Abstract The Manto Verde copper deposit is associated with specularite-rich breccias emplaced during extensional tectonism along brittle faults of the Atacama fault zone (AFZ). The AFZ is a complex sinistral strike-slip/dip-slip system that originated during oblique subduction in a Jurassic to Early Cretaceous magmatic arc. Activity of the AFZ is documented from the Middle Jurassic, with intermittent movements occurring at least into the late Miocene. South of Manto Verde, the AFZ is a major control on the emplacement of apatite-bearing magnetite deposits in the Cretaceous Iron Belt (CIB). The Manto Verde deposit is hosted by cataclased andesitic volcanics and coeval (?) diorite porphyry stocks of Early Cretaceous age affected by low-grade regional metamorphism. The deposit consists of three breccia units paralleling the Manto Verde fault (MVF) for at least 1500 m. The MVF is a 12 km long, north-northwest — striking- 40°-50°E — dipping structure that joins two major branches of the AFZ, and shows evidence of premineralization strike-slip motion followed by postmineralization, east-side-down, dip-slip movement. The hanging - wall Manto Atacama breccia is a ±100 m wide, specularite-rich, matrix-supported hydrothermal breccia that follows the MVF along strike and down dip, although it becomes narrower with depth. A Transition Zone is defined between the Manto Atacama breccia and the andesitic country rocks as brecciation intensity and associated copper mineralization decrease progressively away from the MVF. In contrast, the footwall Manto Verde breccia is represented by a ±20m wide cataclasite displaying a relatively sharp mineralization boundary. These units are all deeply oxidized to 200 m, so that the present mineralogy consists of a suite of copper carbonates, sulfates, and silicates with minor copper chlorides, which occur as fracture fillings, patches, and disseminations. Only a thin, poorly developed supergene enrichment zone exists. Hypogene sulfide mineralization is poorly explored and corresponds to chalcopyrite ± pyrite disseminated in a specularite matrix. District-wide chloritization and K-silicate alteration, characterized by microcline replacement and veining of the andesitic country rock, is recognized. Superimposed sericitic alteration is localized by the MVF and neighboring fracture zones. This late alteration event accompanied the main deposition of specularite and associated copper(-gold) mineralization dated by the KlAr method at 121 ± 3 and 117± 3 Ma. Preliminary studies indicate a predominance of three-phase fluid inclusions as well as the coexistence of vapor-rich and liquid-rich, two-phase inclusions evidencing boiling. Homogenization temperatures for two and three-phase inclusions in translucent quartz veinlets and fragments accompanying early iron - copper - (gold) mineralization range between 180° and 320° C, although they are mainly between 180° and 250° C. Salinities for the three-phase inclusions vary from 30 to 47 wt percent NaCl equivalent, although a decrease in fluid salinity (14-21 wt % NaCl equiv.J is detected in two-phase inclusions from late-mineralization, chalcopyrite-bear-ing calcite veins. The geologic features of the Manto Verde deposit suggest a common magmatic-hydrothermal origin with the apatite-bearing magnetite deposits of the CIB. Manto Verde probably represents the copper-rich, specularite-dominated, and possibly younger end member of a continuum of deposits that extends to copper-poor, magnetite-dominated deposits at the other extreme.

Abstract The Centinela district is a 40-km-long segment of the late Eocene to early Oligocene porphyry copper belt of northern Chile. The main mineralization styles in the district include porphyry copper and associated skarns with gold and molybdenum credits, oxide copper in the form of either exotic deposits or in situ oxidation zones, and supergene copper sulfide enrichment blankets. Structurally controlled, district-wide porphyry copper mineralization formed between 44 and 41 Ma, contemporaneously with transpressional tectonism along the regional-scale Centinela-Limón Verde fault zone, a splay of the major Domeyko fault system. The Domeyko fault system accommodated tectonic uplift of the Cordillera de Domeyko during early stages of the middle Eocene Incaic orogeny. Several productive porphyry copper systems in the district were emplaced syntectonically along reverse- and oblique-slip faults that controlled the shape and attitude of both the dominantly dikelike intrusions and associated alteration mineralization. All the porphyry copper deposits in the district evolved from early potassic alteration with copper mineralization present as chalcopyrite and bornite, through sericite-chlorite alteration with chalcopyrite and pyrite, to late, overprinting pyritic sericitic alteration. Some systems developed advanced argillic lithocaps in their upper parts. Much of the copper was introduced with the potassic alteration, and all the gold-rich deposits display abundant hydrothermal magnetite and a positive correlation between copper and gold. Supergene processes gave rise to oxide copper zones above pyrite-poor, chalcopyrite- and bornite-bearing protore and to chalcocite blankets over less reactive, sericitic zones containing pyrite and chalcopyrite. Coeval regional uplift, erosion, and exhumation resulted in widespread piedmont gravel deposition broadly contemporaneous with the supergene activity, with formation of exotic copper deposits alongside the shallow, actively oxidizing, pyritic parts of porphyry copper deposits. A long exploration history characterizes the Centinela district, from initial prospecting in the 1870s to formal exploration programs by major mining companies approximately 100 years later. However, initial appreciation of the district potential resulted from the discovery of the El Tesoro and Tesoro NE exotic copper deposits between 1990 and 1993 and was confirmed with the discovery of the Esperanza and Telégrafo porphyry copper-gold deposits in 1999 and 2001, respectively. Approximately 18 years of persistent exploration by Antofagasta Minerals resulted in an order-of-magnitude expansion of the district mineral inventory from an original 120,000 metric tons (t) of contained copper at El Tesoro in 1990 to the presently known resources of ~20 Mt of contained copper and ~560 t (18 Moz) gold in the several main deposits. Commercial copper production commenced at the El Tesoro SX-EW plant in 2001, with a cumulative output through 2009 of 790,000 t of cathode copper. With production start-up at Esperanza in late 2010, the district is expected to produce an additional 195,000 t Cu, 7 t (225,000 oz) gold, and 35 t (1,130,000 oz) silver per year in concentrates, whereas Mirador will contribute an additional 95,000 t of cathode copper upon attaining full production in 2012. Early stages of exploration within the district targeted exposed mineralization that had undergone historic, small-scale surface mining. The more recent exploration incorporated geologic mapping and conceptual modeling to test for both extensions to the known mineralization and new deposits as the company’s land holdings expanded. Recent efforts successfully targeted blind mineralization under extensive postmineralization gravel cover through interrogation of comprehensive data sets, use of empirical geologic models, and incorporation of conventional and relatively new geophysical tools to assist with modeling of bed-rock geology. However, the most recent discoveries employed information from nearby drill holes that were completed by previous explorers more than 23 years earlier. Discovery and wealth creation in the Centinela district are considered as products of the company’s long-term commitment to exploration and the exploration team’s view that the district had unfulfilled potential. Persistence overcame at least two cycles of economic turmoil. The most recent discoveries occurred approximately 17 years after initial discovery of El Tesoro and more than 25 years after formal exploration by major mining companies commenced in the district.

