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SalaverryBasin

SechuraBasin

TalaraBasin

LanconesBasin

ProgresoBasin

TumbesBasin

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Talara Basin, overlappedby younger depocentres

TrujilloBasin

PimentalBasin

Tahuin,El Oro

Mid. Cret. ductileshear zones, incl. oph.remnants truncatedby N-S brittle faults

Mid. Cret. shear zone -correlates with Cord.Real? - 250+ km offset?

Cret. back-arc, fold-belttruncated by N-S faults

Cret. Casma arctruncated againstN-S faults

If Celica = Casma Arc300 km offset?

High resolution potentialfield data may help defineburied fault trends

GPS, field data indicates5-7 mm/yr shear on“Dolores-GuayaquilMegashear”, includingneotectonic faults are fareast as rear of Subandes

PiñonTerrane

At least 2-3 mm/yr long termshear must go through N. Peru,including faults which slice up Pet.Sys. elements in forearc areas

Gulf of Guayaquilpull-apart is too smallto take up all of dex-tral shear on “DGM”

“Accreted arc”

“Accreted arc”

“Accreted arc”

ManabiBasin

“Accreted arc”

©Lorcan Kennan and Tectonic Analysis Ltd. 2003

Geology based on published geologicmaps of Peru, Ecuador, Colombia

Figure 1. Outline location and geological elements, NW Peru, SW Ecuador.The map is a composite of National Geologic Maps (sources: INGEMMET, BGS/CODIGEM and INGEOMINAS), on a shaded topography/bathymetry background(source: Sandwell and Smith, 1997). We highlight some of the proposed major shear zones, brittle faults and offsets through the area in relation to the overlappingdepocenters of the Trujillo, Talara, Tumbes and Progreso Basins (red). Dots show oil and gas fields of the Talara, Progreso areas (source: USGS, 2000). Truncations ofbasins and“pre-Caribbean” or “Nazca-related” tectonic trends indicate enormous shape changes in north central South America which continue to the present andwhich must be taken into account when trying to predict the downdip or offshore continuations of source rock or reservoir sand fairways known in onshore areas.Thereare enormous implications for petroleum systems in forearc and inter-montane basins in Peru and Ecuador, and for future exploration in these areas.

Ocean crust

Unstretched

Stretched continent

Volcanic arc

LegendSpreading Ridge

Past position ofpresent coast

Oc. crust post last map

Plateau basalts

CARIBBEANPLATE

FARALLONPLATE

KUL

FARSAm

NAm

CAR

Schematic Vector Nest(56-46 Ma)

Central American Arccollides with Chortis.Note no arc collisionnorth of ridge

Late Paleocene 56 Ma

Antioquia is closeto final position

Volcanic arc flaresup as triple junctionpasses northwards

Talara Basin opens asPiñon moves to north

NW-SE stretching inproto-Yucatán Basin

Oblique openingof Grenada Basin

NOAM-SOAMconvergencedrives thrusting

Foredeep

Forebulge

Eocene arc

Approx.Galap. HSat 56 Ma

This area willbe subducted

Piñon Terrane

Will besubducted

PresentSOAM

(dashed)

Approx.Panamacoastline

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PROTO-KULAPLATE

SOUTHAMERICA

CARIBBEANPLATE

FARALLONPLATE

This area will besubductedbeneath Colombia

Proto-Caribbean

Widespread near axis volcanism(105-125 Ma Java Ontong analog)

Galapagos HS layto the northwest

KUL

FAR

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Schematic Vector Nest(118-101 Ma)

Early Aptian 119 Ma

Antioquia

Andean back-arc

Sechura Block

Andean back-arc closureis driven by flip in Antillessubdn. direction

Piñon

Unstable RRFtriple junction

Pervasive shear(70°) and rotation

Restored all offsets inN. Caribbean region

Initiation of newplate boundaries

This areawill besubducted

Jamaica??

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Schematic Vector Nestfor SW Caribbean

(9.5 - 0 Ma)

Scale: x3 wrt this map

SAm CAR

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Abandonedrift axes

Plate boundary now onPanama Transform

Non-volcanic flat slabzone starts to form

Faulting allowsshape change at rearof Chortis Block

JAMHAI

EV

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ECC

MAC

PAN

GG

This area willbe subducted

Approx.Galap. HSat 9.5 Ma

Ocean crust

Unstretched

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Volcanic arc

LegendSpreading Ridge

Past position ofpresent coast

Oc. crust post last map

Plateau basalts

Palaeo-Equator

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Figure 2. Caribbean region at 120,56, 9 Ma (see Pindell and Kennan,2001). The model satisfies primarygeological and geometric con-straints. The spreading between N.and S. America, and the need tokeep the Andean margin compres-sive since 100-120 Ma placesstrong constraints on the position,rate of migration and rate of rota-tion of the Caribbean Plate.

