December 2008 AGU meeting, San Francisco

NET SLIP ACROSS THE BALLENAS TRANSFORM FAULT MEASURED FROM OFFSET IGNIMBRITE DEPOSITS
J Stock1, A Martin Barajas2, M Martinez-Lopez2, A Chapman1
1- California
Institute of Technology, Pasadena CA USA 2- CICESE, Ensenada, Baja California, Mexico
jstock@gps.caaltech.edu, amartin@cicese.mx, marlopez@cicese.mx, alan@gps.caltech.edu
Abstract
The Ballenas Transform Fault in the Gulf of California separates the Baja California Peninsula (to the west, on the Pacific
Plate) from Isla Angel de la Guarda (to the east, on the North America plate). This active right-lateral fault is thought to
have originated in Pliocene time when a local plate boundary reorganization caused part of the rift margin to jump
westward into the Baja California peninsula, transferring Isla Angel de la Guarda from the Pacific plate onto the North
America plate. The net slip along the Ballenas transform fault system is therefore expected to be less than the 250-300
km of opening of the northern Gulf of California basins since late Miocene time. Here we constrain the amount of net slip
using geological similarities between the west coast of the island and the Baja California peninsula. Reconnaissance
geological mapping on Isla Angel de la Guarda reveals the presence of a middle Miocene high-silica rhyolite ignimbrite
with an Ar/Ar age of 11.8 +/- 0.2 Ma on sanidine (Martin-Barajas et al., 2008). This deposit is preserved in a paleo-low in
Cretaceous (?) granitic rocks in the central part of the island, near the western coast. The major and trace element
compositions are similar to those of the Tuff of San Felipe, identified in Sonora and Baja California, but not previously
recognized on Isla Angel de la Guarda. The deposit from the island exhibits a paleomagnetic remanence vector toward
the SW and nearly horizontal, similar to that known for the Tuff of San Felipe in its outcrop area in other locations, and far
outside of the expected paleomagnetic remanence direction. The ignimbrite deposits on Isla Angel de la Guarda are not
densely welded, and the age is younger than that previously reported for the Tuff of San Felipe, but is within the range of
ages determined for other outcrops of the Tuff of San Felipe in Sonora (Vidal-Solano et al., 2008). We correlate this
ignimbrite to a similar unit preserved in isolated locations northwest of Cataviña on the Baja California peninsula, for
which paleomagnetic work shows a very similar remanence direction. Using the SE limit of these outcrops against the
granitic rocks in both locations yields an estimate of ~130 km right-lateral strike-slip offset along the Ballenas channel.
We expect to refine this estimate further with additional field work.
Baja California Peninsula,
Cataviña region
C
Paleomagnetic Methodology
Densely welded tuff
Brown glassy base
(vitrophyre not well developed)
conglomerate
Sample Preparation:
™ Oriented hand samples were obtained from Isla Angel de la Guarda and the Baja California
Peninsula in the region near Catavina .
™Three to five cores were drilled from each rock; these were re-oriented back to their original field
positions and their orientations measured. They were then sliced into standard lengths.
Demagnetization and analysis at the Caltech Paleomagnetics Laboratory:
™Natural Remanent Magnetization (NRM) was measured.
™Low temperature step: chips were cooled to 77 K (liquid nitrogen treatment) then allowed to
warm up back to room temperature. Magnetization of sample was then measured.
™Alternating Field (AF) steps from 25 mT to 75 mT with increments of 25 mT.
™High AF demagnetization from 100 mT to 800 mT in steps of 50 mT.
™High AF steps are used to obtain the best fit for a linear vector of magnetic remanence for each
core; these are combined for each locality to obtain Fisher and Bingham statistics
POR-08-02
POR-08-03
Marker unit: the Tuff of San Felipe
This processed ASTER satellite image
(short-wave infrared) shows outcrops
of this ignimbrite in light blue
™Peralkaline rhyolitic pyroclastic flow deposit crops out in scattered mesas in coastal and central Sonora and
Baja California, México. Correlated in Baja California by Stock et al. (1999) and on Tiburon Island and coastal
Sonora by Oskin (e.g., Oskin, 2002; Oskin & Stock 2003).
