Helge Alsleben

Driving mechanisms for >40 km of exhumation during contraction and extension in a continental arc, Cascades core, Washington

[1]xa0In the NW North American Cordillera, the Cascades core region of the Coast Plutonic Complex underwent Late Cretaceous (>96 Ma to locally 73 Ma) SW-NE contraction and crustal thickening followed by dextral transpression (∼73 to 55 Ma), then transtension ( 3 mm/yr) by local thrusting in regions undergoing crustal thickening. In the central part of the core (Chelan block), >40 km of exhumation occurred between 91 and 45 Ma, about half of which occurred during early contraction (driven by thrusting) and half during top-to-north, arc-oblique shear during reactivation of a midcrustal Cretaceous thrust, the Dinkelman decollement. The footwall of this thrust consists of the Swakane Biotite Gneiss, a Cretaceous, metaclastic assemblage with recorded pressures of 10–12 kbar, no arc-related magmatism, and structures dominated by pervasive top-to-north shearing. The hanging wall consists of the Napeequa Complex, an oceanic assemblage with recorded pressures of 6–12 kbar, voluminous arc-related magmatism, and complex structures indicating early top-to-WSW shearing, younger top-to-north shearing, and widespread folding. In the Napeequa, top-to-north shearing started by 73 Ma during melt-present conditions at pressures ≥6 kbar. Top-to-north shearing in both hanging wall and footwall continued during exhumation (∼1.6 mm/yr) and cooling to greenschist facies conditions during which slip became increasingly localized, eventually resulting in formation of pseudotachylite on discrete slip surfaces. We suggest that arc-oblique extension was driven by along-arc heterogeneity in displacements/erosion, initially during transpression and underplating of continental sediments, and later during transtension.

Mesozoic tectonic evolution of the Peninsular Ranges of southern and Baja California

The Mesozoic evolution of the Peninsular Ranges of southern California, USA, and Baja California, Mexico, remains a controversial aspect of Cordilleran tectonics with multiple, often mutually exclusive, models potentially viable. A fundamental reason for the lack of agreement between the proposed tectonic models is that they are based on one dimensional, arc perpendicular observations of the batholith from widely separated locations on opposite sides of the ancestral Agua Blanca fault, an active strike-slip fault with an earlier Mesozoic history. North of the ancestral Agua Blanca fault, the Late Triassic through Jurassic was characterized by deep to moderately deep marine sedimentation of continentally derived turbidite sequences of the Bedford Canyon Complex. These strata were deformed within an accretionary prism setting and were subsequently uplifted and beveled by subaerial erosion. During the Early Cretaceous the continental margin arc associated with the earlier-formed accretionary prism migrated westward and developed within and on the Bedford Canyon Complex. South of the ancestral Agua Blanca fault Jurassic strata are only preserved locally in the central zone. During the Early Cretaceous this part of the arc subsided below sea level and became the site of turbidite sedimentation before being uplifted and dominated by the deposition of submarine sediment, succeeded by subaerial volcanics derived from the continental margin arc present in the central and eastern zones. Outboard, the Alisitos arc, developed through and on oceanic crust, began to impinge upon the continental margin in the Early Cretaceous (ca. 115 and 108 Ma). During accretion of the Alisitos arc across the Main Martir thrust and ancestral Agua Blanca fault, the Late Triassic-Jurassic accretionary prism (correlative to the Bedford Canyon Complex) was structurally removed from between the arc and the continent by forcible subduction. If this model is correct, it implies that the Late Cretaceous uplift of the central zone of the Peninsular Ranges batholith, both north and south of the ancestral Agua Blanca fault, was not driven by accretion-related deformation at the trench.

Detrital zircon ages in Palaeozoic and Mesozoic basement assemblages of the Peninsular Ranges batholith, Baja California, Mexico: constraints for depositional ages and provenance

The origin and continuity of Phanerozoic lithostratigraphic terranes in southern and Baja California remain an unsolved issue in Cordilleran tectonics. We present data from eight detrital zircon samples collected across the southern extent of the Peninsular Ranges that help constrain the provenance of detritus and the depositional ages of these basement units. Detrital zircon signatures from units in the eastern Peninsular Ranges correlate with Palaeozoic passive margin assemblages in the southwestern North American Cordillera. Units in the central belt, which consists of Triassic–Jurassic metasedimentary turbidite assemblages that probably deformed in an accretionary prism setting, and Cretaceous metasedimentary and metavolcanic units that represent the remnants of a continental margin arc, were derived from both proximal and more distal sources. The westernmost units, which are locally structurally interleaved with the Triassic through Cretaceous units of the central belt, are Cretaceous deposits that represent a series of collapsed basin complexes located within and flanking the Cretaceous Alisitos volcanic island arc. Cretaceous intra-arc units show little influx of cratonal material until approximately 110 Ma, whereas coeval sediments on the northern and eastern flanks of the Alisitos arc contain abundant cratonal detritus. Intra-arc strata younger than approximately 110 Ma contain large amounts of Proterozoic and older detrital zircons. These data suggest that basins associated with the Alisitos arc were either too distant or somehow shielded from North American detritus before 110 Ma. In the case of the former, increased influx of continental detritus after 110 Ma would support a tectonic model in which the arc was separated from North America by an ocean basin and, as the arc approached the continent, associated depositional centres were close enough to receive input from continental sources.