Transcript PPT

Extent of new, non-oceanic crust in
the northern Gulf of California:
Constraints and Controversies
Joann Stock
Caltech
Key structural characteristics of the N Gulf of California &
Salton Trough
(simplified)
•Similar amounts of net plate
displacement (post-6 Ma) from N to S
(pole of opening far away, and mainly
a transform boundary); ~300 km in
northern basins and Guaymas Basin
•Oceanic crust present in different
amounts in different basins; not simply
related to the net amount of extension
•Extension is less localized in the N
Gulf basins, despite similar amounts of
plate boundary displacement to the N
and to the S
Q. What is the difference between “oceanic” and
“non-oceanic” new crust?
A.Thickness; density; magnetic anomalies; pattern
of active deformation & volcanism. i.e.,
transitional crust
B. Origin: crustal area formed by some process
other than localized seafloor spreading; needs to
be accounted for & removed in order to
reconstruct the pre-rift configuration
Unusual characteristics of N Gulf Basins
Too thick for typical oceanic crust
Gravity modeling – density too low (not
enough basalt in it)
Shallow layers are mostly sediments – not
much magma, no pillow basalts
No lineated magnetic anomalies typical of
oceanic crust
Lower crustal Vp is slow (compared to
oceanic profiles)
Use the geological constraints of the conjugate rifted margins
Volcanic &
sedimentary
stratigraphy of the
margins of the
Upper Delfin Basin
Volcanic sequences of the region
Tuffs of Los Heme
3.3-2.7 Ma
Tuffs of the Northern
Puertecitos Volcanic
Province 6.4-6.1 Ma
Tuff of San Felipe 12.5 Ma
Before 6.1 Ma, all volcanic rocks deposited on land – no evidence for ocean here
Paleomagnetic remanence directions  clockwise
rotations of fault blocks in Baja California
Vector típico para
el mioceno
All of the vectors point SW and slightly up; the tuff
became magnetized during a field excursion or a
field transition of the earth’s magnetic field.
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Red & black dots: regions of outcrops of the ca. 12.5 Ma
Tuff of San Felipe
Conjugate margins of a single basin system (Upper Delfin)
Tie points
Tie points
J. Stock
Summary
reconstructions:
255 +/- 10 km
opening of the
Upper Delfin
Basin, Gulf of
California, since
6.1 Ma (purple
color)
Almost everything
in between the
modern coastlines
would be post-6.1
Ma new crustal
area!!
This figure is from Mike Oskin; published in Oskin & Stock GSAB 2003
Rift margin geology  tight constraints on timing &
amount of rifting
Does this result agree with other data sets?
J. Stock
Gonzalez-Fernandez et al.,
JGR 2005 –
Proyecto Cortes seismic line
(refraction and wide-angle
reflection using OBS and onland stations)
Transect across the Upper
Delfin Basin – Upper
Tiburon basin segment
(slightly oblique to opening
direction)
South Coast Geological Society
Proyecto Cortes transect
gravity
seismic ray paths
Crustal
structure model
Upper Delfin Basin
Upper Tiburon Basin
From Gonzalez-Fernandez et
al., JGR 2005
Magmatic rift;
crust 7-8 km
thick
Wide,
amagmatic;
crust 5.5 km
thick
Segment
includes Rivera
plate; crust 5
km thick
South Coast Geological Society
Arrows are continent-ocean transition (from Lizarralde et al., 2007)
MODELS OF TRANSITIONAL CRUST IN RIFT SYSTEMS
N. Gulf of
California
model
Comparison of the geophysical profiles
from Gonzalez-Fernandez and Lizarralde
New Salton Trough figure (Rymer)
Old Salton Trough density interpretation
(Fuis et al)
J. Stock
Schmitt & Vazquez (EPSL, 2006) model for formation of transitional crust
in the Salton Trough
Analogous regions?
Northern Ross Sea, Antarctica:
Tertiary spreading system between
E and W Antarctica (Adare Trough)
trends into Northern Basin (NB)
J. Stock
Japan Sea – distributed
extension connected to ~200
km of dextral offset on Yangsan
fault
PLATE
BOUNDARY
SLIP BUDGET
vs.
GEOLOGICAL
OFFSETS
Geological offset compared to PAC-NA plate motion since 6 Ma (Oskin
&Stock GSAB 2003)
Net percent of
extension can be
extremely difficult to
estimate using
seismic stratigraphy
in highly extended
basins
From Reston, Geology 2007
J. Stock
Delfin Basin
Tiburon Basin
From Aragon-Arreola 2006 PhD thesis
(CICESE); interpretation of PEMEX
MCS data set
•Condensed section on inter-basin high
•Old rift sediments preserved at
drillable depths (< 3 sec TWTT)
•Rift basement accessible to drilling on
inter-basin high
Timing & Geometry of Marine
Deposition
Earliest marine sedimentary rocks ca. 7 Ma near Santa
Rosalia, 6 to 6.5 Ma near San Felipe
Reports of older (middle Miocene) microfossils in cutting
from PEMEX drill holes in the N. Gulf (Helenes and
Carreno)
PEMEX seismic sections (Aragon-Arreola et al.) show older,
abandoned basins to the E in many segments; basins
have “jumped” westward
Timing of Faulting
West (Baja California)
Some faulting starts around 12
Ma
Time of initiation of faulting varies
from place to place
Rotations about vertical axes are
very important in NE Baja
California
Change from orthogonal
(“Protogulf stage”) to oblique
transtension ca. 6 Ma in many
models (e.g. Umhoefer et al.
GSAB 2002)
East (Sonora to Sinaloa to
Nayarit)
Core complexes e.g. 20-17 Ma
Sierra Mazatan
Regional tilting of pre- and post12.5 Ma units, in coastal Sonora
Faulting < 20 Ma in coastal
Sinaloa
Normal faulting 17-10 Ma in
Nayarit (S. Gulf) (see Ferrari et al.
2005 Bol Soc Geol Mex)
Important results
 Nearly all of the submarine region of the N Gulf basins
may be new crustal area (non-oceanic)
 Timing of formation of the “new” crust agrees well with
the plate motions but not with some microfossil
stratigraphy
 The type & amount of “new” crust varies in different
segments of the plate boundary -- not systematic
 Useful to keep in mind when interpreting other rift
systems