THE COMONDÚ ARC-GULF OF CALIFORNIA RIFT TRANSITION: GEOLOGY AND GEOCHRONOLOGY

The Gulf of California was occupied by a subduction-related extensional continental arc from ca. 20 to 12-11 Ma (Commondú arc), where lithosphere was thinned and thermally weakened. At 11 Ma, subduction ceased abruptly along the length of the Baja California Peninsula, and the Pacific-North American plate boundary jumped inboard across the Baja California Peninsula, from the trench into the Comondú arc axis. This event formed the Gulf of California oblique continental rift, which opened rapidly due to thermal and structural weakening within the arc axis. We have discovered a remarkably pristine, well-exposed, thick (2 km) and laterally extensive (30 km) section through the arc-rift transition in the Concepción Peninsula on the central Baja California margin of the Gulf of California. This section has not been mapped since 1968¹, when it was entirely included in the Comondú arc. This section provides a world-class opportunity to study the magmatic and structural evolution of a continental margin that evolved from convergence to continental rifting.

Our new mapping, petrography, ⁴⁰Ar /³⁹Ar geochronology and geochemistry show that the lower half of the Concepción Peninsula section consists of arc volcanic rocks, and the upper half consists of rift volcanic rocks. The arc volcanic rocks consist of intermediate-composition lavas, block-and-ash-flow tuffs, and hypabyssal intrusions, ranging from ca. 20 Ma to 11 Ma. The rift volcanic rocks are bimodal, consisting of trachydacite and rhyolite welded ignimbrites, alkalic lavas, and basalt lavas, with ages as young as 3 Ma. The rift volcanic section also contains alluvial fan conglomerates, and angular unconformities produced by syn-volcanic extension.

Continental rifts commonly exploit the axes of continental arcs (e.g. Walker Lane), but the timings and precise mechanisms for this are not well understood. This is because few global examples of the arc-rift transition are so well exposed, extensive and pristine as that of the Concepción Peninsula. Ongoing work thus focuses on major- and trace-element geochemistry, and isotope and mineral chemistry, to characterize shifts in source region types, depths and temperatures, as well as magma processing conditions during this transition.

¹McFall 1968 Stanford University Publications, Geological Sciences, X(5) 25 pp.