TectonicAnalysis

Sedimentary Basins in NW Peru and SW Ecuador:the Caribbean connection, and why it matters!

Lorcan Kennan and James Pindell, Tectonic Analysis, Ltd., West Sussex, UK

Email: lorcan@tectonicanalysis.com, jim@tectonicanalysis.com, Web: http://www.tectonicanalysis.com

ABSTRACT

Plate tectonic reconstructions show that from early Cretaceous through to end Eocene time, the basins of NW Peru and SWEcuador (Figure 1) were interacting with the Caribbean Plate and not with the Nazca Plate as they are today. In consequence,the basins were in fact formed in a setting resembling much more strongly a California-type continental borderland than theirpresent day forearc setting. This may directly explain why, for such a small area, exploration for hydrocarbons, and oil espe-cially, has been so relatively successful in the Talara area. It can also explain many aspects of the anomalous structure andstratigraphy of the Talara Basin, and others nearby, and offer insights into those basins that directly affect their futureprospectivity.

Implications for hydrocarbon systems models are significant. First, we should not simply assume an entirely forearc settingfor Talara, Tumbes, Progreso and other basins since the Cretaeceous. Recognition of a more California style of continentalborderland strike-slip has important consequences for parameters such as heatflow - which in the past may have been signifi-cantly higher than at present. We have recognised and measured large-magntitudes which have not to date been consideredby explorationists working in this area. Strike-slip offsets have important consequences for prediction of downdip continua-tion of sand fairways as exploration moves offshore into as-yet-untested areas. Although sand quality and presence is good inthe Talara Basin, finding their downdip continuation farther offshore has so far been a major problem in the few offshoreTalara and Trujillo Basin wells. Rigorous reconstructions of fault offsets and analysis of resulting prediction fairways of highquality sand in the light of the models we are developing should allow us to significantly improve prediction of reservoirquality in the offshore portions of the Talara, Tumbes and Progreso Basins.

Our interpretations are supported by significant geological contrasts throughout Northern Peru, and contrasts with much bet-ter understood areas in nearby Ecuador. An ongoing work program in cooperation with BPZ & Associates and withPerupetro will apply the results of our models to a large seismic and well dataset and directly make exploration predictions.

BACKGROUND

The coastal basins of NW Peru and adjacent Ecuador are much more linked to (1) the mid-Cretaceous collision of the Carib-bean Plate and closure of pre-existing backarc basins, and (2) the subsequent migration of the Caribbean Plate from the west-ern margin of South America to its present position, than is commonly appreciated. Rigorously-tested reconstructions clearlydemonstrate that the entire Caribbean Plate originated west of the Americas, in the eastern Pacific, and paleopositions of theCaribbean Plate (e.g. Figure 2) and bounding arcs (central America and the Antilles), and lateral shear zones, are constrainedto a precision of ca. 100-200 km.

Basin and structural development at the Caribbean Plate’s boundaries with North and South America were strongly con-trolled by regional plate motions associated with the break up of Pangea, opening of the Proto-Caribbean arm of the AtlanticOcean, and then by the progressive engulfment of a swath of Pacific oceanic crust (now part of Caribbean Plate) betweenNorth and South America as they drifted west from Africa (Figures 2a,b,c). Prior to the Aptian, passive margins formedalong northern South America and southern North America and a volcanic arc crossing from North to South America lay far-ther to the west. Subduction polarity must have reversed and become west-dipping during the Aptian allowing Pacific-derived crust of the Caribbean Plate to enter the “Proto-Caribbean Seaway” between the Americas. Many of theallochthonous complexes of southern North America and northern South America, including much of western flank of thenorthern Andes, have been tied through lithology, chronology and/or geochemistry to the Caribbean Plate, to its arcs and toits subduction-accretion complex. However, identifying a terrane as “Caribbean-related” is only the first step to understand-ing how that terrane fits into the long-lived Caribbean–South American interaction. Significant effort is required to recon-struct the various complexes into a coherent and detailed understanding of this progressive “Caribbean Orogeny” and toaddress those aspects of this Caribbean-related history that directly impact on our understanding of basin evolution in theNW Peru, SWW Ecuador region.

SETTING OF NORTHERN SOUTH AMERICA PRIOR TO CARIBBEAN INTERACTION:THE “ANDEAN BACKARC BASIN”

Prior to the middle and late Cretaceous, almost the entire northern Peruvian, Ecuadorian and Colombian margin is thought tohave been essentially a passive margin, facing either the “Colombian Marginal Seaway” (north) or the “Andean BackarcBasin” (south and west). Both of these features were largely destroyed during proposed interaction with the Caribbean Plate(Figures 3a,b,c,d) and thus the key features that indicate their existence and nature warrant brief review:

• The Colombian Marginal Seaway lay outboard of the Colombian passive margin. The arc to the west of this seaway wasintra-oceanic, and comprised the “primitive island arc” rocks of Tobago and islands farther north. The floor of the basincomprised entirely oceanic crust of Proto-Caribbean origin.

