Dear. Munazzan. 

I read that:

Volcanic activity in the western United States today is almost non-existent. However, around 25 to 40 million years ago, an event termed the Mid-Tertiary Ignimbrite Flare-Up produced a massive amount of volcanic ash and lava. Many questions arise from this event, such as where did the volcanic rock come from, what was the journey like to the surface, and what forces within the earth were important to allow this event to happen.

By looking at the chemical structure of the volcanic rocks, earth scientists can estimate the composition of the original "parent" material. The parent material for the flare-up event probably originated at depths of a few to several tens of miles below the surface. Extra heat from an unknown source may have caused the rock to melt a bit, and this liquid rock called magma began to rise toward the surface. Another way to make rock melt is to bring it closer to the surface. As the liquid rock rose toward the surface, the liquid interacted with the rock it was passing through, acquiring and releasing elements. Before the liquid rock reached the surface, the liquid would be held in large subsurface chambers where solid crystals had time to form from the liquid. At the end of the journey, the liquid rock was erupted onto the surface through a large volcano and quickly chilled into solid rock. The San Juan volcanic field in southwestern Colorado was made by several of these volcanoes. Some liquid rock flowed out of the volcano in the form of lava flows. Other high temperature material was pulverized as it blew out of the volcano and then became welded together upon cooling - this is an ignmibrite rock.

Forces similar to the ones that move the earth's surface around today may have played a role in the flare-up event. Prior to the flare-up, a mountain-building event had just occurred in the western United States and volcanoes had been active along the West Coast. The rocks that were melted to produce the volcanoes along the West Coast may have also produced the volcanoes in the flare-up event. Understanding the forces in the earth responsible for the flare-up event is still a challenge.

Dear. Munazzan. I found this paper anexed.

Read it. Ok. 

Good Luck

Valter Vieira

Hi Valter,

thanks,

Munazzam Ali

Hi Munazzam Ali.

Good morning. Now in Brazil is 9:35 PM. Ok.

I tried to help you. Ok

Are you satisfied with my answer? 

Valter Vieira

25 to 40 ma straddles the time the NA plate overrode the E. Pacific Rise, creating the San Andres Fault and changing the stress regime in western N. America from compressional to extensional, as well as ending subduction south of what is now the Juan de Fuca plate.  The overall change in stress regime is thought by some to be one of the factors responsible for the  ignimbrite flareup, and also related to basin-and-range extension.  As the NA plate approached the EP rise, the angle of subduction is thought to have become more shallow, which may have had the effect of moving centers of magma generation eastward, toward the interior.  Finally, the change in stress regime is broadly associated with a change in chemistry, from Andesitic to more bimodal Basatic/Rhyolitic.

Thanks Kavin,

But, I am not sure if a ridge subduction would have anything for initiating SAF, Also, I do not understand if the flat slab can cause extension in the interior? In other words, a flat slab geometry, instead provide the support for overlying crust.

Hi Valter,

Of course, yes the article you attached is very relevant and helpful.

Thanks,

Munazzam Ali

Dear Munazam Ali. Good afternoon.

What is your Thinking about this problem?

Do you have some solution about The orgin of ignimbrite flare up in united states?

My bests regards

Valter Vieira

Q: "I am not sure if a ridge subduction would have anything for initiating SAF?"

A: Add the vector motions of both NA and P plates before and after the ridge subduction for insight into SAF initiation, because the collision was oblique ...

Q: " Also, I do not understand if the flat slab can cause extension in the interior?"

A: Suppose the subducting slab is dipping at a steep angle relative to the overlying NA plate and that partial melting of the slab occurs at depth "x". If the dip of the subducting slab becomes less steep as the NA plate approaches the rise, but depth "x" remains the same, geometry dictates the region of magma generation migrates eastward, into the interior.

Angle of the dipping slab can be attributed to factors as simple as density. Further from the ridge, the subducting oceanic crust is older, colder, and more dense. Closer to the ridge, the slab is younger, warmer, and less dense.

Before the collision with the rise, the stress regime was compressional, which favored trapping magma in the crust. After the collision, extension results from the change in the stress field plus buoyancy of the "pre-loaded" magma reservoirs. Extension creates pathways to the surface. Its not all plate tectonics, some "old-fashioned" ideas of buoyancy and isostacy are at play.

This is probably too simplistic, but it may give you ideas of how to improve the explanation.  Good luck.

Hi Kevin,

Interesting, Thanks for explaining the things

 Dr. kevin. Very Good your anwser.

It is very difficult to prevent what will occur in a ride subduction.

As you said. There are many factors to considerate.

Thanks for the discussions of this theme. 

Valter Vieira

Valter,  Because you are at U.T. El Paso, you should be able to go out and find the change from andesitic to basaltic rocks in outcrop in the Trans-Pecos Volcanic Field, only 100 miles or so east of El Paso.  (Suggest you do this before summer arrives ...)

Mr. Kevin Chesser. I am at Belo Horizonte city, Brazil, not at U.T. El Paso.

My mistake.  If you ever visit Texas, U.S.A. then.  I am sure something equivalent exists in S. America.

Ok my friend. But is a pleasure to have a talk with you. OK 

Yes. I recently studied ignimbrite at Espírito Santo State, Brazil.

I coordenate the studies for the Geologic Map  of  Espírito Santo State, scale 1:400.000 and the ignimbrites occurs at the NW of Espírito Santo Basin, denominated by Petrobras.

I will attach the geologic map for you see the local of the occurrences of the ignimbrites onshore ( they ocurrs offshore, too). OK.

This map wa a published in May, 2014 at VI Brazilian Symposium on Mineral Exploration, by Brazil Geological Survey- CPRM.  

This ignimbrites onshore are the extension of the Abrolhos vulcanism. Ok.

See the NE portion of this geologic map ( portion onshore of the Espírito Santo Basin). See the legend ( Espírito Santo Basin - Abrolhos Formation).

Thanks