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Oblique Rifting Ruptures Continents: Example from the Gulf of California Shear Zone

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Oblique Rifting Ruptures Continents: Example from the Gulf of California Shear Zone Geology, published online on 10 January 2014 as doi:10.1130/G34904.1 Oblique rifting ruptures continents: Example from the Gulf of California shear zone Scott E.K. Bennett* and Michael E. Oskin Department of Earth and Planetary Sciences, University of California–Davis, 1 Shields Avenue, Davis, California 95616, USA ABSTRACT a north-northwest–trending system of right- We show that a belt of clockwise vertical-axis block rotation associated with dextral-oblique stepping, en echelon, strike-slip and oblique- rifting in the Basin and Range province in Mexico hosted the localization of plate-boundary slip faults that transfer strain between narrow strain that led to formation of the Gulf of California ocean basin. Paleomagnetism of Miocene pull-apart basins or spreading centers (Lonsdale, ignimbrites distributed widely across the rift reveals the magnitude, distribution, and tim- 1989). A majority of thinned continental crust on α ≈ ing of rotation. Using new high-precision paleomagnetic vectors ( 95 1°) from tectonically the rifted margins remains above sea level, expos- stable exposures of these ignimbrites in Baja California, we determine clockwise rotations ing the recent record of rift formation (Fig. 1). up to 76° for intrarift sites. Low reference-site error permits isolation of intrarift block rota- Localized extension in the Gulf of California tion during proto-Gulf time, prior to rift localization ca. 6 Ma. We estimate that 48% (locally was preceded by diffuse extension across the 0%–75%) of the net rotation occurred between 12.5 Ma and 6.4 Ma. Sites of large (>20°) Basin and Range province in Mexico (Fig. 1, in- block rotation defi ne an ~100-km-wide belt, associated with strike-slip faulting, herein named set) that initiated in Oligocene time (Henry and the Gulf of California shear zone, which was embedded within the wide rift Basin and Range Aranda Gomez, 1992) within the Sierra Madre province and kinematically linked to the San Andreas fault. After a protracted history of dif- Occidental, and expanded westward behind the fuse extension and transtension, rift localization was accomplished by focusing of Pacifi c– subduction-related volcanic arc, now dissected North America dextral shear into the Gulf of California, which increased strain rates and by the Gulf of California. At the onset of proto- connected nascent pull-apart basins along the western margin of the province. Oblique rifting Gulf time (12.5 Ma), with the southward jump thus helped to localize and increase the rate of continental break up and strongly controlled of the Rivera Triple Junction, the tectonic setting the three-dimensional architecture of the resultant passive margins. evolved from subduction and backarc exten- sion to dextral transtension (Atwater and Stock, INTRODUCTION margins (Huismans and Beaumont, 2011). The 1998). By the end of proto-Gulf time (6 Ma), at Mechanisms of strain localization during con- Gulf of California rift formed by oblique sepa- least 90% of plate motion had become localized tinental rifting play a critical role in formation ration across the Pacifi c–North America plate along the western edge of this asymmetric prov- of ocean basins and the ultimate form of passive boundary, motion currently accommodated by ince (Oskin et al., 2001; Miller and Lizarralde, Figure 1. Extent of Mio- Cerro Prieto 6.4 Ma Tuffs of Mesa Cuadrada (MC, red) Mendocino North 115°W fatlt 114°W cene ignimbrites across (overlapping tuffs in orange) SanS triple A junction America 12.5 Ma Tuff of San Felipe (SF, green) an northern Gulf of Califor- Altar A plate S GULF Basin Basin nndreas fault 40° a and nia. Pacifi c–North Amer- SRB (abandoned) 113°W 112°W dr n e Range as fau 110° ica relative plate motion P Prov. e SFD d N Colorado (large black arrows) trans- r o lt Plateau Wagner- lated Baja California and M SFE Amado fault a Consag 500 KMKM500 r 31°N outcrops of these ignim- t Basin Baja í Pacific r SFB Prev. FIG. 11FIG. 1FIG. 1 f brites to the northwest, a plate MC Ref Site u 30° GulfGulfGulfGulf Basin relative to Sonora (North l t SFF and OF Adair- Sierra MadMadreRange Occidental America). New paleo- 120° Tepoca CaliforniaCa magnetic reference sites SFH Upper Prov. FIGURE 2 Delfin Basin SONORA lif (black dots) are located Basin (abandoned) ornia of SFJ r in distal tuff outcrops of CaliforniaC e Occidental SF al central Baja California, REF SITE SIW Rivera ifornia beyond western limit of triple 30°N junction MC D rift-related faulting. In- REF SITE Lower e Mar fault Delfin Upper CEJ 20° trarift paleomagnetic sites Basin Isla Angel Tiburon de la Basin SB (white dots) are rotated Guarda (abandoned) SL clockwise with respect TiburCALIFORNIA EG SK to these reference sites on BAJA Coastal due to dextral shear- SA