• Paleogeographic evidence for the Colombian passive margin includes no unambiguous remnants of a continental volca-nic arc of younger than late Jurassic age, little or no volcanic material derived from an arc found in the Magdalena Basinor areas to the east, and no indication pre-late Cretaceous of any west-derived sediment of arc derivation.

• The “Andean Backarc Basin” lay east of the Peruvian, Ecuadorian portion of the passive margin, essentially the narrowsouthern end of the Colombian Marginal Seaway. A volcanic arc founded on continental basement lay to the west.Accreted remnants are found in Ecuador through to the island of Margarita, Venezuela. The floor of the basin comprisedhighly stretched continental crust (central Peru) to transitional and possibly oceanic crust in northernmost Peru and Ecua-dor.

• In the Ecuadorian Cordillera Real, as in Colombia, the latest intrusive ages are late Jurassic and only prior to Early toMiddle Jurassic can volcanism with both extensional and subduction affinities be found in the Subandes and inner fore-land basin. Cretaceous volcanic rocks known in the Oriente are of hot-spot affinity and have nothing to do with a nearbysubduction zone. Volcanic rocks of the Cordillera Real show a geochemistry indicative of underlying transitional crust,and still farther west remnants of ophiolite (probable early Cretaceous age) are found with no intervening early Creta-ceous arc. Thus we suggest these are the remnants of “oceanic crust” formed in a back-arc basin and that any arc of earlyCretaceous age lay farther west again.

• To the west of these ophiolites, the “Chaucha Terrane” comprises poorly exposed granite and metamorphic Paleozoicrocks with a Mesozoic clastic fringe on its eastern side (“Guamote Terrane”). In the Cordillera Occidental, Middle Creta-ceous and younger arc rocks are separated from this terrane by a second ophiolite suture. These continental fragments arethe dismembered basement to the arc on the west side of the back-arc basin. To the south, the Celica Volcanics in theLancones Basin are mainly andesites with a geochemical signature indicating underlying continental basement.

Following opening of the back-arc basin, compression dominates northern South America from the Aptian on, with stronguplift and cooling of metamorphic terranes. Back-arc basin closure, and accretion of fragments of arc terranes along Ecuadorand western Colombia throughout the Late Cretaceous, was driven by relative motions of the Caribbean and South AmericanPlates. Large-scale plate tectonic reconstructions indicated that relative motion between the Caribbean and South AmericanPlates was SSW-NNE oriented, consistent with the dextral tranpressive tectonic style in the Cordillera Real indicated by fieldstudies.

CRETACEOUS TO PALEOGENE SETTING OF PERU, ECUADOR, S. COLOMBIA

The lack of younger faulting or volcanism strongly suggests that the Caribbean Plate had reached its present size or larger byabout 90 Ma. Its northern edge was situated south of the Yucatán Block prior to the Campanian and the southern, trailing,edge of the Caribbean Plate was situated in the vicinity of northern Peru at this time (Figures 2 and 3). As the CaribbeanPlate migrated north relative to western South America, the southern edge of the plate also migrated northwards. Followingthe Early Cretaceous passive margin stage, deformation in the northern Central Andes was therefore first driven by Caribbe-an interactions and only later, after the Panama Arc had migrated north of any given point, by interaction with the Farallonand Nazca Plates. North of 4°N, Andean deformation is still controlled by Caribbean interactions today.

There is clear evidence for significant tectonic changes that are probably directly related to the Aptian polarity inversionevent in the Great Arc of the Caribbean. At about 120–115 Ma (Figure 3a) volcanism ceased in the Celica Arc of northern-most Peru and southern Ecuador. Subduction of the newly-isolated Caribbean Plate beneath the active arc along the backarcbasin farther north also ceased, and subduction of “Pacific” (Phoenix, Proto-Farallon) plates began beneath the new Pana-

ma–Costa Rica arc. In Ecuador, the extinct arc, associated plutons and forearc/underlying continental basement were upliftedand eroded prior to deposition of southwest-derived volcaniclastic flysch (Alamór Fm). The oceanic plateau basement of thePiñon Terrane formed the trailing edge of the Caribbean Plate and the presence of large mafic clasts in basal Talara Basinsediments in Peru indicates that until the Paleocene, it still lay 300–500 km south of its present position. Farther south, volca-nism and associated plutonism continued in Peru (Casma Group) during the Albian.

The Caribbean–South America plate boundary remained transpressional during the Cretaceous, causing closure of the Ande-an back-arc basin and driving metamorphism of the Romeral-Peltetec blueschists (Figure 3b). In the model presented here,the Antioquia Block lay at least 300–400 km south of its present position at 120–100 Ma, close to present day Ecuador, asimilar displacement to that proposed for the basement of Talara area (see below). Relative motion of the Caribbean withrespect to South America became more east-west during the late Cretaceous, the resulting increased contraction across theclosed former back-arc basin caused the consistent 85–65 Ma peak in K-Ar cooling ages in the Cordillera Real of Ecuador(Figure 3c) initiating uplift, unroofing and the delivery to basins east and west of quartz-rich sediments.

