A rifted arc-ophiolite assemblage and over-lying volcaniclastic rocks, all of late Middle Jurassic age, represent a fragment of the frontal-arc margin of a back-arc basin, now exposed on Cedros Island in Baja California (Mexico). Ophiolite generation was immediately followed by progradation of a deep- marine volcaniclastic apron (Gran Cañnon Formation) contemporaneous with the growth of island-arc volcanoes. This back-arc apron shows relatively simple, uniform sedimentation patterns that may be recognizable in other back-arc basins isolated from terrigenous sediment influx.

The progradational back-arc apron sequence includes, from base to top, (1) the tuff lithofacies—thin-bedded, well-sorted, laterally continuous tuffs that thicken into basement paleo-lows. These were deposited from dilute sediment gravity flows generated by eruptions from a deeply submerged, nascent arc. (2) the lapilli tuff-tuff breccia lithofacies—medium- to very thick-bedded lapilli tuffs and tuff breccias. These were deposited from debris flows on proximal parts of the apron, and high-density turbidites were deposited on more distal parts. This lithofacies records increasing generation and resedimentation of scoriaceous and hyaloclastic debris as the summits of the arc volcanoes grew nearer to sea level. (3) the primary volcanic lithofacies—monolithologic dacite pyroclastic flows were produced during the eruption of differentiated magmas, which climaxed the growth of the island arc. Basalt lava flows were fed from fissures that extended down the apron within the back-arc basin.

Pyroclastic rocks of the back-arc apron were blanketed with silty basinal turbidites (epiclastic facies) that record abrupt cessation of volcanism and erosion of the arc within about 10 m.y. of ophiolite generation, reflecting the temporal episodicity of back-arc basins.

It is predicted from this study that the frontal-arc side of back-arc spreading centers is preferentially preserved in the geologic record relative to the center of the basin. It is suggested that the "hot" frontal-arc side of the basin is prone to intra-plate volcanism but is unaffected by faulting subsequent to basin initiation. Several factors lead to slumps occurring more frequently on the arc flank of back-arc basins than in any other tectonic setting, except perhaps intra-arc marine basins.

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