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Motagua-Polochic Fault Zone

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  • Manuel Iturralde
    Dear colleagues We had an interesting field trip to Guatemala, well attended, and we learn a lot about that region. We will prepare a resume of some
    Message 1 of 6 , Feb 4, 2002
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      Motagua-Polochic Fault Zone Dear colleagues
      We had an interesting field trip to Guatemala, well attended, and we learn a lot about that region. We will prepare a resume of some observations later.


      By the way, I have received this information from Keith Christensen (brysen@...) and hope that someone can help him in this concerns.
      I was looking for information on the Motagua Fault Zone and just came
      across your site giving notification of this meeting from January 28-31.
      Since you are the project leader I assume that you would know of any
      current information. I am looking for sanswer to some questions
      regarding the Zone. But first I'll give a little background for the
      questions. You probably know all or much of what I say at this point,
      but just to make certain, I'll summarize.
      I was in the Izabal Basin and Motagua valley several years ago so I know
      the area first hand and my questions are not just academic. My interest
      centers on historical aspects of pre-Spanish Central America.
      Dr. Payson Sheets published findings on a "massive" geologic event at
      "about the time of Christ" that involved much of Central America and
      created Lago de Ilopango in El Salvador. (Dr. Payson Sheets, "Ilopango
      Volcano and the Maya Protoclassic, 1975 field season, Dept. of
      Anthropology, Univ. of Colorado). Dr. Sheets later ammended his age
      determination to 260 AD +/- 100 years, the date now typically cited for
      this event.

      Earlier Dr. Barbara Voorhies did field work in the Izabal Basin and
      concluded that the basin was not occupied until shortly before the time
      of Christ. (Doctoral dissertation, Yale Univ., Anthropology Dept., 1969)
      You mention that the zone is "characterised by remarkanble trending
      uplift structures and pull-apart basins." And USGS Bulletin 1034 maps
      show this. Yet I've been told that some consider the Izabal basin in
      this region to be in subsidence.

      My questions acknowledge four things.
      1. The MSZ would have moved up or down at various times and to various
      degrees in the distant past so my questions have to do with the most
      recent uplift.
      2. The channel for the Rio Dulce appears to be a fault that has pulled
      apart, at least in its upper reaches.
      3. There was the great geologic event noted at Ilopango "about the time
      of Christ" that affected much of Mesoamerica.
      4. There was evidently no occupancy in the Izabal Basin until near the
      time of Christ.
      Now my questions.
      A. Do you know of anything that indicates that when the MSZ was lower
      the Gulf of Honduras would have extended into the Izabal Basin and the
      most recent uplift of the MSZ was relatively abrupt, taking place in the
      same "massive" event "about the time of Christ" that extended throughout
      Mesoamerica and rather abruptly created Lago de Ilopango?
      B. Since there was evidently no occupancy of the Izabal Basin until near
      the time of Christ, do you know of anything that indicates that the
      uplift of the zone would have expelled the sea from the Izabal basin at
      that time, allowing occupancy in the basin and leaving dregs of the sea
      which have since been flushed out by rivers to create Lago de Izabal?
      C. Given the characteristics of the Rio Dulce channel, is there anything
      that indicates it is a fault that opened up in the most recent uplift,
      draining the sea from the basin?
      D. You mention the uplift of the zone as "W-E trending." Am I correct in
      assuming that you are saying the greatest uplift is to the east, at the
      Gulf of Honduras coast where there are exposed coral reefs?
      E. Can the Izabal Basin be a graben which is technically a subsidence
      basin and yet be uplifted relative to surrounding terrain?
      The information that I have is fairly old and I don't know what has come
      out in recent years. And probably more will come as a result of your
      meeting and field trip which would likely be helpful.
      I would appreciate hearing from you when possible.
      Keith Christensen
      brysen@...

    • josh rosenfeld
      Motagua-Polochic Fault ZoneKeith, To address your questions, the Ilopango event referred to by Payson Sheets was a major volcanic eruption that buried a large
      Message 2 of 6 , Feb 4, 2002
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        Motagua-Polochic Fault Zone
        Keith,
        To address your questions, the Ilopango event referred to by Payson Sheets was a major volcanic eruption that buried a large part of the southern Mayan area with ash.  The Mt. St. Helen's event is comparable.  This is quite different from the tectonic events that formed, and continue to affect the Izabal Basin.  These forces have operated over the last 20 million years and the spectacular topography and deformation of the rocks in and around the basin are the accumulated result of very gradual processes measured in centimeters per year
         
        The Izabal Basin, being very close to sea level, has also been affected by sea level fluctuations.  The melting of the last great ice sheets between 18,000 and 11,000 years ago caused a rise in sea level of about 120 meters.  It is safe to say that the Izabal Basin, with a present day maximum lake depth of only 15 meters, was a broad, flat valley during the glacial sea level lowstand, with the Polochic River emptying into the sea far to the east.  Sea level actually reached several meters (5?) higher than today at maximum glacial melting.  Therefore, it is conceivable that the floor of the basin was completely covered with water until ~2,000 years ago, as you mention.  However, the area around the lake must have been inhabited.  
         