Interaction of the trailing edge of the Caribbean Plate (Panama Arc) with South America was also clearly diachronous, leav-ing an unmistakable imprint on the forearc basins and the Andean arc system. The San Lorenzo intra-oceanic volcanic arcwas accreted to the western Piñon Terrane during the latest Cretaceous and/or Early Paleocene (Figure 3d), and the PiñonTerrane was then accreted to northernmost Peru and southern Ecuador. The Macuchi Arc was accreted to central Ecuador andsouthern Colombia during the Eocene. Uplift of the extinct early Cretaceous Celica Arc and forearc basement blocks led toforearc limestones giving way to volcaniclastic sediments in the Celica-Lancones area (Copa Sombrero Formation).Maastrichtian continent-derived (from the Cordillera Real) siliciclastic turbidites overlap the accreted terranes of the WesternCordillera and were deposited over marine carbonate and shale sequences in the foreland. All of the western Ecuadoraccreted terranes lie west of the Pallatanga Fault, site of still active dextral faulting (5–7 mm/yr) and were probably accretedto the margin of South America at least 200 km south of their present site, in the vicinity of present day northern Peru.

Once the Panama triple junction passed to the north, subduction of the Nazca Plate beneath South America (which occurredat roughly 5 times the rate of Caribbean subduction) resulted in the re-establishment of andesitic arc volcanism by earlyEocene (Figures 2b, 2c) in S. Ecuador and by middle to late Eocene in S. Colombia. Unmetamorphosed stitching plutons ofEocene age clearly indicate that accretion of basalts and deep-sea sediments had ceased in the Western Cordillera. Subse-quent northward migration of the Piñon Terrane (now separated from the Caribbean Plate) was driven by oblique subductionof the Nazca Plate beneath Ecuador. Onset of migration is probably indicated by initiation of rapid subsidence in the TalaraBasin (NW Peru) during the Eocene. In contrast to other areas of forearc, where sediments are often volcaniclastic-dominated, more quartz-rich sediments, with better reservoir characteristics, of the Talara Basin are derived from the uplift-ing Cordillera Real of Ecuador - sediment pathways were directly controlled by the major fault traces that link the “Dolores-Guayaquil Megashear” to the Talara Basin. Migration of the Piñon Terrane is marked by progressively younger onset of sub-sidence northwards into the Progreso and Guayaquil basins of Ecuador. This subsidence history, and the quartz-rich sedimentwhich found its way into these basins are critical to the success of exploration in the Talara and other basins of this area.

THE BASINS OF NW AND SW ECUADOR WITHIN THIS REGIONAL CONTEXT:

From middle Cretaceous through at least Eocene-Oligocene time, dextral strike-slip at the trailing edge of the CaribbeanPlate linked back westwards to the Peru Trench resulting in the opening of complex arrays of narrow Cretaceous-Paleogenepull-apart basins at several releasing bends (northern Trujillo, Sechura, Talara, Tumbes and Progreso Basins).

Only since Eocene time has NW Peru been in a strictly forearc position and subject to approximately east-directed subduc-tion: post-Eocene strike-slip in Ecuador, related to oblique subduction of the Nazca Plate has linked to the Trench mostlythrough the Gulf of Guayaquil area and active strike-slip fault-related subsidence of forearc basins north of Tumbes is gener-ally younger than farther south. Some dextral strike-slip almost certainly continued on the N-S that pass east of the SechuraBasin but little accommodation space for post-Eocene sediments was created in the Talara Basin or farther south.

Areas of present-day forearc, such as the basement of Talara and Sechura basins, were in the back-arc of the new “Panama-Costa Rica Arc” subduction system during Late Cretaceous. Subsequent northward migration of the Panama arc towardssouthern Colombia drove the strike-slip displacements noted above resulting in the Talara area passing from back-arc toforearc position again by middle to late Paleogene time, when the Andean Arc was re-established.

1

TectonicAnalysis

Sedimentary Basins in NW Peru and SW Ecuador: the Caribbean connection, and why it matters!

Lorcan Kennan and James Pindell, Tectonic Analysis, Ltd., West Sussex, UKEmail: lorcan@tectonicanalysis.com, jim@tectonicanalysis.com, Web: http://www.tectonicanalysis.com

Recognition of middle Cretaceous to early Tertiary interaction between arc-forearc regions and the oceanic Caribbean Platerather than the Nazca Plate is of prime importance. Significant N-S or NNW-SSW trending fault systems (Figure 4) separatediscrete basement blocks and basins, with different stratigraphies and different ages, styles of deformation. These faults havesignificant dextral displacement, and northward block migration in the Peruvian forearc persisted until a least Oligocene time.

