti and Russell Davies for their careful reviews and constructive comments on the manuscript. This work was carried out using Landmark software provided by an LGC University Strategic Alliance Grant.

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