William R. Dickinson

Geologic Evidence and Tectonic Implications

Net Dextral Slip, Neogene San Gregorio-Hosgri Fault Zone, Coastal California

Víctor A. Ramos

Suzanne Mahlburg Kay

"The American Cordilleras form a continuous orogen that extends for 12,500 km along the eastern flank of the Pacific Ocean from Arctic to Antarctic latitudes as an integral part of the circum-Pacific orogenic belt. Following two summary chapters on the overall anatomy and evolution of North and South American segments of the orogenic system, this volume includes ten seminal chapters dealing with salient aspects of the key geodynamic processes that have accompanied Cordilleran geotectonic evolution: forearc terrane accretion, arc magmatism, shallow subduction, and backarc intracontinental deformation. The papers in this volume were selected from those presented at the 2006 Backbone of the Americas Meeting, which was sponsored jointly by multiple North and South American geological societies in Mendoza, Argentina."--pub. desc.

Shallow Subduction, Plateau Uplift, and Ridge and Terrane Collision

Backbone of the Americas

Tertiary Stratigraphy and Structure West of the Arroyo Seco, Monterey County, California

Mid-Tertiary strata exposed as tilted homoclines along the flanks of the San Pedro trough and across broad uplands north of the Catalina core complex are assigned to the following formations, each of which includes informal local members and facies: (a) Mineta Formation, mid-Oligocene redbeds including both conglomeratic fluvial and finer-grained lacustrine deposits; (b) Galiuro Volcanics, including lavas and domes, air-fall and ash-flow tuffs, and intercalated volcaniclastic strata of late Oligocene to earliest Miocene age; (c) Cloudburst Formation, also of late Oligocene and earliest Miocene age but including a sedimentary upper member of conglomeratic strata as well as a volcanic lower member correlative with part of the Galiuro Volcanics; and (d) San Manuel Formation, composed of lower Miocene alluvial fan and braidplain deposits that display contrasting clast assemblages in different areas of exposure. Generally correlative Oligocene-Miocene strata exposed south of the Catalina core complex are assigned to the Pantano Formation, which contains similar lithologic components. Less-deformed Neogene strata of post-mid-Miocene basin fill are assigned to the Quiburis Formation along the San Pedro trough, but stratigraphic equivalents elsewhere lack adequate nomenclature. High benchlands mantled by paleosols mark the highest levels of Neogene aggradation. Successive stages of subsequent erosional dissection are recorded by multiple terrace levels incised into basin fill. Key exposures of syntectonic mid-Tertiary sedimentary sequences in several local subareas reveal typical structural and stratigraphic relationships. Multiple fault blocks expose pre-Tertiary bedrock overlain by tilted mid-Tertiary strata confined to intervening half-grabens. Bounding syndepositional faults dip southwest and associated homoclines dip northeast. Fanning dips and buttress unconformities reflect progressive tilt and burial of eroding fault blocks. Dips of block-bounding faults are inversely proportional to the ages of the faults. Steeper dips for younger faults suggest either progressive erosion of successive listric faults or progressive rotation of successive planar faults. Uniformly moderate to steep dihedral angles between fault surfaces and offset homoclinal bedding imply that the faults dipped more steeply near the surface when syntectonic mid-Tertiary strata were subhorizontal. Although the inference of listric faulting best links apparent strands of the Catalina detachment system, the alternate interpretation of rotational normal faulting is compatible with local structural relationships including tilt of porphyry copper orebodies. Within the San Pedro trough, multiple homo clines of mid-Tertiary strata are exposed locally in tilt-blocks exhumed by Neogene erosion from beneath nearly flat-lying basin fill of the Quiburis Formation. Faults bounding the mid-Tertiary exposures include backtilted strands of the Catalina detachment system, somewhat younger listric or rotational normal faults, and steeper basin-range normal faults that display offsets both synthetic and antithetic to the flanks of the San Petro trough. In Cienega Gap, flanking the Tucson Basin, multiple tilt-blocks of the Pantano Formation form part of the upper plate of the Catalina detachment system. Initial construction of alluvial fans by generally westward paleoflow was followed by ponding of lacustrine environments along the foot of secondary breakaway scarps that also generated massive megabreccia deposits. In summary, syntectonic Oligocene to Miocene sedimentation succeeded a prominent pulse of polymodal mid-Tertiary volcanism and was coeval with mylonitic deformation and detachment faulting along the flank of the Catalina core complex. The headwall rupture for the detachment system migrated westward from an initial position along the range front of the Galiuro Mountains. After mid-Miocene time, accumulation and subsequent dissection of essentially undeformed basin fill was accompanied by basin-range block faulting. The most challenging structural issue is whether fault strands of the Catalina detachment system are interconnected or are disconnected rotational segments.

Tectonic Setting of Faulted Tertiary Strata Associated with the Catalina Core Complex in Southern Arizona

"Following this discovery and further detrital-zircon studies of Mesozoic strata on the Colorado Plateau, Dr. Dickinson began preparing this volume in order to identify key aspects of the sedimentary and tectonic history of Mesozoic strata of the Colorado Plateau and directly adjacent areas. He divided the strata into seven depositional systems"--

Tectonosedimentary Relations of Pennsylvanian to Jurassic Strata on the Colorado Plateau

"Oceanian ceramic cultures making earthenware pottery spread during the past 3500 years through a dozen major island groups spanning 6000 km of the tropical Pacific Ocean from western Micronesia to western Polynesia. Island potters mixed sand as temper into clay bodies during ceramic manufacture. The nature of island sands is governed by the geotectonics of hotspot chains, island arcs, subduction zones, backarc basins, and remnant arcs as well as by sedimentology. Because small islands with bedrock exposures of restricted character are virtual point sources of sand, many tempers are diagnostic of specific islands. Petrographic study of temper sands in thin section allows distinction between indigenous pottery and exotic pottery transported from elsewhere. Study of 2223 prehistoric Oceanian potsherds from 130 islands and island clusters indicates the nature of Oceanian temper types and documents 105 cases of interisland transport of ceramics over distances typically

William R. Dickinson

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