        The upper Rio Dulce between Lake Izabal and the Golfete follows the main trace of the Polochic left lateral strike slip fault.  The lower part of the Rio Dulce, characterized by entrenched meanders cut through the limestones of the gorge between the Golfete and the sea, does not follow the fault which turns to the east near the east end of the Golfete.  This "restraining bend" is responsible for the uplift of Cerro San Gil and the westward tilt of the entire sedimentary basin-fill at the eastern end of the basin.  Interestingly, oil seeps are documented from these upturned beds indicating that a petroleum system is operative in the basin.
         
        Relating the Ilopango event to the Izabal Basin is not warrented by our knowledge of the geological processes involved.  Again, Ilopango was a catastrophe that in a few days nearly wiped out a civilization, while the Izabal Basin has been deforming, as it is today, at a nearly imperceptible pace as measured by human lifetimes.
         
        Best of luck in your investigations,
        Josh Rosenfeld
                    
        ----- Original Message -----
        Sent: Monday, February 04, 2002 11:17 AM
        Subject: [carib] Motagua-Polochic Fault Zone

        Dear colleagues
        We had an interesting field trip to Guatemala, well attended, and we learn a lot about that region. We will prepare a resume of some observations later.


        By the way, I have received this information from Keith Christensen (brysen@...) and hope that someone can help him in this concerns.
        I was looking for information on the Motagua Fault Zone and just came
        across your site giving notification of this meeting from January 28-31.
        Since you are the project leader I assume that you would know of any
        current information. I am looking for sanswer to some questions
        regarding the Zone. But first I'll give a little background for the
        questions. You probably know all or much of what I say at this point,
        but just to make certain, I'll summarize.
        I was in the Izabal Basin and Motagua valley several years ago so I know
        the area first hand and my questions are not just academic. My interest
        centers on historical aspects of pre-Spanish Central America.
        Dr. Payson Sheets published findings on a "massive" geologic event at
        "about the time of Christ" that involved much of Central America and
        created Lago de Ilopango in El Salvador. (Dr. Payson Sheets, "Ilopango
        Volcano and the Maya Protoclassic, 1975 field season, Dept. of
        Anthropology, Univ. of Colorado). Dr. Sheets later ammended his age
        determination to 260 AD +/- 100 years, the date now typically cited for
        this event.

        Earlier Dr. Barbara Voorhies did field work in the Izabal Basin and
        concluded that the basin was not occupied until shortly before the time
        of Christ. (Doctoral dissertation, Yale Univ., Anthropology Dept., 1969)
        You mention that the zone is "characterised by remarkanble trending
        uplift structures and pull-apart basins." And USGS Bulletin 1034 maps
        show this. Yet I've been told that some consider the Izabal basin in
        this region to be in subsidence.

        My questions acknowledge four things.
        1. The MSZ would have moved up or down at various times and to various
        degrees in the distant past so my questions have to do with the most
        recent uplift.
        2. The channel for the Rio Dulce appears to be a fault that has pulled
        apart, at least in its upper reaches.
        3. There was the great geologic event noted at Ilopango "about the time
        of Christ" that affected much of Mesoamerica.
        4. There was evidently no occupancy in the Izabal Basin until near the
        time of Christ.
        Now my questions.
        A. Do you know of anything that indicates that when the MSZ was lower
        the Gulf of Honduras would have extended into the Izabal Basin and the
        most recent uplift of the MSZ was relatively abrupt, taking place in the
        same "massive" event "about the time of Christ" that extended throughout
        Mesoamerica and rather abruptly created Lago de Ilopango?
        B. Since there was evidently no occupancy of the Izabal Basin until near
        the time of Christ, do you know of anything that indicates that the
        uplift of the zone would have expelled the sea from the Izabal basin at
        that time, allowing occupancy in the basin and leaving dregs of the sea
        which have since been flushed out by rivers to create Lago de Izabal?
        C. Given the characteristics of the Rio Dulce channel, is there anything
        that indicates it is a fault that opened up in the most recent uplift,
        draining the sea from the basin?
        D. You mention the uplift of the zone as "W-E trending." Am I correct in
        assuming that you are saying the greatest uplift is to the east, at the
        Gulf of Honduras coast where there are exposed coral reefs?
        E. Can the Izabal Basin be a graben which is technically a subsidence
        basin and yet be uplifted relative to surrounding terrain?
        The information that I have is fairly old and I don't know what has come
        out in recent years. And probably more will come as a result of your
        meeting and field trip which would likely be helpful.
        I would appreciate hearing from you when possible.
        Keith Christensen
        brysen@...



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      • blady9@aol.com
        Dear Josh: Thank you vey much for your interesting comments. I wonder if you would care to comment on the occurrence of the Grenville Complex which outcrops NE
        Message 3 of 6 , Feb 5, 2002
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          Dear Josh:

          Thank you vey much for your interesting comments. I wonder if you would care to comment on the occurrence of the Grenville Complex which outcrops NE of El Progresso, Honduras, as well as in Oaxaca and Chiapas?

