Geología de la margen activa del Perú entre los 3° y 12° de latitud Sur
Authors: Bourgois Jacques , Huchon Philippe , Pautot Guy 
Published: November 2022

The Geology of the Peru Active Margin between 3 and 12° South Latitude

The structure of the Andean type Peruvian margin is defined along three transverse multichannel seismic records located between 5° ans 12° S offshore Paita (CDP3), Chimbóte (CDP2) and Huacho (CDP1) from north to south. Additional data along these transects include extensive SeaBEAM and SeaMARC II surveys, dredging, and core sampling. During ODP Leg 112 the Joides Resolution drilled at 10 sites localed mainly along CDP1 (679, 680, 681, 682 and 683) and CDP2 (683, 684 and 685) transects.

Results of the "Nazca Plate Project" (1972-1980) indicated that continental metamorphic rocks extend at least to the edge of the shelf and that sediment had accreted along the lower continental slope. The subsequent reprocessing of the CDP1, CDP2 and CDP3 multichannel seismic records and the bathymetric SeaBEAM survey off Paita and Chimbóte placed the boundary between continental basement and the accretionary complex along the middle slope-lower slope boundary. This boundary also marks a change in the tectonic regime from tensional upslope to compressional downslope.

Seismic records CDP1 and CDP2 show an unconformity on basement below the Eocene section similar to that drilled at the Delphin and Ballena industry wells. The unconformity is inferred to be the first Incaic unconformity (67 to 57 Ma) documented onshore.

During Leg 112, the Eocene section was sampled at three sites on the middle and lower slopes of the margin. At site 688, the Eocene contains a shallow marine sequence with shelf and upper slope benthic-foraminifera. This Eocene sequence (57 to 40 Ma) is cut by an unconformity along which the uppermost Eocene and Lower Oligocène (40 to 25 Ma) is missing. The unconformity corresponds to the last Incaic unconformity which extends seaward from the shelf to within 15 km of the trench axis and reaches a depth of 6.2 km below sea level. During the past 25 Ma, the minimum rate of subsidence of the outer continental margin off Huacho was 250 m. Ma1.

The sedimentation of the past 25 Ma is marked by several local angular unconformities similar to that know onshore as Quechua 1 to 3 unconformities (18-19 Ma to Recent). At sites 688 and 682 on the lower slope, the basal Pliocene sequence includes finely laminated coastal upwelling facies which are restricted to deposition in water 150 to 500 m deep. Their recovery indicates 3.5 to 4 km of subsidence during the past 5-6 Ma which corresponds to a susidence rate of 675 m Ma1. This is consistent with the subsidence rate of 350 to 650 m Ma1 calculated from samples dredged upslope in the Lima basin.

Seismic records CDP1 and CDP2 image and accretionary wedge stacked against the transition zone. At site 685, a hiatus of at least 4.3 Ma separates the undeformed Pleistocene slope sediment from the underlying late Miocene (6.1 to 6.8 Ma) accreted sediment that exhibits a pervasive compressional fabric. The accretionary began with the subduction to the trailing edge of the Nazca Ridge 6 Ma ago.

Along records CDP3 off Paita, SeaBEAM bathymetry and additional seismic reflection records reveal mass wasting in the form of an avalanche debris on the lower slope. Two curved scarps cut the middle slope and mark slip surfaces seaward of which a 20 km by 33 km block was displaced 800 to 1000 m and back rotated 5° as it moved downslope. If this slip was catastrophic as was the avalanche debris documented downslope, a local 50 m high tsunami was generated during the past 50,000 yr.

Block faulting of the middle slope area destroyed a smooth flat surface that may represent a seaward extension of a flat structure known onshore as the Tablaw. The Tablazo is of Quaternary age. The middle slope area along the CDP3 line may have, subsided as much as 3 km during the past 2 Ma at a rate of 1,500 m.Ma1.

Estimates of the volume of material eroded from the Peru margin can be derived from the amount of subsidence along CDP1 and CDP3. Along CDP1 the estimated erosion during the past 5-6 Ma is 450 km3 per km along trench.

Along CDP3, a more speculative value of erosion during the past 2 Ma is 300 km3 per km along trench. This yields rates of erosion-bet ween 75 and 150 km3, km1 Ma1 consistent with the 345 to 535 km3 calculated along the CDP1 line by von Huene et al. (in preparation) using the Coulomb wedge model. The lowest flux value of material removed by subcrustal erosion equal more or less the greatest flux value of sediment coming from the Nazca Plate.