Structural framework of mineralisation, Maranon fold-thrust belt, Peru

The mineral-rich Eocene Marañón fold-thrust belt (FTB) in the Andes of central Peru is generally portrayed as a classic continental margin thin-skinned FTB verging away from the oceanic plate and with numerous thrusts and associated fault bend and propagation folds detaching from a basal décollement at the boundary between basement and sedimentary cover. The FTB is known to be broadly Eocene in age, but known mineralisation ages (zinc, gold, and silver) tend to be some 30 m.y. younger, and more closely related to a vaguely understood younger phase of basement-involved (thick-skinned) deformation. The ~30 m.y. disparity in ages between thin-skinned FTB formation and mineralisation occurs commonly in other FTBs around the world and this study is based on addressing the question as to why it is so, and whether there are fundamental and globally applicable reasons for it to occur. This study documents the stratigraphic framework, structural architecture, and evolution of a ~130 km-long segment of the Marañón FTB in an area that includes several anomalous topographic highs, and a 50 km-long, slight oroclinal kink in the general Andean structural trend. The study addresses the problem in four stages: • Development of a model of the stratigraphic Late Cretaceous basin framework that existed at the onset of the Cainozoic Andean Orogeny; • Modification of existing 1:100,000 maps to produce a well-constrained surface stratigraphic and structural map, and analysis of the surface structures across the FTB; • Determination of the 3-D architecture of the FTB by development of seven balanced cross-sections across the FTB, projecting to depths that include a model of the basement/supracrustal sequence; • Determination of a geochronologic framework by Apatite Fission Track, Zircon Fission Track, and (U-Th)/He thermochronometry of cooling and exhumation. The major result of the field analysis is that a steep major median reverse fault, called here the Chonta Fault, delineates marked changes in both the original Permian-Cretaceous basin architecture and the final structural architecture, and is spatially associated with mineralisation. The Chonta Fault marks a major E-W change in both facies and thickness in the Cretaceous foreland basin, and is interpreted as originating as an intra-basinal growth fault, that may have been active since the Permo-Triassic. The pre-Andean stratigraphic architecture consisted of a thick pile of Permian-Cretaceous sedimentary units in the western half of the fold belt separated by the steeply westdipping normal fault from a much thinner pile of similar, easterly thickening, units. This boundary provided a strong rheological buttress during subsequent contractional deformation. Two distinct orogenic pulses define the Andean Orogeny (Incaic and Quechuan), and both of these have temporal and spatial variations: • The Andean Orogeny in the latest Cretaceous/Early Palaeocene started with east-vergent contraction (≡ Incaic I Phase) that produced a narrow fold belt in the supracrustal rocks west of the rheological buttress created by the Chonta Fault. A syntectonic red bed sequence, the Casapalca Fm, formed in the foreland to the east of this early mountain front at the Chonta Fault. • As the folds amplified and tightened, the restraining buttress of the Chonta Fault was breached by breakthrough thrusts that propagated as a thin-skinned nappe complex into the old foreland. Sparse thermochronologic data suggests that this occurred at about 40 Ma (Incaic II Phase). • The combination of original basin architecture and the partitioning of deformation has produced two distinct domains within the Marañón FTB: A western fold-dominated domain, and an eastern thrust-dominated domain. • Apart from arc-related volcanism, the mountain system remained structurally quiet until the mid- to Late Miocene. At this stage the undeformed foreland began to buckle westward against the Eocene mountain belt and locally formed a broad arch exposing the basement. As this arch developed and tilted the old nappe structure, the footwall of that structure was underthrust back beneath the old fold thrust belt. Thermochronological data suggest that this began at about 20-15 Ma (Quechua I Phase) and continued, perhaps diachronously to the west, until the Pliocene (and perhaps is continuous to the Recent). • The youngest (Pliocene) cooling (U-Th)/He ages indicate that the final stages of uplift occurred to the west in the old fold belt in what are now the anomalous topographic highs of the Cordilleras Raura and Huayhuash (Quechua II Phase). Zn, Au, and Ag mineralisation ages (10-7.7 Ma) in the area correspond to the time of Late Miocene west-vergent basement arching and underthrusting. Deposits are spatially related to the Chonta Fault, or to faults that may link with this structure, and this is considered to be a major mineralising conduit driven by and requiring active deformation. However this fault was also active as a basement structure during the Eocene deformation, with no associated mineralisation, so the fertility required must be related to other mantle and slab processes specific to the Late Miocene