Total northward motion of the Talara basement, for example, is estimated to be up to ca. 300 km. Thus, present day distribu-tion of pre-Oligocene rocks of hydrocarbon significance (e.g. ?Cret. source rocks, or Eocene reservoir rocks) is not a reliableindicator of their extent at time of deposition, nor of their original relationships to syn- deposition structures, nor is outcropor known subsurface extent a reliable guide to where they may be found in undrilled areas across these major faults.

SOME IMPLICATIONS OF THE TECTONIC MODELS:

Numerous features of the North Peruvian and South Ecuadorian forearc and arc regions (Figures 1, 4) indicate possibleexploration problems and solutions in the NW Peru and SW Ecuador area:

• In Peru, Carboniferous to Jurassic “basement” of the Amotape-Tahuín Block (ATB) is overlapped by sandstones and car-bonates as old as early Cretaceous. In contrast, in SW Ecuador, a similar basement mapped as part of the same block wasdeformed and metamorphosed during the middle Cretaceous and is bounded on its northern flank by a blueschist-bearinghigh-pressure subduction-related metamorphic belt. Overlap assemblages are of Campanian or younger age only and thussome of the key source units identified in Peru are missing in southernmost Ecuador.

• It is of primary importance to assess why the Amotape and Tahuín sections of the ATB appear to have such different his-tories, whether they have been juxtaposed by later significant strike-slip faulting and whether the “Tahuín” segment con-tinuation is traceable offshore as the ridge separating Talara and Tumbes basins.

• The relationship of the ATB and Talara/Tumbes basins to the Celica-Lancones Basin farther east is not yet clear. At leasttwo significant lineaments separate the forearc from the Celica arc volcanics of early-middle Cretaceous age. Strike-slipfaulting on these may explain dramatic stratigraphic contrasts between the ATB and Celica-Lancones Basin and signifi-cant differences between the east and west of the Celica-Lancones Basin itself.

• Protoliths, metamorphic facies and dominantly dextral ductile deformation fabrics of the Tahuín section of the ATB beara striking similarity to those of the Ecuadorian Cordillera Real. In both areas, peak metamorphism appears to be of mid-dle Cretaceous age and unroofing occurred from ca. 120 Ma-80 Ma. Structures in the Tahuín area are near West-East andare sharply truncated by N-S trending faults along the western flank of the Cordillera Real, requiring significant displace-ment between the two. This displacement must be traceable southward into the Trujillo or Salaverry Basins.

• Palaeomagnetic data show ca. 60° clockwise rotation has occurred in the forearc regions and also suggests signicantnorthward migration of the deep Carboniferous basement of the ATB compared to coastal and interior Peru farther south.The shear zones of the Cordillera Real have not been mapped in detail in Peru - published geological maps of Peru tendto show rocks within the major ductile shear zones as “Precambrian” when they are almost certainly of Cretaceous age.

• N-S faults also separate and displace two distinct fragments of the early to middle Cretaceous Andean volcanic arc.Southeast of Chicama (8°S), the Casma Group comprises andesites and basaltic-andesites, and north of 5°S, the Celica-Lancones formations comprise similar volcanics. The ca. 300 km region between has no known volcanic arc rocks ofCretaceous age and the N-S lineaments noted above juxtapose Palaeozoic schists of the forearc and Cretaceous clasticsand carbonates of the “West Peruvian Trough” back-arc basin, with no intervening Aptian-Albian arc rocks.

• Consequently, we infer a ca. 300 km northward displacement of the Celica block associated with block rotation in forearcareas. Displacement continued through to Eocene time, when the N-S lineaments were overlapped by extensive ignim-brite shields. If proved correct, this implies that the Casma arc does not continue in any part of the subsurface of theSechura and Salaverry basins and that basement should be deformed continental crust comparable to the ATB.

• Restoration of this proposed displacement places the Tahuín segment of the ATB at the southern termination of the Ecua-dorian Cordillera Real at approximately 6°S and southern Talara Basin (Paita) at approximately 8°S, consistent with theinference from larger-scale tectonic models that the tail of the Caribbean Plate lay almost as far south as Lima at approxi-mately 120 Ma.

• The Tahuín and Cordillera Real belts appear to define a single major ductile shear zone which allowed the CaribbeanPlate to migrate N or NE with respect to northern South America during Aptian-Santonian time, and defining the general-ly SW-NE trend of the northern Andes. Similar protoliths, metamorphic/unroofing ages and shear fabrics characterize theCentralCordillera of Colombia and suggest that it too may be part of the same Caribbean lateral shear zone - we havealready built in substantial N-ward migration of Antioquia Terrane into our paleogeographic models.