ABSTRACT The Los Angeles basin formed in late Neogene time on a continental margin previously shaped by Cretaceous and early Paleogene subduction, Paleogene terrane accretion, and mid-Miocene rifting and block rotation. During Neogene time, the boundary between the Pacific and North American plates shifted progressively eastward beneath the Los Angeles region, creating the broad San Andreas transform zone. As reviewed in this paper, structures and rocks within the Los Angeles basin document each stage of that Neogene evolution. The Los Angeles basin began to take its present shape in late Miocene time (ca. 7 Ma) by subsidence between the right-oblique Whittier and Palos Verdes fault zones and the left-oblique Santa Monica fault system. The principal phase of basin opening involved early Pliocene extension in a northwest direction, which accompanied the opening of the Gulf of California and the eastward shift of the southern San Andreas fault to its present position. Most of the structural traps that hold the basin's oil fields began to form during this latest Miocene-early Pliocene deformation. Since mid-Pliocene time, many of these traps have been altered and enhanced—and a few have been breached—by Pasadenan deformation, involving southward shortening, the uplift of the Transverse Ranges, and the propagation of blind thrusts beneath the northern Los Angeles basin. The rapid transition from early Pliocene extension to late Pliocene contraction was associated temporally with a change in relative plate motion dated at 3.9- 3.4 Ma. In analyzing Pasadenan deformation, the flake-tectonics model is more appropriate than the fold-and-thrust-belt model, although both models incorporate aseismic detachment at midcrustal depths. The flake-tectonics model is valid for all phases of Neogene deformation, both transtensional and transpressive, in the Los Angeles region. Fields discovered to date in the Los Angeles basin will yield an ultimate 10.4 billion oil-equivalent barrels (GOEB) of petroleum. Of this, approximately 73% is trapped in faulted anticlines, 12% in simple anticlines, 10% in fault traps, and 5% in stratigraphic traps. Folding has been controlled primarily by preexisting structural hingelines and sedimentary wedge belts and secondarily by en echelon folding associated with wrench faults. Oil seeps and Quaternary topographic uplifts led to most of the discoveries prior to 1925 along the Whittier and Newport−Inglewood fault zones and in the Coyote Hills. Most later discoveries, including the 3-billion-barrel Wilmington oil field, were in structures with little or no Quaternary expression.

Abstract Continental transform plate boundaries are broad, composed of numerous active and subparallel strike-slip fault zones. Irregular geometry along the major transform structure creates convergence and divergence zones within the plate boundary where other strike-slip faults terminate. Some prominent irregularities result from microplate interactions. Relative fault displacement, diminishing to zero at fault terminations, must be accommodated or transferred to other structures, laterally or vertically, away from the fault end-point. Distinct styles of strike-slip fault termination may represent different degrees of vertical strain partitioning within the plate boundary. The Western Transverse Ranges (WTR) of California mark a major structural discontinuity that cuts at high angle across the Pacific–North America transform boundary. Within the California Continental Borderland, two end-member classes of right-slip fault termination against the WTR are apparent. (1) Several major faults, including the San Clemente, San Pedro Basin, Ferrelo and Newport–Inglewood, intersect the southern boundary of the WTR at high angles, with negligible to minor local deflection and minor dissipation of right shear at the Earth’s surface. These faults are inferred to cut through the entire borderland crust and continue in the lower crust beneath the WTR, as evident in geophysical data. These ‘blind’ near vertical faults may control segmentation and earthquake activity on the overlying west-trending WTR structures. (2) In contrast, the Palos Verdes and possibly Whittier faults appear broadly deflected westward to merge at low angle with WTR structure. NW-trending faults rotate counterclockwise, away from the axis of principal shortening as observed in pure shear models, and slip is dissipated through folding, thrust transfer and rotation. Deflected faults are inferred to be predominately upper crustal features, detached from the lower crust and unable to underthrust the WTR. These two distinct right-slip fault termination styles, and associated convergent structures, suggest that basal shear drives vertical-axis rotation of the WTR block over the underthrust Inner Borderland plate. Furthermore, the lower plate, slivered by these right-slip faults, is incompletely coupled with the Pacific plate.