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SDAG Monthly Meeting
Wednesday, August 17, 2016

Location: Andres Restaurant
1241 Morena Blvd
San Diego, CA 92110
Tel: 619.275.4114

FROM INTERSTATE 5: Take Exit 21 or Sea World Drive/Tecolote Road and head east. Make a right onto Morena Boulevard and head south for approximately 0.2 miles. The restaurant is on the left (or east side of the street) (see the Map).
FROM INTERSTATE 8: Take Exit 2C Morena Boulevard prior to getting to Interstate 5. Make a right onto Morena Boulevard and head north for approximately 0.7 miles. The restaurant will be on the right (or east side of the street) (see the Map).
happy hour
5:30pm -
Social hour  
Cash Bar.

Menu: Pollo a la Plancha (grilled marinated chicken and onions), Ropa Vieja (shredded beef with onions, green peppers & tomatoes), rice, black beans, green salad, with Cash Bar.
6:30pm -

Cost: $30.00 for non-members, $25.00 for members, $15.00 for students and professors. if pre-registered by the deadline, $5 extra if you did not make a reservation. Click the SDAG member checkbox on the reservation form if you are a member.

Reservations: Make your reservation online by clicking the button below no later than NOON, Monday, August 15. RESERVATIONS CANNOT BE ACCEPTED AFTER Monday at noon. Late reservations/cancellations are preferred over walk-ins or no-shows. Fees payable at the meeting or pre-pay with PayPal.
As a new payment option, there will be a phone credit card reader at the meeting.

If you are a current SDAG member and are not getting e-mail announcements,
make sure the SDAG secretary has your correct e-mail address.

7:30pm -

Speaker: Monte Marshall

"The Important Contribution that the Linear Marine Magnetic Anomalies made to the Discovery of Seafloor Spreading, and then to the Neogene Plate Tectonics of Southern and Baja California"


This talk is a special one for me. As time slips more and more quickly by, I realize that there are fewer and fewer geologists remaining whose careers have spanned the great plate tectonic revolution of the late nineteen sixties. And there are even fewer left whose research involved this revolution. Since I am one of them, I want to share with the audience a personal trip down the memory lane of one of the greatest paradigm shifts in earth sciences.

When I was in my last undergraduate year at SDSU, we were taught that Wegener's proposal that the Atlantic Ocean had formed by the spreading apart of the adjacent continents was "stupid". Just three years later, in December of 1968, I was sitting in the audience at the annual meeting of the American Geophysical Union in San Francisco. Marine geophysicists were showing slides of the linear marine magnetic anomalies that they had measured over all the world's oceans. Those squiggly lines, by their symmetry, worldwide sameness, and, most importantly, perfect match to the recently developed geomagnetic reversal timescale, proved without a doubt that the seafloor did spread away from the oceanic ridges and rises and, in the case of the Atlantic Ocean, carried the adjacent continents with them. We all sat there stunned - realizing that Wegener was basically right! Two powerful new words were added to the geologic vocabulary: "plate tectonics". But, it's the whole upper 100 kilometers of the earth, i.e., the plates that move around, not just the continents. This was the part of the story that Wegner had gotten wrong--and that became a stumbling block for most geologists for almost 50 years.

As mentioned above, the most convincing proof for seafloor spreading was the match between the marine magnetic anomaly pattern and the pattern in geologic time of the just-discovered geomagnetic reversals. As the newly created oceanic crust cools and spreads away, it acts just like a double headed tape recorder and forms alternately normally and reversely magnetized strips parallel to the ridges and rises. Because the reversals are random, a true barcode is created. Much of the definition of the geomagnetic reversal time scale (GRTS) for the last 4 My was done in the 1960ies by a team of three USGS scientists in Menlo Park, CA. Allan Cox was the brilliant theoretician, Dick Doell was the master instrumentalist who designed the magnetometers that measured the direction of magnetization in oriented rock cores, and Brent Dalrymple was a pioneer of K-Ar radiometric age dating. Allan Cox left the USGS about 1967 and joined the faculty of the geophysics department at Stanford, and became my thesis advisor. Because he had not yet set up his paleomagnetism lab at Stanford, he made it possible for me to use the facilities at his former USGS lab. In the course of my research I found that the seafloor pillow basalts were much more strongly magnetized than subaerial basalts, and that they are the major source of the marine magnetic anomalies.

The next step was to date and number the squiggles (bars in the code) recorded by the ship-towed magnetometers so that the marine magnetic anomalies could be used to determine when various parts of the Atlantic and other oceans formed. One of the first persons to apply these new insights to the Pacific Ocean was a graduate student at Scripps Institute of Oceanography, Tanya Atwater, in a classic paper in 1970. Using the magnetic anomaly profiles collected by Scripps over many years in the northeastern Pacific off the coast of North America, she was able to show that subduction off our coast ended about 30 Ma in the vicinity of Los Angeles, when a projection of the Pacific-Farallon spreading center came into contact with the North American Plate. This point of contact lengthened into a line/plate boundary with largely strike slip relative motion, until at about 5 Ma the southern end reached Cabo San Lucas/Puerto Vallarta and the Baja Peninsula was rifted from mainland Mexico, and the modern San Andreas system was born. Three years later, she and Peter Molnar showed that there has been about 1000 km of relative motion between the Pacific and North American Plates. Since the current San Andreas has only about 250 km of offset, their 1000 km estimate gave rise to a still-unresolved debate about the other 750 km-offshore faulting, Basin and Range extension, etc.?

The marine magnetic anomaly pattern offshore of the two Californias and in the Gulf has continued to be studied and refined by Tanya and others. Probably the most significant refinement for us is the finding that some of the thin, leading edge of the North American Plate overrode the Pacific Plate. Then, as the Pacific Plate continued to move to the northwest, it rifted and rotated parts of this crustal slab. This is the tectonic model proposed by Atwater and others for the rotation of the Transverse Ranges and the deformation and widening of the former continental margin beginning at about 18 Ma and extending from Point Conception to the Vizcaino Peninsula - which is why it has been given the special name of "continental borderland".

I was born on a pre-plate tectonic day in San Diego, CA. I received my BS in geology and geophysics from SDSU in 1966, and my PhD from Stanford in 1971. My PhD research involved determining what rocks in the oceanic crust cause the marine magnetic anomalies. After several years of research at the USGS in Menlo Park, I returned to San Diego and to SDSU in 1975. I taught courses in structural and petroleum geology, geophysics, and paleomagnetism and plate tectonics - as well as an occasional "rocks for jocks". My research with my students involved using paleomagnetism to study the plate tectonics of the Southwest, and detailed gravity surveys to study faults, especially in the San Diego metropolitan area.

I retired, theoretically, in 2005 and since then I have been heavily involved in geologic community service - giving talks, leading field trips, teaching volunteers at the SD Natural History Museum, writing articles, advising students, etc.. And, on occasion, I travel to other parts of the world to learn about their cultures and rocks! :>)

Upcoming SDAG meetings - 2016

September 21 (may change to 9/14): Peter Gold - Precursor to the SDAG Field Trip - The Agua Blanca Fault

October 22-24: THE 2016 SDAG Field Trip - Northern Baja California

November 16: TBD

Meetings are usually scheduled for the 3rd Wednesday evening of the month. Meeting information on this website is normally updated the second week of the month.

If you have any information, announcements, ads or suggestions for an upcoming newsletter, please submit it to Chris Livesey, (2016 SDAG Secretary). Any news regarding upcoming events that may be of interest to the Association or news of your business can be submitted. The submittal deadline for the next SDAG newsletter is the last Friday of the month.
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