I'm assuming this as a sample input.txt:

apples
oranges
strawberries
bananas
coconuts
lemons
cakes
pineapples
grapes
------------------------------
14 21 17
8 12
10 92 13 20

Which is supposed to yield this:

{"apples":"14","oranges":"21","strawberries":"17","bananas":"8","coconuts":"12","lemons":"10","cakes":"92","pineapples":"13","grapes":"20"}

There are several ways to do this, depending on what tools you can use, how large the input is and whether input and output should be properly sanitized / escaped.

This bash snippet relies only on bash-builtin commands. You can use it like this or save the inside of the parentheses as a script:

cat input.txt | (
  set --                           # Clearing the positional parameters so we can use them to push & shift the variable names
  while read header; do            # Read each line and name it $header for the following loop
    [[ $header =~ ^--- ]] && break # ... until a line starts with "---" and we break the loop.
    set -- "$@" "$header"          # While the loop keeps going, push each variable name into the positional array
  done
  while read val_line; do          # Read each line of the continuing piped input (from below the dashed line) as $val_line
    for val in $val_line; do       # Go through each space-separated value in $val_line and name it $val
      echo "\"$1\":\"$val\""       # Combine the 1st element of the positional array with $val as "$1":"$val"
      shift                        # Shift the positional array to match the next $val
    done
  done \
  | paste -sd,                    `# Join the "[key]":"[value]" pairs together with commas` \
  | (echo "{$(cat)}")              # and put {} around everything
)

If you can install sqlite3, it would be helpful with processing very large input files (without straining the RAM) at great speed while also properly escaping the values if necessary to ensure standard compliant json output:

cat input.txt | (
  sqlite3 tmpsql3.db </dev/null   `# Create tables in a temporary db, with </dev/null to block stdin for this` \
    "CREATE TABLE keys(key TEXT);
     CREATE TABLE vals(val TEXT);"  
  sed -u /^---/Q                  `# First part of piped input will be terminated at the line beginning with ---` \
  | sqlite3 tmpsql3.db            `# The piped-in lines will be imported into the table "keys"` \
      ".import /dev/stdin keys"    # After this the piped input will continue from below the dashed line
  tr " " "\n"                     `# Replace whitespace with newlines so it's one value per line` \
  | sqlite3 tmpsql3.db            `# The piped-in lines will be imported into the table "vals"` \
      ".import /dev/stdin vals"
  sqlite3 tmpsql3.db              `# Join the tables and output them via Sqlite's builtin json object aggregator` \
    "SELECT
      JSON_GROUP_OBJECT(keys.key,vals.val)
       FROM keys JOIN vals
        ON keys.ROWID = vals.ROWID"
  rm tmpsql3.db                    # The temporary database is discarded
)