PACIFIC CALIFORNIA Ballenas PC driven block rotation. Of SMH OCEAN NWH 11 paired sites, 7 show PR BK statistically signifi cant 29°N PRS Isla Sonora LC larger magnitude rotation 0° Rotation Tiburón HE C SM Fracture La Cruz fault of older tuff. Site locations W and error: Transform fault zone? ? CCW BV are provided in Table DR1 SF (green) Zone ? -90°SITE 90° EP111°W (see footnote 1). Colored MC (red) stars show hypothesized ignimbrite vents from Oskin and Stock (2003b); thin black lines indicate rift-related strike-slip and normal faults; thick black lines indicate rift segment axes; white polygons outline sedimentary pull-apart basins created during formation of Gulf of California. CW—clockwise; CCW—counterclockwise. Inset: Present-day tectonic setting of western North America, showing diffuse Pacifi c–North America plate bound- ary and provinces of mid- to late Cenozoic extension (tan) (modifi ed from Oskin and Stock, 2003b). Prov.—Province. *Current address: U.S. Geological Survey, Geologic Hazards Science Center, 1711 Illinois Street, Golden, Colorado 80401, USA; E-mail: [email protected]. GEOLOGY, March 2014; v. 42; no. 3; p. 1–4; Data Repository item 2014074 | doi:10.1130/G34904.1 | Published online XX Month 2013 GEOLOGY© 2013 Geological | March Society 2014 of| www.gsapubs.orgAmerica. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 Geology, published online on 10 January 2014 as doi:10.1130/G34904.1 2013), an event broadly synchronous with ma- paleomagnetic reference sites for both tuffs in 92) collected at the new reference sites. At sites rine incursion and formation of a continuous undeformed exposures in north-central Baja where both the Tmr3 and Tmr4 cooling units of Gulf of California seaway ca. 6.5–6.3 Ma (Os- California, west of the San Pedro Martír fault the MC were drilled, their rotations are similar. kin and Stock, 2003a). Focused dextral shear (Fig. 2; see the GSA Data Repository1). Rema- However, due to higher rotation errors for Tmr4 is well documented within the rifted margins nence directions determined here (declination, (Table DR1), we include only the results from of the northern Gulf of California (Lewis and DSF = 212.4°, inclination, ISF = –3.0°; DMC = Tmr3 in our analysis. At paired sites, where both Stock, 1998; Oskin and Stock, 2003b; Seiler 15.6°, IMC = 56.2°) are consistent across tens of SF and MC are present in the same fault block, α et al., 2010; Bennett et al., 2013); however, the kilometers. Confi dence cones ( 95) for multiple clockwise rotation that occurred prior to 6.4 Ma extent of this deformation and the amount that cores (SF = 1.3°, MC = 1.0°) are signifi cantly is detectable by differential rotation between occurred prior to rift localization are unknown. smaller than those reported previously from SF and MC. At 10 of 11 paired sites, SF is ro- Mesa Cuadrada (SF = 4.1°, MC = 8.9°) (Fig. 2 tated clockwise by a greater amount than MC PALEOMAGNETISM OF REGIONAL inset; Table DR1 in the Data Repository). (Fig. 3). Of these 10 sites, 7 are precise enough IGNIMBRITES Comparisons of new paleomagnetic rema- to statistically isolate differential rotation. At To quantify dextral shear prior to rift local- nence directions from central Baja California paired sites, as much as 48° (weighted mean ization, we compiled a transect of vertical-axis with directions from 26 intrarift sites (2 new, 24 of 16°) of clockwise rotation occurred prior to block rotations across the Pacifi c–North Amer- previously published) indicate clockwise rota- 6.4 Ma. By weighting all paired site results by ica plate boundary at lat 29°–31°N using pa- tions for SF and MC to 76° ± 11° and 40° ± 3°, the differential rotation error, we estimate that leomagnetism of extensive ignimbrite (welded respectively (Figs. 1 and 3; Table DR1). Mean 48% (locally 0%–75%) of the net rotation oc- Δ ash-fl ow tuff) deposits. Two widespread ignim- magnitudes of the rotation errors ( RSF = 4.9°, curred prior to 6.4 Ma. Δ brite markers that loosely bracket proto-Gulf RMC = 6.3°) using these new paleomagnetic Overall, we fi nd that clockwise vertical-axis time, the 12.5 Ma Tuff of San Felipe (SF) and sites are lower than the rotation errors using rotation was widely distributed across the Basin the 6.4 Ma Tuffs of Mesa Cuadrada (MC), are the previous reference sites at Mesa Cuadrada and Range province in Mexico (Fig. 3). The low- Δ Δ offset by similar amounts across the northern ( RSF = 6.0°, RMC = 13.2°), owing greatly to est rotation values (error-weighted mean of 10°) Gulf of California (Oskin et al., 2001; Oskin the larger number of cores (SF, n = 48; MC, n = are observed in central Sonora, where as many and Stock, 2003b) (Fig. 1). Dextral shear may result in clockwise vertical-axis rotation, detect- 115°15'W 10 KM Equal Area able by comparing paleomagnetic vectors pre- Mesa El Pinole 115°10'W served by the alignment of magnetic minerals, normal MC4 n=48 primarily magnetite (Nagy, 2000), in these tuffs.
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