Thomas Mourier

Geodynamic evolution of the northern and central Andes during early to middle Mesozoic times : a Tethyan model

The early to middle Mesozoic sedimentary and magmatic history of the northern and central Andes (i.e. the NNE-trending Colombian-Ecuadorian segment, and the NW-trending Peruvian segment) exhibits a spatially contrasted evolution involving several successive tectonic and geodynamic settings. The late Triassic to late Liassic period began with a widespread marine transgression. From the latest Triassic, the Colombian marine shelf was progressively destroyed by southward propagating extensional tectonic activity but marine sedimentation continued in Peru. During this time, no significant magmatic activity is recorded except in the emerging Colombian area. This period is interpreted as the result of rifting of a Tethyan oceanic arm which separated the Colombian and palaeo-Mexican margins. From latest Liassic, an important calc-alkaline magmatic arc developed along the emergent Colombian segment. Further south, the north-Peruvian shelf probably emerged, but marine sedimentation continued in southern Peru. During middle and early Late Jurassic times, the Colombian segment was characterized by important magmatic activity and by coarse clastic continental sedimentation. Along the Peruvian segment, a turbiditic trough, emergent areas, and continental basins were created, and the scarcity of calc-alkaline magmatism suggests that only very local subduction took place. This period is regarded as one of the southeastward subduction beneath the Colombian segment, and of Tethyan oceanic crust originating in the newly formed ‘Colombian’ oceanic arm. This pattern would have induced a chiefly left-lateral transform motion along the Peruvian segment. By Kimmeridgian-Tithonian times, the palaeogeographic framework had drastically changed. Along the Colombian segment, magmatic activity ceased, and continental accretions occurred along dextral strike-slip sutures. In Peru, tectonic activity was recorded by the creation of a new turbiditic trough and by the resumption of detrital sedimentation. In the coastal area, arc-related volcanism indicated that subduction took place beneath this segment. This geodynamic change is interpreted as the result of a sharp decrease in the spreading activity of the Tethyan ridges and replacement by Pacific spreading centres inducing a roughly northeastward convergence direction.

Cenozoic stratigraphy, magmatic activity, compressive deformation, and uplift in northern Peru

Integrated field study and radiometric dating in the Llama-Bambamarca area of the Western Cordillera of northern Peru have resulted in major revisions in the stratigraphic and structural relations and ages of thick sequences of Tertiary volcanic rocks and constrain the timing of compressive deformation, magmatic activity, and uplift. Rocks of the upper part of the Llama Formation yield K-Ar mineral dates of 54.8 ± 1.8 and 44.2 ± 1.2 Ma; altered volcanic rocks composing the lower part of the formation in the western part of the area may be older. Eocene K-Ar dates have been reported for several granitic plutons in the region, and other plutons of probable similar age intrude the Llama Formation and are overlain by upper Eocene strata. The pervasive alteration that affects much of the Llama Formation may be the result of large hydrothermal systems driven by a coeval, and presumably cogenetic, batholith. The Chota Formation, which is mostly volcaniclastic, has yielded ages of about 50 and 44 Ma. The Chota Formation thus does not predate the Llama Formation but rather is an eastern distal-facies equivalent of rhyolitic ash-flow sheets and dacitic volcanic rocks of the Llama Formation. The major unconformity at the base of the Llama Formation reflects deformation during Late Cretaceous (Peruvian tectonic phase) and/or Paleocene (Incaic I) time. The time of Incaic II deformation is bracketed between the 44.2 ± 1.2 Ma date on the Llama Formation and the age of about 39 Ma of a thick ash-flow sheet that comprises the lower part of the unconformably overlying Huambos Formation. Dates on rocks postdating Incaic II tectonism in central Peru suggest that deformation had ceased by about 41 Ma. Incaic II tectonism in northern and central Peru appears to have been a short but intense compressive event that peaked about 43 m.y. ago. The Incaic II event is coeval with formation of the Hawaiian-Emperor bend and with a period of rapid convergence of the Nazca and South American plates and therefore may reflect a major plate change in lithospheric plate movement patterns. A largely volcaniclastic sequence of early Miocene age shows that the early Neogene pulse of volcanic activity recognized throughout the Central Andes is represented in northern Peru. Beds of conglomerate within this sequence are probably the result of Quechua I tectonism, although the 23.2 ± 1.5 Ma date obtained on tuff from the unit appears somewhat older than the time of about 19 Ma recognized for Quechua I tectonism in central and southern Peru. Units of partly welded and unaltered ash-flow tuff that fill deeply incised paleovalleys have ages of about 8.2 and 11.4 Ma, showing that the Western Cordillera of northern Peru was uplifted before late Miocene time.

Paleomagnetic Rotations in the Coastal Areas of Ecuador and Northern Peru

We report a paleomagnetic study of about 850 cores from over 80 sites sampled in Paleozoic to Tertiary volcanic, plutonic and sedimentary formations in western Ecuador and Northern Peru. Most of the sampled lithologies carry a stable primary remanent magnetization whose direction is significantly different from that of coeval formations of stable South America. In western Ecuador the results are consistent with the progressive disappearance of a marginal basin accompanied by a clockwise rotation of about 70°. In Northern Peru four sites from Late Carboniferous (Pennsylvanian) formations from the Amotape-Tahuin range in the Piura province show a 110° clockwise rotation and yield evidence for a northward displacement. The results from the Cretaceous to Paleogene formation from the same region indicate a clockwise rotation ranging from 90° for the lowermost units to 25° for the uppermost ones. When considered together with previous geological studies, these data are consistent with the hypothesis of the accretion of an Amotape-Tahuin continental terrane to the Peruvian margin in Neocomian times. The accretion was followed by in-situ rotation, suggesting a dextral shear regime. Preliminary results obtained here from Cretaceous and Paleogene formations in the Central Andes and published results from other authors indicate that this pattern of clokwise rotation changes to anticlockwise south of the major Huancabamba deflection (4° S). This rotational pattern could be related to a general shear regime, dextral north of the deflection and sinistral south of it, or to a recently proposed mechanism involving along-strike variations in the amount of late Cenozoic shortening.