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Ancient transforms

Ancient transforms. GEOS425/ 525 20 slides – 40 mins. Ancient systems. Difficult to interpret unless major transcontinental truncations; Not clear how a San Andreas type fault looks at 20 km; The subcretion vs translation controversy;

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Ancient transforms

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  1. Ancient transforms GEOS425/525 20 slides – 40 mins

  2. Ancient systems • Difficult to interpret unless major transcontinental truncations; • Not clear how a San Andreas type fault looks at 20 km; • The subcretion vs translation controversy; • Can we track translation with no uplift markers (WILL NOT DO THIS DUE TO LACK OF TIME)

  3. Tornquist-Teisseyre fault Major suture separating Baltica from mobile Europe Vertical structure imaged across the TESZ - suggests strike slip; Timing: late Cenozoic WHY Amount of slip - unknown, plate configuration- unknown

  4. TESZ in map view

  5. Other ancient transforms • Mojave Sonora megashear • Nacimiento fault Countless of other linear structures- but where they big transform faults at one time ????

  6. Mojave Sonora (Silver and Anderson ca. 1972) • Premise- truncation of the western NA margin; structures that are truncated are pre Cretaceous: • Debate regarding age - Jurassic, some speculate Permian; • Debate regarding offset - left lateral but how much; • Lack of outcrop and obliteration by younger structures and magmatism- a problem; • Overall- one of the most mysterious “necessary” structure in NA geology.

  7. The Nacimiento fault - mega thrust or transform fault? • Separates eastern arc from Franciscan type wedge rocks - about 150 km (width) of subduction zone rocks are missing- fairly large structure; • Age of structure - not known but has to be pre - latest Cretaceous - Cenozoic seds cover both sides of the fault (seal) • Fundamental question- where are the missing rocks? Were they sliced by a strike slip fault or were they subducted?

  8. Discrepancies in interpreting the Nacimiento fault • Hall, 1991, considers it the Sur-Nacimiento, which is interpreted as the main thrust fault over which the so Cal “allochton” moved westward during the laramide orogeny; • Dickinson, 1982, makes a distinction between Sur and Nacimiento faults and considers the NF a major left lateral transform fault (500 mi displacement) that was active in the late K, prior to the onset of the Laramide orogeny.

  9. The fault has been reactivated during the Cenozoic, no question. Both of these hypotheses explain the out-of-place location of these rocks even after a pre-San Andreas palinspastic reconstruction has been applied; Dickinson states the geologic obvious that is the fault looks strike slip, Hall sees the other obvious - these rocks are floating over franciscan.

  10. Nacimiento fault

  11. Brittle fault, sense of shear inconsistent

  12. 500 miles of left lateral displacement, huh?????? All we see is a pile of messed up (I.e. brittle deformation) rocks. And 150 km (width) of missing margin,…..

  13. Strike slip horse or klippe????

  14. Significance • Present day continental margins are dominated by transform regimes or oblique subduction; • There is a 16% probability that under similar circumstances, margin parallel transport would be on the scale of >400-500 km. • Therefore…. Assembly of continents by lateral or oblique transport is probably very significant throughout the evolution of the continents.

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