• Subsequent brittle displacements on N-S lineaments, which can be traced through the Chicama area into the SalaverryBasin, appear to cut across this ductile shear zone and were apparently driven by northward migration of the trailing edgeof the Caribbean Plate (future Panama Arc) and of the Piñon Terrane, in part driven by very oblique northward motion ofthe future Nazca Plate with respect to the central Andean region.

• The Talara Basin is the largest of several pull-aparts developed where the N-S trending lineaments step west to the Ande-an trench. Dextral wrenching and pull-apart formation also plays a role in developing the pre-Eocene basement fabric ofthe Salaverry and Trujillo basins.

• During the late Paleocene-Eocene there was a significant change in relative motion between the Caribbean, Nazca Platesand western South America. Subduction direction turned to near W-E and where the Andean Trench trended more or lessN-S strike-slip displacement stopped. North of Tumbes, the Cordillera Real – Tahuín shear zone defined a more NE trendto South America and oblique wrenching continued, opening the Progreso and Tumbes basin at the same time as W-Ecompression started to deform the Talara area.

HYDROCARBON SYSTEMS IN LIGHT OF TECTONIC MODELS:

The tectonic models and, in particular, the restoration of fault offsets have significant implications for how we assess the dep-ositional setting and extent of key source and reservoir units, for the likely thermal regime in the basins after deposition, andthe structural geometries developed subsequently:

• Following cessation of volcanism and local deformation of the Aptian-Albian arc, thin carbonate and shale sequencesequivalent to Goyllarisquizqa group were deposited across the former Casma arc and forearc, including potentiallyimportant source rocks in the Talara Basin (Muerto Fm). Reconstruction of fault displacements places Talara sections far-ther south during the Cretaceous than at present and > 100 km south of the Celica-Lancones area, and much closer to theWest Peruvian Trough. Source facies were probably widespread and continuous in back-arc, extinct arc and forearc areas.

• In the absence of arc volcanism and associated volcaniclastic sedimentation they may have extended far west on theforearc platform west of the extinct Aptian-Albian arc. Talara area outcrops are probably good analogs for inferred buriedand undrilled Cretaceous in parts of the Trujillo and Salaverry basins.

• The Campanian arc was located approximately 400 km west the former arc. As a result the Talara basin and nearby areaswere by then in a retro-arc position. Basement heatflow would have risen (over 10-20 M) because there was nosubducting cold oceanic slab beneath and because mid-crustal rocks were brought to the surface nearby.

• The northern edge of this carbonate-shale province was strongly sheared by the migrating Caribbean Plate prior to theCampanian, explaining why equivalents of the Muerto Fm are absent or overlain unconformably by late Cretaceous strataand why sediments with clasts derived from the Amotape-Tahuín Block are found in the Celica-Lancones Basin. As aresult quality (quartz content) of sandstones and Albian source rock potential are generally better in the southern Talaraarea than in the Celica-Lancones Basin. Ductile shearing was largely complete in this area by Campanian time, whenunconformably strata were deposited. Declining volcaniclastic and volcanic influence and subsidence in the Amotape-Tahuín area lead to intermittent deposition of shale-carbonate units (Redondo Fm) with significant source potential.

• From Campanian time on, there was a significant change in fault style in northwestern Peru. Northeastward migration ofthe Panama arc along the Tahuín-Cordillera Real shear zone began to pull the Talara and Tumbes areas into the forearcagain. Brittle transtensional faulting on N-S to SW-NE trending started to dismember the area into a series of SW-NEtrending deep basins (Talara, Tumbes) and intervening elevated horst blocks (parts of Sechura Basin, Salaverry High).

• This broke the continuity of the former Late Cretaceous carbonate-shale platform and allowed the deposition of thickdelta to slope turbidites with mixed continental-volcanic provenance during Maastrichtian to middle Eocene time. Thereis significant risk of eroded Cretaceous source rock over any horst blocks. Because much of the sediment was sourcedfrom the rising Cord. Real of Ecuador, sand content and quality is higher in the Talara Basin than in basins farther south.

• The opening of these basins was driven by both migration of the Panama Arc and of the Piñon Terrane, and relativemotion of Farallon Plate was north or northeast with respect to South America. The transtensional basins young fromsouth (Talara) to north (Tumbes-Progreso and Ancón), marking the position of the tail of the Piñon Terrane at any giventime. Intermittent hiatuses in sedimentation allowed the deposition of upwelling-related source rocks within thePaleogene section at Talara, while focused high rates of sedimentation and deformation allowed penecontemporaneoustrap formation (complex strike-slip related geometries) and maturation/migration.

• The trailing edge of the Piñon Terrane allowed Andean areas farther south to “see” the subducting Farallon Plate. Sub-duction direction also turned more West-East and these two factors allowed the re-establishment of the Andean volcanicarc by ca. 55 Ma at 6°S, migrating northwards towards 3°S by ca. 40 Ma.