          Bill Lady


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        • josh rosenfeld
          Bill, I m not that knowledgable of Honduras, but others who read this page certainly are. As a quick answer, Honduras, as part of the Chortis Block, was
          Message 4 of 6 , Feb 5, 2002
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            Bill,
            I'm not that knowledgable of Honduras, but others who read this page certainly are.  As a quick answer, Honduras, as part of the Chortis Block, was attached to southwestern Mexico before sliding across the southern Yucatan (Maya) Block in an ongoing left-lateral sense since the Late Cretaceous.  The details are still a bit murky but I believe this will addressed in detail in a soon-to-be-released paper in an AAPG Memoir by Burke Burkart.
            Best wishes,
            Josh 
             
          • Agustin Cardona
            Dear Bill, in relation with you question of the Grenville exposures along Honduras and Mexico, extensive work and discussions by Mexican workers
            Message 5 of 6 , Feb 5, 2002
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              Dear Bill, in relation with you question of the
              Grenville exposures along Honduras and Mexico,
              extensive work and discussions by Mexican workers
              (Ortega-Gutierrez, Keppie and others) as well as
              Arizona group (Ruiz and others) have considered this
              exposures as an alloctonous terrane that may have been
              accreted to NW Amazon Craton or southernmost Arequipa
              region in peru during Grenville times (~990MA) as a
              Microcontinent or as and Island Arc, its suggested
              thats its main arc magmatic event between 1250 and
              ~1150 is not find in Laurentia Grenville Province so
              its outboard Laurentia genesis is supposed, also early
              Paleozoic covers match with a South America position,
              finally its Pb and Nd isotopic constrains seems to
              correlated with Colombian Garzon-Santa Marta Massifs
              and are not exactly matched with the Amazon Craton,
              altough this could be discussed.
              Altough its possible that this exposures may represent
              a single unit, it may also be a composite terrane as
              suggest by Pb isotopics (Ruiz et al., 1999) that
              define some difference north and SOuth of the
              TransMexican volcanic belt, what seems more acceptable
              is its alloctonous character with respect with
              Laurentia and its probabble SOuth America source. Its
              subsequent accretion or transfer to Laurentia seems to
              be a Late Paleozoic event.
              Manton (1996) als suggest based on the character of
              the Honduras Grenville exposures and is strong
              correlation with the Mexican exposures that thier
              relation in an strong element for the analysis of the
              amount of strike-slipe motion of the Montagua-Polochic
              fault system.
              The best
              Agustin Cardona

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            • blady9@aol.com
              Dear Augustin: This is fascinating information, and I appreciate the time you have taken to respond. A geochemical exploration crew under my supervision found
              Message 6 of 6 , Feb 5, 2002
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                Dear Augustin:

                This is fascinating information, and I appreciate the time you have taken to respond. A geochemical exploration crew under my supervision found the Honduran exposure during the 1960's, but we did not recognize what it was at that time. The exposures in Oaxaca and Chiapas do seem very similar. I suspect there may be another exposure in the Sierra de Omoa, in Honduras, truncated by the Sula Valley graben, but I have not yet confirmed this. We did not cover that area during our regional explorations. Aside from this possibility, I haven't seen other Grenvillian exposure anywhere in the country.

                My very best wishes to you,

                Bill Lady

                Subj: Re: [carib] Motagua-Polochic Fault Zone (Grenvillian Exposures)
                Date: 2/5/02 10:11:15 AM Eastern Standard Time
                From: aguscm@...
                Reply-to: carib@yahoogroups.com
                To: carib@yahoogroups.com
                Sent from the Internet (Details)



                Dear Bill, in relation with you question of the
                Grenville exposures along Honduras and Mexico,
                extensive work and discussions by Mexican workers
                (Ortega-Gutierrez, Keppie and others) as well as
                Arizona group (Ruiz and others) have considered this
                exposures as an alloctonous terrane that may have been
                accreted to NW Amazon Craton or southernmost Arequipa
                region in peru during Grenville times (~990MA) as a
                Microcontinent or as and Island Arc, its suggested
                thats its main arc magmatic event between 1250 and
                ~1150 is not find in Laurentia Grenville Province so
                its outboard Laurentia genesis is supposed, also early
                Paleozoic covers match with a South America position,
                finally its Pb and Nd isotopic constrains seems to
                correlated with Colombian Garzon-Santa Marta Massifs
                and are not exactly matched with the Amazon Craton,
                altough this could be discussed.
                Altough its possible that this exposures may represent
                a single unit, it may also be a composite terrane as
                suggest by Pb isotopics (Ruiz et al., 1999) that
                define some difference north and SOuth of the
                TransMexican volcanic belt, what seems more acceptable
                is its alloctonous character with respect with
                Laurentia and its probabble SOuth America source. Its
                subsequent accretion or transfer to Laurentia seems to
                be a Late Paleozoic event.
                Manton  (1996) als suggest based on the character of
                the  Honduras Grenville exposures and is strong
                correlation with the Mexican exposures that thier
                relation in an strong element for the analysis of the
                amount of strike-slipe motion of the Montagua-Polochic
                fault system.
                The best
                Agustin Cardona




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