Scott E.K. Bennett

Oblique rifting ruptures continents: Example from the Gulf of California shear zone

We show that a belt of clockwise vertical-axis block rotation associated with dextral-oblique rifting in the Basin and Range province in Mexico hosted the localization of plate-boundary strain that led to formation of the Gulf of California ocean basin. Paleomagnetism of Miocene ignimbrites distributed widely across the rift reveals the magnitude, distribution, and timing of rotation. Using new high-precision paleomagnetic vectors (α 95 ≈ 1°) from tectonically stable exposures of these ignimbrites in Baja California, we determine clockwise rotations up to 76° for intrarift sites. Low reference-site error permits isolation of intrarift block rotation during proto-Gulf time, prior to rift localization ca. 6 Ma. We estimate that 48% (locally 0%–75%) of the net rotation occurred between 12.5 Ma and 6.4 Ma. Sites of large (>20°) block rotation defi ne an ~100-km-wide belt, associated with strike-slip faulting, herein named the Gulf of California shear zone, which was embedded within the wide rift Basin and Range province and kinematically linked to the San Andreas fault. After a protracted history of diffuse extension and transtension, rift localization was accomplished by focusing of Pacifi c– North America dextral shear into the Gulf of California, which increased strain rates and connected nascent pull-apart basins along the western margin of the province. Oblique rifting thus helped to localize and increase the rate of continental break up and strongly controlled the three-dimensional architecture of the resultant passive margins.

Transtensional rifting in the proto–Gulf of California near Bahía Kino, Sonora, México

Continental rifts require focused strain to rupture and form an ocean basin. In oblique rifts, such as the Gulf of California, focused transtensional strain associated with strike-slip faulting may serve as a catalyst for rupture. To test this hypothesis, we analyzed structural mapping, geochronology, paleomagnetism, and fault kinematics of pre- and synrift rocks exposed in an ∼200 km 2 coastal mountain belt flanking the eastern rift margin of the northern Gulf of California. This coastal Sonora region hosts the onshore portion of the transform boundary between the Upper Tiburon and Adair-Tepoca marine basins—two early-formed oblique rift segments. Extension commenced here between 11.5 Ma and 7 Ma, resulting in 25°–40° of E-NE tilting, initiation of clockwise vertical-axis rotation of fault-bounded blocks, and minor basin sedimentation. Rates of deformation prior to 7 Ma are unconstrained due to a lack of exposed syntectonic deposits. Deformation after 7 Ma was associated with rapid tilting and the majority of observed clockwise vertical-axis rotation and strike-slip faulting. Nonmarine sedimentary basins accumulated coarse sediments above an unconformity eroded across older, tilted strata. By 5–6 Ma, deformation in coastal Sonora must have largely ceased and migrated westward into the Upper Tiburon marine basin. We document up to 120% total extension and total clockwise block rotations up to 53°. In portions of the study area, extension and rotation were supplanted by strike-slip faulting as deformation proceeded. We develop a tectonic model for this Coastal Sonora fault zone, which is bounded by major NW-striking transform faults with >10 km of displacement. Internal to the Coastal Sonora fault zone, the majority of an estimated 6.2 ± 1.1 km of dextral deformation, associated with up to 5.7 km of WNW-directed extension, occurred over the final 1–2 m.y. of its life span, at a strain rate approaching 10 –14 s –1 . This activity occurred as the plate boundary localized along nascent pull-apart basins in the northern Gulf of California, consistent with the hypothesis that late proto–Gulf of California dextral shear zones, such as the Coastal Sonora fault zone, acted to focus lithospheric-scale strain and promoted continental rupture in the wide-rift setting of the Mexican Basin and Range.

Stratigraphy and structural development of the southwest Isla Tiburón marine basin: Implications for latest Miocene tectonic opening and flooding of the northern Gulf of California

Accurate information on the timing of earliest marine incursion into the Gulf of California (northwestern Mexico) is critical for paleogeographic models and for understanding the spatial and temporal evolution of strain accommodation across the obliquely divergent Pacific–North America plate boundary. Marine strata exposed on southwest Isla Tiburon (SWIT) have been cited as evidence for a middle Miocene marine incursion into the Gulf of California at least 7 m.y. prior to plate boundary localization ca. 6 Ma. A middle Miocene interpretation for SWIT marine deposits has played a large role in subsequent interpretations of regional tectonics and rift evolution, the ages of marine basins containing similar fossil assemblages along ∼1300 km of the plate boundary, and the timing of marine incursion into the Gulf of California. We report new detailed geologic mapping and geochronologic data from the SWIT basin, an elongate sedimentary basin associated with deformation along the dextral-oblique La Cruz fault. We integrate these results with previously published biostratigraphic and geochronologic data to bracket the age of marine deposits in the SWIT basin and show that they have a total maximum thickness of ∼300 m. The 6.44 ± 0.05 Ma (Ar/Ar) tuff of Hast Pitzcal is an ash-flow tuff stratigraphically below the oldest marine strata, and the 6.01 ± 0.20 Ma (U/Pb) tuff of Oyster Amphitheater, also an ash-flow tuff, is interbedded with marine conglomerate near the base of the marine section. A dike-fed rhyodacite lava flow that caps all marine strata yields ages of 3.51 ± 0.05 Ma (Ar/Ar) and 4.13 ± 0.09 Ma (U/Pb) from the base of the flow, consistent with previously reported ages of 4.16 ± 1.81 Ma (K-Ar) from the flow top and (K-Ar) 3.7 ± 0.9 Ma from the feeder dike. Our new results confirm a latest Miocene to early Pliocene age for the SWIT marine basin, consistent with previously documented latest Miocene to early Pliocene (ca. 6.2–4.3 Ma) planktonic and benthic foraminifera from this section. Results from biostratigraphy and geochronology thus constrain earliest marine deposition on SWIT to ca. 6.2 ± 0.2 Ma, coincident with a regional-scale latest Miocene marine incursion into the northern proto–Gulf of California. This regional marine incursion flooded the northernmost, >500-km-long portion of the Gulf of California shear zone, a narrow belt of localized strike-slip faulting, clockwise block rotation, and subsiding pull-apart basins. Oblique Pacific–North America relative plate motion gradually localized in the >1000-km-long Gulf of California shear zone ca. 9–6 Ma, subsequently permitting the punctuated south to north flooding of the incipient Gulf of California seaway.