• This more “normal” subduction zone setting saw uplift and subsidence dominated by localised subduction-accretion orsubduction-erosion up to the present. Strike-slip faults were reactivated by northward migration of fore-arc terranesstopped by ca. 40 Ma. Open to the Pacific, upwelling-related strata are common, with delivery of clastic sediments to thebasins dropping as the area became the rain-shadow of the rising Andes.

• Hydrocarbon systems are predicted to be discontinuous in the Trujillo and Salaverry basins. Maturation of undrilled Cre-taceous section is likely in deeper parts of these basins. The model outlined above will allow us to predict the extent offacies belts and deformation zones. Because deformation and basin initiation appears to young north, there is higher riskof there being no viable pre-Oligocene petroleum systems in north of Talara, but this remains to be confirmed - we donote that there are oil as well as gas shows and fields in Ecuadorian forearc areas. There is also significant risk attachedto inferring underlying continental basement anywhere north of the Tahuín-Cordillera Real shear zone.

SUMMARY and CONCLUSIONS:

• The forearc basins of northern Peru have produced at least 1.6 BBO and the USGS estimates that at least this muchremains as undiscovered reserves.

• The basins have an undeserved reputation for complex faulting and sands distribution based mostly on drilling restrictedto two areas where exploration has been driven by historic surface oil seeps. The limited area of exploration and concen-trated drilling has also biased reserve estimates. When we look at deeper (drill or stratigraphic) we see that these basinsare still amazingly immature, and basins offshore such as Trujillo, Tumbes are essentially untested.

• Offshore drilling HAS demonstrated that all the elements of a successful petroleum system are present, but only rigorousstructural mapping and paleographic reconstruction along the lines of the above, but at a more detailed scale, will allowus to find the sands and the source rock pods. Once they are found, logistical and political difficulties will be far easier tosurmount than in foreland areas.

• Our preliminary work on reconstructing this area suggests that the present day gives a very poor impression of the shapeof the central-northern Andes at time of deposition of petroleum systems elements. For instance, back-arc basins are trun-cated, slivers of arc and fore-arc rocks have been displaced several hundred kilometres from their site of formation, andonly recently has a typically low-heatflow fore-arc setting been superimposed on the area.

• Similarities between the pre-Eocene of the Talara-Progreso area and the continental borderland tectonic style of Califor-nia may indicate far more hydrocarbon potential than appears to be the case. Despite a relative lack of success in offshoredrilling so far, there may be far more undiscovered reserves than current estimates.

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V

V V VOcean crust

Unstretched

Stretched continent

Volcanic arc

LegendSpreading Ridge

Past position ofpresent coast

Oc. crust post last map

Plateau basalts

PROTO-KULAPLATE

SOUTHAMERICA

CARIBBEANPLATE

FARALLONPLATE

This area will besubducted beneathColombia

Proto-Caribbean

Widespread near axis volcanism(105-125 Ma Java Ontong analog)

Galapagos HS layto the northwest

KUL

FAR

SAm

NAm

CAR

Schematic Vector Nest(118-101 Ma)

Early Aptian 119 Ma

Antioquia

Andean back-arc

Sechura Block

Andean back-arc closure isdriven by flip in Antillessubdn. direction

Piñon

Unstable RRFtriple junction

Pervasive shear(70°) and rotation

Restored all offsets inN. Caribbean region

Initiation of newplate boundaries

This areawill besubducted

Jamaica??

Central CubaPalaeo-Equator

120 110 100 90 80 70

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-10

20

10

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-10

120 110 100 90 80 70

From Pindell and Kennan, 2001

80° 75° 70°95° 90° 85°

5°

0°

-5°

-10°

-15°

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5°

0°

-5°

-10°

-15°

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80° 75° 70°95° 90° 85°

Tobago

Margarita

L. Antilles

This area of CaribbeanPlate will be subductedbeneath Colombia

Piñon docksopposite Paitaregion

Paita

Antioquia

Celica Arc

Casma Arc

After 120 Ma, arcmigrates rapidly westtowards new trench

Andeanback-arcbasin

ChauchaTerrane

Cor

dille

raR

eal

Shea

rZ

one

Colombian“Piñon”fragments

Formerarc andtrench

CARIBBEANPLATE

FARALLONPLATE

Thin or no known deposition. Depositionoften inferred in areas of later erosion.

Palinspastically relocated geographic boundaries(coastlines, national borders)

Present geographic boundaries and lat/long ticks

Palinspastically relocated latitude/longitude lines

Palinspastically relocated locations of cities/fields

Significant granitoids - stocks are not shownsince they are associated with all majorvolcanic outbursts

Marine carbonates deposited on otherwiseshale-dominated shelf

Sandy shallow water (delta front; innertidal shelf).

Deep water.

Coastal fringe (delta plain, lakes, bays,swamps. estuaries, lagoons).

Fluvial/alluvial, locally conglomeratic.

