Hi so I have some questions for a review guide that I’m not 100% certain on. I’m

Question

Hi so I have some questions for a review guide that I’m not 100% certain on. I’m working on it now.
1) geologic map interpretation: How do outcrop patterns show the orientation of dipping rock units?
2) Proterozoic construction of the core of Laurentia: How could you tell the polarity of a past subduction zone?

Thanks Hi so I have some questions for a review guide that I’m not 100% certain on. I’m working on it now.
1) geologic map interpretation: How do outcrop patterns show the orientation of dipping rock units?
2) Proterozoic construction of the core of Laurentia: How could you tell the polarity of a past subduction zone?

Thanks
1) geologic map interpretation: How do outcrop patterns show the orientation of dipping rock units?
2) Proterozoic construction of the core of Laurentia: How could you tell the polarity of a past subduction zone?

Thanks

Explanation / Answer

1)Outcrop patterns usually represent the intersection of the 3-dimensional shape of the rock with the land surface. Where the rock units are flat but the land is not, then the boundaries will outcrop along topographic contour lines. Outcrop patterns also provide information on the dip direction of simple sedimentary units. Vertical beds can outcrop only as straight lines but when the rocks are dipping, they outcrop along lines that do not follow the contours, nor are they straight lines. They then mark the intersection of the shape of the boundaries between beds, with the landscape surface in 3-dimension. Gently dipping beds outcrop in patterns that are almost the same as topographic contours, but the bed boundaries occasionally cross them while steeply dipping beds outcrop in lines that are nearly close to straight, cutting across many topographic contours. The shape of the outcrop can also help us to access the dip of the bed. Also, we can understand the direction of dip from the outcrop if we know the relative ahes of the rock units. This is because in an unfolded and undeformed area, the dip direction is generally from older to younger. The outcrop patterns change in response to changes in dip, thickness and the topography of the land surface.

2) Subduction zone polarity in ancient arc-systems, in some instances, may be deduced from compositional polarity of igneous rocks in the arc, asymmetry of nappes and thrusts along the collision boundaries (directed towards the converging plate) and zonation of metamorphic belts (increasing Pressure -Temperature assemblages in dip direction of the descending slab).

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