™ 40Ar /39Ar geochronology on sanidine and U-Pb geochronology on zircon crystal found within the tuff date
the unit at around 12-12.5 Ma (Vidal-Solano et al., 2005; Vidal-Solano et al., 2007a; Vidal-Solano et al.,
2007b).
™Unit exposed over a distance of at least 430 km; it is a key horizon in the reconstruction of the Gulf
Extensional Province because it was erupted at the time of the onset of extension in part of the Gulf of
California (Stock et al., 1999).
NORTH AMERICA
C
IAG
PACIFIC
Ballenas Transform Fault
Cataviña
Ignimbrites form flat-topped mesas
overlying conglomerates above the
granitic rocks
Figure 2: Tie points on opposite
sides of the Gulf of California
(Oskin & Stock, 2003).
™Crystal-rich, strongly indurated, lithic-lapilli pyroclastic flow
deposit with black basal vitrophyre, grading down into a narrow
zone of brown, moderately welded tuff, and grading up into a red
spherulitic horizon followed by a zone of densely welded magenta
colored cliff forming ignimbrite (Stock et al., 1999).
™ It has 5-15% alkali feldspars and can be up to 180 m thick in
some locations near the vent (Stock et al., 1999).
™Distinctive mineralogical association with fayalite, Fe-rich augite,
alkali feldspar phenocrysts and zircon as a common trace mineral
(Vidal-Solano et al., 2008).
™Unit has unique, low-inclination, reverse magnetization vital for
correlation of tuff along the region (Stock et al, 1999).
Lower hemisphere projection of POR-08 demagnetization vectors
between 300 mT and the origin, for 8 cores. Because a strong and
anomalous NRM was removed by the earlier demagnetization steps,
more samples need to be studied to get reliable uncertainties for this
location.
N=9
Decl = -242.2; Incl=-9.5
α95=3.6
Isla Angel de la Guarda
Conclusions
Trachyte inclusion
Trachyte inclusion
Trachyte inclusion
Matrix
Figure 1: Map of the Sonora and Baja California region of Mexico,
separated by the Pacific-North America plate boundary in the Gulf of
California. Black dots: outcrops of the Tuff of San Felipe previously
reported. Red dots: newly correlated locations. C= Cataviña; IAG= Isla
Angel de la Guarda.
IAG
Tilt-corrected vectors
of paleomagnetic
remanence, for
sample IAG-07-44,
from Isla Angel de la
Guarda, Gulf of
California, México.
Sum of the probabilities of
the ages of Tuff of San
Felipe, coastal Baja
California
The paleomagnetic remanence direction of the ignimbrite discovered on Isla
Angel de la Guarda, and that exposed in mesas near Cataviña, Baja California,
is nearly horizontal towards the SSW. This is similar to the direction found in
the Tuff of San Felipe in Baja California and coastal Sonora. Geochemical
analyses from Isla Angel de la Guarda, geochronology from the same location,
and field observations from both regions suggest that these two localities
belong to a previously unrecognized lobe of pyroclastic density current
deposits (ignimbrite) from the ca. 12.5 Ma eruption of the Tuff of San Felipe.
This lobe of the eruption appears to have filled a depression with thin
conglomerates overlying basement granitic rocks. The southern edge of the
exposures in both locations are offset ca. 130 km parallel to the strike of the
active Ballenas Transform Fault, giving a maximum of 130 km for the net slip
on this fault. This is less than the ca. 300 km of net post-6.3 Ma slip in the
Northern Gulf of California because the Ballenas Transform Fault is a relatively
young structure; it formed in Pliocene time, due to a plate boundary
reorganization and cessation of activity on the Tiburon Fracture Zone.
Acknowledgments:
This research was supported by the U.S. National Science Foundation under Grant No. EAR-0610011 and by Consejo Nacional de Ciencias
y Tecnología, Mexico, under Project CONACyT P 46600- F. Gianna Hernandez-Mendez assisted with paleomagnetic measurements.
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