These are skin wrinkles specifically caused by sun damage. They are deeper and thicker than typical age-related wrinkles.

http://www.skinsight.com/adult/cutisRhomboidalisNuchae.htm
Cutis rhomboidalis nuchae refers to the manifestation of long-term, prolonged sun exposure and resultant damage to the skin occurring on the back of the neck. Long-term, chronic sun exposure causes thickening of the most superficial layer of the skin (the epidermis) and abnormalities in the composition of the middle layer of the skin (the dermis).

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Note added at 13 mins (2011-01-26 02:13:21 GMT)
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http://blog.sumiyoshi-clinic.com/?eid=787587
菱形皮膚（りょうけい ひふ）」とは、光老化に伴う皮膚の慢性障害の一種です。
Apparently, 菱形皮膚 = "glyphic wrinkles." "Cutis rhomboidalis nuchae" is a particular subcategory of glyphic wrinkles (項部菱形皮膚). So I'd actually go with "wrinkles (glyphic)," but that might not be the best wording in context.

「項部菱形皮膚（こうぶ りょうけい ひふ）」とも呼ばれ、紫外線の当たりやすい項部（首の後ろ）にのみ現れる症状です。


http://books.google.com/books?id=HtkQURlm4d0C&pg=PA16&lpg=PA...

http://www.abateit.com/wrinkles/
Glyphic wrinkles are wrinkles that we all normally have, but which become accentuated with aging. If you have taken to wearing turtlenecks or scarves to hide those lines on your neck that didn't used to look so bad, you know what a glyphic wrinkle is. These tend to become more pronounced with muscle contraction or with facial expressions.

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Note added at 15 mins (2011-01-26 02:14:53 GMT)
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Sorry, I pasted my note incorrectly.

This is my message, and the Japanese parts are from the sumiyoshi link:

Apparently, 菱形皮膚 = "glyphic wrinkles." "Cutis rhomboidalis nuchae" is a particular subcategory of glyphic wrinkles (項部菱形皮膚). So I'd actually go with "wrinkles (glyphic)," but that might not be the best wording in context.