Breaching of strike-slip faults and successive flooding of pull-apart basins to form the Gulf of California seaway from ca. 8–6 Ma

The geologic record of the formation of marine basins during continental rifting is uncommonly preserved. Using GIS-based paleotectonic maps, we show that marine basin formation in the Gulf of California–Salton trough oblique rift (Mexico and the United States) occurred in a stepwise manner as crustal thinning lowered elevations within the Gulf of California Shear Zone, and subsidence along strike-slip and transtensional faults linked isolated pull-apart basins. At 8 Ma, the earliest marine conditions in the Gulf of California were restricted to an embayment at its southern mouth. Farther north, the plate boundary was a set of continental strike-slip faults and linked pull-apart basins, similar to the modern Walker Lane in Nevada and California. By ca. 7 Ma, a series of marine incursions breached across strike-slip faults to the Pescadero and Farallon basins. Marine waters then breached a 75–100 km-long transtensional fault zone between the Farallon and Guaymas basins, with intermittent flooding that led to accumulation of extensive evaporite deposits in the Guaymas basin. Marine incursion north of the Guaymas basin via breaches across the Guaymas and Tiburón strike-slip faults and transtensional zones resulted in flooding of the northern >500 km of the oblique rift by 6.5–6.3 Ma. Thus, strike-slip and transtensional faulting promoted localization of plate boundary strain and guided punctuated marine flooding of the Gulf of California seaway. Inception of the narrow, 1500-km-long Gulf of California at ca. 6.3 Ma was followed by complete continental rupture in the Guaymas basin at ca. 6.0 Ma. INTRODUCTION Continental lithospheric rupture and formation of a new ocean basin represent one of the most fundamental tectonic and physiographic transitions on Earth (McKenzie, 1978). Transformative effects of these events include the development of marine depositional environments, reorganizations of topography, climate patterns, and drainage networks (Weissel and Seidl, 1998), and biodiversification through the introduction of marine ecosystems and isolation and evolution of species (e.g Dolby et al., 2015). The stratigraphic record of the transition from early rifting to seafloor spreading is often concealed beneath thick sedimentary sequences along subsided passive margins, or exhumed and destroyed along convergent plate boundaries. Therefore, the processes that occur during initial stages of rifting and seaway development are difficult to constrain and poorly understood. Many continental rift systems open oblique to the trend of the incipient plate boundary. Models of highly oblique divergent plate boundaries predict that continental rupture is preceded by a focused thinning/rifting stage that involves a series of pull-apart basins (Brune et al., 2012). Conceptually, oblique rifting should lead to stepwise expansion of a nascent ocean basin by linking pull-apart basins (Fig. 1). We define this process of flooding of marine waters into an adjacent basin as breaching (Figs. 1B and 1C). We suggest that breaching of strike-slip fault-controlled topographic sills is a fundamental process in the formation of a narrow seaway that, due to focused crustal thinning and subsidence, leads to continental rupture on geologically short time scales (Umhoefer, 2011). In the Gulf of California (GOC, Mexico), dextral-oblique rifting between the North America and Pacific plates (Baja California microplate) (Fig. 2) initiated after ca. 12.5 Ma (Lonsdale, 1989; Dorsey and Umhoefer, 2012; Bennett and Oskin, 2014) following ca. 25– 15 Ma extension along and east of the future GOC (Ferrari et al., 2002). By ca. 8 Ma, an initial marine embayment had formed in the southernmost GOC, and by ca. 6.3 Ma, a narrow marine seaway was fully developed from the mouth of the GOC in the south to the Salton Trough in the north (California, USA; Bennett et al., 2015). Seafloor spreading GEOLOGY, August 2018; v. 46; no. 8; p. 695–698 | GSA Data Repository item 2018250 | https:// doi .org /10 .1130 /G40242 .1 | Published online 12 July 2018