Volcanic arc

Lakes

Muddy outer shelf.

KEY

Carbonateplatform

WESTPERUVIANTROUGH

MARANONGEANTICLINE

EASTPERUVIANTROUGH

ANDEANBACK-ARC

BASIN

Figure 4. Preliminary reconstruction of the northern Central Andes at approximately 120 Ma. We show the Jurassic-Early Cretaceous “Andean back-arc basin” (width not constrained, may not have beenfloored by oceanic crust south of Ecuador), and the Caribbean Plate at the time of differentiation from other subducting “Pacific” plates. As the Caribbean Plate formed, subduction at the old trench stopped and arcmagmatism shut off. A new arc developed at the west and southwest edge of the new Caribbean Plate and the Caribbean-South America boundary became dominated by N-S strike-slip rather than subduction, with aseries of pull-apart basins forming in a California-type setting, with possible much higher heatflow than in a typical forearc, but also benefiting from source-rocks related to upwelling at the continental margin. For-mer arc areas were drowned by carbonates and shales and merged with the facies of the Western Peruvian Trough back-arc basin. This map shows a retro-deformed lat-long. grid for areas where we have some con-trol on magnitudes of shortening and shear. It also attempts to account for South to North shearing of forearc terranes (very preliminary) and undo some of the effects of Cordillera Oriental shearing in Ecuador. Theheavy dashed line is the edge of the future Cordillera Real, a major shear zone formed at the right edge of the Caribbean Plate as it migrated north.

SO WHAT HAS ANY OF THIS GOT TO DO WITH HYDROCARBON POTENTIAL IN “FOREARC” AND OTHER BASINS?

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0

-10

708090

70809010

0

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Volcaniclasticflysch pathways

Peru Lst Shelf

Orquideas,Toachi Fms

Antioquia

ColombiaPassiveMargin

ca. 105-100 MaEmergent

Sandy shallow sea

Deep water

Coastal fringe

Alluvial plain

Carbonates

Muddy outer shelf

SEDIMENTARY FACIES

Cherts

Condensed seqs.

Casma Fm

Alamor Fm

Carib. Platepivots nearPiñon

Unroofing of qtzsources in westCelica Basin -10

0

80

70

90

CoastalBatholith

No unambiguousgranite intrusive ages

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708090

?ca. 75-70 Ma

Emergent

Sandy shallow sea

Deep water

Coastal fringe

Alluvial plain

Carbonates

Muddy outer shelf

SEDIMENTARY FACIES

Cherts

Volcanics

Vivian Fm

San Lorenzo

Yunguilla FmChoco-Baudo

Pazul Fm

Monserrate Fm

Lr. Tena Fm

Colón Fm

Umir Fm

Strongly emergentAnqioquia and E. Cord.

Chota Fm

8090

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70809010

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ca. 120-115 Ma

Piñon about tocollide, shut offCelica arc, causemild uplift

New PlateBoundary

Peru LstShelf

?Early Casma Arc

Peru back-arc isabout to close

ColombiaPassiveMargin

0

-10

8070

Emergent

Sandy shallow sea

Deep water

Coastal fringe

Alluvial plain

Carbonates

Muddy outer shelf

SEDIMENTARY FACIES

Cherts

Condensed seqs.

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708090

Rotation of Farallon aboutpole nearby to south keepsPeru transcurrent (tension?)without volcanism

Head-on subductiondrives Panama Arc?

CopaSom.Fm

Pilatón,Cayo Fms

PararinFm ??

Piñon still shieldedfrom quartz sand

IncipientEast Cord.unroofing

Antioquiaemerges at endof this interval?

Passive marginEustasy driven

ca. 90-80 MaEmergent

Sandy shallow sea

Deep water

Coastal fringe

Alluvial plain

Carbonates

Muddy outer shelf

SEDIMENTARY FACIES

Cherts

Condensed seqs.

Last incrementof rotation (70°)

0

-10

8090

Widespread volcanismfrom dispersed hotspots,accreted soon after toColombian margin

Figure 3. Snapshots of northern Central Andes development from 120–70 Ma. Positions of the trailing edge of the Caribbean Plate off northern Peru are con-strained by position of its leading edge off Yucatán, by equatorial paleomagnetic data from allochthonous Ecuadorian terranes, and by absence of arc volcanismfarther north in the Ecuadorian and Colombian autochthon. Note the north-northeast-directed diachronous transpression of the former Andean back-arc and north-wards migration of the Panama triple junction. Enhanced rate of uplift and erosion in the Andes from 84 Ma is driven by increased subduction of Caribbean crustbeneath the Andes, related to the end of oceanic spreading between the Americas.

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Find out more about our work, and our Exploration Framework Atlas for the CentralAndes by visiting our website (www.tectonicanalysis.com) or by contacting LorcanKennan directly (email: lorcan@tectonicanalysis.com, phone, fax: 44-1798-831110